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SpringerBriefs in Earth System Sciences
SpringerBriefs Seaways and Landbridges: Southern

Hemisphere Biogeographic Connections Through Time
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Gerrit Lohmann
Lawrence A. Mysak
Justus Notholt
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Vikram Unnithan
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Susana E. Damborenea •
Javier Echevarría Sonia Ros-Franch

•
Southern Hemisphere
Palaeobiogeography
of Triassic-Jurassic
Marine Bivalves
123
Susana E. Damborenea Sonia Ros-Franch
Departamento Paleontología Invertebrados Departamento Paleontología Invertebrados
Museo de Ciencias Naturales La Plata Museo de Ciencias Naturales La Plata
La Plata La Plata
Argentina Argentina
Javier Echevarría
Departamento Paleontología Invertebrados
Museo de Ciencias Naturales La Plata
La Plata
Argentina
ISSN 2191-589X ISSN 2191-5903 (electronic)

ISBN 978-94-007-5097-5 ISBN 978-94-007-5098-2 (eBook)
DOI 10.1007/978-94-007-5098-2
Springer Dordrecht Heidelberg New York London
Library of Congress Control Number: 2012944966
Ó The Author(s) 2013

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Preface
The study of global biodiversity changes is a strong issue these days, as we

become aware of the fragility of the Earth system and the urgent need to under-
stand it better to keep it working. One of the key aspects of biodiversity is the
distribution of organisms, and biogeography is the discipline which tries to rec-
ognize and characterize the causes of that distribution. It is closely linked to
ecology, since the distribution of organisms is related to ecologic factors, but it
cannot ignore other matters, such as species origin, dispersal, and extinction, and
thus it becomes a historic science. Introducing the important time dimension,
scientists are turning their interest to the past distribution of organisms, and
paleobiogeography is now a complex subject which processes information pro-
vided by both Biology and Earth Sciences. It is conceptually and philosophically
equivalent to neobiogeography. Nevertheless, its methods are somewhat different,
since it is seriously limited by the incompleteness of the fossil record. On the other
hand, it has direct access to the time involved, a key ingredient of organic
evolution.
This book is a synthesis of many years of research on Mesozoic bivalve mol-
lusks from South America. The task of updating and processing all the information
was triggered by a Symposium on ‘‘Seaways and landbridges: Southern Hemi-
sphere biogeographic connections through time’’, organized by Dr. Silvio Casadío
(CONICET and Universidad Nacional de Río Negro) and Dr. Miguel Griffin
(CONICET and Universidad Nacional de La Plata), held at the Universidad
Nacional de Río Negro, General Roca, Río Negro Province, Argentina, April
26–May 1, 2011, and who kindly encouraged us to complete this contribution.
After that memorable meeting we put together our knowledge and skills to produce
this book together, and present a picture of present-day knowledge on Triassic and
Jurassic bivalve distribution in the Southern Hemisphere. Benthonic bivalves have
been widely used in biogeography and paleobiogeography, but data from the
Southern Hemisphere Mesozoic are still few and scattered. We trust this book will
encourage future research on this subject to build better databases and allow
the application of more sophisticated methodology. Through the book we analyze
the subject from several viewpoints. From a merely descriptive perspective,
v
vi Preface
bivalve distribution shows latitudinal gradients and distributional patterns, both at

regional and global scales, which are the basis for the recognition of biochoremas
(i.e., paleobiogeographic units of different ranks). Moving forward toward a causal
paleobiogeography, these organisms also provide interesting insight into particular
biogeographic questions, such as bipolarity and its origin. The evolution in time of
the recognized biochoremas can be discussed in relation to paleoclimates and
paleogeography. Finally, some of the results obtained from the analysis of the
distribution of past bivalve biotas were even used to propose and discuss the
development of marine corridors and argue about the distribution of continents in
the past.
Susana E. Damborenea
Javier Echevarría
Sonia Ros-Franch
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Paleobiogeography and Neobiogeography. . . . . . . . . . . . . . . . . 3
1.2 Why Bivalves? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Time Frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Paleogeography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Paleoclimates and Water Temperatures . . . . . . . . . . . . . . . . . . 8
1.6 Paleocurrents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1 The Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Quantification: A Difficult Approach . . . . . . . . . . . . . . . . . . . . 15
2.3 Analytic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.1 Latitudinal Distributions . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 Recognition of Biochoremas
and Their Characterization . . . ................... 19
References . . . . . . . . . . . . . . . . . . . . . . . ................... 20
3 A Bivalve Perspective. . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . 23

3.1 Previous Research: A Northern Hemisphere Affair . . . . . . . . . . 24
3.2 Some General Issues . . . . . . . . . . . . . . . . ..... . . . . . . . . . . 25
3.2.1 Larval Types and Dispersion . . . . . ..... . . . . . . . . . . 25
3.2.2 Swimming Habit . . . . . . . . . . . . . . ..... . . . . . . . . . . 27
3.2.3 Pseudoplanktonic Habit . . . . . . . . . ..... . . . . . . . . . . 27
3.2.4 Reef-Forming Bivalves . . . . . . . . . ..... . . . . . . . . . . 28
3.2.5 Low-Oxygen Tolerance . . . . . . . . . ..... . . . . . . . . . . 29
3.3 Paleobiogeographic Affinities . . . . . . . . . . ..... . . . . . . . . . . 32
References . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . 37
vii
viii Contents
4 Regional Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 45

4.1 Facies and Bivalve Distributions: Examples
From the Neuquén Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.1.1 Late Triassic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1.2 Early Jurassic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2 Latitudinal Gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2.1 Diversity gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2.2 Latitudinal Distribution of Biogeographic
Affinities Through Time . . . . . . . . . . . . . . . . . . . . . . . 58
4.2.3 Systematic Latitudinal Variations . . . . . . . . . . . . . . . . . 69
4.3 Distribution Patterns and Boundaries . . . . . . . . . . . . . . . . . . . . 71
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5 Hemispheric Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.1 Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.1.1 South America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.1.2 Antarctica, New Zealand-New Caledonia . . . . . . . . . . . . 85
5.1.3 Australia-New-Guinea . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.1.4 Southern Asia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.1.5 Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.1.6 Near East . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.2 Biochorema Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.2.1 Biogeographic Units and Their Characterization . . . . . . . 87
5.3 Evolution of Biochoremas . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.3.1 Triassic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.3.2 Early Jurassic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.3.3 Middle Jurassic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.3.4 Late Jurassic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.3.5 Early Cretaceous. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
5.4 Congruence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5.5 Paleobiogeographic Units and Mass Extinctions . . . . . . . . . . . . 106
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6 Global Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

6.1 Bipolarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
6.1.1 Triassic–Jurassic Bipolar Bivalves. . . . . . . . . . . . . . . . . 122
6.1.2 Discussion of Triassic–Jurassic Bipolarity . . . . . . . . . . . 127
6.2 Seaways: the Hispanic Corridor. . . . . . . . . . . . . . . . . . . . . . . . 128
6.3 Oceanic Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.4 Evolution of Global Biogeographic Boundaries . . . . . . . . . . . . . 134
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Chapter 1
Introduction
Abstract Bivalves have proven to have a great potential for paleobiogeographic

analyses due to their relatively complete fossil record, especially for Mesozoic and
Cenozoic times. Being mostly benthonic, they have a large variety of life habits
which should be taken into account, particularly in detailed paleobiogeographic
studies. We will analyze marine bivalve distribution in the Southern Hemisphere
during several successive time slices within the Triassic and Jurassic, an epoch
marked by critical geologic and biotic events. This period covers both the biotic
recovery after the harshest diversity crisis ever (the Permian/Triassic extinction
event), and later also the biotic reaction to another severe crisis at the Triassic/
Jurassic boundary. This allows the opportunity to evaluate the response of pale-
obiogeographic patterns to such events. The Earth’s configuration drastically
changed from a concentration of land masses in a unique supercontinent (Pangea)
and two oceans (Tethys and Panthalassa), to a fragmented series of continental
land masses. These began to disperse, opening sea corridors which largely affected
not only the global distribution of biotas but also paleoclimate and sea paleocur-
rents as well. This dynamic paleogeography adds an interesting ingredient to the
study of past distributions of benthic organisms making it possible to frame them
into a physically and biologically changing scenario.
‘Every naturalist who has directed his attention to the subject

of the geographical distribution of animals and plants, must
have been interested in the singular facts which it presents…
Of late years, … a great light has been thrown upon the subject
by geological investigations, which have shown that the
present state of the earth, and the organisms now inhabiting it,
are but the last stage of a long and uninterrupted series of
changes which it has undergone, and consequently, that to
endeavour to explain and account for its present condition
without any reference to those changes (as has frequently been
done) must lead to very imperfect and erroneous conclusions.’
Alfred Russel Wallace 1855
S. E. Damborenea et al., Southern Hemisphere Palaeobiogeography of 1

Triassic-Jurassic Marine Bivalves, SpringerBriefs Seaways and Landbridges:
Southern Hemisphere Biogeographic Connections Through Time,
DOI: 10.1007/978-94-007-5098-2_1, The Author(s) 2013
2 1 Introduction
Alfred Russel Wallace, considered the nineteenth century’s leading expert on the
geographic distribution of animal species and sometimes called the ‘‘father of
biogeography’’, already recognized the importance of studying the history of
biotas long before moving continents and plate tectonics were heard or even
thought of. On the other hand Darwin, who initially recognized the importance of
geographic isolation to speciation in his unpublished notebooks (see Lieberman
2003, 2008), did not mention this in his later publications.
The study of global biodiversity changes is a hot issue these days, as we humans
become aware of the fragility of the Earth system and the urgent need to under-
stand it better to keep it going. One of the key aspects of biodiversity is the
distribution of organisms, and biogeography is the discipline which tries to rec-
ognize and characterize the causes and patterns of distribution. Biogeography is
closely linked to ecology, since the distribution of organisms is governed by
ecologic factors, but it cannot ignore other matters, such as the origin of species,
their dispersal, and extinction, and thus it can be considered a historic science.
Biologists are beginning to investigate the causes of the great global biodi-
versity changes that are now taking place on the Earth. But paleontologists, who
possess a much more extensive time perspective, are constantly observing and
surveying the changes in biodiversity produced at various times in the past, and
they have the most precise access possible to this very important dimension: time.
Thus, paleobiogeography, which studies the distribution of organisms in the past,
is a very complex subject that combines information from both biology and the
Earth sciences (Cecca 2002), and ‘‘paleobiogeographers can actually monitor how
the Earth and its biota have evolved and coevolved’’ (Lieberman 2000). The data
provided by the fossil record are increasingly being used in combination with other
sorts of data in modern biogeographic analysis. The relationship between geology
and biogeography is then unavoidable, and should be based on a reciprocal
illumination approach (Morrone 2009).
Similarly to biogeography, different approaches can be recognized for paleo-
biogeography (Rosen 1992, 1995):
1. Descriptive paleobiogeography: recognition and description of the distribution
of organisms. The outcome is the definition of biogeographic units or bio-
choremas. Both quantitative (more frequent in neobiogeography) and qualita-
tive (more subjective) methods can be used.
2. Causal paleobiogeography: examines the causes of the observed distributions.
There are many arguments related to theoretic biogeography and the philosophic
approaches, which will not be discussed further here (for a good synthesis see
Cecca 2009). According to the temporal scale of the processes involved, two
main viewpoints allow distinction between ecologic biogeography, with a tem-
poral scale related to biologic cycles, and historic biogeography when long-term
processes are analyzed.
3. Applied paleobiogeography: the distribution of organisms can be used to infer
the role of ecologic factors, the relation between phylogeny and provinciality,
or paleogeographic patterns.
1 Introduction 3
Through the book we will follow all these three approaches using Mesozoic
bivalves from the Southern Hemisphere, and try to apply the resulting knowledge
to the discussion of regional and global issues. Since scale is a key element in
paleobiogeography, the chapters will be ordered progressively according to the
regional (Chap. 4), hemispheric (Chap. 5) and finally (Chap. 6) some global issues
involved.
During a long time of research on the distributions of Mesozoic bivalves, we
became increasingly aware of the huge gaps in our knowledge. At this point we
fully agree with Rex et al. (2005, p. 2288) statement that ‘‘making sense of large-
scale marine biogeography remains a difficult challenge’’, and we dare say it is
certainly much harder when we plunge into deep time as we will try to do here. In
this mood, we pick up the gauntlet and offer this review as a small but (we hope)
significant contribution to the fascinating subject of the evolution of the Earth’s
biotas.
In this first chapter, the necessary framing concepts about the organisms and the
time involved, some aspects on Mesozoic paleoclimate, and marine paleogeog-
raphy and paleocurrents, will be laid out briefly.
1.1 Paleobiogeography and Neobiogeography
Although when we refer to the distribution of living organisms we are doing

biogeography, some authors have introduced the term neobiogeography to clearly
distinguish it from paleobiogeography (see Rosen 1995). These two disciplines are
conceptually and philosophically equivalent, and the only important differences
concern the object of study and the methodology applied, both aspects highly
conditioned by the availability and limitations imposed by the data, as extensively
discussed by many authors (see for instance Rosen 1992; Lieberman 2000; Cecca
2002) and summarized in Table 1.1. New techniques are continually being pro-
posed and tested to overcome these limitations, and paleobiogeography and neo-
biogeography are now converging fast into a unified discipline (Cecca et al. 2011).
1.2 Why Bivalves?
Benthonic mollusks, and among them especially the bivalves, have a great
potential for paleobiogeographic analyses. They are usually considered in marine
neobiogeography studies (see Flessa and Jablonski 1995 and references therein),
and they were also frequently used in paleobiogeography, particularly for Meso-
zoic and Cenozoic times (e.g. Hayami 1961, 1984, 1987; Hallam 1967, 1969,
1977; Kauffman 1973; Zinsmeister 1979, 1982; Crame 1986, 1987, 1993; Liu
1995; Niu et al. 2011; among many others). They are generally well preserved,
which is reflected in a relatively uninterrupted paleontological record, they are
4 1 Introduction
Table 1.1 Some practical differences between neobiogeography and paleobiogeography

Neobiogeography Paleobiogeography
Easier access to terrestrial environments Mostly restricted to marine environments
Deals with barriers, isolated regions Almost no physical barriers
The distributions can be quantified Neither the areas nor the abundance can be easily
(geographical areas, abundance) quantified
Quantitative methods are used, can be Very incomplete data, hard to be checked
checked
Molecular biology data can be used No molecular biology data available
Geographic and environmental factors are Geographic and environmental factors can only be
known and can be measured inferred by indirect evidences
Only short processes can be studied There is direct access to the time dimension and
thus long processes can be studied
Evolutionary histories are inferred without The evolutionary histories are confirmed by the
taking into account the time dimension fossil record and are referred to the
stratigraphic time scale
It is strong on processes It is strong on patterns
It may be used to infer past geographic It is usually used when discussing paleogeography
scenarios
Difficulties arise when long-term processes Long-term processes, such as changes in
are involved distribution patterns, can be followed through
time
abundant in all marine environments, they are highly diverse and have a great
dispersal potential at the larval stage. Despite their age, Mesozoic bivalve faunas
display a clearly modern composition, being very different to Paleozoic ones, and
bivalves are one of the main components of the Modern Evolutionary Fauna in
Sepkoski’s (1981) sense.
It is well known that many bivalves are highly facies-dependant, and this fact
should be taken into account when analyzing their distribution for paleobiogeo-
graphic purposes (Hallam 1969, 1971). Substrate type is a key factor for bivalve
distribution, and it may change locally and regionally according to the bottom
conditions, sediment supply, and water movement. Nevertheless, if adequately
dealt with, facies dependency can turn from an apparent drawback to an asset in
paleobiogeographic studies, since it allows us to interpret faunal distribution in a
large range of marine habitats.
Bivalves have a wide variety of life habits, mostly related to substrate type, and
these can fortunately be inferred from shell shape (see Stanley 1970). The main
life habits common among Mesozoic bivalves are depicted in Fig. 1.1, and these
are the categories being used throughout the text. They can be grouped or further
qualified according to different criteria: (1) if using relation to the substrate into
epifauna, semi-infauna, and infauna; (2) if using mobility into relatively mobile
and immobile; (3) if using feeding type into detritus feeders and suspension
feeders.
Also, the geographic distribution patterns of bivalves are widely ranged, from
extremely local for some species to nearly global for others. Nevertheless, all
authors agree that cosmopolitan species evidently dominated Mesozoic bivalve
1.2 Why Bivalves? 5
Fig. 1.1 Block diagram of a littoral marine environment showing main bivalve ecomorphologic
groups represented in the Mesozoic, sketches not to scale. a Epibyssate (e.g. Lycettia,
Camptonectes, pterioids), b endobyssate (e.g. Modiolus, Pinna), c cemented (e.g. oysters),
d recliner (e.g. Weyla), e swimmer (e.g. Parvamussium), f shallow burrower (e.g. nuculoids,
trigonioids), g deep burrower (e.g. lucinoids, Thracia, pholadomyids), h borer (e.g. pholadoid),
i nestler (e.g. Ctenostreon). Modified from Damborenea in Camacho et al. 2008
biotas (to a higher degree than in modern biotas), at least during Triassic and
Jurassic times, leaving only a few taxa to be used to define and characterize
paleobiogeographic units.
1.3 Time Frame
The time frame for this book is summarized in Table 1.2. We will refer to the
Triassic and Jurassic periods, which extend just after the major biotic crisis ever,
the Permo-Triassic extinction event, to the end of the Jurassic, approximately
between 251 and 145 Ma. This is a very interesting period of time because it
shows first the dramatic recovery of biotas after the crisis, and also marks the
development and establishment of modern marine faunas (Ros et al. 2012). The
period is marked by critical geologic (see Sect. 1.4) and biotic events. The biota
underwent several diversity crises during this interval: a severe one at the end of
the Triassic and several minor ones.
6 1 Introduction
Table 1.2 International stratigraphic units for the Triassic and Jurassic, according to Interna-
tional Commission on Stratigraphy 2009, and New Zealand timescale (according to Cooper
2004). Vertical scale represents time
A stratigraphic frame is preferred over the absolute dating for several reasons.
Knowledge of precise absolute ages is still very unstable for this period of time
and this is especially critical for the Southern Hemisphere. Consequently, instead
of absolute dates, we prefer the widespread use of stages and biostratigraphically
based time units, which allow the use of data compiled at different times, and are
not seriously affected by new developments in absolute dating techniques.
Table 1.2 shows the approximate equivalence of globally recognized stage units
1.3 Time Frame 7
with those established for New Zealand, since these last have a wide use in the area
(they are also applied to New Caledonia and Antarctic sequences, for instance) and
thus will also be frequently referred to in this book.
Most of the analyses pursued and discussed in this book were developed within
time slices coinciding with the different stages (or group of stages) of the Inter-
national Stratigraphic Chart, as a way of obtaining comparable results in a time
succession, thus amenable to further analysis to understand general processes. The
results obtained for each of these units were then used as reference for the historic
evolution of general paleobiogeographic patterns at a very broad scale. In this first
approach, our time slices are admittedly very ‘‘thick’’ (i.e., long), and, what is
worse, of very uneven ‘‘thickness’’ (length) if stages are used, as is evident from
Table 1.2, in which the vertical scale is proportional to time in Ma. The great
difference in stage duration (cf. for instance Norian and Rhaetian) introduces
another distorting factor and hinders comparison. For the moment there is no way
of building a comprehensive database applying time slices below the stage level.
1.4 Paleogeography
Several major geologic events with significant paleogeographic consequences took

place during the time interval considered here, the main ones being the opening of
the North Atlantic Ocean and the Mozambique Corridor (Fig. 1.2).
The Earth’s configuration thus drastically changed from a concentration of land
masses in a unique supercontinent (Pangea) and two oceans (Tethys and Pantha-
lassa or Paleo-Pacific) at the Triassic and earliest Jurassic, first into Gondwana and
Laurasia and later into a fragmented series of continental land masses which began
then to disperse (Fig. 1.2b). The geodynamic processes involved are numerous and
complex, and are still being the subject of research. Some of the still hotly con-
troversial points refer to the dates of the rifting and initial splitting along plate
boundaries and so-called ‘‘marine corridors’’ and will be further discussed later
(Sect. 6.2). A huge amount of geologic and geophysic evidence was accumulated
and discussed in the last few decades. In the context of this book, which deals with
the distribution of (mostly shallow) marine organisms, it should be reminded that
frequently the paleobiogeographic data yield older dates than the earliest
demonstrable occurrence of oceanic crust, since shallow-water or intermittent
connections suitable for the dispersal of littoral benthonic faunas can occur even
during the initial rifting phase.
The evolution of the Earth’s configuration during the time here considered was
accompanied by climatic changes triggered by the disruption of the monsoonal
circulation (Parrish 1992), and directly implied a significant reorganization of
water movements in the oceans, with the appearance of new barriers and the
disappearance of previous ones. These facts should be taken into account when
analyzing the distribution of marine benthic organisms.
8 1 Introduction
Fig. 1.2 Global paleogeography at the beginning (a, Triassic) and end (b, Late Jurassic) of the
period of time considered in this book, compiled from various sources. Note that between the two
moments the fragmentation of the Pangea supercontinent started, with the development of several
marine corridors, among them the Hispanic Corridor, which connected the western Tethys with
the Panthalassa (or Paleo-Pacific) and then originated the North Atlantic. These drastic changes
affected oceanic circulation, climate, and the distribution of marine benthonic organisms
(paleogeographic maps compiled from several sources, base map from Smith et al. 1981)
1.5 Paleoclimates and Water Temperatures
Most authors agree that the Jurassic was a period characterized by temperature
gradients less evident than at the present (Hallam 1975, 1994), and although
possible glaciations during the Jurassic were investigated, evidence is not con-
vincing. A lot of geologic information backed by paleontologic knowledge of
terrestrial faunas and floras supports this general statement. Research on this
aspect is heavily dependent on the development of models (Parrish 1992).
The paleoclimatic aspect which interests us most in view of the subject of this
analysis is sea-surface water temperatures. Most of the previous research was
developed on Northern Hemisphere data, based on diverse isotopic analyses, and
extrapolated to the whole Earth. According to Golonka and Ford (2000), green-
house conditions prevailed during the Sinemurian-Toarcian, with a warm, humid
environment, and moderate temperatures into high latitudes with no evidence of
significant continental glaciation. On several evidences, Price (1999) concluded
that the extent of polar ice during the Mesozoic was probably only one-third the
size of the present day. Kiessling and Scasso (1996) suggested that Antarctic
surface waters may have been warmer in average than those in equivalent northern
high latitudes, according to the distribution of Pantanelliidae radiolarians.
The recent application of new techniques (TX86) to Callovian to Late Jurassic
deposits from the Southern Ocean (Jenkyns et al. 2012) supports the existence of
1.5 Paleoclimates and Water Temperatures 9
Fig. 1.3 Sketch of inferred global ocean surface circulation patterns for the Late Triassic-Early
Jurassic (a) and the Late Jurassic (b), simplified from many sources. Only the inferred summer
circulation pattern is shown for the western Tethys, a winter countercurrents pattern was strong
during the Triassic but was not so clear after the opening of the Hispanic Corridor. Base map as in
Fig. 1.2
relatively warm sea-surface conditions (26–30) for that period. According to these
studies, there was a general warming trend through the Late Jurassic, and the
Callovian-Oxfordian boundary had slightly colder seawater temperatures (though
never below 25). These results strongly favor an equable tropical to subtropical
environment up to the poles, contrary to earlier studies. It is now suggested that
most paleotemperature studies based on belemnites, which give consistently lower
temperatures, in fact reflect the water conditions below the termocline.
1.6 Paleocurrents
Paleoceanographic global models developed for the Late Triassic and Early
Jurassic (Fig. 1.3a) propose that Panthalassa surface water circulation pattern
appeared to be hemispherically symmetric, there was no circum-equatorial current,
and circulation within the Tethys ocean had seasonal countercurrents (see revision
in Parrish 1992; Arias 2006, 2008, and references therein). The annual surface
circulation in Panthalassa had two main large subtropical gyres rotating anti-
clockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere.
The circulation within the Tethys Ocean was quite different; in western Tethys the
strong monsoonal regime produced an alternating change in water circulation
directions between summer and winter.
The major paleogeographic global change produced during the Jurassic was the
separation of Gondwana and Laurasia and the development of the Hispanic Corridor
10 1 Introduction
(giving rise later to the North Atlantic). This produced a drastic reorganization of the
equatorial circulation pattern (Fig. 1.3b), which was previously interrupted by the
Pangea landmass, and seasonality within the Tethys became less evident.
The consequences of these great changes in ocean circulation patterns on global
climate were discussed extensively by Parrish (1992) with relation to the
Panthalassa (or Paleo-Pacific) ocean.
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Arias C (2008) Palaeoceanography and biogeography in the early Jurassic Panthalassa and Tethys
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(eds) Los Invertebrados Fósiles. Tomo I. Fundación de Historia Natural Félix de Azara,
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Taylor & Francis, London
Cecca F (2009) La dimension biogéographique de l0 évolution de la Vie. CR Palevol 8:119–132
Cecca F, Morrone JJ, Ebach MC (2011) Biogeographical convergence and time-Slicing.
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Crame JA (1986) Late Mesozoic bipolar bivalve faunas. Geol Mag 123:611–618
Crame JA (1987) Late Mesozoic bivalve biogeography of Antarctica. Proceeding 6th Gondwana
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Crame JA (1993) Bipolar molluscs and their evolutionary implications. J Biogeogr 20:145–161
Flessa KW, Jablonski D (1995) Biogeography of recent marine bivalve molluscs and its
implications for paleobiogeography and the geography of extinction: a progress report. Hist
Biol 10:25–47
Golonka J, Ford D (2000) Pangean (Late Carboniferous-Middle Jurassic) paleoenvironment and
lithofacies. Palaeogeogr Palaeoclimatol Palaeoecol 161:1–34
Hallam A (1967) The bearing of certain palaeozoogeographic data on continental drift.
Palaeogeogr Palaeoclimatol Palaeoecol 3:201–241
Hallam A (1969) Faunal realms and facies in the Jurassic. Palaeontology 12:1–18
Hallam A (1971) Provinciality in Jurassic faunas in relation to facies and palaeogeography. In:
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Hallam A (1975) Jurassic environments. Cambridge University Press, Cambridge
Hallam A (1977) Jurassic bivalve biogeography. Paleobiology 3:58–73
Hallam A (1994) Jurassic climates as inferred from the sedimentary and fossil record. In: Allen J,
Hoskins B, Sellwood B, Spicer R, Valdes P (eds) Palaeoclimates and their modelling with
special reference to the Mesozoic Era. Chapman & Hall, London
Hayami I (1961) On the Jurassic pelecypod faunas in Japan. J Fac Sci, UnivTokyo, Sect II Geol
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Jenkyns HC, Schouten-Huibers L, Schouten S, Sinninghe Damsté JS (2012) Warm Middle
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Climate Past 8:215–226
Kauffman EG (1973) Cretaceuos bivalvia. In: Hallam A (ed) Atlas of palaeobiogeography.
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1-2:317-326
Lieberman BS (2000) Paleobiogeography. Using fossils to study global change, plate tectonics,
and evolution. Topics in Geobiology 16, Kluwer Academic, Plenum Publishers, New York
Lieberman BS (2003) Paleobiogeography: the relevance of fossils to biogeography. Ann Rev
Ecol Systemat 34:51–69
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Rosen BR (1995) Neobiogeography versus palaeobiogeography. In: Matteucci R, Carboni MG,
Lee MST (eds) Studies on ecology and paleoecology of benthic communities. Boll Soc
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the changing environment. Oregon State University Press, Cambridge
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circum-Pacific. J Paleontol 56:84–102
Chapter 2
Techniques
Abstract Databases used for the analysis of past biotas should be as internally
consistent as possible taking into account the incompleteness of the fossil record
and the taxonomic distortions due to the history of their knowledge. A compre-
hensive and critically updated database of Southern Hemisphere bivalve occur-
rences through the Triassic and Jurassic was built. Most of paleobiogeographic
analyses were performed within time slices to obtain comparable results in a time
succession. Analytical methods were used for both (a) the analysis of latitudinal
ranges along the South American paleo-coasts, and (b) the recognition of paleo-
biogeographic units for the Southern Hemisphere. a) The first approach to the
study of species latitudinal ranges was cluster analysis, but this method, although
useful, imposes a hierarchical structure on the data. Thus, to check for faunal
changes along latitude, the distribution limits of species were explored using a
technique similar to that considered for origination/extinction analysis, substitut-
ing first and last appearances by northernmost and southernmost geographical
occurrences. Generalized linear models were used to look for changes on the
proportional values of different species categories related to systematic and
paleobiogeographic kinships. b) For the recognition of biochoremas, the incom-
plete and uneven nature of the data precludes the application of methods which
may group areas according to the common absence of data, and we followed a
traditional approach based on endemicity. In order to check the biogeographic
structures without assuming a hierarchical or gradational arrangement, a Boot-
strapped Spanning Network was calculated.
2.1 The Data
Occurrences of Triassic and Jurassic bivalve species were compiled from various
published sources as well as the authors’ own data, and plotted stage by stage from
Induan (Early Triassic) to Berriasian (Early Cretaceous). Ros’ (2009; also

DOI: 10.1007/978-94-007-5098-2_2, Ó The Author(s) 2013
14 2 Techniques
Ros et al. 2012) updated database was used for Triassic occurrences. The study
area is restricted to the Paleo-Southern Hemisphere, but the initial database was
compiled on a global scale, not only to provide the necessary framework for the
detailed analysis of southern regions, but also to adequately recognize patterns of
general distribution and endemism. The species distribution data compilation was
systematically and stratigraphically updated as far as possible and dubious records
were excluded. The most serious problems related to such global databases are the
incompleteness of the fossil record and the taxonomic distortions introduced by
different authors working in different areas at different times. The first problem has
no immediate solution, while the second can be somewhat reduced by critical
evaluation of the data. This cannot always be done, for several reasons, but internal
consistency was sought whenever possible, with careful reappraisal of both tax-
onomy and age of the records taken from the previous literature. In order to obtain
a sound foundation for biogeographic considerations, only species personally
examined or adequately described and figured were included; uncritical listing of
taxa from sources lacking illustrations was avoided.
Presence-absence data were used throughout, since reliable quantitative records
are only available for a small fraction of the occurrences.
Theoretically, the species is the most objective of taxonomic units; however,
when species lists are compiled from studies made by various authors and at
different times they become intensely subjective to the point that compilations at
the generic or familial levels are preferred for global analysis (Stehli et al. 1967).
Furthermore, the use of genus-group taxa increases the consistency of the data-
base, as generic and sub-generic concepts have more consensus than species
among different authors. In this book, genera and subgenera are used for the larger
scales analyses (hemispheric and global), while species are preferred for per-
forming regional analysis within the area studied by the authors, where first-hand
knowledge facilitates identification and consistency.
Although it is evident that there are serious gaps in our knowledge of Triassic
and Jurassic faunas from certain regions and for some bivalve groups, which have
not been systematically reviewed or updated yet, it is believed that the data are
comprehensive enough for the general purpose of this study.
In order to rationalize the study, the biogeographic affinities of the species
(according to the categories discussed in Sect. 3.3) were recorded as well. Since
the assignment of species to a definite type is based on its known distribution and
relative abundance, and this knowledge is constantly being improved, this task
proved difficult in some instances but relatively straightforward in others (such as
the pectinaceans, see Hayami 1989; Damborenea 1993). Though many of the
species have local distributions restricted to South America, they may have strong
relations with other species or genera belonging to the paleobiogeographic affin-
ities categories recognized here, and these were listed and used in the analysis.
Relative abundance was regarded as an important factor too; sporadic occur-
rences outside the main area of distribution are to be expected and, if adequately
recognized as such, should not obscure the picture.
2.2 Quantification: A Difficult Approach 15
2.2 Quantification: A Difficult Approach
The application of any analytical technique in biogeography implies the accep-

tance of a defined biogeographic theory; biogeographic patterns are the result of
ecological and historical processes acting on them, and if one accepts that such
patterns actually do exist, then some analytical approach is required (Lieberman
2000). Quantitative analysis are claimed to be necessary in order to make bio-
geography a more rigorous science; these kinds of approaches should act as
arbiters to choose between competing hypotheses. Nevertheless, these analytical
techniques on their own do not guarantee valid results (Lieberman 2000), and the
quality of the outcome largely depends of the quality of the available data.
The basic unit of any biogeographic study is the geographic range of a species,
but its characterization is complicated by problems of defining and mapping them;
what we usually see as maps of geographic ranges are simplifications of complex
historic and ecologic patterns (Brown et al. 1996). Species ranges are usually
mapped as irregular continuous areas, although in some cases they are presented as
more precise ‘‘dot maps’’ that plot each location where a species has been
recorded; most published range maps attempt to define the historical range of a
species, encompassing localities where a species has regularly occurred in the past
and recently colonized areas (Brown et al. 1996). To all these problems, common
in the analysis of modern biota, the imperfection of the fossil record and the time
averaging are added on paleobiogeographic analyses.
We use reconstructions of Mesozoic paleogeography to discuss our results. As
pointed out by Smith (2011), although we now know a great deal more than
50 years ago, available Mesozoic paleogeographic maps are still rudimentary,
showing only where continents and oceans were in the past, without bathymetry
data, for instance. For the general purposes of this book they are nevertheless
adequate.
Like any other model, a biogeographic model is a simplification of reality, and
hence, as pointed out above, considered geographic ranges are simplified from
actual geographic ranges; the kind of simplification applied will depend on the aim
of the study.
In Sect. 4.2, an analysis of bivalve distributions along the west paleo-coast of
South America is addressed; this coast presents a north–south orientation nowa-
days and it was similarly disposed during the Jurassic. This peculiarity allows for
an analysis of latitudinal gradients on bivalve distribution. These gradients are
associated with climatic gradients, imposing ecological restrictions to species
distribution. A total of 13 areas were defined between parallels 20° and 46° S, each
with a latitudinal range of 2°, as considered by Damborenea (1996). The presence
of a species in one locality was computed as a presence for the whole area. During
this analysis, discontinuity of the outcrops and differences in preservation proba-
bility were interpreted to be biasing a somewhat continuous distribution (Fig. 2.1),
and so species ranges were interpolated between two localities with occurrence
16 2 Techniques
Fig. 2.1 Latitudinal range data exemplified with the Hettangian species data along a section of
the eastern Paleo-Pacific coast. The actually observed occurrences (left) were used in most of the
analyses performed, while the graphic presentation of data in Sect. 4.2.2 (Figs. 4.7, 4.9, 4.11 and
4.13) was simplified to show only the extended ranges (right)
(although on certain analyses, explained below, only the observed data were
considered).
In Sect. 5.2, a hemispheric-scale analysis was carried out, and for this the units
considered were well defined and bounded regions established a priori based on
the current literature on the topic. Presence or absence of taxa (genera group taxa
in this case) was computed for each region as a whole, disregarding the internal
structure of distribution. The detail lost with this approach is not relevant at the
scale considered for the purposes of this review.
Concerning time, most of the analyses performed were developed within time
slices coinciding with the different stages (or group of stages), as a way of
obtaining comparable results in a time succession. If stages are used, time slices
are very unevenly ‘‘thick’’ (see Table 1.2, in which the vertical scale is propor-
tional to time in Ma). One way to overcome in part the unevenness of the time
duration of each slice is to group stages as done for some of the analyses (see
Table 5.1), a method which also has some other practical advantages during the
data gathering process.
On top of this, the time averaging within each of these time slices is consid-
erable. The effects of time averaging are difficult to remove, especially in small-
scale studies; as mentioned before, ‘‘instantaneous’’ geographic ranges cannot be
obtained even for modern species, and this is yet less likely for fossil species. To
deal with this, time bins were considered, and stratigraphic ranges are bounded by
2.2 Quantification: A Difficult Approach 17
their upper and lower limits. The geographic ranges presented here include all the
localities, where a species was present at any time of that time bin, in this case a
stage; in some cases, the paucity of data forced the grouping of stages analyzed.
2.3 Analytic Methods
Phenetic approaches are common techniques in biogeographic analysis; in these

methods information about distribution of taxa is entered into a data matrix for
each of the regions or areas analyzed (Lieberman 2000). For this particular work
only presence or absence of taxa (species or genera, depending on the analysis)
was considered; abundance was indirectly considered solely when establishing
biogeographic affinities.
All calculations were done using the different facilities provided by the soft-
wares PAST (Hammer et al. 2001) and R (R Development Core Team 2008), as
well as standard spreadsheet software.
Analytical methods further used to deal with both the latitudinal distribution
and the biochoremas recognition are shortly described below.
2.3.1 Latitudinal Distributions
2.3.1.1 Cluster Analysis
The first explorative technique applied here is the hierarchical cluster analysis, for
which a distance or similarity measure must be defined (Hammer and Harper
2006). Our main goal was to group together the localities according to their species
content, so the Simpson’s coefficient of similarity (Simpson 1943; see also Shi
1993) was used. This index is defined as the number of shared species between two
localities divided by the number of species in the smaller sample. This index is
totally insensitive to the size of the larger sample, which makes it suitable when
sampling is considered to be incomplete (Shi 1993; Hammer and Harper 2006), as
is the case for our database. The localities considered here were not equally treated
in the literature, neither have the same abundance of fossils; hence, they cannot be
considered as equally sampled, making the Simpson’s coefficient the most ade-
quate available index of similarity to use. Cluster analysis is an ordination method,
grouping elements according to their similarity; clusters or groups have no sta-
tistical significance associated. A support value can be obtained for the nodes by
simply resampling taxa (in this case species) and building a new dendrogram; the
proportion of times the node appears on the dendrograms resulting from the
resampled matrices is the support value for the node. Although the general
grouping and disposition of the localities are evaluated on each analysis, special
value is given to groups with similarity values of 0.50 or higher (i.e., 50 % of
18 2 Techniques
species shared or more) and to groups with support values of 0.50 or higher, as
considered in other paleobiogeographic studies (Brayard et al. 2007; Dera et al.
2011).
On the analysis of latitudinal gradients the main interest focuses on distribution
limits, so for species that appear at two distant localities it is usual to extend their
ranges along the intermediate latitudes, and most data here are presented graphi-
cally in that way for clarity (Fig. 2.1). Nevertheless, for this analysis that meth-
odology would result in a circular reasoning, since the latitudinal gradient would
be analyzed presuming its existence; nearby localities would be similar because
we assume they share species for being close to each other. To avoid this, cluster
analysis was performed on the actually observed presence/absence data; this may
produce some sensitivity to differences in knowledge between localities, but that is
why we use Simpson’s coefficient.
2.3.1.2 Distribution Limits of Species
To check for the faunal changes along a latitudinal gradient, we analyzed the
distribution limits of the considered species through that gradient. Cluster analy-
ses, although useful, are hierarchical ordination methods and hence they impose a
hierarchical structure on the data, whether this exists or not. If a gradation among
localities is to be expected, as happens in a latitudinal gradient, other independent
approaches should be considered to check for it. A first graphic and very simple
approach is to analyze the distribution limits of the considered species through that
gradient. The methodology applied is similar to that considered for origination/
extinction analyses, counting the first and last appearance data (FAD and LAD
respectively) on each stage (Hammer and Harper 2006), although in this case the
stages are substituted by the latitudinal intervals, while the FADs and LADs are
replaced by the northern distribution limit datum (NDL) and the southern distri-
bution limit datum (SDL). If faunal turnover presents a gradational pattern, then
high values of SDL and NDL are expected in all areas. On the other hand, sudden
changes in faunal distribution will be recognized as peaks on the graphic; par-
ticularly significant will be the coincidence of peaks on both curves since they will
show a major faunal turnover at that latitude (i.e., there will be a lot of species that
appear only to the north and a lot that appear only to the south of that point). Peaks
on only one curve indicate a reduction on general diversity on one direction (either
north or south) and may be informative depending on the nature of data. This
reduction could be spurious, if it only represents a sampling bias. For example, on
the graphic for the Pliensbachian stage (Fig. 4.14), there is a high peak on the NDL
curve between 24° and 26° S, but data for the areas between 20° and 24° S are
scarce, and hence many of the considered species may have a broader range,
extending northwards; something similar may be happening on the SDL peak
between 40° and 42° S.
2.3 Analytic Methods 19
2.3.1.3 Generalized Linear Models
A second approach to check for gradational patterns is to look for changes on the
proportional values of different species categories; data such as biogeographic
affinities or systematic kinship (for instance, superfamilies) are good raw material
for this kind of analysis. Generalized linear models (GLMs) are useful for data on
proportions (Crawley 2007). The software R (R Development Core Team 2008)
carries out a weighted regression, using the individual sample sizes as weights and
the logit link function to ensure linearity (Crawley 2007). As a result a linear
predictor is obtained together with its significance; the significance level used here
was 0.05, but significance values between 0.05 and 0.10 were also considered as
potentially explanatory. Positive linear predictors will imply positive associations
between variables (i.e., an increment in the independent value, on this case lati-
tude, is associated to an increment on the dependent value, on this case the pro-
portion of species of the analyzed group). Negative linear predictors will imply the
opposite trend, i.e., an increasing proportion of species of the group toward the
north. This same analysis was applied to other groupings, such as superfamilies; on
this last case the analyses were formed both on observed data and on extended
range data. Localities poorly sampled may introduce noise instead to clear up a
pattern, and so were removed from the analysis. When this happened it was made
clear in the discussion.
2.3.2 Recognition of Biochoremas and Their Characterization
The recognition and characterization of ‘‘conventional’’ biogeographic units, i.e.,

based on area and hierarchy, was dismissed by some authors because ‘‘objective’’
criteria are lacking and methods used are not rigorous enough. One of the aims of
biogeography is to infer historical connections among biotas or geographical areas
on the basis of the distribution of organisms (historic biogeography). A variety of
analytical methods were developed to compare the biota of different localities with
this purpose (see a discussion and references in Posadas et al. 2006), one of the most
popular being parsimony analysis of endemicity (PAE) developed by Rosen (1988).
The incomplete and uneven nature of our data precludes the application of
methods which may group areas on the basis of the common absence of taxa. So at
this stage we have mostly followed a traditional qualitative approach based on
endemicity, combined with other features, such as diversity, to characterize bio-
choremas. In the same vein, we have consciously avoided the use of ranking names
for the units so discussed (such as Realm, Province, etc.), and prefer to refer them
simply as ‘‘biochoremas’’, sometimes specifying their relative ranking only.
Cluster analysis was applied to these data too, but only as a first approach since,
as explained before, this kind of analysis imposes a hierarchical structure.
20 2 Techniques
2.3.2.1 Bootstrapped Spanning Network
In order to check biogeographic structures between regions but without assuming

either a hierarchical or a gradational structure, we calculated a Bootstrapped
Spanning Network or BSN (Brayard et al. 2007) for the localities involved. The
construction of a BSN is a straightforward procedure; starting from a matrix of
observed occurrences a dissimilarity matrix was obtained, using as distance
measure the complement value of Simpson’s coefficient (1-S). From this second
matrix a minimum spanning network (MSN) was obtained, by connecting local-
ities from lower to higher dissimilarity value until all localities are directly or
indirectly connected. Then, the original data matrix of observed occurrences was
randomly resampled with replacement, and new dissimilarity matrix and MSN
were calculated; this whole procedure was repeated 1000 times. For each edge on
the original MSN a bootstrapped support value was obtained by simply calculating
the proportion of times each edge was obtained on the original and resampled
MSNs. Starting from the observed MSN, edges were removed (starting by those
with the least support value) if and only if the network was still connected after its
removal and the overall product of bootstrapped supports was increased; the
resulting network is the bootstrapped spanning network (Brayard et al. 2007),
obtained here for the more complete data sets.
BSN may be sensitive to the presence of sampled areas with low diversity. Lets
consider as an extreme case a locality with only one taxon; if this taxon is endemic
to that region, then the distance value of this area will be maximum (i.e. 1) with
any other area. During the construction of the resampled MSN, if this taxon is
sampled, all values lower than 1 (almost all edges) will be chosen, hence incre-
menting the support values of many edges that most probably should be considered
as low supported. To avoid this, areas with less than 10 known taxa for the stage
involved were removed from the analysis.
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clusters, gradients and networks. Geobios 40:749–765
Brown JH, Stevens GC, Kaufman DM (1996) The geographic range: size, shape, boundaries, and
internal structure. Ann Rev Ecol Syst 27:597–623
Crawley MJ (2007) The R book. Wiley, NY
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Damborenea SE (1996) Palaeobiogeography of Early Jurassic bivalves along the southeastern
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5:151–167
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tions. Global Planet Change 78:92–105
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and evolution. Topics in Geobiology 16, Kluwer Academic, Plenum Publishers, New York
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Chapter 3
A Bivalve Perspective
Abstract Mesozoic bivalves have been the subject of many paleobiogeographic

studies, either with the aim of recognizing units, to argue about the proposal of
opening of seaways and exotic terranes movements, or even to relate biogeography
with extinction and evolution. With a few notable exceptions, Northern Hemi-
sphere data were used and frequently conclusions extrapolated worldwide. In the
analysis of bivalve geographic distribution, some special issues should be taken
into account, such as larval type, mode of life, and tolerance to certain environ-
mental factors, which are here briefly discussed for Southern Hemisphere bivalves.
Special attention is paid to the proposed pseudoplanktonic habit as an aid to
dispersal, to reef-building bivalves, and to those with special low-oxygen toler-
ance. For some of the various analyses performed, Triassic-Jurassic bivalve genera
were classified according to their paleobiogeographic affinities in truly cosmo-
politan, low-latitude (Tethyan), high-latitude (austral or bipolar), trans-temperate
(Pacific), and endemic.
The study of past bivalve distributions has many applications, both geologic and
biologic. Perhaps, the geologic ones have been explored more and are better
known, but biologic implications are also diverse. For instance, it has been argued
that geographic range in mollusks is significantly heritable (Jablonski and Hunt
2006) and thus it should be considered in evolutionary dynamic interpretations of
these groups. In this chapter, we will try to discuss some general aspects which
will be used in the analysis of bivalve distribution performed in the following
chapters.
When dealing with fossil taxa, we have to bear in mind that we should refer all
our observations to the past paleogeography, and frame them in the past climate
and past ocean currents. Since all these features are mostly inferred, in many cases
using the distribution of fossil taxa as evidence, it is indeed very difficult to avoid
circular reasoning when arguing about these issues. Nevertheless, a general

DOI: 10.1007/978-94-007-5098-2_3, Ó The Author(s) 2013
24 3 A Bivalve Perspective
discussion of each for the Triassic-Jurassic time interval and the Paleo-Southern
Hemisphere is offered below.
3.1 Previous Research: A Northern Hemisphere Affair
Bivalves have provided data for paleobiogeographic studies almost since their
earliest representatives (see Sánchez and Babin 2001; Cope 2002 on a summary of
Ordovician bivalve paleobiogeography). The diverse Late Paleozoic bivalve fau-
nas have particularly fueled interesting discussions related to regional and global
paleogeography and paleoclimates (Runnegar and Newell 1971; Runnegar 1975;
Dickins 1993; Shi and Grunt 2000, among many others).
The interest on Mesozoic bivalve distribution is also very old, and it was
refreshed in the second half of the past century by new approaches based on
abundant data, largely through the work of Anthony Hallam, Erle Kauffman and
Graeme Stevens, who promoted a wide discussion on different (mainly geologic)
related aspects. To make a complete account of previous research on the subject is
well beyond the objectives of this book, and we will only mention a few significant
contributions. Triassic and Jurassic bivalve geographic distributions were analyzed
in many ways, not only from the descriptive point of view and the recognition of
paleobiogeographic units (Hallam 1969, 1971, 1977; Fürsich and Sykes 1977;
Kobayashi and Tamura 1983a, b; Silberling 1985; Hayami 1989, 1990; Tamura
1990; Liu et al. 1998; Grant-Mackie et al. 2000; Damborenea 2002; Shurygin
2005), but also as the main arguments to propose terrane movements (Newton
1983, 1987; Silberling et al. 1997; Aberhan 1998, 1999), migration routes
(Marwick 1953), the opening of seaways (Damborenea and Manceñido 1979,
1988; Hallam 1983; Newton 1988; Liu 1995; Damborenea 2000; Aberhan 2001,
2002; Sha 2002), or even to relate biogeography with evolution (Ando 1987) and
extinctions (Kiessling and Aberhan 2007), to explain the restriction to certain
facies (Broglio Loriga and Neri 1976), or add to the geographic history of vast
regions (Hayami 1961, 1984; Hallam 1967; Stevens 1967, 1977, 1980; Chen 1982;
Hallam et al. 1986; Smith et al. 1990; Niu et al. 2011). They have been particularly
instrumental to the discussion of general biogeographic issues such as bipolarity
(Crame 1986, 1993, 1996; Damborenea 1993, 1998; Sha 1996) and latitudinal
gradients (Damborenea 1996; Crame 2002; Niu et al. 2011) and the different
factors governing distribution (Hallam 1981; Hayami 1987; McRoberts and
Aberhan 1997).
Cretaceous bivalves and biogeography have also been the subject of many
papers (for instance Kauffman 1973, 1975; Dhondt 1992, 1999), sometimes
restricted to particular groups, such as rudists (Douvillé 1900; Coates 1973;
Skelton and Wright 1987; Voigt et al. 1999; and many others) or specific biologic
issues, such as evolutionary dynamics (Jablonski and Hunt 2006). There are also
some recent contributions specifically related to the Southern Hemisphere:
Aguirre-Urreta et al. 2008.
3.1 Previous Research: A Northern Hemisphere Affair 25
Cenozoic and living bivalves provide a wealth of information which has been
used from the biogeographic viewpoint, either descriptively (e.g., Hall 1964;
Emerson 1978; Zinsmeister 1979, 1982; Darragh 1985; Masse 1992) or method-
ologically (e.g., Jablonski and Valentine 1990), and the issue of latitudinal gra-
dients in particular has been the subject of many discussions (e.g., Jablonski et al.
1999, 2000; Roy et al. 2000). In this case, the biologic significance of bivalve
distribution is being addressed from many points of view, such as extinction
(Flessa and Jablonski 1995).
3.2 Some General Issues
Despite the fact that most adult bivalves are not very mobile (except for some
swimmers), many taxa have a remarkably wide biogeographic distribution. In
living bivalves this is due to several biologic factors (already discussed by
Kauffman 1975), as follows:
1. induced spawning when conditions are optimum for fertilization and larval
survival,
2. a large egg yield,
3. a long-lived, planktonic larval stage, which may be extended if high-stress
environments are imposed or suitable substrates are not encountered during
settling,
4. a broad environmental tolerance in late larval and adult stages, including var-
iable substrates and temporary, high-stress conditions,
5. effective adult mobility in some (swimmers),
6. ‘‘pseudoplanktonic’’ or epi-planktonic habits,
7. tolerance to extreme low-oxygen conditions.
On the whole, these factors are more difficult to evaluate when dealing with
fossil taxa, but among them a planktotrophic larval stage can sometimes be
directly observed in well-preserved material, while the swimming and pseudo-
planktonic habits can be inferred from the morphology of the shell and/or taph-
onomic conditions, and tolerance to low-oxygen conditions is now supported by
geochemic studies. For this reason, only these four factors will be discussed here in
relation to Triassic and Jurassic taxa, with the addition of a few remarks about
reef-forming bivalves during that time.
3.2.1 Larval Types and Dispersion
Bivalves have a wide variety of larval and developmental types, but most marine
taxa have either planktotrophic or lecitotrophic-shelled larval stages, and these can
sometimes be recognized in well-preserved specimens with preserved protoconch

(see Jablonski and Lutz 1983 for a good review on the subject).
Knowledge on larval type distribution of living bivalves is scarce, among other
things because they are difficult to identify since they usually have few distinctive
features. For fossil species, information is still too incomplete. Unfortunately,
present data on larval types of Mesozoic bivalves are limited to a small number of
species, and there are even important groups for which direct evidence is not
available yet. Sometimes the scanty information on living species is extrapolated
and referred to the fossil taxa belonging to long-ranging families. However, this
approach is still unsafe, since variability of this condition within groups is not
sufficiently known. Moreover, Triassic and Jurassic marine bivalve faunas contain
many groups with no living representatives, such as monotoids and inoceramoids.
Within the first, an extended planktotrophic larval stage may have aided long-
distance dispersal in halobiids (McRoberts 1997). Some information is also known
about inoceramids: planktotrophic larvae in well-preserved Cretaceous material
were described by Knight and Morris (1996) and Tanoue (2003); they also dis-
cussed the biogeographic significance for the dispersal of the group. The wide
distribution of Triassic daonellids was attributed to long-term planktotrophic lar-
vae (Campbell 1994; McRoberts 1997). For the Jurassic, larval shells are known in
oxytomids and oysters (Palmer 1989), and bakevelliids (Malchus 2004).
Dispersal of the long-lived planktotrophic larvae is generally considered more
effective than rafting of adults or egg masses (Kauffman 1975). However, this is
still a controversial issue and cannot be applied without duly considering the
0
particular conditions (e.g., O Foighil 1989). Based on the study of two gastropod
species, Johannesson (1988) argued that even if transport of benthic stages hap-
pens very rarely, this may be more influential than larval dispersal over long
distances. Liu et al. (2007) studied the distribution of bivalve larval types along
latitude in the east Pacific, and concluded that the average geographic dispersal
levels of planktotrophic and non-planktotrophic larval types do not show great
differences.
Pelagic planktotrophic larvae spend a large time (up to several weeks) in the
water column before settling, and are thus mainly distributed by currents. During
this time, food availability is one of the factors affecting their development and
metamorphosis capacity. The long potential survival of holopelagic larval stage
might allow for enough time to disperse even across large oceans (Scheltema
1977, 1988; Scheltema and Williams 1983) and although rare in living forms,
extreme holopelagic adaptations were favored for several fossil taxa (Oschmann
1993), such as Triassic halobiids, for instance (McRoberts 1997).
Another aspect almost impossible to be considered for fossil species is the
vertical distribution of planktotrophic larvae, which is normally stratified, and the
associated depth is related to differences in water movement velocity. Bivalve
larvae are capable of swimming and adjusting their vertical position in the water
column (Raby et al. 1994).
In synthesis, although we recognize the great interest and potential of including
larval types in the analysis of bivalve distribution, there is yet not enough
3.2 Some General Issues 27
information to do so without resorting to large-scale extrapolation. For these

reasons no attempt will be done here to use inferred larval types in the analyses.
3.2.2 Swimming Habit
In the context of bivalve large-scale geographic dispersion, we are not interested in

occasional swimmers but on truly nektoplanktonic bivalves, and these seem to
have been particularly uncommon. The swimming habit is a late acquisition in
bivalve evolution, but already in the Silurian some species had adaptive conver-
gent features with Mesozoic Posidoniidae, which were regarded as nektoplank-
tonic (Kříž 1996).
However, at least a facultative swimming habit seems to have been compara-
tively widespread among Triassic bivalves, when members of the Posidoniidae
(Bositra and others) and Halobiidae (Halobia, Daonella) were frequently men-
tioned as ‘‘pelagic’’ bivalves. The external morphology of some halobiid species
could fit into a swimming mode of life (subcircular form, equivalve, and short
hinge line), but the adductor muscle scar is small and falls just below the umbo,
and the shells are too thin (Schatz 2005). All the discussion is far from settled, and
it is possible that the different morphologies of members of the group indicate
slightly different modes of life.
Several authors argued that at least some species of Bositra were nektonic
(Jefferies and Minton 1965; Hayami 1969; Duff 1978). Also, Posidonotis can be
included in this group, as it is frequently found in facial equivalents to the
‘‘Posidonienschiefer’’ of central Europe forming shell pavements. These bivalves
are often called ‘‘paper-clams’’ (Wignall 1990) or ‘‘flat-clams’’ (see Aberhan and
Pálfy 1996) and occur abundantly in dark shales. For most of them reclining and
pseudoplanktonic habits were also proposed (see discussion below).
3.2.3 Pseudoplanktonic Habit
Exclusive pseudoplanktonic species are expected to be rare faunal elements, but

preserved in a very wide range of environments and facies. Some epibyssate
bivalves can have a facultative pseudoplanktonic habit if they fix to floating
objects. There are several groups of bivalves which thrived during the Triassic and
Jurassic and for which a mostly pseudoplanktonic habit has been proposed
(Wignall and Simms 1990), though for few of them this is currently accepted as
their main mode of life. These are thought to be mostly benthonic bivalves which
occasionally could fix themselves to swimming organisms or floating objects, and
so the role of this mode of life for geographic dispersal is not regarded as
significant by some authors. On the other hand, in view of the extremely rapid
dispersal (nearly instantaneous at geologic scale) experienced by some extant
invasive bivalve species after accidental settlement of a few individuals in suitable

habitats, rapid dispersal aided by pseudoplanktonic rafting cannot be dismissed. A
few of the Triassic-Jurassic genera which could have facultative pseudoplanktonic
species will be briefly mentioned below, and it is interesting to note that many of
these species have a particularly wide geographic distribution:
Posidonotis Losacco: A pseudoplanktonic mode of life was proposed by
Hillebrandt 1980, 1981) and extensively discussed by Aberhan and Pálfy (1996)
on the basis of functional morphology, abundance, and facies distribution. These
authors concluded that species of this genus had a mainly benthic mode of life.
Bositra de Gregorio: Several authors postulated the possibility of a pseudo-
planktonic mode of life for this bivalve, on account of its preferred occurrence in
dark, organic-rich mudstones (see for instance Stanley 1972), while others
regarded it as nektoplanktonic. Etter (1996) re-evaluated the life habits of this
bivalve and concluded that B. buchi (Roemer) was a truly benthic species and
probably lived byssally attached to the substrate. An updated discussion on Bositra
mode of life was provided by Caswell et al. (2009, see references therein).
Pseudomytiloides Cox: A mainly benthic mode of life was also favored for P.
dubius (Sowerby) by Etter (1996), a species which is nevertheless known to occur
as facultative pseudoplanktonic, as revealed by its occurrence in clusters arranged
in lines (Caswell et al. 2009).
Halobia Bronn and Daonella Mojsisovics: Several modes of life were assigned
to species of this group (Campbell 1994), although Schatz (2005) supported an
epibenthic pleurothetic mode of life on soft substrate for Daonella species.
However, other authors, such as McRoberts (1997) and Campbell (1994), sug-
gested that a pseudoplanktonic mode of life cannot be ruled out, although Schatz
(2005) argued that the morphology exhibited by halobiids does not support this
hypothesis. McRoberts (1997) favored a pseudoplanktonic or epibenthic mode of
life for this extremely diverse and short-lived bivalve group. He suggested that
halobiids were probably not obligate pseudoplanktonic but, being opportunistic,
could attach themselves to any available firm substrate (drifting or not).
3.2.4 Reef-Forming Bivalves
During severe biotic crises reef-forming organisms have particularly suffered, but
the reef ecosystem has proven its resilience by being successively renovated after
every extinction event. This is extensively documented in the interesting history of
reef-building organisms through time, and in our particular time interval bivalves
played a prominent role after the end Triassic extinction, which decimated corals,
and just before the establishment of scleractinian coral reefs.
The classic deposits are collectively known as ‘‘Lithiotis’’ facies (named from
the peculiar genus Lithiotis Gümbel) of the Calcari Grigi, and were first described
from Pliensbachian-Toarcian beds in the southern Alps (Tausch 1890; Böhm
1906). The fossil associations are not diverse, and, besides particular gastropods
and brachiopods, they include a variety of thick-shelled bivalves such as Lithiotis,

Cochlearites Reis, Lithioperna Accorsi Benini, Opisoma Stoliczka, Gervilleio-
perna Krumbeck, Mytiloperna Ihering (in the sense of Accorsi Benini and Broglio
Loriga 1982), Pseudopachymytilus Krumbeck, and some megalodontids, as
Pachymegalodon Gümbel (Accorsi Benini 1979, 1981, 1985).
The geologic setting and both the geometry and the internal structure of the reef
bodies were compared to the Cretaceous rudist limestones (Rey et al. 1990). The
Alpine reefs are up to 5 m thick and tens of meters wide, and reef builders were
Lithiotis, Lithioperna, and Cochlearites species. These exhibit a wide range of
adaptations to this habitat and life habit, including very thick shells with discordant
valves (one of them acting as a thin elastic operculum), allometric growth of the
apical parts, and remarkable phenotypic variability (see discussion in Chinzei 1982
and Savazzi 1996). The possibility of symbiosis with green algae was proposed
(Accorsi Benini and Broglio Loriga 1977) and later challenged on the basis of the
analysis of growth increments (Accorsi Benini 1985), the large size being attrib-
uted to long life span (up to 30 years) at a normal growth rate.
Of special interest in the context of this book is the geographic distribution of
the reef builders Lithiotis and Cochlearites (see maps in Broglio Loriga and Neri
1976; Nauss and Smith 1988; Krobicki and Golonka 2009), which were confirmed
from Italy, Austria, Slovenia, Croatia, Albania, Morocco, Spain, Oman, Arab
Emirates, Iran, Timor Indonesia, USA (where Lithiotis was called Plicatostylus
Lupher and Packard), Peru, and northern Chile, and also reported from France and
the Himalayas. This is a characteristic low-latitude, southern Tethyan distribution
(see Sect. 3.3), and can be compared in latitudinal extension to the distribution of
modern coral reefs. The Lithiotis bioherms and biostromes extended mainly along
the southern margin of Tethys and reached both South and North America, but
never extended to high paleolatitudes.
This particular bivalve association is the best Tethyan marker for Pliensba-
chian/Toarcian times and will be mentioned in several places of this book. It helps
to define the boundaries of the Tethyan biochorema, and has also been used in the
discussion of the establishment of the Hispanic Corridor.
3.2.5 Low-Oxygen Tolerance
Some Mesozoic bivalves are usually found associated to oxygen-poor sedimentary

environments, their abundance in such settings being probably related to an
opportunistic behavior (Etter 1996). These bivalves are collectively and informally
called ‘‘paper pectens’’ or ‘‘flat clams’’ due to their usually very thin shell. The
better studied are those from the lower Jurassic Posidonienschiefer in Europe. Data
of the detailed relationship of such bivalves with true anoxic events are just being
developed, and show that each species may have slightly different occurrence
patterns. For instance, in Yorkshire Bositra radiata occurs at the beginning and the
end of the main carbon-positive excursion but it is absent during the most severe
Fig. 3.1 Geographic distribution of Posidonotis cancellata (Leanza) in the Neuquén basin,
Argentina. Inset map shows the location of the logged sections, the sketches are leveled to the top
of the Posidonotis-bearing beds. Local bivalve biozonation indicated between sections A and
B. See detail of the Arroyo Lapa section (K, extreme right) in Fig. 3.2
conditions during the Toarcian Oceanic Anoxic Event (TOAE), while Pseud-
omytiloides dubius is the only benthic taxon then present (Caswell et al. 2009).
In the western Americas, species of Posidonotis form shell pavements (see
Fig. 4.4) in dysaerobic environments (Damborenea 1989; Aberhan and Pálfy
1996), and were qualified as low-oxygen tolerant by these last authors. Posidonotis
cancellata (Leanza) has a wide geographic distribution in the Neuquén basin
(Argentina) (Fig. 3.1).
A very detailed analysis of its distribution around the TOAE at the Arroyo Lapa
section (Al-Suwaidi et al. 2010) shows that Posidonotis became abundant, forming
dense monospecific shell pavements, just before the negative carbon isotopic
excursion, but was not present either during or after the harshest conditions
(Fig. 3.2).
McRoberts (1997) discussed the adaptations that enabled Halobia to live in
poor-oxygen environments but concluded that there is only circumstantial evi-
dence for a chemosymbiotic association with sulfur-reducing bacteria.
Fig. 3.2 Arroyo Lapa section, Neuquén basin, Argentina. General section to the left, detailed
section of the Pliensbachian-Toarcian boundary beds to the right. Beds with Posidonotis
cancellata (Leanza) shell pavements (arrows) related to the total organic carbon and d13Corg.
Chemostratigraphic data from Al-Suwaidi et al. (2010)
Fig. 3.3 General patterns of paleobiogeographic distribution recognized here for Triassic and
Jurassic marine bivalves, plotted on a Pliensbachian-Toarcian paleogeographic map based on
Smith and Briden (1977) and Scotese (1997)
3.3 Paleobiogeographic Affinities
For some of the various analyses exposed later in this book, bivalve genera were
classified according to their paleobiogeographic affinities, following Kauffman
(1973); Stevens (1980) and Damborenea (1993), as follows (Fig. 3.3):
Pandemic (cosmopolitan): A large number of genera are widespread bivalves,
truly cosmopolitan forms, such as Nuculana, Palaeoneilo, Mytilus, Pseudolimea,
Daonella, Halobia, Entolium, Camptonectes, Eopecten, Bositra, Meleagrinella,
Oxytoma, Neoschizodus, Trigonia, Septocardia, Pleuromya, Pholadomya, etc.
Several species of these genera have surprisingly wide geographic distributions.
On the other hand, it is interesting to note that many genera traditionally referred
to as ‘‘cosmopolitan’’ have not been reported from the Southern Hemisphere
during the time interval considered. These include, for instance, Atrina, Cirto-
pinna, Ctenoides, Limopsis, Linotrigonia, Malletia, Martesia, Megalodon,
Mytiloides, Palaeomya, Paratancredia, Protocardia, Septifer. Other so-called
‘‘cosmopolitan’’ genera are present only in the northern regions of the Southern
Hemisphere but are not known from the southernmost areas. Examples are
Hippopodium, Neomegalodon, Pinguiastarte, and Rollieria. All genera mentioned
3.3 Paleobiogeographic Affinities 33
Fig. 3.4 General map of low-latitude distribution pattern and some examples of Tethyan bivalve
genera: a Opisoma, b Lycettia, c Gervillaria. Specimens illustrated are: a Opisoma sp., MLP
18460, early Toarcian, Argentina; b Lycettia cf. lunularis (Lycett), MLP 19091, early Bajocian,
Argentina; c Gervillaria pallas (Leanza), MLP 19079, Pliensbachian, Argentina. Scale bars:
10 mm. MLP: La Plata Natural History Museum
with such distributions were excluded from the list of ‘‘cosmopolitan’’ taxa for the
analysis.
Low-latitude or Tethyan: Genera restricted to low paleolatitudes (Fig. 3.4).
During the time involved the Tethyan sea extended along the low paleolatitudes.
Tethyan bivalve faunas were characterized by their high diversity and the abun-
dance of large, thick-shelled forms. Some examples are Rhaetavicula, Curionia,
Trichites, Spondylopecten, Radulopecten, Lycettia (Fig. 3.4b), Anningella, Caen-
odiotis, Krumbeckia, Wallowaconcha, nearly all megalodontids, many arcoids and
pterioids (Fig. 3.4c), and those restricted to Lithiotis reef facies as Cochlearites,
Lithioperna, Opisoma (Fig. 3.4a), Gervilleioperna, Pseudopachymytilus, and
Pachymegalodon.
High-latitude: Several Triassic and Jurassic bivalve genera show a geographic
distribution restricted to areas that, according to most paleogeographic recon-
structions, were at high latitudes during those times. As could be expected most of
these high-latitude taxa were also restricted to the Panthalassa or Paleo-Pacific, the
only ocean displaying the whole latitudinal range at that time. These genera were
previously thought to be either East Asian or Southwest Pacific endemics, but now
the following patterns can be recognized (Damborenea 1993):
Fig. 3.5 General map of austral distribution pattern and some examples of Maorian bivalve
genera: a Oretia, b Pseudaucella, c Malayomaorica. Specimens illustrated are: a Oretia coxi
Marwick, NZGS-TM 2283 (holotype), Oretian, New Zealand (reproduced from Speden and
Keyes 1981, pl. 9, Fig. 23); b Pseudaucella marshalli (Trechmann), OU 3501, Ururoan, New
Zealand; c Malayomaorica malayomaorica (Krumbeck), NZGS-TM 5783, Heterian, New
Zealand (reproduced from Speden and Keyes 1981, p. 14, Fig. 21). Scale bars: 10 mm. OU:
Otago University. NZGS: New Zealand Geological Survey
1. Austral (= Maorian or paleoaustral). Only very few genera were widespread but
restricted to high latitudes in the Austral regions (Fig. 3.5), as Oretia,
Malayomaorica, Pseudaucella. Besides, there are a lot of genera which were
endemic to particular austral regions, notably New Zealand and/or New
Caledonia.
2. Boreal. No genera with such distribution reached the Paleo-Southern Hemi-
sphere, and thus will not be discussed in detail here, but there were some
bivalve genera restricted in distribution to boreal regions, as Ochotochlamys
and Amuropecten.
3. Didemic, antitropical, or bipolar (Fig. 3.6), restricted to high latitudes and
present in both hemispheres, being absent from the low-latitude intervening
areas (Crame 1993; Damborenea 1993; Sha 1996). Examples are Aparimella,
Maoritrigonia, Minetrigonia, Ochotomya, Triaphorus, Asoella, Kalentera
(Fig. 3.6c), Retroceramus (Fig. 3.6a). Also, various pectinacean taxa, previ-
ously thought to be restricted and characteristic of Boreal regions, have been
found in southern South America (Damborenea 1993) and their distribution was
Fig. 3.6 General map of bipolar distribution pattern and some examples of bipolar bivalve
genera: a Retroceramus, b Kolymonectes, c Kalentera. Specimens illustrated are: a Retroceramus
stehni Damborenea, MLP 14672 (holotype), Early Callovian, Argentina; b Kolymonectes weaveri
Damborenea, Late Pliensbachian, field photograph, Chubut, Argentina; c Kalentera riccardii
Damborenea, MLP 24308, Pliensbachian, Argentina. Scale bars: 10 mm. MLP: La Plata Natural
History Museum
in fact bipolar instead (Fig. 6.1), for instance Palmoxytoma, Kolymonectes

(Fig. 3.6b), Radulonectites, Agerchlamys, Arctotis. This particular distribution
pattern and representative genera will be discussed more extensively on Sect. 6.1
in relation to bipolarity.
Trans-temperate (Kauffman 1973): this category includes taxa common in
temperate regions, but with a distribution not latitudinally limited, and at the same
time not pandemic. For instance, bivalves with the so-called ‘‘Pacific’’ distribution
(Fig. 3.7), i.e., along the margins of the Paleo-Pacific ocean both in low and
medium (rarely also high) paleolatitudes, only occasionally present in other areas,
such as Otapiria (Fig. 3.7a), Weyla (Fig. 3.7c), Lywea, Posidonotis (Fig. 3.7b),
Cardinioides, and some trigonioidean genera, at least during part of their time
range.
Endemic: These are the key elements to recognize and characterize biogeo-
graphic units. Only true endemics (from each region considered here compared at
the global scale) were counted as such (Fig. 3.8). Endemism was high in various
Southern Hemisphere areas in the Late Triassic, almost disappeared after the
Triassic/Jurassic extinction, and was again significant for late Early Jurassic, to
Fig. 3.7 General map of trans-temperate distribution pattern and some examples of Pacific
bivalve genera: a Otapiria, b Posidonotis, c Weyla. Specimens illustrated are: a Otapiria
neuquensis Damborenea, MLP 16480 (holotype), Pliensbachian, Argentina; b Posidonotis
cancellata (Leanza), Early Toarcian, field photograph, Argentina; c Weyla angustecostata
(Philippi), MLP 19076, Early Toarcian, Argentina. Scale bars: 10 mm. MLP: La Plata Natural
History Museum
decline toward the end of the Jurassic. There are many genera which were endemic
to New Zealand and/or New Caledonia, for instance Etalia, Marwickiella,
Agonisca, Caledogonia, Hokonuia (Fig. 3.8c), Praegonia, Oretia, Ouamouia in
the Triassic; Notoastarte, Haastina, Kanakimya, Moewakamya, Austrocardilanx in
the Jurassic. Isopristes, Perugonia, and Schizocardita (Fig. 3.8a) were endemic
to the central Andean region in South America during the late Triassic, while in
the Jurassic Quadratojaworskiella, Gervilletia, Gervilleiognoma, Neuquenitri-
gonia, Andivaugonia (Fig. 3.8b), Eoanditrigonia, Anditrigonia, and perhaps a few
others were characteristic of that region. During the late Triassic some peculiar
genera were exclusively known from Australia-New Guinea, as Gervillancea,
Guineana, Somareoides (Fig. 3.8d), and Krumbeckiella. Other set of genera
characterized the late Triassic bivalve faunas from Iran, such as Antiquicorbula,
Healeya, Modesticoncha, Primahinnites, Triasoperna, and Umbrostrea.
Only pandemic and endemic taxa can be objectively defined, reference of some
genera to either of the other categories may involve necessarily a debate. This is
not a serious drawback in this context, however, since these last categories are only
accessory elements in this study. From the nearly 500 bivalve genera known from
Fig. 3.8 Some examples of endemic bivalve genera from different regions and ages:
a Schizocardita, b Andivaugonia, c Hokonuia, d Somareioides. Specimens illustrated are:
a Schizocardita cristata Körner, Norian, Perú (reproduced from Körner 1937, Fig. 4);
b Andivaugonia radixscripta (Lambert), MLP 6710, Bajocian, Argentina; c Hokonuia limaefor-
mis (Trechmann), OU 17487, Otamitan, New Zealand; d Somareioides hastatus (Skwarko),
Norian, New Guinea (reproduced from Skwarko 1983, p. 1, Fig. 9). Scale bars: 10 mm. MLP: La
Plata Natural History Museum; OU: Otago University
the Jurassic, 52 % can be regarded as low latitude, 21 % as high latitude, 6 % as

trans temperate, and 21 % as pandemic (Damborenea 2002, p. 53). It is interesting
to note here that for the whole Jurassic, and despite the paleogeographic asym-
metries mentioned above, endemic genera are proportionately equally frequent in
high-latitude and in low-latitude regions.
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Chapter 4
Regional Scale
Abstract First-hand knowledge of Triassic and Jurassic bivalve faunas from

western South America can be applied to the discussion of several regional issues
related to paleobiogeography, such as the close relationship of bivalve local dis-
tribution with sedimentary facies. Special attention is paid to the analysis of
bivalve species distribution along a 25° latitudinal range of the Panthalassa paleo-
coast, done in time slices corresponding to the Jurassic stages. The results did not
show the expected decrease in species diversity toward higher latitudes. A local
diversity increase between 30 and 40° S Lat., especially evident for Pliensbachian
and Toarcian times, may be due to the establishment of a variety of habitats within
the extensive Neuquén basin. The latitudinal distribution pattern was also explored
using cluster analysis and faunal turnover analysis. These two analytical methods,
together with the distribution of species with high-latitude biogeographic affinities,
consistently show the existence of a faunal turnover area, here interpreted as the
boundary between Tethyan and South Pacific faunas, which migrated southwards
with time. This shift amounts to about 700 km (about 8° paleolatitude) between
Hettangian and Toarcian times. The analysis for changes in proportion of different
superfamilies along this latitudinal range indicated a decreasing diversity trend
toward higher latitudes for Trigonioidea, Limoidea, and Pholadomyoidea, while
the opposite trend was shown for some of the time slices by Nuculanoidea, Mo-
notoidea, Pectinoidea, and Crassatelloidea.
Even at a regional scale, paleobiogeographic studies are complex and it is nec-

essary to have good paleogeographic base maps and a reliable and independently
based time frame. Unfortunately, this is not the case for several regions included in
this study, in many areas of the Southern Hemisphere the geologic history is not
known in sufficient detail yet. For this reason, and also because it is the area we
know best, we will mostly exemplify this approach using data from western
Argentina/Chile (Fig. 4.1).

DOI: 10.1007/978-94-007-5098-2_4, Ó The Author(s) 2013
46 4 Regional Scale
Fig. 4.1 General paleogeographic context for the late Early Jurassic at different scales. a Global
scale, compiled from several sources. b Continental scale, showing western South American
paleogeography (modified from Vicente 2005). c Regional scale, generalized paleogeography of
the Neuquén Basin during Pliensbachian-Toarcian times (modified from Legarreta and Uliana
2000). The extent and distribution of the different paleoenvironments changed with time, see
discussion in text. Note that in a the inferred paleolatitudes are indicated, while in c present-day
latitudes are referred to instead
4.1 Facies and Bivalve Distributions: Examples from

the Neuquén Basin
The Mesozoic South American basin system developed along the eastern Pan-
thalassa margin. By Norian times these basins probably had no less than two
connections with the open ocean, while by Early Jurassic times at least three
passages were established (Vicente 2005; See Fig. 4.1b), the southernmost
(Curepto) connecting the Neuquén basin with the Paleo-Pacific. The Neuquén
Basin extended at the western margin of South America; it formed a wide eastward
embayment between 36 and 40° present S latitude. It was a typical back-arc basin,
4.1 Facies and Bivalve Distributions: Examples from the Neuquén Basin 47
and its paleogeography has been the subject of many studies since the pioneer
synthesis by Groeber (1946). Paleogeographic maps depicting the extension and
facies distribution of marine deposits in the basin are now available at stage-scale
intervals (Gulisano 1992; Riccardi et al. 1992, 2011; Legarreta and Uliana 1996,
2000).
This local basin mostly developed in the present territories of San Juan,
Mendoza, and Neuquén provinces in Argentina and in part of central Chile
(Fig. 4.1c), and accumulated a thick marine sedimentary succession from late
Triassic to late early Cretaceous times. During the late Early Jurassic it extended
temporarily to the South into Chubut province in Argentina.
The history of this extensive basin was complex but is relatively well known
(see Legarreta and Uliana 1996 for a good synthesis), in part due to the economic
interests derived from oil production. As a result, the biostratigraphic (Riccardi
2008a, b), paleogeographic, and paleoenvironmental frames are good enough to
attempt the analysis and interpretation of regional bivalve distribution.
The available paleogeographic maps allow us to examine the distribution of
bivalves in the context of the environmental conditions at each stage. The gathered
information on the detailed geographic range of more than a hundred Triassic and
Jurassic bivalve species from the Neuquén Basin demonstrates once again the
well-known fact that lithofacies (mainly related to substrate type) and distribution
are highly correlated, although this is not the only factor to account for. Other
aspects, such as water circulation patterns, oxygenation, paleosalinity, and
paleotemperature, should also be considered. Of the wealth of data at hand, only a
few examples, placed at different time intervals, will be mentioned to exemplify
the importance of an adequate evaluation of facies when dealing with paleogeo-
graphic distributions of bivalves.
4.1.1 Late Triassic
An interesting example at a large regional scale derives from the affinities of the
late Triassic fauna from southern Mendoza in Argentina (Damborenea and
Manceñido 2012). Many of its bivalve species (e.g., belonging to the genera
Cassianella, Palaeocardita, Septocardia, Minetrigonia?, Liostrea) are more
related to those known from the late Triassic of Perú and northern Chile than to
those from the Chilean coast at the same latitude (Curepto area). There is an
evident environmental distinction between the two sets of species, a western belt
being dominated by open ocean species (probably outer shelf to off-shore), mostly
monotoids and limids. On the other hand, the eastern fauna developed in a marine,
well-oxygenated, littoral environment, with shallow water depth and type of
substrate as the critical factors limiting faunal distribution. It is evident here that
both main facies are widely distributed latitudinally in western South America as
two belts roughly parallel to the paleoshore (Fig. 4.2).
48 4 Regional Scale
Fig. 4.2 Main facies and

faunal distribution for latest
Triassic (Norian–Rhaetian)
bivalve bearing deposits in
western South America. Only
main areas are mentioned, see
discussion in text
Goodwin (1997) discussed a similar situation in the Northern Hemisphere:

Norian faunas from Sonora (Mexico) are very close to those from Nevada (USA)
despite latitudinal differences in present-day location. Only after an adequate
evaluation of paleoenvironmental conditions a discussion of paleogeographic
affinities can be attempted. In our case, littoral faunas are related to northern ones,
while open marine faunas have high-latitude affinities, in agreement with inferred
surface paleocurrents (see Sect. 1.6, Fig. 1.3a).
Fig. 4.3 An example of the close relationship between faunal distribution and paleogeography in
the Neuquén Basin. The localities where four Pliensbachian-Toarcian pectinoid species occur are
indicated on the corresponding paleogeographic map (paleogeography modified from Legarreta
and Uliana 2000). Specimens figured not to scale. References to facies in Fig. 4.1
4.1.2 Early Jurassic
The analysis of the distribution of four pectinoid species during the Pliensbachian-
Toarcian within the Neuquén embayment (Fig. 4.3) again shows that there is a
close correspondence between distribution and environment. The thin-shelled and
probably swimming species Posidonotis cancellata (Leanza) and Kolymonectes
weaveri Damborenea are only abundant in inner basin areas, although they may
occasionally appear elsewhere. On the other hand, the recliner Weyla bodenben-
deri (Behrendsen) and the byssate Radulonectites sosneadoensis (Weaver) are
usually found in sublittoral to platform sites.
In this context, and extending beyond the region, it is interesting to further
discuss the distribution of species of the peculiar genus Posidonotis. This pectinoid
clearly qualifies as a ‘‘paper-clam’’ or ‘‘flat-clam’’ and occurs abundantly in dark
shales (Fig. 4.4), in facial equivalents to the ‘‘Posidonienschiefer’’ of central
Europe. In South America, the genus ranged from latest Pliensbachian to earliest
Toarcian times (Damborenea 1989), forming highly concentrated monospecific
50 4 Regional Scale
Fig. 4.4 The paper-clam Posidonotis cancellata (Leanza), field photograph at Arroyo Lapa,
Neuquén Basin, earliest Toarcian. Slab showing the usual preservation as dense shell pavements
in dark shales. Inset nearly complete valve
shell pavements in thin-bedded sediments related to the Early Toarcian anoxic

event (Al-Suwaidi et al. 2010). The species P. dainellii Losacco is known from
Japan in beds of the same age (Hayami 1988). In North America, Posidonotis is
known from older deposits (Sinemurian), concentrated at a few levels also prob-
ably related to poorly oxygenated bottom conditions (Aberhan and Pálfy 1996).
Posidonotis was apparently absent from extreme Boreal and Austral regions, and
was restricted to low and mid-paleolatitudes of the circum-Pacific, sporadically
occurring in the Tethys (Damborenea 1989; Aberhan and Pálfy 1996). Throughout
its geographic range, Posidonotis thrived in dysaerobic conditions (see Sect. 3.2.5,
Figs. 3.1 and 3.2), but it could survive too in more oxygenated environments,
where it was never abundant.
The distribution of Kolymonectes weaveri Damborenea also extended beyond
the Neuquén embayment, since this propeamussiid with clear bipolar affinities was
very abundant in several localities further South, in Chubut Province as well
(Fig. 4.5). The genus did not extend to low latitudes although the right facies do
occur further north, but it is also known from late Triassic-Early Jurassic deposits
in the Northern Hemisphere mid- and high-latitude regions (Damborenea 1998, see
references therein). In Argentina, the species occurs in very fine-grained, mostly
light-colored fine-grained sandstones and shales, and is rare in dark shales; it
Fig. 4.5 Mean size attained by left valves of Kolymonectes weaveri Damborenea at different
regions (1–5) of the Neuquén Basin: 1 Atuel/Portezuelo Ancho, 2 Puchenque/Serrucho, 3 Chacay
Melehue, 4 La Pintada, 5 Agnia. Only complete specimens were measured. Mean size: shaded,
minimum and maximum size shown as broken lines. Inset left and right valves of complete
specimen (MLP 23807), Pliensbachian of Chacay Melehue, Neuquén. Right valve shows shallow
byssal notch and the ventral margin broken along the boundary of inner and outer shell layers.
References to facies in Fig. 4.1 (Modified from Damborenea 1998)
clearly preferred well-oxygenated, open sea conditions. The abundance of the local
species increases to the south (southernmost Neuquén and Chubut). Populations
from different localities differ greatly in mean shell size but not latitudinally. It is
probably related to depth and/or oxygen availability rather than temperature or
other latitude-related factors. Larger sizes are attained in deposits of low water
energy, the only accompanying fauna being ammonites. Young individuals were
probably byssally attached but the byssal notch became obsolete in adults and was
then probably not functional; adults most likely lived resting on the substrate and
might have been good occasional swimmers. It is also interesting that propea-
mussiids (or ‘‘glass scallops’’) in general seem to be relegated nowadays to safe
places in the deep sea (Waller 2011), whereas they clearly inhabited shallower
waters in the Triassic and Jurassic. They are also regarded as living relicts due to
their primitive soft anatomy and shell microstructure (Waller 1971, 2006).
52 4 Regional Scale
4.2 Latitudinal Gradients
Bivalves have been fundamental to investigate the nature and origin of marine
diversity gradients (Crame 2000a, b; Valentine and Jablonski 2010, and references
therein). One of the most obvious relationships between biogeography and ecology
is the existence of latitudinal gradients in species diversity. This is generally
understood to mean an increase in species richness from the poles to the tropics,
and is well documented for terrestrial faunas. This pattern was extrapolated to the
marine biotas (Sanders 1968), and later verified for prosobranch gastropods (Roy
et al. 1998) and bivalves, both from the continental shelf (Jablonski et al. 2000)
and deep sea (Rex et al. 1993, 2000) of the Northern Hemisphere. Latitudinal
gradients are thus a remarkable large-scale biotic pattern, which is shared by
terrestrial and marine organisms. Roy et al. (1998) analyzed various previous
hypothesis to explain the origin of this major pattern, and, based on marine gas-
tropods living on the shelves of the eastern Pacific and western Atlantic, they
concluded that sea surface temperature (as the result of solar energy input) is
significantly correlated to the strikingly similar latitudinal gradients observed.
Marine bivalves show clear latitudinal diversity gradients as well, but the
observed pattern is not simple and it does not appear to be symmetric in both
hemispheres (Rex et al. 1993; Crame 2000a, b). Even an inverse gradient is also
regionally known for some groups (Valdovinos et al. 2003; Kindlmann et al. 2007)
along the Chilean coast. The latitudinal gradients in biodiversity are not easy to
interpret because they are strongly influenced by local conditions and the history of
the regions concerned (Crame 2000b), but it was proposed that it is maintained by
high tropical origination rates (Valentine and Jablonski 2010).
The various hypotheses proposed to explain the origin of this pattern are of
general nature and thus imply that this feature should have been present in past
biotas as well. Crame (2000a, b) proved that latitudinal gradients in bivalve tax-
onomic diversity can be traced back to the late Paleozoic in both hemispheres,
though they were not symmetric. Furthermore, he observed that late Paleozoic and
late Jurassic diversity gradients were weaker than present ones (Crame 2001,
2002), and there was a dramatic increase in these gradients during the Cenozoic.
Apart from this well-known and universally recognized latitudinal gradient in
diversity, knowledge about other latitudinal gradients (related for instance to
taxonomy, functional groups, size or intraspecific variability) is still patchy, but
again bivalves provide good data for their discussion.
Based on the analysis of latitudinal distribution of eastern Pacific bivalves, Roy
et al. (2000) concluded that latitudinal patterns of species richness are decoupled
from patterns of body size (modal size, average size and size range). Size-
frequency distributions are significantly undistinguishable over more than
5,000 km of continental shelf, from the equator to the polar sea, although the
family level composition of the modal size class varies considerably with latitude.
On the other hand, concerning bivalve taxonomy, Crame (2000a) argued that
the steepest latitudinal biodiversity gradients for bivalves are related to the
4.2 Latitudinal Gradients 53
youngest clades. Thus, the present-day latitudinal gradient in marine bivalves is

influenced by a tropical and low latitude concentration of infaunal taxa (mainly
heteroconchs), while the gradient in epifaunal pteriomorphs is far less marked.
These results were compared with a similar analysis of late Jurassic bivalve dis-
tribution (Crame 2002), and the differences observed were attributed to a large
Cenozoic heteroconch diversification, which caused a steepening of the latitudinal
gradient, more evident in the Northern Hemisphere. The Anomalodesmata are
peculiar in this context, since they lack a diversity maximum in the tropics
(Krug et al. 2007), and have two maxima in temperate Northern and Southern
Hemispheres.
The latitudinal diversity gradients in living marine bivalves are also evident for
both infauna and epifauna, and for most major functional groups (Roy et al. 2000),
except for the deposit-feeding protobranchs.
Latitudinal diversity gradients in marine Mesozoic bivalves have been recog-
nized and described at a global scale, but little is known about regional patterns in
detail. The paleogeographic position of South America during the Early Jurassic
and the availability of outcrops make this part of Gondwanaland a privileged place
to study the latitudinal variation of benthonic faunas. The South American Paleo-
Pacific margin had a wide range of paleolatitudes, and the available data allow the
examination of bivalve biodiversity (species richness) along a strip of over 20°
paleolatitude (presently between 20 and 45° S lat.).
The first results of such an analysis (Damborenea 1996) showed that within that
paleolatitudinal range there was no apparent diversity gradient referable to latitude
for three different Early Jurassic time intervals then considered. On the other hand,
the latitudinal distribution of the species strongly suggested the presence of a wide
area of mixed faunas between Tethyan (to the North) and Austral (to the South)
faunas, a subject which will be more extensively discussed later (Sect. 5.4).
A new analysis performed on the basis of an updated database (sources listed in
Sect. 5.1.1) of the bivalve species distribution provided the following results,
discussed here in general and then discriminated by systematic and paleobiogeo-
graphic affinities (in the sense discussed in Sect. 3.3).
4.2.1 Diversity gradients
The distribution of bivalve species in about 200 localities from Chile and
Argentina between 20 and 45° (Table 4.1) was recorded for the four Early Jurassic
stages: (a) Hettangian (Fig. 4.7), (b) Sinemurian (Fig. 4.9), (c) Pliensbachian
(Fig. 4.11), and (d) Toarcian (Fig. 4.13). Though it is possible to analyze shorter
time intervals for the distribution of Argentinean Early Jurassic bivalves from the
Neuquén Basin, where their time ranges are determined accurately by accompa-
nying ammonites, the same precision is not yet possible for some of the other
areas. The Neuquén Basin time ranges cannot be extrapolated to the whole area
since differences may be expected due to the large geographic distances involved.
Table 4.1 Main localities and data sources for the latitudinal analysis along western South America. Data are arranged according to 13 areas with a 2°
54
present-day latitudinal range each

Region Present- Main localities Sources of data
day
latitude
range
0 20–22° Socosani, Longacho, Pampa Soledad, Quillagua Pérez and Reyes 1994; Pérez et al. 2008
1 22–24° Cerritos Bayos, Moctezuma, Limón Verde, Calama, Caracoles, Steinmann 1881; Möricke 1894; Pérez and Levi 1961; Harrington
Cuevitas, Cochrane, Azabache 1961; Pérez and Reyes 1977, 1994; Hillebrandt 1990; Aberhan
1994; Pérez et al. 2008
2 24–26° Domeyko, Alto Varas, Bonita, Chaco Sur, Incahuasi, Tres Hillebrandt 1971, 1973, 1977, 1980, 2000; Pérez and Reyes 1977,
Hidalgos, Oreganito, Burros, Aspera, Profeta, Mulas, Paposo, 1994; Covacevich and Escobar 1979; Chong and Hillebrandt
Vaquillas Altas, Carretas, Cachina 1985; Hillebrandt et al. 1986; Quinzio 1987; Aberhan 1994;
Pérez et al. 2008
3 26–28° Doña Inés Chica, Minillas, Pan de Azúcar, Pedernales, San Juan, Möricke 1894; Philippi 1899; Hillebrandt 1973; Pérez and Reyes
Asientos, Caballo Muerto, Tamberías, El Peñón, La Chaucha, 1977; Hillebrandt and Schmidt-Effing 1981; Mercado 1982;
Paipote, El Bolito, El Patón, El Carbón, Cortaderita, La Sepúlveda and Naranjo 1982; Chong and Hillebrandt 1985;
Ternera, Potrerillos, Vaca Muerta, Yerbas Buenas, San Hillebrandt and Westermann 1985; Hillebrandt et al. 1986;
Pedrito, Larga, Noria, Llareta, San Miguel, Figueroa, Jorquera, Quinzio 1987; Hillebrandt 1990, 2000; Aberhan 1992, 1993,
La Guardia, Calquis, Los Eucaliptus, Las Vizcachas, Las 1994, 2004; Pérez et al. 1995, 2008; Aberhan and
Trancas Hillebrandt 1996
4 28–30° Manflas, Amolanas, La Iglesia, Pulido, Juntas del Tolar, Salto del Bayle and Coquand 1851; Burmeister and Giebel 1861;
Toro, El Tránsito, El Corral, La Totora, Chanchoquín, Möricke 1894; Philippi 1899; Groeber et al. 1953; Thiele
Paitepén, Plaza, Tatul, Las Pircas, Pinte, Picudo, La Plata, La 1964; Hillebrandt 1971, 1973, 1977, 2002; Pérez and
Papa, Los Cuartitos, Calabocito, La Punilla, Doña Ana, Elqui Reyes 1977; Hillebrandt and Westermann 1985; Aberhan
1992, 1994, 2004; Pérez et al. 1995, 2008; Aberhan and
Hillebrandt 1996, 1999
5 30–32° Matahuaico, Tres Cruces, Mostazal, Los Pingos, Los Erizos, Bayle and Coquand 1851; Conrad 1855; Philippi 1899; Dediós
El Pachón 1967; Mpodozis et al. 1973; Pérez and Reyes 1977; Ramos
et al. 1993; Aberhan 1994, 2004; own data
(continued)
4 Regional Scale
Table 4.1 (continued)
Region Present- Main localities Sources of data
day
latitude
range
6 32–34° Las Flores, La Honda, Los Molles, La Laguna, La Ligua, del Rigal 1930; Thomas 1958; Cecioni and Westermann 1968; Pérez
Pobre and Reyes 1977; Volkheimer et al. 1978; Damborenea 1987a,
b, 2002a; Pérez et al. 2008; own data
4.2 Latitudinal Gradients
7 34–36° La Manga, Malo, La Horqueta, Tinguiririca, Blanco, Pedrero, Behrendsen 1891; Philippi 1899; Jaworski 1925; Groeber et al.
Los Caballos, Las Chilcas, Araya, La Brea, La Bajada, 1953; Damborenea 1987a, b, 2002a, 2004; Riccardi et al.
Curepto, Portezuelo Ancho, Deshecho, Santa Elena, Salado, 1988, 1991; Pérez et al. 1995; Damborenea and Lanés 2007;
Troncoso, El Infiernillo, Serrucho, Puchenque, Tricolor, own data
Barda Blanca, Chacayco, Poti-Malal
8 36–38° Los Baños, Tocuyo, Ñiraico, Rajapalo, Perfil, Lista Blanca, Damborenea 1987a, b, 2002a; own data
Chacay Melehue,
9 38–40° Del Gringo, Los Toldos, Ñireco, Pichi Picún Leufú, Granito, Weaver 1931; Groeber et al. 1953; Damborenea 1987a, b, 2002a;
Puruvé Pehuén, Vuta Picún Leufú, Lonqueo, Ibáñez, Espinazo Pérez et al. 1995; own data
del Zorro, Llao–Llao, Aluminé, Lapa, Charahuilla,
Keli Mahuida, Los Molles, Picún Leufú, La Jardinera,
Catán Lil, Santa Isabel
10 40–42° Carrán Cura, Salitral Grande, Sañicó, Los Chilenos, Los Pantanos, Leanza 1942; Manceñido and Damborenea 1984; Damborenea
Roth, Mesa, La Pintada, del Vasco, Corona, Piltriquitrón 1987a, b, 2002a; Pérez et al. 1995; own data
11 42–44° Gualjaina, Pescado, Cuche, Peña, La Carlota, Agnia, Currumil, Piatnitzky 1936; Robbiano 1971; Lesta et al. 1980; Lage 1982;
Nahuelquir, Chapingo, Carnerero, Plate, Lomas Chatas, Nullo 1983; Benito and Chernicoff 1986; Vizán 1988;
El Córdoba Massaferro 2001; own data
12 44–46° Puelman, Negro, Altamirano, Piedra Shotle, Parra, Betancourt, Piatnitzky 1933, 1936; Feruglio 1934; Wahnish 1942; Robbiano
La Trampa, Nueva Lubecka, Aguada Loca, Ferrarotti, 1971; Malumián and Ploszkiewicz 1976; Blasco et al. 1980;
Loncopán, Salazar, Guadal, Colorado Lesta et al. 1980; Nullo 1983; Cortiñas 1984; Pérez et al. 1995;
Damborenea 2002a; Pagani et al. 2012; own data
55
56 4 Regional Scale
Table 4.2 Number of bivalve species present in each latitudinal segment for each of the four
stages considered
Regions 0 1 2 3 4 5 6 7 8 9 10 11 12
Hettangian 1 6 7 12 0 0 1 18 – – – – –
Sinemurian 1 8 10 23 54 3 3 67 1 – – – –
Pliensbachian – 5 2 84 57 6 24 87 32 50 67 14 50
Toarcian – 3 1 30 55 3 34 54 24 23 3 4 8
Regions numbered as in Table 4.1
Fig. 4.6 Total bivalve species richness through time for the whole latitudinal range considered
in western South America (20–46° present-day S latitude)
Only eight localities belong to the Coastal Cordillera of Chile; all the others are in
the Andes.
The database for the analysis is a species list showing the distribution of 233
bivalves in 13 areas (0–12), each with a latitudinal range of 2o, spanning a north–
south strip from 20o to 46o S latitude. This database also includes information on
each species taxonomic and paleobiogeographic affinities. A summary of data is
shown on Table 4.2. Though the purpose is the consideration of paleobiogeo-
graphic issues, data were initially plotted on their present-day positions to avoid a
priori bias and circular reasoning. As pointed out by Rosen (1992), present-day
positions are the only universally objective reference for fossil locations available
so far.
It was preferred at this stage to use all the available information, regardless of
the paleoecologic types of the bivalve species. These comprise several life-habit
types, though most of them are shallow platform dwellers. It is well known that
facies control may significantly affect distribution of some bivalves (see Sect. 4.1),
and this should be distinguished from regional factors determining provincialism.
Nevertheless, it is thought that the large number of records and localities taken into
account (comprising a wide range of facies within each area) make this ‘‘noise’’
factor less of a problem.
In central Chile and Argentina the occurrence of typically Tethyan bivalves in
the same area as high-latitude bivalve species is a consequence of a mid-latitude
paleoposition of this region during the Early Jurassic without significant barriers
along the East Paleo-Pacific margin, the probable pantropic nature of Tethyan
faunas, and a shallow sea connection with the western Tethys from middle Early
Jurassic times onwards (Damborenea 1993).
Some general trends through the time involved will be discussed first, before
treating the paleolatitudinally related features of the faunas. Within the study area,
there is a slight decrease in the percentage of ‘‘local’’ species through time from
the Hettangian (67 %) to the Toarcian (60 %). This decline of restricted species
during the Early Jurassic is in agreement with similar trends observed in several
areas of the northern hemisphere (see Hallam 1977) for endemic bivalve genera. It
is interesting to note that Hallam (1977, Fig. 2) recorded an opposite trend for
South America but then correctly attributed it to poorly documented data.
We can now add plots of overall bivalve diversity (number of species) through
time (Fig. 4.6) along the whole studied area in western South America, which
shows a sharp maximum in the Pliensbachian. This fact is in agreement with plots
of the number of bivalve genera worldwide along this same time interval (Hal-
lam 1977, Fig. 1). When data are discriminated according to the biogeographic
affinities of each species, it is evident that all types participate in this diversity
increase except ‘‘Pacific’’ species, which are equally numerous in Sinemurian,
Pliensbachian, and Toarcian times. For this reason, there is almost no change in the
proportion with which these types contribute to the general composition of faunas
through time.
For living bivalves taxonomic diversity at family, genus, and species levels are
covariant with latitude (Stehli et al. 1967; Stehli 1968), and this can be extrapo-
lated to fossil faunas, even during times when climatic belts were apparently ill-
defined (Stehli et al. 1969) as seems to have been during the Early Jurassic.
Concerning diversity latitudinal gradients, the data do not show the expected
decrease in species diversity toward higher latitudes in the geographic range
considered here, but, instead, a local diversity increase between 30° and 40°,
which is especially evident for Pliensbachian and Toarcian times. This local
increase may be due to the establishment of favorable conditions and an increased
variety of habitats within the extensive Neuquén Basin, which at that time was a
quasi-isolated shallow water epeiric sea (see Sect. 4.1 for paleogeographic
reconstructions of this area).
58 4 Regional Scale
4.2.2 Latitudinal Distribution of Biogeographic Affinities

Through Time
Latitudinal species ranges could be sensitive to the variety of physical conditions

displayed through the stretch of coast, including water current systems,
geomorphology of the coast, input of freshwater, local oxygenation conditions,
and so on. At the scale of our data, discontinuities are recognizable by the
concurrence of latitudinal breaks of different species. Our data set can be ana-
lyzed quantitatively to investigate first the presence of latitudinal locality groups
characterized by their fauna. The northern and southern limits of species ranges
were analyzed to recognize sudden changes in faunal composition (see
description of methodology applied in Sect. 2.3.1). Data were then discriminated
by superfamilies and also by paleobiogeographic affinities, to complete the
characterization of the latitudinal distribution patterns for each of the time
intervals. In this way, the evolution of the observed patterns through the Early
Jurassic (c. 25 Ma) can be described as well.
4.2.2.1 Hettangian
No deposits of this age bearing marine bivalves are known to the south of 36° S,
and thus the analysis is constrained to the northern regions of our range
(Fig. 4.7).
Cluster analysis for the Hettangian (Fig. 4.8) shows certain latitudinal gradient,
discriminating between northern (20°–288 S) and southern (32o–36o S) bivalve
faunas. Although northern and southern limits of distribution show some turnover
between 26° and 28° S, it must be pointed out that there is no data for the latitudes
between 28° and 32° S, so the peak for southern limits of distribution may be
overestimated.
With reference to the biogeographic affinities of the analyzed species
(Fig. 4.15a), there are no strictly significant changes along this latitudinal gradient,
but there is a relative reduction toward the south of species with Pacific affinities
(linear predictor: -0.48) that is in the limit of significance (p = 0.083); this
biogeographic grouping disappears by 288 S. Species with high-latitude (bipolar
plus austral) affinities are present and relatively abundant through the whole of the
range (Fig. 4.15a). They also show a not strictly significant (p = 0.098) trend,
with their proportion increasing slightly and evenly toward the South (linear
predictor: 0.12), whereas in absolute numbers the maxima are at the intervals
26–28° (3 species) and at 34–36° (7 species). Species of this group which reach the
northernmost regions are: Palmoxytoma cf. cygnipes (Young and Bird) and
Agerchlamys? sp. The southern diversity peak for high-latitude taxa also contain
Palmoxytoma cf. cygnipes, but to these, Otapiria pacifica Covacevich and
Escobar, and species of Parainoceramus?, Kalentera and Astartidae are added.
Fig. 4.7 Latitudinal ranges

of Hettangian bivalve species
(each vertical line represents
one species), discriminated
by paleobiogeographic
affinities. The 13 latitudinal
areas used for this analysis
(spanning 2° each) are
numbered, and each locality
is represented by a black dot.
Notice that localities south of
36° do not bear Hettangian
faunas. Extended ranges are
shown, but analysis was
mostly developed on actual
occurrence data
On the other hand, species with warm-water affinities are few: Eopecten velatus
(Goldfuss) and a Parallelodon species.
With these data, it is evident that high-latitude species are relatively abundant
south to 32° but extend their influence at least to 22°. This is here regarded as the
range of mixed faunas between the Austral and Tethyan Realms for Hettangian
times (about 900 km wide).
60 4 Regional Scale
Fig. 4.8 Hettangian: cluster and faunal turnover analyses along the studied latitudinal range,
locality map in the center. Left Cluster analysis for latitudinal gradient, values on each node
represent the support value for the node obtained by bootstrapping. Similarity measure:
Simpson’s coefficient; algorithm: paired group; number of iterations for the bootstrapping: 1,000.
Right Faunal turnover analysis
4.2.2.2 Sinemurian
During the Sinemurian (Fig. 4.9) there seems to be a southwards shift in the main
turnover region, as indicated by the cluster analysis (grouping the zones between
26° and 32° S on one hand, and those between 32° and 36° S on the other,
Fig. 4.10) as well as by the limits of distribution (showing a peak of northern and
southern limits between 28° and 30° S). The minor inconsistency between both
types of data may be due to scarcity of records in some regions, being more
reliable the limit suggested by the faunal turnover.
Fig. 4.9 Latitudinal ranges of Sinemurian bivalve species (each vertical line represents one
species) discriminated by paleobiogeographic affinities (line key in Fig. 4.7). The 13 latitudinal
areas used for this analysis (spanning 2° each) are numbered, and each locality is represented by a
black dot. Notice that localities south of 36° do not bear Sinemurian faunas. Extended ranges are
shown, but analysis was mostly developed on actual occurrence data
Once again there is a strong southwards reduction (Fig. 4.15b) in the relative
number of species of Pacific affinities (linear predictor: -0.23; p \ 0.001), while
cosmopolitan species seem to increase in that direction (linear predictor: 0.08;
p = 0.045).
Species with high-latitude affinities (austral or bipolar) which reach the
northernmost regions (up to 24°–26°) are: Agerchlamys? sp., Otapiria pacifica,
O. neuquensis Damborenea, Kalentera sp., Parainoceramus apollo? (Leanza), and
Harpax rapa (Bayle and Coquand). The southern diversity peak for high-latitude
taxa also contains some of these species, but to these, species of Astartidae,
Asoella, and Kolymonectes are added. On the other hand, the number of species
with warm-water affinities is reduced south of 32° (the southernmost increase is
due to the occurrence of just one isolated species in that region).
62 4 Regional Scale
Fig. 4.10 Sinemurian: cluster and faunal turnover analyses along the studied latitudinal range,
The dendrogram was drawn in three dimensions to fit it to the geographic relative positions of the
analyzed areas. Right Faunal turnover analysis
4.2.2.3 Pliensbachian
Reliable data to the north of 26° are very scarce and have only been included for
the sake of completeness. Otherwise, bivalve faunas of this age are by far the best
known for the Early Jurassic of the southern Andean region (Fig. 4.11).
As already said, bivalve faunas show a sharp rise in overall diversity during the
Pliensbachian (see Fig. 4.6), which may be only partially attributed to intensity of
studies. All elements of the fauna participate in this increase in species numbers.
Fig. 4.11 Latitudinal ranges of Pliensbachian bivalve species (each vertical line represents one
species) discriminated by paleobiogeographic affinities (line key in Fig. 4.7). The 13 latitudinal
black dot. Notice that localities north of 26° have very few data. Extended ranges are shown, but
analysis was mostly developed on actual occurrence data
Along the whole study area, bivalves with Tethyan affinities maintain a steady
percentage and coexist with high-latitude taxa.
The complete Pliensbachian database allowed for the most detailed analysis of the
paleobiogeography of the west margin of southern South America. Cluster analysis
(Fig. 4.12) allowed for the discrimination of northern latitudes (22°–32° S) from
southern ones (32°–46°), although the best defined biogeographic region is between
34° and 44° S (i.e., coinciding with the Neuquén embayment at the time). According
to the limits of distribution (Fig. 4.12), and in coincidence with the cluster analysis,
the main biogeographic turnover seemed to be between 34° and 36° S during this
stage, showing an even greater displacement toward south. It is noteworthy that the
64 4 Regional Scale
Fig. 4.12 Pliensbachian: cluster and faunal turnover analyses along the studied latitudinal range,
analyzed areas. Right Faunal turnover analysis. On the map BSN analysis; edge thickness
indicating dissimilarity: thick line: dissimilarity lower than 0.1, intermediate line: dissimilarity
equal or higher than 0.1 but lower than 0.25, thin line: dissimilarity equal or higher than 0.25 but
lower than 0.5; values on the edges: support value for each edge (value is 1 when there is no
indication)
peak of southern limits of distribution is strongly affected by species of Tethyan

affinities (8), while the one of northern limits is affected by species with high-latitude
affinities (12) and also by species of Tethyan affinities (12). The main change at this
boundary is marked by a clear increment toward the south (linear predictor: 0.12;
p 0.001) in the proportion of species with high-latitude affinities (either austral or
bipolar) associated with a reduction (linear predictor: -0.03; p = 0.047) in cos-
mopolitan species (Fig. 4.15c).
For this stage a BSN was constructed (Fig. 4.12), obtaining similar results; a
southern region can be recognized, mostly due to the similarity of many latitudinal
bins with that between 34° and 36° S. This points once again to a biogeographic
turnover at that region. Also, at this stage a significant positive correlation was
found between dissimilarity value and latitudinal separation among samples
(Spearman’s rs = 0.61, p 0.01) indicating a clear latitudinal gradient.
High-latitude taxa are relatively abundant south of 34° (Fig. 4.15c), but Harpax
rapa, Radulonectites sosneadoensis (Weaver), and a species of Palaeopharus?, all
of them with bipolar affinities, reach the 26–28° region. High-latitude faunas south
of 34° are more varied, including 12 species south of 40°. The more conspicuous
elements of this Austral fauna are: Parainoceramus apollo (Leanza), Kolymo-
nectes weaveri Damborenea, Agerchlamys wunschae (Marwick), Otapiria
neuquensis Damborenea, and Kalentera riccardii Damborenea.
4.2.2.4 Toarcian
Toarcian faunas, though widespread and relatively abundant, are less well known
than Pliensbachian ones, especially south of 40° (Fig. 4.13). Consequently, the
results for the Toarcian seem a little obscure, at least for the ordination methods.
The cluster analysis (Fig. 4.14) shows no clear pattern, while on the graphics for
the limits of distribution there are several peaks. Nevertheless, some significant
trends were recognized on the proportional representation of species with
different biogeographic affinities (Fig. 4.15d). There is a significant increase in
high-latitude species (both austral and bipolar) toward the south (linear predictor:
0.21; p = 0.001), and probably also in Pacific species (linear predictor: 0.05;
p = 0.092); on the other hand, species of Tethyan affinities significantly decrease
in relative number in that same direction (linear predictor: -0.06; p = 0.033).
These changes seem to be gradual, occurring between 32° and 42° S.
There is a decrease in the total number of taxa with high-latitude (bipo-
lar ? austral) affinities, from 7 in the Hettangian, 15 in the Sinemurian, and 22 in
the Pliensbachian, to only 5 species in the Toarcian (Fig. 4.6). By Toarcian times,
true high-latitude taxa, such as Harpax rapa, Arctotis? frenguellii Damborenea, an
austral species of Entolium (E. mapuche Damborenea), and a species of Inoce-
ramidae, do not extend north of 34°, and only Meleagrinella, which has bipolar
affinities but is nevertheless rather cosmopolitan in distribution, reaches 32°. On
the other hand, several elements of the Lithiotis reef facies occur in the northern
part of the range, including Lithiotis down to the 28°–30° region. Gervilleioperna
and Opisoma extend even further south.
66 4 Regional Scale
Fig. 4.13 Latitudinal ranges of Toarcian bivalve species (each vertical line represents one
species), discriminated by paleobiogeographic affinities (line key in Fig. 4.7). The 13 latitudinal
black dot. Notice that localities north of 26° and south of 40° have very few data. Extended
ranges are shown, but analysis was mostly developed on actual occurrence data
Fig. 4.14 Toarcian: cluster and faunal turnover analyses along the studied latitudinal range,
Simpson0 s coefficient; algorithm: paired group; number of iterations for the bootstrapping: 1,000.
analyzed areas. Right Faunal turnover analysis
In summary, this brief account of the temporal changes shown by the latitudinal
distribution of bivalves indicates that both the faunal turnover analysis (Figs. 4.8,
4.10, 4.12, and 4.14) and the proportion of species with high-latitude biogeo-
graphic affinities (Fig. 4.15) consistently confirm a southward migration of the
boundary between northern and southern faunas through time (from Hettangian to
Toarcian). This will be discussed in detail in Sect. 4.3, see also Table 4.3.
68 4 Regional Scale
Fig. 4.15 Latitudinal variations in the proportion of species discriminated by their biogeo-
graphic affinities for the four time intervals analyzed. The northernmost reach of species with
high-latitude affinities is also shown, and shows a steady migration toward the south with time
Table 4.3 Comparison of the results obtained using three different methods to recognize the
latitudinal position of the boundary between Tethyan and South Pacific faunas for each of the
Early Jurassic stages
Cluster Faunal turnover Northernmost reach of species with
analysis (here) analysis (here) high-latitude affinities
HETTANGIAN 30° 26–28° 22–24°
SINEMURIAN 32° 28–30° 24–26°
PLIENSBACHIAN 32° 34–36° 26–28°
TOARCIAN ? ? 32–34°
Fig. 4.16 Latitudinal variations in the proportion of species among superfamilies; only those
superfamilies with a significant variation for each stage are shown (explanation in the text)
4.2.3 Systematic Latitudinal Variations
The analysis for changes in proportion of different systematic groups was applied
to the superfamilies represented on each stage (Fig. 4.16), but in most cases, due to
the low number of species in each group, there were no significant results, espe-
cially for the Hettangian and Sinemurian. Nevertheless, it is noteworthy the trend
of decreasing relative diversity at higher latitudes for the superfamily Trigonioidea
in the four considered stages (Fig. 4.16) only when range extension was applied
(linear predictor: -1.09; p = 0.025 for the Hettangian; linear predictor: -0.10;
p = 0.067 for the Sinemurian; linear predictor: -0.06; p = 0.033 for the
70 4 Regional Scale
Fig. 4.17 Late Pliensbachian/Early Toarcian bivalve species richness discriminated by super-
families, comparing faunas from Mendoza-Neuquén provinces (present-day 32–41° S lat.) with
those from Chubut province (present-day 42–45° S lat.), see location of these regions in Fig. 4.1b.
The arrows point to superfamilies which show significant differences in species richness between
the two areas: N richer in Neuquén basin, C richer in Chubut
Pliensbachian excluding interval areas 1 and 2; linear predictor: -0.09; p = 0.026

for the Toarcian).
During the Pliensbachian, the superfamilies Arcoidea (linear predictor: 0.06;
p = 0.036), Nuculanoidea (linear predictor: 0.10; p = 0.025) and Inoceramoidea
(linear predictor: 0.30; p = 0.042) showed a southward increasing trend in
proportion of species, while Pholadomyoidea (linear predictor: -0.05;
p = 0.008) and possibly Trigonioidea showed an opposite trend. The low values
on the proportion of species of the different families are remarkable for this stage
(Fig. 4.16), and they were not restricted only to the mentioned superfamilies. For
the Toarcian the superfamilies Limoidea (linear predictor: -0.09; p = 0.048),
Lucinoidea (linear predictor: -0.31; p = 0.027), and Trigonioidea decreased
southwards in relative number of species, while Monotoidea (linear predictor:
0.15; p = 0.001), Pectinoidea (linear predictor: 0.07; p = 0.009) and Crassat-
elloidea (linear predictor: 0.09; p = 0.033) tend to increase their relative
diversity in that same direction (Fig. 4.16); superfamily Crassatelloidea showed
the same trend during the Sinemurian (linear predictor: 0.41; p = 0.048) despite
its low overall diversity.
At a regional scale in western Argentina, a general comparison of the late
Pliensbachian-early Toarcian faunas from Mendoza/Neuquén and those from
Chubut (Damborenea et al. 2010) done as part of an ongoing detailed systematic
revision of the latter shows that species richness within most taxonomic groups
was almost equal in both areas (pectinoids and trigonioids, for example, to name
diverse Jurassic groups). On the other hand, it is also evident that a few clades had
notable differences in diversity between both areas (Fig. 4.17). Some superfamilies
were significantly more diverse in Chubut (such as nuculanoids and crassatelloids),
while several others (for instance mytiloids, arcoids, pterioids and pholadomyoids)
were more diverse in the northern region. Due to the limited time span of the
Chubut extensive marine deposits, these trends can only be noticed at this par-
ticular time slice.
Knowledge on the latitudinal distribution of living bivalves (Crame 2000a, b,
2001, 2002) just shows that some of the mentioned superfamilies are nowadays
latitudinally limited in their distribution, or have very steep diversity gradients
toward the poles. Extant mytiloids, arcoids, pholadomyoids, and pterioids have a
steep decrease toward high latitudes, while protobranchs show no significant lat-
itudinal gradient. The observed pattern in our data suggest that this particular
trends may be considerably older than previously thought for some groups, and it
is difficult to understand them as being originated solely by clade age as postulated
by Crame. It is also interesting to note that the Jurassic was characterized by
temperature gradients less evident than at the present (see Sect. 1.5), and even so
these selective diversity gradients are revealed.
4.3 Distribution Patterns and Boundaries
Throughout the Jurassic, as just seen, the boundary zone between South Pacific and
Tethyan paleobiogeographic units fluctuated in position through time (Damborenea
2002b). The approximate latitudinal location of the transitional boundary area and
its shift through time were long ago recognized on the basis of faunal composition
along the Andean region (Damborenea 1996, 2002b). Those early studies are now
confirmed by the quantitative analysis presented here: along the South American
western margin, the faunal change boundary shifted southwards between 8 and 10
latitude degrees, and this is consistently indicated (though figures obtained are not
strictly coincident) by the three methods used so far (Table 4.3).
It is well known that biogeographic boundaries are not sharp borders but
transitional zones between different faunas, and therefore it is difficult to determine
them precisely. These transitional zones have also been reported and studied in the
Early Jurassic deposits of the Northern Hemisphere based both on bivalves (for
instance Hayami 1990) or ammonites (see Dommergues and Meister 1991, with
further references), and will be discussed later (Sect. 5.4).
As already mentioned, the results presented here agree to indicate that the
boundary zone shifted latitudinally southwards during the Early Jurassic
(Table 4.3). This observation holds true either if the northermost boundary of this
mixed-fauna fringe is taken into account (26° ? 28° ? 34° for the turnover
analysis, or 22° ? 24° ? 26° ? 32° for the high-latitude species reach) or the
southernmost limit is considered (28° ? 30° ? 36° for the turnover analysis, or
24° ? 26° ? 28° ? 34° for the high-latitude species reach). This southwards
72 4 Regional Scale
shift in the boundary between the Austral and Tethyan Realms amounts, therefore,
to about 700 km (about 8°) along the margin of the Jurassic Pacific.
This observed shift (recorded on present-day maps) of an Early Jurassic pale-
obiogeographic boundary in this area of the Pacific could be interpreted as a result
of either: (a) drifting of continental masses without significant changes in climatic
conditions, (b) changing of overall climatic conditions without significant latitu-
dinal continental drift, or, more likely, (c) a combination of factors involving both
climatic change and continental latitudinal drift. At the present state of knowledge,
it is very difficult to identify and test separately the two main possible causes. Pole
paleopositions based on paleomagnetic studies and water paleotemperatures
inferred from isotopic distributions are instrumental to this discussion.
Paleomagnetic data (Iglesia-Llanos et al. 2006; Iglesia-Llanos 2012) and
‘‘absolute’’ paleogeographic reconstructions of Pangea for the Early Jurassic
suggest that it moved northwards, attaining its northernmost position by Late
Pliensbachian-Early Toarcian, and shifted several latitude degrees from Hettan-
gian to Toarcian times. Despite the paucity of available paleomagnetic data, it is
evident that the results presented here are broadly consistent with them. This is in
general agreement with the observed shift in the bivalve distribution boundaries,
and will be treated in more detail in a global-scale discussion (Sect. 5.4).
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Chapter 5
Hemispheric Scale
Abstract On the basis of the distribution of benthonic Jurassic bivalve genera in

the Southern Hemisphere paleobiogeographic units (biochoremas) were charac-
terized according to their biologic contents (mainly levels of endemism and BSN
analysis). Two first-order paleobiogeographic units are recognized for this region:
Tethyan and South Pacific. Their evolution through time is followed from the
Triassic to the earliest Cretaceous. The Tethyan unit was undoubtedly the most
mature and persistent, with five subordinate units in this part of the world at dif-
ferent times: an Australian unit restricted to the Late Triassic, a North Andean unit,
which appeared sporadically as an endemic center, a South Tethyan unit (Iran and
Arabia), an SE Tethyan unit (Himalayas) already present in the Triassic but well
established from Middle Jurassic, and an East African unit which is recognizable
from Bajocian times onwards. From Late Triassic times a South Pacific first-order
unit is also evident, with a persistent Maorian biochorema and a South Andean unit
identifiable through most of the Jurassic. Being a transitional biogeographic setting
between Tethyan and South Pacific first-order units, the South Andean one is
included in the South Pacific due to the common presence of antitropical genera.
The East African unit is included within the Tethyan during the Jurassic, but later,
in Early Cretaceous times, it split into two units, one of which was regarded as part
of the ‘‘South Temperate Realm’’ by Kauffman. The rank of all these units changed
with time. The evolution of the paleobiogeographic patterns with time also reveals
the nearly complete disruption as a consequence of the Triassic/Jurassic biotic
crisis, with almost no units recognizable for the earliest Jurassic.
In this chapter, we will discuss the distribution in time and space of marine
Triassic and Jurassic bivalves from the Paleo-Southern Hemisphere, using the
available data for the recognition, description, and study of the evolution of
paleobiogeographic units or biochoremas.
The term biochorema (biochore in the sense of (Makridin 1973; Westermann
2000a and Cecca and Westermann 2003) applies to a biogeographic unit of any rank.

DOI: 10.1007/978-94-007-5098-2_5, Ó The Author(s) 2013
80 5 Hemispheric Scale
Biochoremas are highly dynamic units; they may appear and disappear, expand or
shrink in their geographic range, and also change in rank through time. Those units
based on benthonic faunas do not necessarily have a coincident history with those
based on pelagic groups. All these aspects are clearly exemplified by the history and
evolution of Southern Hemisphere biogeographic units during Triassic and Jurassic
times (Damborenea 2002b).
As already pointed out in Sect. 3.1, the traditional knowledge of global bivalve
distribution during the Mesozoic was based mainly on data from the Northern
Hemisphere. Global studies on the distribution of Mesozoic bivalves used to be
largely based on databases which had a very poor coverage of the Southern
Hemisphere, but this has changed now, and a wealth of recently published
information from this region certainly added substantial evidence to be analyzed
(see references in Damborenea 2002b).
To this historic bias, another important factor should be added: mainly as a
consequence of the unevenness of land masses distribution on the Earth, the
northern–southern asymmetry evident in some aspects of marine faunal distribu-
tion patterns is well known (see discussion in Crame 1996a, 2000a, b). Never-
theless, three major biogeographic units based on benthonic invertebrates (one
low-latitude and two high-latitude) are recognized for Permian (see Shi and Grunt
2000 and references therein), Triassic (e.g., Diener 1916; Stevens 1980),
Cretaceous (e.g., Fleming 1963; Kauffman 1973; Sohl 1987; Stevens 1980), and
Cenozoic times (e.g. Fleming 1963; Hayami 1989).
In contrast, the just mentioned hemispheric asymmetry of biogeographic units
during most of the Mesozoic led many authors to recognize only two first-order
paleobiogeographic units for the Jurassic, mainly on the basis of the distribution of
ammonites: the Boreal and Tethyan Realms (Hallam 1969, 1971, 1977; Stevens
1980, 1990; Doyle 1987; Challinor et al. 1992; Hillebrandt et al. 1992; and many
others, see discussion and references in Westermann 2000a, b). As Kauffmann
(1973) aptly expressed, ‘‘poor knowledge of south temperate [bivalve] faunas has
led many authors to assume no ‘anti-Boreal’ realm existed south of Tethys during
the Mesozoic’’ and he demonstrated that this is clearly not true for the Cretaceous,
although he admitted that ‘‘this was possibly true in the Early Jurassic’’.
Furthermore, Ager (1975, p. 17) said that ‘‘it is often commented that there was no
southern counterpart’’ of the Boreal Realm in the Jurassic, but certain invertebrates
do seem to be ‘‘restricted to that region’’. In fact, lack of data and proper analysis
for the Southern Hemisphere pervaded also studies of other fossil groups (see for
instance Dommergues et al. 2001), and was explicitly acknowledged by Crame
(1986) in relation to bivalves.
This assumed twofold division, most probably heavily influenced by leading
opinions based on ammonite distribution (for instance Arkell 1956) hindered the
determination of the possible role that the austral regions may have had in the origin
and diversification of the biota (see Crame 1997) during the Jurassic. Widespread
phenomena such as austral endemism or antitropicality were either ignored or their
relative importance was disregarded. Some authors who work on Southern Hemi-
sphere faunas (especially bivalves) have been challenging this twofold division for
5. Hemispheric Scale 81
a long time (see Stevens 1980 and references therein; Crame 1986, 1987, 1992,
1993, 1993; Damborenea 1993, 1996, 2002a, b; Enay and Cariou 1997 and refer-
ences therein). As a result, several marine paleobiogeographic units of low rank
were proposed for the Southern Hemisphere Jurassic on the basis of the known
distribution of different marine organisms. The relationships, rank, and history of
these units were reviewed by Enay and Cariou (1997) and Westermann (2000b).
Jurassic paleobiogeographic provinces based on bivalves were analyzed
quantitatively for the European Tethyan and Proto-Atlantic (Liu 1995; Liu et al.
1998). For the Southern Hemisphere, several contributions related to paleobiog-
eographic issues using these organisms were already available for the South
Pacific (Stevens 1967, 1977, 1980, 1989, 1990; Hayami 1984, 1987; Grant-Mackie
et al. 2000), Antarctica (Crame 1987, 1992, 1996a, b), and the South American
margin of the Pacific (Damborenea and Manceñido 1979, 1988; Hillebrandt 1981;
Hallam 1983; Damborenea 1993, 1996). A first comprehensive paleobiogeo-
graphic analysis based on late Triassic–Jurassic bivalves for the Southern Hemi-
sphere was provided by Damborenea (2002b). We discuss here the issue of
paleobiogeographic units based on the distribution of bivalves in the light of new
data and the application of new techniques.
5.1 Data
Occurrences of Triassic and Jurassic bivalve species were compiled from various
published sources as well as the author’s own data, and plotted stage by stage from
Induan to Berriasian.
Data were then gathered within wide areas, each containing a large variety of
habitats. About 14 such areas were chosen for the present analysis (located in
Fig. 5.1). On the whole, they represent a wide coverage of the Southern Hemi-
sphere Triassic and Jurassic seas, but some important gaps still exist, limited by the
availability of data. Both the amount and quality of data are, unsurprisingly, very
uneven, but although this hinders serious detailed quantitative analysis, the data-
base provides enough information to obtain a broad framework.
For this analysis data were processed at the genus group level and according to
the following nine time intervals: Induan-Anisian, Ladinian-Carnian, Norian-
Rhaetian, Hettangian-Sinemurian, Pliensbachian-Toarcian, Aalenian-Bajocian,
Bathonian-Callovian, Oxfordian-Kimmeridgian, and Tithonian-Berriasian
(Table 5.1). Admittedly, this implies a loss of detail in the information for some
regions, but on the other hand it allows the use of some occurrences with uncertain
stratigraphic provenance. The age slice including Tithonian and Berriasian has the
extra drawback that in this way any event related to the Jurassic-Cretaceous
boundary may pass unnoticed. Data are summarized in Table 5.2.
Endemic genera were recognized as such for each time interval. Since geo-
graphic ranges of taxa may change through time, sometimes this results in a
different categorization for the same taxon. For instance, a genus may be endemic
Fig. 5.1 Location of the geographic units used in this study. 1 NW South America (Colombia), 2
Perú and northernmost Chile, 3 Central Argentina and Chile, 4 Southern Argentina and Chile, 5
Antarctica, 6 New Zealand-New Caledonia, 7 Western Australia-New Guinea, 8 Himalayas (N
India, S Tibet), 9 Western India, 10 Madagascar, 11 SE Africa (Kenya, Tanzania), 12 E Africa
(Eritrea, Ethiopia, Somalia), 13 Arabian Peninsula (Saudi Arabia, Yemen, Oman); and 14 Iran.
Base map as in Fig. 1.2
to a certain region during a stage and then become widespread. An example is

Kalentera, which was endemic to the Maorian Province in Late Triassic times but
became antitropical during the Early Jurassic. Similarly, Gryphaea was restricted
to high latitudes during the Triassic but later became pandemic. Conversely, a
previously widespread taxon may have a temporarily restricted distribution. The
recognition of these subtle changes is highly dependent on a very detailed
knowledge of recorded distributions, and is thus not always easy to establish in the
fossil record. Nevertheless, these changes were herein recognized and taken into
account as far as possible.
A few comments about each region and the data sources are given below;
numbers refer to those located in Fig. 5.1 and used throughout in the text.
5.1.1 South America
1. NW South America: (Colombia and northernmost Perú): Though relatively

diverse faunas are known to occur, the published information from this region
is very scarce and is not comparable with the other data sets, being confined to a
few records from Norian to Toarcian and Tithonian-Berriasian. The time
location of some of these data is still controversial, as clearly exemplified by
the fact that the Batá Formation, which contains bivalves originally referred to
the Jurassic by Bürgl (1961) and Geyer (1973), is now regarded as Cretaceous
in age (Etayo Serna et al. 2003). Data used here were critically compiled from
Geyer (1973, 1979) and Guzmán (1984) and references therein.
5.1 Data 83
Table 5.1 Time frame used

in this study for the
discussion of
paleobiogeographic units and
their evolution, right hand
column indicates the letter
symbols used throughout the
text to indicate the time slices
used in the analysis
2. Perú and Northern Chile: Comprises most of Perú and the northernmost regions
of Chile (up to about 268 S present-day latitude). References are few and not
updated, and there is evidence to suggest that they do not accurately reflect the
actual bivalve diversity in the area. Data sources for the Early Jurassic of Chile
listed in Damborenea 1996, with the addition of data from other ages and from
Perú (and some new references) in: Jaworski 1915, 1922; Körner 1937; Cox
1949, 1956; Harrington 1961; Pérez and Reyes 1977, 1983, 1985, 1986, 1991;
Hayami et al. 1977; Westermann et al. 1980; Chong and Hillebrandt 1985;
Prinz 1985; Riccardi et al. 1990a, b; Pérez et al. 1987; Romero et al. 1995;
Aberhan and Hillebrandt 1996, 1999; Rubilar 1998; Aberhan 2007.
3. Central Argentina and Chile: This region extends from 26° to 41° S present-day
latitude, and the southern part includes what is regionally known as the
Neuquén basin (s.l.). The geology of this area is well known and paleogeo-
graphic maps at different moments of the time interval are available. Bivalve
faunas are very well documented for the whole Jurassic, but Triassic ones are
not so well known. Early Jurassic references listed in Damborenea 1996, with
the addition of: Burckhardt 1900a, b, 1903; Haupt 1907; Jaworski 1914, 1915;
Stehn 1923; Fuenzalida Villegas 1937; Leanza 1941; Lambert 1944;
Table 5.2 Chronologic and geographic distribution of Triassic and Jurassic bivalve genus group
taxa in the areas and age intervals considered in this study
a Regions
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Time I – 2 – – – 4 3 12 5 – – – 4 3
L – 3 – – – 26 3 9 1 – – – – 1
N 13 50 19 – – 37 26 49 1 – – – 21 69
H 15 63 76 – 5 13 – 17 – – – – – –
P 3 75 102 53 2 25 5 10 – 15 18 – 4 25
A – 33 69 – 20 51 24 12 10 13 30 – 23 10
B – 1 31 – 23 38 1 23 130 50 49 57 66 21
O – 5 11 – 34 30 7 13 32 16 76 49 45 –
T 15 3 70 27 25 19 4 23 15 3 29 – 9 –
b Regions
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Time I – 2 – – – 1 3 8 3 – – – 4 3
L – 2 – – – 14 1 6 1 – – – – 0
N 8 32 15 – – 18 11 20 0 – – – 10 33
H 12 37 41 – 4 10 – 14 – – – – – –
P 2 42 54 30 2 16 3 8 – 11 19 – 1 17
A – 22 44 – 14 31 19 12 3 9 20 – 16 8
B – 1 22 – 13 21 1 18 57 31 30 37 38 11
O – 4 9 – 19 20 3 6 18 10 47 32 26 –
T 8 0 39 16 15 10 2 15 8 2 17 – 8 –
c Regions
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Time I – 0 – – – 2 0 1 0 – – – 0 0
L – 0 – – – 6 0 0 0 – – – – 0
N 1 3 0 – – 8 4 4 0 – – – 0 8
H 0 3 3 – 0 0 – 1 – – – – – –
P 0 6 5 1 0 2 0 0 – 0 0 – 0 0
A – 2 4 – 0 7 0 0 1 0 0 – 1 0
B – 0 2 – 1 5 0 1 8 1 2 1 3 1
O – 1 2 – 1 2 0 2 3 2 5 1 0 –
T 0 1 5 1 1 1 0 3 1 0 3 – 0 –
a Total number of genera, b Number of cosmopolitan (s.s.) genera, c Number of endemic genera.
Regions: key to numbers in Fig. 5.1; time slices: key to letters in Table 5.1
Sokolov 1946; Levy 1967; Thiele Cartagena 1967; Cecioni and Westermann
1968; Hallam et al. 1986; Lo Forte 1988; Damborenea 1990, 1998, 2002a,
2004; Leanza and Garate 1987; Riccardi 1988; Riccardi et al. 1990a, b, c, 1997,
2004, 2011; Damborenea et al. 1992; Leanza 1993; Aberhan 1994, 2004, 2007;
Malchus and Aberhan 1998; Rubilar 1998; Damborenea and Manceñido 2005,
2012; Damborenea and Lanés 2007; Pérez et al. 2008; and unpublished data.
5.1 Data 85
4. Southern Argentina and Chile: Includes early Jurassic (Pliensbachian-Toarcian)

deposits in Chubut Province (Argentina), which are considered by some authors
as a southern extension of the Neuquén basin, and late Jurassic-early Creta-
ceous (Tithonian-Berriasian) beds from the Austral Basin of southernmost
Chile and Argentina. Very few data are known, references for Chubut listed in
Damborenea 1996, with the addition of: Feruglio 1936; Leanza 1968; Riccardi
1977; Olivero 1988; and unpublished data.
5.1.2 Antarctica, New Zealand-New Caledonia
5. Antarctica: This region comprises mainly the Antarctic Peninsula and adjacent
areas. Data from the Falkland Plateau have also been included here. Knowledge
is abundant but very patchy and scattered, taking into account the large size of
the area. Data were compiled from (Stevens 1967; Thomson and Willey 1972;
Thomson 1975a, b, 1981, 1982; Willey 1975a, b, c; Jones and Plafker 1976;
Quilty 1978, 1982, 1983; Edwards 1980; Crame 1981, 1982a, b, 1983, 1984,
1985, 1996c; Jeletzky 1983; Medina and Ramos 1983; Thomson and Tranter
1986; Riccardi et al. 1990c; Doyle et al. 1990; Thomson and Damborenea
1993; Crame et al. 1993; Crame and Kelly 1995; Kelly 1995; Riley et al. 1997).
There is a lot of information which remains unpublished in Hikuroa’s thesis
(2005).
6. New Zealand-New Caledonia: Although these regions are now widely sepa-
rated in space, in the past they were closely related and their Mesozoic bivalve
fauna was very alike and studied by the same authors. New Zealand faunas are
by far better known, only few data are available from New Caledonia, and thus
data from these two areas are pooled together for the analysis. Bivalve diversity
is very well documented for the whole time interval of the present study. Some
bivalves are often used in biostratigraphy and they define several units. Buchiid
species, for instance, are used to subdivide and correlate within the Puaroan
Stage (Hikuroa and Grant-Mackie 2008 and references therein). Data were
compiled from Trechmann 1918, 1923; Marwick 1935, 1953, 1956; Avias
1953; Fleming 1959, 1962, 1964, 1987; Grant-Mackie 1960, 1976a, b, 1978a,
b, c, d, 1980a, b, 2011; Speden 1970; Stevens 1978; Freneix et al. 1974; Speden
and Keyes 1981; Begg and Campbell 1985; Crampton 1988; Grant-Mackie and
Silberling 1990; Damborenea and Manceñido 1992; Damborenea 1993;
Gardner and Campbell 1997, 2002, 2007; Gardner 2005, 2009; Hikuroa and
Grant-Mackie 2008; and unpublished data (in Hudson 1999).
5.1.3 Australia-New-Guinea
7. Western Australia (Bonaparte, Carnarvon, and Perth Basins), New Guinea, and
Sula Islands: Although there are a few scattered data from most of the time
intervals considered, only those for Norian-Rhaetian and Aalenian-Bajocian
faunas are used here. Data were compiled from: Etheridge 1910; Whitehouse
1924; Teichert 1940; Skwarko 1967, 1973, 1974, 1981a, b, 1983; Coleman and
Skwarko 1967; Skwarko et al. 1976; Sato et al. 1978; Grant-Mackie 1994;
Waterhouse 2008.
5.1.4 Southern Asia
8. Himalayas (northern India, Afghanistan and southern Tibet): Rossi-Ronchetti

and Fantini-Sestini 1961; Rossi-Ronchetti 1970; Li and Grant-Mackie 1988,
1994 and references therein; Li 1990; Yancey et al. 2005.
9. Western India: No data for late Triassic and Early Jurassic are available. On the
other hand, knowledge of middle Jurassic bivalve faunas is very good and has
been the subject of several recent monographs. Data were compiled from
Kitchin 1903; Cox 1935b, 1940, 1952; Agrawal 1956a, b; Kanjilal and Singh
1973; 1980; Singh and Kanjilal 1974, 1977, 1982; Agrawal and Rai 1978;
Kanjilal 1979a, b, 1980a, b, 1981; Singh and Rai 1980; Singh et al. 1982; Jaitly
and Singh 1983; Pandey and Agrawal 1984; Jaitly 1986a, b, c, 1988, 1989,
1992; Jaitly et al. 1995; Pandey et al. 1996; Fürsich and Heinze 1998;
Fürsich et al. 2000.
5.1.5 Africa
10. Madagascar: Reliable data are available only from Toarcian times onwards.
Due to the scarcity of data, some records that appear to be trustworthy were
included despite the fact that they are not yet backed by descriptions and
figures (for instance in Mette 2004). See Geiger and Schweigert (2006) for a
revision of Jurassic sedimentary cycles and tectonic history in western
Madagascar. Data compiled from Newton 1889, 1895; Douvillé 1904;
Thevenin 1908; Barrabé 1929; Besairie 1930; Besaire and Collignon 1972;
Nicolaï 1950–1951; Mette 2004.
11. Southeast Africa (Kenya and Tanzania): Again, there are no data older than
Toarcian, and were compiled from Weir 1930; Dietrich 1933; Cox 1965;
Aberhan et al. 2002; Bussert et al. 2009.
12. East Africa (Ethiopia, Somalia, and Eritrea): Reliable data only from
Pliensbachian times onwards, but some of the units are not precisely dated yet;
see for instance the discussion for the Tendaguru Formation units in Bussert
5.1 Data 87
et al. (2009). Data for the analysis were compiled from Fütterer 1897; Dacque
1905; Basse 1930; Díaz-Romero 1931; Cox 1935a and references therein;
Stefanini 1939; Venzo 1949; Jaboli 1959; Ficcarelli 1968; Jordan 1971;
Abbate et al. 1974; Kiessling et al. 2011.
5.1.6 Near East
13. Southern Arabian Peninsula: Yemen, Oman, Saudi Arabia: Published data are
very uneven for southern Arabic peninsula. Bivalves from this region have
general affinities with those from eastern Africa, but they are in need of
revision. The geology is well known in relation to oil industry; see for instance
(Ziegler 2001) for paleogeographic synthesis and tectonic history. Data were
compiled from (Basse 1930; Arkell 1956; Hudson and Jefferies 1961;
El-Asa’ad 1989; Manivit et al. 1990; Howarth and Morris 1998; Krystyn et al.
2003; Yancey et al. 2005).
14. Iran: Bivalve faunas from central Iran beds are well known, especially those
from the late Triassic. The territory was in the Southern Hemisphere during
that time, but it then migrated northwards to the paleoequator. Bivalve data
were compiled from (Fischer 1915; Cox 1936; Fantini-Sestini 1966; Geyer
1977; Kluyver et al. 1978; Kristan-Tollmann et al. 1980; Kalantari 1981;
Schairer et al. 2000; Hautmann 2001a, b; Fürsich et al. 2005).
5.2 Biochorema Recognition
5.2.1 Biogeographic Units and Their Characterization
The scarcity of data for the Early and Middle Triassic makes it impossible to
attempt a paleobiogeographic analysis based on endemism. Most of the regions
had less than 10 genera for each time interval from Induan to Carnian (see
Table 5.2), and were not included in the analysis, which was performed for Norian
times onwards.
Although always low, at different times through the Late Triassic and Jurassic,
endemism within the different areas of the Southern Hemisphere varied, and was
used here to recognize and characterize paleobiogeographic units. For the
following analysis, percentage of endemism was calculated excluding cosmopol-
itan forms (as done by Kauffman 1973) and including only strictly endemic taxa.
In the following discussion, all percentages quoted are calculated over total minus
cosmopolitan genera.
A maximum of seven basic biochoremas (not all of them persistent during the
whole time range) and two units of higher rank were recognized for the different
time slices considered according to the distribution and percentage of endemic
taxa (Table 5.3) (Damborenea 2002b, modified here). Some of the basic units are
regarded as belonging to the Tethyan first-order biochorema, while others can be
grouped in another high rank unit which following Westermann’s (2000b)
recommendations is called South Pacific (Challinor 1991, Austral Realm in
Damborenea 1993). Overall endemism for second-order biochoremas of this last
high rank unit is proportionally high during the latest Triassic, between 8 and 22 %
for the Early Jurassic, and over 14 % for Middle and Late Jurassic. These fluc-
tuations suggest its change in rank through time, as will be discussed later.
These percentages are low according to normalized scales based on species
distribution (see discussion in Westermann 2000a), but are of the same order as
those used by other authors to define paleobiogeographic units on the basis of
fossil bivalve genera (e.g., Kauffman 1973), and they allow the recognition of units
of different rank. Nevertheless, taking into account the uneven nature of the
database, no attempt is made here to establish a threshold of minimum values for
each rank. It is interesting to remember that the distinction between Tethyan and
Boreal Realms based on bivalves in the Northern Hemisphere rests on only a few
taxa. According to Liu (1995), for instance, for the Pliensbachian only the pres-
ence of Hippopodium and Meleagrinella is characteristic of the Boreal Realm and
Weyla and Lithiotis of the Tethyan Realm in Europe.
All units are further characterized by other aspects, such as the presence and
relative abundance of taxa with high-latitude or strictly low-latitude affinities
(Table 5.3), overall diversity, and the presence/absence of certain higher rank
taxonomic groups. Antitropical taxa were common within monotoids, pectinoids,
inoceramoids, and other bivalve groups, and add character to some of these units.
On a worldwide scale, this present arrangement implies the presence of three
first-order units during the Jurassic based on bivalve data: Boreal, Tethyan, and
South Pacific (= Austral). The distribution of bivalves and the corresponding
proposed paleobiogeographic zonation is evidently not symmetric relative to the
paleoequator. In fact, in view of the unbalanced distribution of land/water masses
and the related uneven oceanic current patterns (which are mostly hypothetic at
this stage), such a paleobiogeographic asymmetry is only to be expected. Never-
theless, it is quite clear that the analyzed data do not support the alternative of
overstressing this asymmetry to the point of reducing the paleobiogeographic
zonation for the Jurassic to only two first-order units (Damborenea 2002b).
At the family level, all three first-order paleobiogeographic units are well
characterized. During the Triassic, Jurassic, and Early Cretaceous most genera of
Anomiidae, Burmesiidae, Ceratomyopsidae, Cuspidariidae, Diceratidae,
Dicerocardiidae, Isoarcidae, Lithiotidae, Mactromyidae, Malleidae, Megalodonti-
dae, Myalinidae, Myopholadidae, Myophoricardiidae, Mysidiellidae, Ostreidae,
Pergamidiidae, Protocardiidae, Ptychomyidae, Pulvinitidae, Requieniidae, Sower-
byidae, and Unicardiopsidae were restricted to low latitudes and characterized the
Tethyan Realm. To these, a group of families with more than average low-latitude
taxa (Fig. 5.2) should be added: Laternulidae, Pinnidae, Prospondylidae, Tancre-
diidae, Arcticidae, and Myophoriidae. The first four of these have no high-latitude
taxa, and the others contain less than average high-latitude genera.
Table 5.3 Southern Hemisphere second-order paleobiogeographic units recognized and their percentage of endemic generic level taxa through time (bottom
row)
Time Biochoremas Percentages (%)
First order Second order Low-latitude High-latitude Trans-temperate Endemics
Norian–Rhaetian South Pacific Maorian [6] 5 37 16 42
Andean transitional [3] 40 40 20 0
? North Andean [1 ? 2] 60 15 10 15
Tethyan S Tethyan [13 ? 14] 80 0 0 20
SE Tethyan [8 ? 9] 86 0 0 14
5.2 Biochorema Recognition
Australian [7] 73 0 0 27
Hett–Sinem South Pacific Maorian [5-6] 25 25 50 0
South Andean [2 ? 3] 59 19 14 8
Tethyan North Andean [1] 33 0 67 0
SE Tethyan [8] 67 0 0 33
Pliens–Toarc South Pacific Maorian [5 ? 6] 0 45 33 22
South Andean [4] 48 22 26 4
Tethyan North Andean [1 ? 2 ? 3] 54 18 15 13
S Tethyan [13 ? 14] 80 0 20 0
SE Tethyan [7 ? 8] 75 0 25 0
E African [10 ? 11] 89 0 11 0
Aalenian–Bajocian South Pacific Maorian [5 ? 6] 40 24 8 28
Tethyan North Andean [2] 73 9 0 18
S Tethyan [13 ? 14] 87 0 0 13
SE Tethyan [7 ? 8 ? 9] 84 0 8 8
East African [10 ? 11] 100 0 0 0
Bath-Call South Pacific Maorian [5 ? 6] 61 5 5 29
Tehyan East African [10 ? 11 ? 12 ? 13] 82 2 6 10
89
SE Tethyan [8 ? 9] 82 1 5 12
(continued)
Table 5.3 (continued)
90
Time Biochoremas Percentages (%)

First order Second order Low-latitude High-latitude Trans-temperate Endemics
Oxfor-Kimm South Pacific Maorian [5 ? 6] 29 41 12 18
South Andean [3] 0 0 0 ?
Tehyan East African [10 ? 11 ? 12 ? 13] 74 9 4 13
SE Tethyan [8 ? 9] 52 19 5 24
Tithon-Berr South Pacific Maorian [5 ? 6] 29 36 21 14
South Andean [3 ? 4] 67 8 11 14
Tehyan East African [10 ? 11 ? 13] 72 0 7 21
North Andean [1] 100 0 0 0
SE Tethyan [8 ? 9] 59 7 7 27
The percentage of low-latitude, high-latitude, and trans-temperate genera is also indicated. All percentages are calculated over the number of total minus
cosmopolitan genera. Numbers in brackets behind the name of each biochorema refer to the geographic units used here, see location in Fig. 5.1
5 Hemispheric Scale
5.2 Biochorema Recognition 91
Fig. 5.2 Percentage of

genera with different
paleobiogeographic
distribution during the
Triassic and Jurassic within
selected bivalve families.
Families arranged according
to the percentage of high-
latitude genera. Average
composition for the whole
time interval at the top,
background diagonal lines
indicate average percentage
of low-latitude (orange) and
high-latitude (red) genera.
Own data based on nearly
500 bivalve genera
Families strictly restricted to high latitudes in the Jurassic are fewer; among
them Asoellidae, Minetrigoniidae, and Palaeopharidae are present in both Boreal
and South Pacific Realms whereas Sportellidae and Yoldiidae are known only
from the Boreal Realm. There is a consistent group of families which have more
than average high-latitude taxa and at the same time less than average low-latitude
genera (Fig. 5.2), all of them diverse and abundant in both Boreal and South
Pacific Realms: Kalenteridae, Monotidae, Inoceramidae, Oxytomidae, Trigonii-
dae, Retroceramidae, and Buchiidae. It should be pointed out here that updated
systematic knowledge of bivalve faunas is still very uneven in the Southern
Hemisphere and that future revisions are likely to alter this picture slightly.
5.3 Evolution of Biochoremas
As biochoremas change with time in rank, geographic spread, and even realm to
which they are assigned, the discussion below will follow a stratigraphic order and
will be restricted to the Southern Hemisphere. The discussion will include different
qualitative aspects, mainly endemism, as well as the quantitative analysis
performed around the Triassic-Jurassic boundary (cluster analysis and BSN). In
the following account, quoted percentages of different elements of the faunas were
calculated exclusive of cosmopolitan genera, as done by Kauffman (1973).
5.3.1 Triassic
As already said, the scarcity of data for the Early and Middle Triassic makes it
very difficult to recognize paleobiogeographic patterns in the Southern Hemi-
sphere. This is one of the consequences of the vast end-Permian extinction event,
which had dramatic consequences for the systematic and ecologic composition of
benthonic faunas. Diversity loss after the extinction resulted in a reduction to less
than 10 bivalve genera in most of the Southern Hemisphere regions from Induan to
Carnian (Table 5.2). As a result of a similar analysis of bivalve generic distribution
for Tibet and adjacent areas, Niu et al. (2011) recognized three provinces (NE
Tethyan, SE Tethyan, and Himalayan), all of them Tethyan. Only the last two were
still placed in the Southern Hemisphere during the early Triassic, they all migrated
northwards later. The unit here called SE Tethyan roughly corresponds to Niu et al.
(2011) Himalayan and SE Tethyan.
Nonetheless, already in the early Middle Triassic two endemic genera are known
from the Etalian of New Zealand (Etalia Begg and Campbell and Marwickiella Sha
and Fürsich) in an otherwise low diverse fauna. These indicate the incipient
appearance of an endemic center in this circum-polar area, which would develop
later on. Not surprisingly, the most diverse Southern Hemisphere fauna for this time
was located in low paleolatitudes, in the Himalayan region, but endemism there was
restricted to only one doubtful genus. Niu et al. (2011), mostly based on relative
diversity, proposed the continuity of the three provinces (NE Tethyan, SE Tethyan,
and Himalayan) in the Tibetan area during the whole Triassic.
This emerging pattern became stronger by Ladinian and Carnian times, when
all the endemic genera known so far for the Southern Hemisphere lived in the
southernmost region (New Zealand-New Caledonia): Praegonia Fleming and
Agonisca Fleming from the early Kaihikuan, later Balantioselena Speden and
Manticula Waterhouse, while the first Hokonuia Trechmann are already Oretian.
This supports the definite establishment of a Maorian biochorema by mid-Triassic
times. Other faunas of that age are very poorly known. The three Tibetan provinces
are still recognized by Niu et al. (2011), but by this time only the southern one
(Himalayan) remains in the Southern Hemisphere.
5.3 Evolution of Biochoremas 93
Fig. 5.3 Cluster analysis of South Hemisphere regions for the Norian-Rhaetian interval, values
on each node representing the support value for the node obtained by bootstrapping; cophenetic
correlation: 0.694. Similarity measure: Simpson0 s coefficient; algorithm: paired group, number of
iterations for the bootstrapping: 1000. Regions code-numbered as in Fig. 5.1. To the right:
paleobiogeographic units as recognized here
Endemism significantly increased in the whole hemisphere toward the Late

Triassic, and up to six basic units are recognizable (Damborenea 2002b). The
South Pacific first-order biochorema includes a very distinctive Maorian unit, to
which perhaps an Andean Transitional unit could be added. All other biochoremas
are included in the Tethyan Realm, with an Australian unit and two eastern
Tethyan units (here called S Tethyan and SE Tethyan, this last one corresponding
to the Himalayan province of Niu et al. 2011).
The affinities of the North Andean endemic center are obscure; this unit could
be related to the South Pacific on the basis of the obtained dendrogram (Fig. 5.3)
and BSN analysis (Fig. 5.4), but has also many links to the Tethyan unit.
The Maorian (Diener 1916) biochorema presents very high endemism: the
genus Hokonuia persists from the previous stage and becomes abundant, Cale-
dogonia Freneix and Avias and other local genera (Heslingtonia Fleming,
Kalentera Marwick, Maorimonotis Grant-Mackie, Oretia Marwick, Ouamouia
Campbell, Torastarte Marwick) are also endemic to New Zealand-New Caledonia
during this time. This biochorema is very well characterized during the Middle and
Late Triassic, not only by the percentage of endemics (42 %), but also by the
abundance (37 %) of genera with antitropical or austral distribution, such as
Eomonotis Grant-Mackie, Inflatomonotis Grant-Mackie, Maoritrigonia Fleming,
Minetrigonia Kobayashi and Katayama, and Triaphorus Marwick. Strictly low-
latitude genera are very few (5 %).
An Australian biochorema had very low diversity but is also well characterized
by several endemic genera (27 %), among them Gervillancea Skwarko, Guineana
Skwarko, and Somareoides Skwarko. Low-latitude genera are more numerous, but
Fig. 5.4 BSN of South

Hemisphere regions for the
Norian-Rhaetian interval.
Continuous thin lines:
distance value C0.25 and
\0.50 (75 % to more than
50 % of genera shared);
dashed lines: distance value
C0.50 (50 % of genera
shared or less); support
values written on each line
high-latitude and trans-temperate forms are absent, clearly indicating a fully

Tethyan affinity of the faunas of this region.
A North Andean endemic center extends along the Andes from Colombia to
northern Chile. Most of the bivalve fauna has low-latitude affinities, but the
endemic genera Isopristes Nicol and Allen and Schizocardita Körner are abundant
elements of the Late Triassic fauna from Peru and northern Chile (totaling 15 %),
and some genera with high-latitude distribution are also present (15 %).
The central Argentina-Chile area represents a transitional zone, with no ende-
mic taxa but with a high percentage (40 %) of high-latitude genera, including
Maoritrigonia and Minetrigonia. The abundance of trans-temperate taxa (20 %) is
mainly due to genera distributed along the eastern margins of the Paleo-Pacific in
both hemispheres. Reference of both the North Andean and Andean Transitional
biochoremas to the Tethyan or the South Pacific Realm is thus still uncertain on
these evidences alone.
Apart from endemism, the quantitative analysis also shows that by the end-
Triassic there was a clear differentiation among regions. Cluster analysis (Fig. 5.3)
resulted in three minor groups with similarity coefficients higher than 0.50:
northern South America (= North Andean endemic centre), southern South
America ? New Zealand, and Australia ? Himalayas, although this last grouping
is not strongly supported (support value just lower than 0.50). The dendrogram
supports the idea of two major biochoremas: the first one grouping all the Pacific
localities (west coast of South America and New Zealand) while the other includes
the Tethyan localities (Iran, Arabia, Himalayas and Australia). On this evidence,
the Andean Transitional biochorema (and perhaps also the North Andean one) can
be related to the South Pacific Realm.
Bootstrapped Spanning Network (BSN) (Fig. 5.4) seems to perform better on
these data, since it clearly separates both first-order biochoremas and also reflects
the strong similarities among northern and southern South America. The best
connection between both first-order biochoremas shows a high dissimilarity value
(0.58, meaning only 42 % of genera shared between localities) and a low support
value (0.42).
Fig. 5.5 Paleobiogeographic

sketch for the Norian-
Rhaetian time slice, showing
inferred extension of second-
order biochoremas. Thick
broken line: approximate
position of the boundary zone
between Tethyan and South
Pacific first-order
biochoremas. Base map as in
Fig. 1.2
Extensive areas without late Triassic outcrops or reliable paleontologic data in

the key regions prevent adequate discussion of the boundary between biochoremas
at this time. Nevertheless, it is evident that paleobiogeographic patterns were
mature by Norian-Rhaetian times (Fig. 5.5).
5.3.2 Early Jurassic
At the beginning of the Jurassic, the percentage of endemism was remarkably low
for all regions. Although it is difficult to point a definite cause, this situation is
probably a consequence of the end-Triassic extinction (see Sect. 5.5). It is inter-
esting to note that, on the available data, the low latitude regions of the Southern
Hemisphere also contain almost no endemic genera. The seemingly high value
(33 %) in the Himalayas is in fact based on the apparent persistence into the
Hettangian of only one endemic genus (Persia Repin) in an otherwise low
diversity fauna with high proportion of cosmopolitan taxa. On the other hand, in
transitional zones, such as the South Andean areas (2 and 3 here), an endemic
center developed during Sinemurian times, with a few endemic genera (8 %), such
as Gervilletia Damborenea, Groeberella Leanza and Quadratojaworskiella Reyes
and Pérez. This endemic center is regarded here as part of the South Pacific unit
despite the relatively high proportion of low-latitude taxa, due to the common
occurrence of genera with antitropical or austral distributions (19 %), such as
Agerchlamys Damborenea, Asoella Tokuyama, Harpax Parkinson, Kalentera,
Palaeopharus Kittl, and Kolymonectes Milova and Polubotko (Damborenea 1998).
The quantitative study also suggests that the end-Triassic extinction event
noticeably affected bivalve biogeography. Although a few localities had enough
data to perform the analysis, the results show a clear homogenization of bivalve
faunas. Cluster analysis on the Hettangian-Sinemurian data could not reveal major
groupings among regions, with all nodes with similarity values higher than 0.50
and none of them with a significant support (values below 0.50). The BSN
(Fig. 5.6) shows an evident decrease in dissimilarity values while all edges appear
strongly supported (even some of the removed edges).
Fig. 5.6 BSN of South

Hemisphere regions for the
Hettangian-Sinemurian
interval. Continuous lines:
distance value C0.10 and
\0.25 (90 % to more than
75 % of genera shared);
support values of 1 unless
otherwise indicated on each
line
By Pliensbachian-Toarcian times the Maorian unit (= South West Pacific Prov-

ince in Hallam 1977) had recovered (Fig. 5.8), having 22 % of endemic genera,
including Torastarte and Pseudaucella Marwick (Grant-Mackie et al. 2000) and
44 % of high-latitude genera. A South Andean endemic center persists with a low
percentage of endemics (4 %), but with a high percentage of high-latitude taxa
(22 %), which include Asoella, Harpax, Kolymonectes, Palaeopharus, Och-
otochlamys Milova and Polubotko, Praebuchia? Zakharov, and Radulonectites
Hayami (Damborenea and Manceñido 1992; Damborenea 1993, 2002b). This
interval is also characterized by a very high percentage of high-latitude taxa (22 %)
and also the highest levels of trans-temperate (mainly east Pacific) taxa for the whole
Jurassic (26 %). A North Andean endemic center is again discernible (13 % of
endemic genera), with its southern limit extending farther south than earlier in the
Lower Jurassic (see discussion in Sect. 4.3 and Fig. 5.11). The peculiar genera
Lithiotis Gümbel and Opisoma Stoliczka, characteristic of the Jurassic tropical
regions during Pliensbachian times (see discussions in Sect. 3.2.4), reached northern
Chile during the Toarcian. While Lithiotis was absent further south in the South
Andean unit, other elements of this association, such as Opisoma (Fig. 3.4a) and
Gervilleioperna, extended further south (Aberhan and Hillebrandt 1999).
Cox (1965) mentioned that the presence of the species Weyla ambongoensis
(Thevenin) in Toarcian beds of eastern Africa ‘‘affords a somewhat meagre evidence
that a faunal sub-province comprising the western part of the present Indian Ocean
region and extending over northern Africa had come into existence’’. This is a
remarkable observation, taking into account the evolution of paleobiogeography of
this part of Africa later in the Jurassic. Nevertheless, data from this area are scarce,
and no endemic bivalve genera have been reported so far for the Early Jurassic.
Interestingly enough, the quantitative analysis suggests that by the Pliensba-
chian-Toarcian interval there was no clear differentiation of biochoremas yet.
Cluster analysis shows one major group with similarity values higher than 0.50,
encompassing South America, the Himalayas, Madagascar, and Iran, with South
American localities more related to Madagascar.
Furthermore, BSN shows no clear pattern either, with all regions presenting high
similarities with Central Argentina and Chile (Fig. 5.7a). This phenomenon has two
possible explanations: on one hand, a sampling bias cannot be disregarded: in a better
Fig. 5.7 Four BSN analyses of South Hemisphere regions for the Pliensbachian-Toarcian
interval. a Including all Southern Hemisphere regions with enough data, b Excluding region 3
(central Argentina and Chile) from the analysis, c Including all Southern Hemisphere regions plus
three regions situated in the Paleo-Northern Hemisphere: Mexico, Iberian Peninsula, and
southern France, d Same as c excluding region 3. Continuous thick lines: distance value \0.10
(more than 90 % of genera shared); continuous lines: distance value C0.10 and \0.25 (90 % to
more than 75 % of genera shared); continuous thin lines C0.25 and \0.50 (75 % to more than
50 % of genera shared); support values of 1 unless otherwise indicated on each line
known region there may be higher chances of finding genera shared with other
regions. Table 5.2 shows that South America (especially region 3) is by far the area
with more known genera. On the other hand, the results of the BSN may be showing
an actual pattern, consequence of a major biogeographic event: the aperture of the
Hispanic Corridor. The development of this migration route may have allowed for
faunal mixing among some regions; if this were the case, then those regions should be
highly similar to all other regions since it would be lumping genera common to both
main realms. Both of these explanations may be responsible for the pattern observed
here; thus, another BSN was built excluding central Argentina and Chile, resulting in
the scheme of Fig. 5.7b. As can be seen dissimilarity is still low (although higher than
in Fig. 5.7a), but an incipient differentiation can be seen between northern and
southern regions. When some Northern Hemisphere localities are added to the

sketch for the Pliensbachian-
Toarcian time slice, showing
Pacific first-order
Fig. 1.2
analysis, the Tethyan realm appears better differentiated (Fig. 5.7c–d); this is more
obvious when locality 3 is excluded (Fig. 5.7d).
The strong affinity of Madagascar (and to a lesser extent South–East Africa) with
South America in all the analyses is remarkable. This similarity did not escape early
global studies of Jurassic bivalve diversity, and was even used to support the exis-
tence of a direct marine connection between East Africa and South America as early
as the Early Jurassic (Hallam 1977), a hypothesis challenged also on the basis of
bivalve distribution (Damborenea and Manceñido 1979) and subsequently dropped.
Nevertheless, there is a seemingly disjunct distribution of some bivalve genera in
both areas, which is probably heavily related to their sharing similar paleolatitudes at
the time. Although initial rifting between eastern and western Gondwana probably
began late in the Early Jurassic, actual sea-floors spreading apparently started only in
the Late Jurassic (Crame 1999), which is in agreement with our results (see
discussion below under ‘‘Early Cretaceous’’). Figure 5.8 shows a graphic summary
of the Pliensbachian-Toarcian paleobiogeographic units in the Southern
Hemisphere, with the hypothetical paleoposition of the boundary zones.
5.3.3 Middle Jurassic
During Middle Jurassic (Fig. 5.9) and early Late Jurassic times the Southern
Hemisphere bivalve benthonic faunas maintained their high proportion of
cosmopolitan genera, always above 40 % of all taxa, but still some endemic high-
latitude genera were present.
The two South Pacific subunits maintain their level of endemism between 16 and
29 % for the whole Middle Jurassic (see Table 5.3), although at the same time they
experienced the proportional decrease of high-latitude taxa. The influx of high-
latitude genera reached 24 % for the Maorian and 33 % for the South Andean units,
and in the Maorian biochorema it was exceeded during the Late Jurassic. On this
evidence, many authors (see Grant-Mackie et al. 2000) concluded that the Maorian
Province ceased to exist in the early Middle Jurassic. Nevertheless, a

sketch for the Aalenian-
Bajocian time slice, showing
Pacific first-order
Fig. 1.2
paleogeographic unit is still recognizable (though probably with lower rank)

throughout the Middle Jurassic (Damborenea 2002b). This unit can either be
regarded as an extension of the Maorian Province, or as the origin of the Austral
Province recognized by Kauffman (1973) for Early Cretaceous times. To name it for
the Middle Jurassic the first alternative was followed by Damborenea (2002b), but
regardless of how we chose to call it, the continuity of an austral biogeographic unit
from Triassic (Maorian Province) to Cretaceous (Austral Province) is evident from
our results. This unit is recognized on the basis of the following endemic genera:
Haastina Marwick (Bajocian), Malagasitrigonia Cox (Aalenian-Bajocian), Kana-
kimya Campbell and Grant-Mackie (Aalenian-Bajocian?), and Moewakamya
Campbell and Grant-Mackie (Bathonian-Oxfordian). For the South Andean unit
endemic genera include Anditrigonia Levy (Bajocian-Tithonian), Andivaugonia
Leanza (Bajocian-Callovian), Eoanditrigonia Leanza (Bajocian-Callovian), Neu-
quenitrigonia Leanza and Garate (Aalenian-Bajocian), Lambertrigonia Leanza
(Callovian). High-latitude taxa in these two units are proportionately fewer; they
include Retroceramus Koshelkina (Bajocian-Oxfordian) and Scaphogonia Crick-
may (Bajocian-Tithonian) for both units, Praebuchia? (Aalenian) for the South
Andean, and Fractoceramus Koshelkina (Bajocian) and Hijitrigonia Kobayashi
(Bajocian) for the Maorian unit. Despite the early Toarcian extinction event,
endemism in the Maorian unit climbed from 22 % in the late Early Jurassic to 28 %
in the early Middle Jurassic. In this connection, in the Andean region endemic
bivalve species were more affected than others by the ‘‘Pliensbachian-Toarcian’’
extinction event (Aberhan and Fürsich 2000).
The North Andean endemic center is still faintly distinct during the Aalenian,
with the presence of the endemic Gervilleiognoma Aberhan and Hillebrandt.
Published data from younger times in this area appear very scarce and incomplete,
thus precluding confident use in this context.
A new SE Tethyan unit (Fig. 5.9) appears to be recognizable from Bajocian
times onwards on the basis of its edemic genera Indolucina Fürsich et al.
(Bajocian-Oxfordian), Agrawalimya Singh et al. (Bathonian-Callovian), Indocor-
bula Fürsich et al. (Bathonian-Callovian), Indomya Jaitly (Bathonian-Callovian),
Indoweyla Fürsich and Heinze (Bathonian-Callovian), and Venilicyprina Fürsich

et al. (Bathonian-Callovian).
For Bathonian-Callovian times yet another new unit began to be discernible:
East African (= Provincia Etiopico-Indo-Malgascia in Ficcarelli 1968; Ethiopian
Province in Hallam 1977). This unit contains, however, a high percentage of
strictly low-latitude genera (82 %) and only one probably high-latitude genus. For
these reasons, it is here regarded as part of the Tethyan Realm at this time.
By the end of the Middle Jurassic, just four biochoremas are identifiable in the
Southern Hemisphere, two South Pacific (Austral and South Andean), and two
Tethyan (East African and SE Tethyan) units. The South Tethyan unit was still
recognizable but it was then placed in the Northern Hemisphere as a result of
tectonic plate migration to the north.
5.3.4 Late Jurassic
The broad situation observed for Bathonian-Callovian times is generally maintained

for the Late Jurassic (Fig. 5.10). The Maorian unit is extended geographically to
include the Antarctic and South Atlantic Plateau regions, and even some western
Pacific localities (Timor, Sula, Buru, Ceram, see Hayami 1984), but is somewhat
less strongly recognizable, and its rank should be lowered, since its overall ende-
mism at the generic level diminishes to 18 % during Oxfordian-Kimmeridgian and
to only 14 % during Tithonian-Berriasian times. Endemic genera include
Moewakamya (Oxfordian), Jeletzkiella (Oxfordian-Kimmeridgian), and Praeau-
cellina (Tithonian-Berriasian), while Malayomaorica (Oxfordian-Kimmeridgian)
has an austral distribution reaching Australia-New Guinea. Retroceramus, Scaph-
ogonia, Lyapinella, and Anopaea are high-latitude genera, mostly didemic in
distribution.
On the other hand, the South Andean unit maintained its levels of endemism.
The anomalous very high percentage of endemism for the Oxfordian-
Kimmeridgian is due to the small number of taxa reported so far, and this figure is
not statistically significant. Endemic genera include the trigonioideans Anditri-
gonia (Bajocian-Tithonian), Antutrigonia (Tithonian-Berriasian), Notoscabrotri-
gonia (Tithonian), and Splenditrigonia (Tithonian-Berriasian), and Retroceramus
and Scaphogonia represent the high-latitude taxa.
The East African unit maintained a percentage of endemic taxa between 13 and
21 %, including Africomiodon and Eoseebachia (Oxfordian-Kimmeridgian), with
a very high percentage (just over 70 %) of low-latitude taxa. It is interesting to
note here that the influence of high-latitude taxa in this area increased from 2 % in
the Bathonian-Callovian to 9 % in the Oxfordian-Kimmeridgian.
Fig. 5.10 Paleobiogeographic sketch for the Oxfordian-Kimmeridgian time slice showing inferred
extension of second-order biochoremas. Thick broken line: approximate position of the boundary
zone between Tethyan and South Pacific first-order biochoremas. Base map as in Fig. 1.2
5.3.5 Early Cretaceous
By the end of the Jurassic and beginning of the Cretaceous the situation changed
and antitropical bivalve genera were again well established at least within
inoceramoids and monotoids (Crame 1986, 1993; Dhondt 1992). According to
Kauffman (1973), recognition of a relatively mature ‘‘South Temperate Realm’’ is
clear, with well-defined ‘‘Austral’’ and ‘‘East African’’ provinces as subordinate
biogeographic units by the beginning of the Cretaceous. The change of the East
African unit from the Tethyan (during most of the Jurassic) to the South Pacific
realm is already hinted by the relatively high percentage (9 %) of high-latitude
taxa during Oxfordian-Kimmeridgian times, possibly as a direct consequence of
the opening of the Mozambique corridor or trans-Gondwana seaway, which
communicated this part of Africa with the South Pacific (see discussion in Crame
1999). Kauffman (1973) referred the East African province to the Indo-Pacific
Region in the Early Cretaceous, according to its content of southern Pacific bivalve
lineages, with a geographic range now restricted to Southern Africa, Madagascar,
and Tanzania. To the north, a transition zone developed (India, Arabia, NE Africa)
as continuation of the Jurassic SE Tethyan biochorema, with strong Tethyan
influence (i.e., the north Indian Ocean subprovince in Kauffman 1973).
During the Early Cretaceous (Berriasian) the South Andean unit contained a
few endemic trigoniodeans, such as Antutrigonia, Splenditrigonia, and Transitri-
gonia. Anopaea was a genus with didemic distribution, and during the Berriasian it
was present in New Zealand and Antarctica, while the endemic Praeaucellina
lingered from Tithonian times in the Maorian unit. According to Kauffman (1973),
the Austral Province of the Indo-Pacific Region (his South Temperate Realm) was
strongly developed at the beginning of the Cretaceous, including Australia, New
Zealand and New Guinea.
Fig. 5.11 Evolution of the second-order paleobiogeographic units discussed here, recognized for
the Southern Hemisphere using marine benthonic bivalve faunas. Triassic and Jurassic data
discussed here, Cretaceous data from Kauffman (1973). The width of the bars is proportional to the
percentage of endemic genera through time, which is an indication of their change in rank. Solid
color indicates development in the Paleo-Southern Hemisphere; diagonal pattern shows units
which migrated to the Paleo-Northern Hemisphere (undated from Damborenea 2002c, Fig. 7)
From Middle Cretaceous times onwards the North Andean unit was clearly
recognizable as a Caribbean subprovince/province (Kauffman 1973), and an
Austral Realm was proposed by Fleming (1963) on the basis of bivalve and
gastropod distribution.
Integrating our results (Damborenea 2002b, Fig. 6, updated here) with those
outlined by Kauffman (1973), the overall history of the paleobiogeographic units
just discussed during Triassic to Cretaceous times shows some interesting features
(Fig. 5.11). The disruption of the pre-existing pattern at the Triassic/Jurassic
boundary, coinciding with the severe biotic crisis, is evident, as well as the rela-
tively rapid recovery. Apart from this post-extinction period, the Maorian-Austral
unit shows a remarkable continuity in time and may be regarded as precursor of
the Late Cretaceous-Early Cenozoic Weddellian Province (Crame 1999).
If a ranking related to degree of endemism is used, it is evident that units
frequently changed rank (represented by the changing width of their graphic
representation on Fig. 5.11). Moreover, it is clear that some units changed their
relation to higher order units, too. This happened with the East African unit, which
first developed as part of the Tethyan Realm but toward the end of the Jurassic
began to receive more influence from southern faunas to eventually be regarded as
part of the South Temperate Realm by Kauffman (1973) in the Cretaceous.
5.4 Congruence
Since this analysis was only based on one group of organisms, it is interesting to
compare the results with those obtained from other groups. Different groups of
organisms sometimes show congruent patterns of biogeographic distribution, and
these are extremely important to the recognition of general patterns which could be
related to common causes. Nevertheless, most other benthonic macro-invertebrate
groups (gastropods, corals and echinoderms) are still insufficiently known to be
used in paleobiogeography for the Jurassic at a global scale. The only groups with
comparable level of analysis for the time considered here are ammonites and
belemnites, but they had mostly nektonic or nekto-benthonic habits and thus their
distribution patterns may be partly influenced by somewhat different phenomena.
From the scattered available data it is evident that some groups of restricted
tropical affinities, such as reef building corals and certain sponges like Stylothal-
amia, are completely absent from the Southern Hemisphere low-latitude areas
belonging to the South Pacific paleobiogeographic unit as understood here
(Hillebrandt 1981; Crame 1987; Beauvais 1992).
One of the most interesting questions that need further discussion is the
observed differences in the first-order paleobiogeographic units for the Southern
Hemisphere between those based on bivalves as described here and those based on
ammonites. Differences between distribution of benthonic and pelagic organisms
are to be expected (see discussion in Masse 1992). According to the pattern of
ammonite distribution most authors consistently recognize only two Realms
(Boreal and Tethyan) for the Jurassic (see Arkell 1956, and more references in
Cecca 1999; Westermann 2000b; Grant-Mackie et al. 2000).
Hillebrandt (1981; Hillebrandt et al. 1992) distinguished an Eastern Pacific and
an Indo-SW Pacific sub-realm during some time lapses within the Early and
Middle Jurassic, but they admit that for the Late Jurassic lack of data severely
constrains the discussion of ammonite provincialism in circum-Pacific regions.
Dommergues et al. (2001) reviewed the distribution of Early Jurassic ammonoids
trying to match different morphologic sets previously recognized with analytic
distribution patterns, defined by considering the distribution and abundance of
species, but they did not use data from the southern high-paleolatitude regions. In
fact, a different picture seems to be emerging as new data become known.
Westermann (1981) recognized an East Pacific ammonite realm (sub-realm in
Westermann and Hudson 1991) for late Bajocian to early Callovian times
extending from Arctic Canada along the Pacific coasts of America and west
Antarctica to New Zealand. Riccardi (1991) found a nearly continuous presence of
endemic ammonites from Pliensbachian to Oxfordian in South America, with a
sharp increase during Aalenian, and almost no endemics during Kimmeridgian-
Berriasian. Westermann (1996) recognized a high proportion of endemic taxa for
the Bajocian of New Zealand, with a marked increase in Andean affinities for the
Bathonian-Callovian (see Grant-Mackie et al. 2000 for references). According to
Enay and Cariou (1997), an austral ammonite fauna of low diversity became
progressively better established around east and south Gondwana from Oxfordian
times onwards.
An Indo-Pacific paleobiogeographic unit was recognized on the basis of bel-
emnite distribution from Callovian times onwards, initially given realm rank
(Stevens 1963), though linked to (or derived from) the Tethys, but only Boreal and
Tethyan Realms were later accepted (Stevens 1973). The contrast between the
sharp and stable Boreal/Tethyan boundary and a diffuse and unstable Tethyan/
Indo-Pacific boundary of belemnite paleobiogeographic units is emphasized by all
authors. According to Challinor (1991) and Challinor et al. (1992) a South Pacific
Province (referred to the Tethyan Realm) may be distinguished during the Jurassic.
This faunal province extended along the coast of Gondwanaland from southern
South America to New Zealand and possibly New Caledonia in the Jurassic
(Challinor and Hikuroa 2007). Belemnitina (which defines the Boreal Realm) was
dominant in Boreal regions throughout the Jurassic and was also known in South
America-New Zealand-New Caledonia up to the Middle Jurassic (Challinor et al.
1992; Doyle et al. 1997). During the Middle and Late Jurassic trans-Gondwana
migrations introduced Tethyan elements to southern South America and Antarctica
(Challinor and Hikuroa 2007). In the Jurassic there was no belemnite species in
common between Tethyan and Pacific coasts of Gondwanaland, and two provinces
are recognized, respectively; Indo-Tethyan and South Pacific (Challinor in Grant-
Mackie et al. 2000).
Gastropods have been used to recognize paleobiogeographic units for the
Cretaceous (Sohl 1987), but Triassic and Jurassic gastropods are poorly known at a
global scale yet. Within these limitations, Tong and Erwin (2001) noted that the
5.4 Congruence 105
relationships between Triassic Tethyan and American gastropods are weak, even at
the generic level.
Triassic and Early Jurassic brachiopods from austral regions do show a con-
siderable degree of endemism which supports the recognition of a Maorian
Province (MacFarlan 1992; Manceñido and Dagys 1992; Manceñido 2002 and
references therein). During the Late Triassic a strong Maorian unit was well
characterized by (among others) the spiriferide Rastelligera, the athyride Clavi-
gera, and the rhynchonellide Sakawairhynchia, the last one widely distributed in
East Asia, New Zealand, New Caledonia (MacFarlan in Grant-Mackie et al. 2000),
and west central Argentina (Damborenea and Manceñido 2012). Close similarities
of some rhynchonellides and spiriferinides from New Zealand and South America
persist into the Early Jurassic but austral endemism was lower, though there were
probably also some bipolar genera. Maorian brachiopod endemism extended
during Middle and Late Jurassic in New Zealand-New Caledonia (MacFarlan in
Grant-Mackie et al. 2000). Brachiopod diversity diminished considerably toward
the Late Jurassic in the whole austral regions.
Knowledge of Jurassic radiolarians from the Southern Hemisphere is just
emerging (see Kiessling and Scasso 1996; Aita et al. in Grant-Mackie et al. 2000),
but clearly points to non-Tethyan affinities for the Late Jurassic faunas from
Antarctica and the Waipapa Terrane (New Zealand). These data support a pale-
olatitudinal zonation for Late Jurassic radiolarian faunas in the Southern Hemi-
sphere, with a distinct austral paleobiogeographic unit (which is not symmetric
with the boreal one), characterized by low endemism and dominated by Parvi-
cingula-Praeparvicungula (Kiessling and Scasso 1996).
Other microfossils which have been discussed in this context are ostracods
(Boomer and Ballent 1996; Mette 2004; Arias 2006) and interesting results are
emerging, though information is still too fragmentary to show comparable detail
with extensive data from bivalves.
Brief references to paleoclimate and paleogeography can be made to help
explain these differences. During the Jurassic, climates are said to have been
milder, with temperatures more equable across latitude, but with some seasonality
due to the different day lengths through the year (see Sect. 1.5). This may have
been a particularly important factor affecting marine benthonic biota at high lat-
itudes. Pelagic taxa are probably less affected by seasonality than benthonic
inhabitants of littoral environments, such as bivalves. But apart from climate,
purely geographic factors, affecting the ocean circulation patterns, may have had
considerable influence, too. For instance, the distribution of land and sea during
the Jurassic in high latitude regions: while in boreal regions there was a polar
ocean, nearly completely surrounded by land or shallow seas, the austral regions
were part of the proto-Pacific ocean, which was continuous with the eastern
Tethys. Enay and Cariou (1997) emphasized that the lack of such geographic trap
in austral regions would explain why the austral ammonite faunas were never as
distinct from the Tethyan as the Boreal ones. In addition, an important migration
route for benthonic organisms since the Early Jurassic, the Hispanic Corridor
(Damborenea and Manceñido 1979; Smith 1989; Boomer and Ballent 1996; etc.),
remained as a barrier for most pelagic animals until late Middle or even Late
Jurassic times (Elmi 1993; Damborenea 2000; Aberhan 2001).
Regarding the evidence from terrestrial organisms, Balme (in Grant-Mackie
et al. 2000) indicated that there is general agreement that latitudinal zonations
based on megafossil plant distribution existed in the Jurassic, but these do not
support the existence of any substantial climatic barriers.
5.5 Paleobiogeographic Units and Mass Extinctions
The largest biotic crises did not only affect faunal taxonomic diversity, but also
had serious impacts on other aspects as well, the ecologic one being the most well
known. Several authors had also emphasized that one of the most direct results of
such severe taxonomic and ecologic disruption is the loss of previous paleobi-
ogeographic structures (see discussion and references in Ros and Echevarría
2011). It is well-known that endemic taxa seem to be positively selected by
extinction and suffer more than widely distributed taxa (Hallam and Miller 1988).
As the recognition of paleobiogeographic structures is largely based on the pres-
ence of endemic taxa, the disruption of the previous pattern is the most likely
result.
As expected, the then existing late Paleozoic paleobiogeographic patterns were
seriously disrupted by the Permo-Triassic extinction event. This has been already
noticed by previous authors, and it is clearly seen in Shi and Grunt 2000 (Fig. 8),
where the Permian paleobiogeographic regions recognized for the Anti-Boreal (or
Gondwana) areas disappeared altogether at the beginning of the Triassic. Our
analysis shows that there was no recognizable paleobiogeographic pattern for all
Early and part of Middle Triassic times, and endemism in the Southern Hemi-
sphere re-appeared only by Etalian times (i.e., late Anisian) in New Zealand. It
became stronger by the Ladinian, when already a Maorian unit was well charac-
terized, and was definitively settled during the late Triassic, when other endemic
centers also appeared (see Fig. 5.11).
This apparently well-established late Triassic paleobiogeographic pattern, with
at least five biochoremas of various ranks, was again almost totally wiped out by
the next biotic crisis, the Triassic-Jurassic extinction event. The earliest Jurassic
low endemism level was not restricted to the southern hemisphere, but is also
apparent at a global scale: Liu (1995) concluded that provinciality in the northern
hemisphere during Hettangian, Sinemurian and even Pliensbachian times was
unrecognizable. Our data clearly show this pattern for the end Triassic extinction,
and it is also evident that the biota and endemic centers took some time to recover
(see discussion in Hallam 1996), but recovery appears to have been somewhat
faster than after the Permo-Triassic event. This is particularly clear in the previ-
ously well-defined Maorian biochorema, as the lack of endemic genera makes this
unit impossible to recognize during Hettangian and Sinemurian times, though it
recovered by Pliensbachian times.
5.5 Paleobiogeographic Units and Mass Extinctions 107
Macchioni and Cecca (2002) register two drops in ammonite diversity in the
Early Toarcian of Europe, the first one producing the disruption of Tethyan-Boreal
provinciality, the second one coinciding with the onset of OAE. They further
proposed that trends in endemism may be reversed during transgressions or
regressions. Our data are not refined enough to pick any consequence of the Early
Toarcian biotic crises which is known to have affected bivalve biodiversity
worldwide.
Another quite different but interesting relationship between biogeography and
extinction events is just now emerging. The new developments in paleobiogeo-
graphic analysis suggest that extinction may negatively influence the retrieval of
biogeographic patterns from living organisms (Lieberman 2003). For this reason,
the careful analysis of distribution of extinct taxa acquires an extra interest.
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Chapter 6
Global Scale
Abstract Southern Hemisphere bivalves have provided arguments for the anal-
ysis of some interesting topics of global significance, such as bipolarity and the
establishment of seaways. The records of Triassic and Jurassic bivalves with
bipolar distribution are numerous, and show that bipolarity was a persistent phe-
nomenon in marine environments for hundreds of million years. These examples
are potentially very enlightening for the discussion about the causes of this global
disjunct distribution pattern, for which an integrative approach is still pending.
Bivalves were also significant for the early proposal of a marine connection
between western Tethys and eastern Panthalassa in the Early Jurassic, now known
as Hispanic Corridor. The evolution of similarity coefficients during time has
proven to be a good tool for detecting changes in the relationships of bivalve
faunas from western Tethys and eastern Panthalassa. Within regions from the same
side of the passage, similarity was high at all times and increased slightly with
time during the Early and Middle Jurassic. When comparing regions from either
side of the Hispanic Corridor, a sudden increase in similarity beginning very early
during the Early Jurassic from low values during Late Triassic times is evident,
indicating that the first marine connection between them was probably established
by Pliensbachian times. The shallow connection acted first as a screen, being an
effective barrier for ‘‘neritic’’ species, while allowing the passage of benthonic
littoral species. This is also related to the change in water current pattern. The
observed southwards shift of the boundary zones between Tethyan and South
Pacific along the western South American coast during the Early Jurassic is
coincident with a symmetric northwards shift of the Tethyan/Boreal boundary in
the Northern Hemisphere, suggesting a common cause, such as global climate
change.
Though this book is not treating Triassic/Jurassic bivalve distribution at a worldwide

scale, the already discussed arguments about the persistence of a South Pacific
biochorema through time (Chap. 5) have undoubted global interest for

DOI: 10.1007/978-94-007-5098-2_6, Ó The Author(s) 2013
122 6 Global Scale
understanding the broad distribution patterns of marine biotas and their evolution
through time.
Furthermore, there are a few extra topics of universal significance which will be
addressed here, since Southern Hemisphere bivalves provided, and will certainly
continue to offer, considerable insight for their analysis. This chapter will deal
mainly with two worldwide issues: bipolarity and the establishment of major
seaways, but water circulation patterns and the boundaries of the main biochor-
emas will also be shortly discussed in relation to the evidence provided by the
distribution of marine bivalves.
6.1 Bipolarity
Disjunct distributions have long interested biogeographers, since the existence of

taxonomically related taxa living in areas separated by barriers calls for a historic
explanation of their development. Among disjunct distributional patterns, one of
the first to be recognized is that of taxa with northern and southern high-latitude
components separated by a distinct low-latitude gap, which was observed both in
terrestrial and marine biotas. This was already discussed by Darwin (1859), and is
generally known as bipolarity.
Bipolarity and antitropicality are relatively common but yet poorly known
large-scale biogeographic phenomena. Their explanation has been, and still is,
highly controversial, since they have been interpreted as the result of either dis-
persal events from a center of origin or vicariant disruption (Cecca 2009), ratio-
nales which are directly linked, respectively, to two bitterly antagonist schools:
dispersalism and vicariance. An integrative approach to understand bipolarity is
certainly needed (Donoghue 2011), and examples from the past such as those
discussed here can be very enlightening.
But let us leave the explanation of the causes of this phenomenon for the
moment, and concentrate on the observed facts first.
6.1.1 Triassic–Jurassic Bipolar Bivalves
The characterization of marine faunas into a tropical (peri-equatorial) and two

antitropical (polar) belts (these last frequently containing bipolar taxa) was dem-
onstrated beyond doubt for the Cretaceous–Cenozoic (Kauffman 1973; Zinsmeister
1982) and for the Permian–Triassic (Shi and Grunt 2000), in addition to being
revealed for the Triassic–Jurassic on the basis of bivalve distribution (Crame 1986,
1987, 1992, 1993; Damborenea and Manceñido 1992; Damborenea 1993, 2002a, b,
and here).
Late Mesozoic (Late Jurassic–Cretaceous) bipolar bivalve faunas were exten-
sively discussed as a consequence of the development on the knowledge of faunas
6.1 Bipolarity 123
from Antarctica, New Zealand, and South America (Crame 1986, 1987, 1992 and
references therein), which revealed the presence of several taxa previously thought
to be restricted to the northern high-latitude regions, such as Buchia, Arctotis,
Anopaea, Retroceramus, and Aucellina.
A detailed account of the distribution of Early Jurassic pectinaceans from South
America revealed the presence of at least eight genera with bipolar distributions
during that time: Otapiria, Palmoxytoma, Arctotis, Asoella, Kolymonectes, Rad-
ulonectites, Agerchlamys, and Harpax (Damborenea 1993) and these were soon
included in more general discussions on the subject by Crame (1993, 1996a, b).
Bipolarity appears to have been less evident during the Middle Jurassic, though
Retroceramus had very similar species in high latitudes from both hemispheres
(Damborenea 1990) from Bathonian to Tithonian times.
Triassic and Jurassic bipolar bivalve genera had very similar species on both
high-latitude areas (Fig. 6.1), and sometimes antitropical populations are even
regarded as conspecific. It is also interesting to note that known examples are
increasing as thorough and open-minded systematic revisions are performed, and
many taxa originally thought to be restricted to the polar or high-latitudes regions
of one hemisphere have proven to have a bipolar distribution instead as research
progresses. On the other hand, the absence of these taxa in low-latitude areas
cannot be blamed on poor knowledge, since the faunas from those areas are the
best known.
It should also be noted that the distribution pattern of a particular genus may
change with time, bipolarity being only apparent during part of the whole genus
stratigraphic range. Detailed knowledge of the occurrences may add interesting
points to the debate, since not always these observed changes can be attributed
solely to an insufficient fossil record. To the pectinaceans, some other groups are
now known to have had a bipolar distribution during the time interval involved
(Crame 1993, Table 1), and a few examples will be shortly mentioned below.
Aparimella Campbell was an Anisian to Carnian halobiid, known from New
Zealand during its whole time range, but reported also from the early Carnian of
Svalbard (Campbell 1994).
Asoella Tokuyama (Fig. 6.1b, l) spanned the Triassic–Jurassic boundary; it
ranged from Anisian to Pliensbachian and had mostly a circum-Pacific distribution
(see discussion and references in Damborenea and Manceñido 2012 and Ros et al.
2012), though it was certainly more abundant both in the northern (Japan) and
southern (New Zealand, Argentina) areas during its whole time range. It was also
reported from China. Species of this genus were byssally attached and there is
some taphonomic evidence of a facultative pseudoplanktonic habit for some of
them.
Maoritrigonia Fleming was a late Triassic (Carnian–Rhaetian) trigoniid first
regarded as endemic of the Maorian region (New Zealand–New Caledonia), but
later its geographic range was extended to South America (Chile) and SW Alaska
(see details and references in Ros et al. 2012). The poorly known Minetrigonia
Kobayashi and Katayama was another late Triassic trigoniid with records in high-
latitude regions of both hemispheres: Japan, Siberia, Alaska, British Columbia,
124 6 Global Scale
6.1 Bipolarity 125
b Fig. 6.1 Some examples of bipolar bivalves, a–j, specimens from northern high latitude regions.
a Agerchlamys propius (Milova), Pliensbachian, NE Russia; b Asoella confertoradiata
(Tokuyama), Carnian–Norian, Japan; c Palmoxytoma cygnipes (Phllips), Sinemurian, W Canada;
d Kolymonectes staeschei (Polubotko), Sinemurian, NE Russia; e Harpax spinosa (Sowerby),
Pliensbachian, NE Russia; f Arctotis sublaevis Bodyl., Bajocian, NE Russia; g Radulonectites
hayamii Polubotko, Pliensbachian, NE Russia; h Ochotochlamys bureiensis Sey, Pliensbachian,
NW Russia; i Kalentera brodnayaensis Milova, Pliensbachian, NE Russia; j Retroceramus
tongusensis (Lahusen), upper Bajocian, NE Russia. k–t, Specimens from the southern high
latitude regions. k Agerchlamys wunschae (Marwick), MLP23658, Pliensbachian, Argentina;
l Asoella campbellorum Damborenea, MLP32730, Norian–Rhaetian, Argentina; m Palmoxytoma
sp., OU17810, Aratauran, New Zealand; n Kolymonectes weaveri Damborenea, Pliensbachian,
Argentina, MLP23686; o Harpax rapa (Bayle and Coquand), MLP16511, Pliensbachian,
Argentina; p Arctotis frenguellii Damborenea, MLP10418, Early Jurassic, Argentina; q Radu-
lonectites sosneadoensis (Weaver), MLP22321, Pliensbachian, Argentina; r Ochotochlamys sp.,
MLP27592, Pliensbachian, Argentina; s Kalentera mackayi Marwick, MLP 24873, Aratauran,
New Zealand; t Retroceramus stehni Damborenea, MLP14672, Callovian, Argentina. b from
Hayami 1975; c from Frebold 1957; d, e, f, g, h, j from Sey and Polubotko 1992; i from Milova
1988. Figures not to same scale, but proportional size differences are roughly shown
Chile, New Zealand, and probably Argentina (see Ros et al. 2012). A systematic
revision may extend this distribution, since apparently some late Triassic species
referred to Trigonia or Myophoria may actually belong to Minetrigonia.
Triaphorus Marwick was a late Triassic Kalenteridae originally described for
New Zealand but later reported from New Caledonia, and also from Japan and NE
Russia. It was probably a shallow burrower belonging to the shallow infauna or
semi-infauna (see Ros et al. 2012). Kalentera Marwick (Fig. 6.1j, s) was also a
shallow burrower Kalenteridae which lived from Norian to Toarcian times.
Although initially thought to be restricted to Austral regions, later records show that
it was endemic to the Maorian biochorema during the late Triassic, but had a bipolar
distribution at high paleolatitudes during the early Jurassic (Damborenea 2004 and
references therein), being known from NE Siberia, and reaching to northern Chile,
but no species are known below 30° paleolatitude in either hemisphere.
Ochotomya Polubotko was a Norian–Rhaetian ceratomyid genus known both
from northern (NE Siberia) and southern (New Zealand) regions. It was most
probably a shallow burrower (see Ros et al. (2012) for references).
The early Jurassic genus Palmoxytoma Cox (Fig. 6.1c, m) was originally pro-
posed for northern European species, but was later recorded from New Zealand and
Argentina in austral regions; and from Canada, Japan, NE Siberia, and eastern
Russia in boreal areas. It had a clear antitropical distribution during the Hettangian
(New Zealand, Argentina and Chile in the South, NE Siberia in the North), while
during Sinemurian and Pliensbachian times it seems to have been restricted to the
Northern Hemisphere (Damborenea 1993 and references therein). Sporadic
occurrences out of its high-latitude range have been used as indication of influx of
cool seawater (see Arp and Seppelt 2012 for German Pliensbachian Palmoxytoma).
The genus was reported from Europe (England, Sweden, France, Switzerland) and
North America (Canada) but never from low paleolatitude regions.
126 6 Global Scale
Kolymonectes Milova and Polubotko (Fig. 6.1d, n) was traditionally mentioned

as a typical element of Triassic-early Jurassic Boreal faunas, known from NE
Russia and NW Canada. Moreover, it was used by some authors to define and
outline the Arctic subprovince of the Boreal Realm. But at least during the late
Pliensbachian the species K. weaveri Damborenea was very abundant in western
Argentina and a related species was probably also present in New Zealand
(Damborenea and Manceñido 1992; Damborenea 1993 and references therein).
Similarly, Ochotochlamys Milova and Polubotko (Fig. 6.1h, r) was originally
thought to be endemic to the late Triassic–early Jurassic of NE Asia, but it was
later found in Canada and the Pliensbachian deposits of southern Argentina
(Damborenea 2002a and references therein), and thus a bipolar distribution for the
late part of its range is proposed. Species of this genus were byssally attached.
Agerchlamys Damborenea (Fig. 6.1a, k) includes late Triassic–Early Jurassic
species known from circum-Pacific localities. During most of this time range, but
at least during the Pliensbachian, the genus had a clear bipolar distribution, with
records from NE Russia, New Zealand, and west-central Argentina. Species of this
genus are found in the Andes and in New Zealand in very fine-grained sediments,
from marls to tuffaceous siltstones, but never in laminated dark mudstones.
Although locally abundant in certain beds, they were not widespread, and at least
A. wunschae (Marwick) seems to have been a stenotopic species limited to very
low-energy but well-oxygenated environments. It was a byssally attached pectinid,
and it was never associated to thick-shelled epifaunal bivalves or corals, but
usually occurs with sponge spicules instead, probably sponges were their substrate
(Damborenea 1993, 2002a and references therein).
Species of the genus Harpax Parkinson (or subgenus of Plicatula for some
authors) were conspicuous elements of Boreal late Triassic and bipolar Early
Jurassic (mainly Hettangian–Pliensbachian) bivalve faunas (Fig. 6.1e, o), reported
from southern South America, New Zealand, NE Russia and Canada (Damborenea
2002a and references therein). This genus included species which cemented to
hard substrates (mainly other shells) at least early in ontogeny, and adults could
have been free recliners.
Radulonectites Hayami (Fig. 6.1g, q) was a byssally attached Early Jurassic
pectinid genus. Since its original description for Japanese species, it was reported
from the Hettangian–Sinemurian of the eastern Tethys, but during the Pliensba-
chian it was clearly bipolar (Argentina, East Siberia, Japan) (Damborenea 1993,
2002a and references therein).
Arctotis Bodylevsky (Fig. 6.1f, p) was also first thought to be typically boreal,
known from Arctic regions from latest Early Jurassic to Late Jurassic times, but the
discovery of late Jurassic Arctotis in Austral regions has changed this view, and a
bipolar distribution for this genus during that time was proposed by Kelly (1984)
and is now fully accepted with records from Antarctica (Crame 1993). The Toar-
cian/Aalenian species Arctotis? frenguellii Damborenea from western Argentina
may indicate that the bipolar distribution extended in fact over the whole time range
of the genus (Damborenea 1993 and references therein).
6.1 Bipolarity 127
The bipolar occurrences of Retroceramus Koshelkina, Anopaea Eichwald and

Buchia Rouillier (s.l.) were discussed by Crame (1993, for Anopaea see also
Crame 1981 and Dhondt 1992).
6.1.2 Discussion of Triassic–Jurassic Bipolarity
The simplest explanation for such global pattern seems to be a latitudinally related
climatic control, even if temperature gradients changed a lot with time and for the
Jurassic they were milder than at present (thus generating wide transitional zones).
This matter was extensively discussed by Crame (1986, 1987, 1992, 1993),
Damborenea (1993, 2002a, b), and Sha (1996, 2003) with direct reference to
Jurassic faunas, and little else can be added here. Sha (1996) suggested that this
was the result of some degree of repeated interbreeding due to the regularity of the
larval exchange and proposed both cold seawater temperatures and suitable sub-
strate as the main environmental parameters which could control such past anti-
tropical distributions.
It is also evident that this distribution pattern was not restricted to a particular
mode of life; although many of the genera were probably byssally attached or free
epifaunal recliners, some were cemented and some burrowers. Several could have
been pseudoplanktonic and some facultative swimmers. They lived in a variety of
normal marine environments, not being restricted to any particular substrate,
energy, or water depth. Most of them probably had planktotrophic larval stages but
actual data are still few.
On the other hand, they are not randomly distributed systematically; they do
belong to a few families, notably Monotidae, Oxytomidae, Pectinidae, Kalenter-
idae, and Trigoniidae in the Triassic–Early Jurassic, with the addition of
Buchiidae, Inoceramidae, and Retroceramidae in the Middle and Late Jurassic.
Interestingly enough, these families are also those which have been already
mentioned as generically more diverse than average at high latitudes (Fig. 5.2).
Moreover, the Triassic–Jurassic bipolar bivalve set does not include a single genus
of any of the families more diverse than average at low latitudes.
The records just discussed show that bipolarity was a persistent large-scale
phenomenon in marine environments for hundreds of million years. Crame (1993)
explained Jurassic and Cretaceous examples as the result of vicariance due to the
disintegration of Pangea, but emphasized that phylogenetic studies of critical
groups, as the Monotoidea, are needed to test this hypothesis. Such studies could
also indicate whether convergent evolution occurred in unrelated stocks, but
although this possibility cannot be altogether discarded, repetition of bipolarity
makes this explanation hardly applicable to all known examples. In contrast, a
mainly dispersalist explanation was favored by Sha (1996), who emphasized that
planktotrophic larvae could use cool deep water currents for dispersal and even to
maintain interbreeding between northern and southern populations in those
examples where bipolarity is recognized at the specific level. Sha (2003) also
128 6 Global Scale
argued that the pseudoplanktonic inferred habit for most of the bivalve genera
involved was an efficient dispersal medium.
It is evident that there was a peak of bipolarity among bivalves during Early
Jurassic times, which can be related to the disruption of the pre-existing water-
movement patterns and the establishment of a circum-equatorial current which
could have acted as a new barrier for the dispersal of some groups. Nevertheless,
the repetition of the phenomenon through time both before and after this event
makes it difficult to explain such global scale disjunct distributions just as a result
of vicariance alone. Phylogenetic evidence of the groups involved and the Earth’s
tectonic history at these large spatial and time scales are still hard to match, and the
question of the relentless repetition of bipolarity claims for an integrative and
imaginative proposal. Crame (1993) recognized three main phases of bipolarity:
Jurassic–Cretaceous, Paleogene–Early Neogene, and Plio-Pleistocene-Recent, and
related each of them to different major causes, respectively, the disintegration of
Pangea, global climatic change, and glacial cooling, with different emphasis on
vicariance and dispersal.
It is evident that much more work is required in several fields of knowledge to
advance in the integrated explanation of bipolarity. Data from the fossil record are
essential, but a higher detail in both geographic and time ranges of the taxa
involved is needed.
6.2 Seaways: The Hispanic Corridor
The opening of the North Atlantic, which triggered off the fragmentation of Pangea,
and the establishment of a marine connection between the western Tethys and the
eastern central Pacific (Hispanic Corridor) is one of the most important paleo-
geographic events which occurred during the Jurassic. New biogeographic, geo-
logic, paleomagnetic and geophysic data are constantly providing rich arguments
for the discussion of this event. Among them, biogeography is a most powerful tool
to understand its nature and timing, and was one of the first to be used in this
connection.
The distribution patterns of bivalves during the late Triassic–Middle Jurassic
time interval is now well known, both along the eastern Pacific (Damborenea
1996; Aberhan 1993, 1994, 1998a, b and references therein) and the western
Tethys (Hallam 1977; Liu 1995; Liu et al. 1998 and references therein).
The distribution of certain bivalves was used to propose the hypothesis of the
existence of a shallow marine connection (now called Hispanic Corridor) since
Early Jurassic times (Damborenea and Manceñido 1979) and has been widely
discussed since then on the basis of the distribution of bivalves and also other
marine invertebrates (Hallam 1983; Smith and Tipper 1986; Nauss and Smith
1988; Newton 1988; Smith 1989; Smith et al. 1990; Riccardi 1991; Damborenea
2000; Aberhan 2001), especially in connection to the age of the establishment of a
continuous marine corridor.
6.2 Seaways: The Hispanic Corridor 129
Fig. 6.2 Location of the

regions used for the analysis
of Hispanic Corridor. Base
map as in Fig. 1.2. 1,
Colombia; 2, Perú and N
Chile; 3, Central Chile and
Argentina; 4, Mexico; 5,
Cratonic USA and Sonomia
terrane; 6, North American
Boreal craton; 7, S France; 8,
Spain and Portugal; 9, Mor-
occo and Algeria; 10, Japan;
11,Wallowa; 12, Wrangellia;
13, Stikinia; 14, Quesnellia;
15, Cadwallader
Some authors even proposed that the Hispanic Corridor might have been open
already in the Late Triassic, but supporting proof is doubtful and also there is strong
evidence against this idea, such as the paleobiogeographic distribution of the aberrant
bivalves included in the genus Wallowaconcha, known only from eastern Tethys and
eastern Panthalassa, with no occurrences in western Tethys (Yancey et al. 2005).
The quality of the database is critical in biogeographic approaches such as this.
A large amount of the data used here was compiled from papers published during
more than a century, and is necessarily uneven. Most published information was
re-evaluated, and for this reason the data were treated only at generic/subgeneric
level. This admittedly results in loss of information but is common practice in other
recent paleobiogeographic papers using bivalve data (see Liu 1995; Liu et al. 1998).
The distribution of 236 bivalve genera and subgenera was recorded for 15 geo-
graphic areas (Fig. 6.2), stage by stage from the Norian/Rhaetian to the Bajocian,
from the sources listed in Damborenea (2000) with the additions mentioned below.
Since the aim of the analysis was to register changes across the Hispanic Corridor,
some of the areas were pooled together: South America: (1 ? 2 ? 3), references
already mentioned in Sect. 5.1, and unpublished data; Mexico(4) can be pooled
together with cratonic North America, but it was not because this is a key area for the
discussion, data from Jaworski 1929, Sandoval and Westermann 1986, Stanley et al.
1994, Damborenea and González-León 1997, McRoberts 1997, Stanley and Gon-
zález-León 1997, and Scholz et al. 2008; ‘‘Cratonic’’ North America: (5 ? 6);
Southwestern Tethys: (7 ? 8 ? 9). Apart from these regions, directly concerned
with the question of the Hispanic Corridor, data from Japan (10) were analyzed to
test the behavior of bivalve distribution across the Pacific. Although not directly
involved in the main subject, data from several suspect terranes of the western North
American margin were likewise included (11–15).
130 6 Global Scale
For each area, data were distributed within seven time slices corresponding to
stages, as follows: Norian/Rhaetian, Hettangian, Sinemurian, Pliensbachian,
Toarcian, Aalenian, and Bajocian. Only records with reliable age assignment to the
stage level were included. Data are lacking or very scanty for some areas/time
combinations, and those with few data were discarded for the analysis. Unfortu-
nately, data are still scarce for key areas, such as northern South America
(Colombia, Perú).
Simpson’s similarity coefficients were calculated for every pair of areas with
enough data for the seven time intervals considered. This set of coefficients is the
basis for the following analysis.
Two background patterns should be taken into account: the overall increase in
bivalve diversity at the Pliensbachian and paleolatitudinal control, which was
recognized along the Pacific margins both for the northern (Tozer 1982) and
Southern Hemispheres (see Sect. 4.2).
Several observations can be drawn from the analysis of this set of similarity
coefficients, but only a few will be discussed here.
When analyzing the composition of the fauna between localities, there seems to
be a general trend to increase the similarity value through time when localities
situated at both extremes of the Corridor are compared (Fig. 6.3b), while between
localities from the same side of the corridor values trend to be high but less variable
(Fig. 6.3a). In order to test the significance of these observations, a GLM was
performed. Since Simpson’s coefficient of similarity is nothing but a proportion, and
we are looking for general trends in the values of those proportions through time, a
GLM is perfectly applicable. As a result, many of the localities (although not all of
them) at both ends of the Corridor showed an increasing significant trend in simi-
larity; some of these significant trends are shown in Fig. 6.3b.
On the other hand, among the comparisons between localities at the same
extreme of the Corridor only a few showed some significant change through time,
standing out Central Argentina and Chile, which tends to significantly increase the
similarity with the Pacific localities of the Northern Hemisphere (Fig. 6.3a).
Considering the expansion of tropical faunas during the Early Jurassic (Sect. 6.4),
and the position of this locality close to the boundary between biogeographic units,
this result was to some extent expected. But most important, this result shows that
there might be many factors influencing the similarity between localities, and
hence the observed patterns, though providing auxiliary support for the develop-
ment of the Corridor, cannot be regarded as conclusive evidence on their own.
Across the Hispanic Corridor (Fig. 6.3b) there was a general pattern of sudden
increase in similarity beginning very early during the Early Jurassic (Sinemurian)
from low values during Late Triassic times, reaching similarity peaks at about
Pliensbachian–Toarcian times, followed by a decrease by the end of the Early
Jurassic. The maximum values then observed are similar to those obtained within
regions and along the eastern Pacific for earlier times. Maximum similarity peaks
across the Corridor fluctuate between the Pliensbachian and Toarcian for different
regions.
Fig. 6.3 Evolution of Simpson’s similarity coefficients with time when the areas located in
Fig. 6.2 are compared to each other. a Comparison of areas located within the same side of the
Hispanic Corridor. b Comparison of areas situated on both sides of the Hispanic Corridor; dashed
lines represent time intervals with 1 genus or none in one of the compared areas
The first marine connection was probably well established by Pliensbachian-

Toarcian times, and thus preceded rifting by millions of years, but allowed an
important faunal interchange of benthonic organisms such as bivalves. It is also
evident that the shallow connection first acted as a screen, being an effective barrier
for ‘‘neritic’’ species, while allowing the passage of benthonic littoral species
(Damborenea and Manceñido 1979; Damborenea 2000; Aberhan 2001). It was
postulated that the corridor might have been open much earlier, since early
Hettangian times (Sha 2002), or even as early as Norian (Sandy and Stanley 1993,
based on the distribution of brachiopods), but these last proposals are not conclu-
sive, and in fact there is evidence against them (see discussion in Sect. 6.3).
Of special interest for this subject are taxa restricted to the western Tethys and
the eastern Pacific and their mutual relationships. Several benthonic bivalve taxa,
and particularly epifaunal stocks, can be followed ‘‘step by step’’ along this
migration route, either eastwards or westwards, examples being Weyla and some
other pectinoid species (Damborenea and Manceñido 1979, 1988; Aberhan 2001),
132 6 Global Scale
Fig. 6.4 Hypothetic dispersal routes proposed for early Jurassic bivalves belonging to different
life-habit groups, exemplified with the sublittoral benthonic recliner Weyla (left) and the
swimmer or pseudoplanktonic Posidonotis (right). First occurrences in the different regions are
marked with letters: H, Hettangian, S, Sinemurian, P, Pliensbachian and T, Toarcian. Base map as
in Fig. 1.2
and species of the Lithiotis reef fauna (Broglio Loriga and Neri 1976; Hillebrandt
1981; Nauss and Smith 1988; Aberhan and Hillebrandt 1999). Weyla migrated
eastwards (Fig. 6.4) but other pectinoids and species of the Lithiotis-Opisoma
association probably followed the same route at approximately the same time but
in opposite direction. Other bivalve taxa which were not restricted to these regions
but may have migrated along this corridor, according to the known distribution
with time, are: from Tethys to eastern Pacific: Palaeolopha, Pseudopecten, Atreta,
Terquemia, Cardinia, Pteromya, Goniomya; from eastern Pacific to Tethys
Gryphaea, Actinostreon, Preaexogyra, Aguilerella, Gervillaria, Lycettia, and
some trigoniids, such as Frenguelliella, Jaworskiella, Vaugonia, and
Psilotrigonia.
Other possible consequences of the opening of the Hispanic Corridor for
bivalve diversity dynamics were explored. Aberhan and Fürsich (1997) proposed
that the preferential disappearance of endemic bivalves at the early Toarcian
extinction event in South America could be partly explained by immigration of
cosmopolitan species via the corridor and subsequent competitive replacement. In
addition, it was suggested that the rise in NW European bivalve diversity during
Toarcian–Aalenian times was a direct consequence of taxa immigration through
the corridor from South America, presumably filling the vacated ecospace after the
early Toarcian extinction event (Hallam 1983; Hallam and Wignall 1997). Based
on a detailed survey of time ranges at the species level for bivalves from each side
of the corridor Aberhan (2002) concluded that neither of these hypothesis can be
sustained, and suggested that the biotic recovery in both regions was largely
controlled by increasing within-region origination rates rather than by

immigration.
6.3 Oceanic Currents
The distribution of some Triassic and Jurassic bivalve species was also used as
evidence to propose or favor certain ocean circulation models. Muller and Ferguson
(1939) pointed out that Norian faunas of western North America were more similar
to those from Mediterranean and especially Alpine regions than to those from the
westernmost Tethys areas. To explain this pattern, Kristan-Tollmann and Tollmann
(1981) envisaged a westerly directed ‘‘Tethyan current’’ for late Triassic times, and
proposed such direction for the migration of the marine fauna. Some bivalve data
are indeed in agreement with this hypothesis. For instance, during Triassic times
Palaeolopha was common in eastern Panthalassa and central Europe, but was
absent from Western Europe. The distribution of the peculiar late Triassic genus
Wallowaconcha, abundant in eastern Panthalassa and also present in central and
eastern Tethys (Arabia and south Asia), suggests westwards migration across
Panthalassa (Yancey et al. 2005). This Triassic water-movement pattern also
implies the absence of a direct connection between western Tethys and eastern
Pacific at that time.
Bivalves were again used as arguments for the distribution of oceanic surface
currents during the Jurassic, sometimes in a contradictory manner. For instance,
while Nauss and Smith (1988) advocated a late Pliensbachian migration through
the Hispanic Corridor for the main elements of the Lithiotis fauna, Krobicki and
Golonka (2009) proposed an eastwards migration from western Tethys through
Panthalassa up to the western margin of North and South America. This last
proposal would mean the existence of a strong eastwards surface equatorial cur-
rent, while the prevailing ideas favor an opposite-directed current for that time (see
Sect. 1.6).
Data analyzed here strongly suggest that the Hispanic Corridor was the most
probable migration route for benthonic bivalves. Nevertheless, when ‘‘neritic’’
bivalve species are analyzed separately from the benthonic ones, and although data
are still few, their distribution during the Early Jurassic can be alternatively
explained using a pantropic model, implying planktotrophic larvae and fast ocean
currents. In this case, data agree better with a westwards direction of prevailing
currents (in agreement with the proposal of Kristan-Tollmann and Tollmann 1981
for Late Triassic times), since some taxa which are abundant along the eastern
Pacific, such as Posidonotis and Otapiria, do have isolated late records in eastern
(but not western) Tethys (Fig. 6.4).
134 6 Global Scale
6.4 Evolution of Global Biogeographic Boundaries
As previously mentioned, transitional zones occurred between the Tethyan and

South Pacific first-order units. To study the evolution of these boundary zones
requires a very detailed set of data, which is not available for the key areas in the
Southern Hemisphere, except for a few, such as the already described ones along
the western South American margin (Sect. 4.3). Nevertheless, some broad features
will be pointed out, since it is interesting to compare them with the evolution of
Boreal/Tethyan boundary zones, which are somewhat better known. The evolution
of the boundary between Tethyan and Boreal Realms has been the subject of many
contributions, most of them based on the distribution of ammonite taxa (e.g. Imlay
1965; Hallam 1969, 1981, 1994; Fürsich and Sykes 1977; Hayami 1990;
Dommergues and Meister 1991; Hillebrandt et al. 1992), and on the basis of
bivalve distribution in Europe by Liu (1995).
For the northwest Pacific, the Tethyan–Boreal boundary based on bivalves was
between 36 and 34° during the Late Triassic in Japan (Tamura 1990) but was
attributed to accretion, and it was situated in the lower Amur region of Siberia
during the Early Jurassic (approximately 48° N) according to Hayami (1990). Sey
and Kalacheva (1985) suggested that the Boreal–Tethyan ammonite boundary was
about 60° N during the Sinemurian, a few degrees further north during the
Pliensbachian and reached about 75° N during Bajocian/Bathonian times.
For the northeast Pacific Taylor et al. (1984) record a northwards shift (about
8–10°) of the Boreal–Tehyan boundary and a 30° northwards shift in the reach of
east Pacific faunas between Sinemurian and Pliensbachian times. For the
Pliensbachian the boundary is marked by the extension of the Lithiotis reef faunas.
Dommergues and Meister (1991) also reported a northwards shift in the boundary
zone between Euroboreal and Tethyan ammonite faunas from Sinemurian to
Domerian times.
All data indicate that a northwards shift of the Boreal/Tethyan boundary from
Pliensbachian to Bathonian times took place, and that the Bathonian can be rec-
ognized as the time of the greatest Tethyan spread during the Jurassic. The
boundary then moved southwards during the Callovian (Liu 1995, p. 49), when the
distinctiveness of the Boreal bivalve fauna strongly increased and was maximum
at the Oxfordian (Liu 1995, p. 50). The Callovian and early Oxfordian marked the
widest spread of the Boreal Realm according to Hallam (1971).
From the possible transitional zones located in paleotemperate regions of the
Southern Hemisphere, the relationship between Maorian-type and other faunas in
the western Pacific was studied by Hayami (1984). In the eastern Pacific, the South
Andean boundary zone has been recognized for Early Jurassic times (Sect. 4.2).
Based on species distribution along the eastern Paleo-Pacific margin, a southwards
shift of this boundary of about 8–10° latitude has been proposed for the Hettan-
gian–Toarcian interval. It is interesting to compare these observations with the
evolution of the boundary areas between Tethyan and Boreal Realms in the
Jurassic. To make this comparison, the boundary areas proposed on the basis of
6.4 Evolution of Global Biogeographic Boundaries 135
Fig. 6.5 Position of the

boundary zone between
Tethyan and high-latitude
first-order paleobiogeographic
units based on bivalve
distribution for the
Pliensbachian and Toarcian.
Sources discussed in text. Base
map as in Fig. 1.2. (From
Damborenea 2002b)
bivalve distribution (Hayami 1984, 1990; Smith and Tipper 1986; Smith 1989;
Liu 1995; Damborenea 1996; Liu et al. 1998; Aberhan 1999), which are not
always coincident with the limits proposed on the basis of ammonites, have been
plotted on Fig. 6.5 for the Pliensbachian and Toarcian. The results show a high
congruence between the behavior of the boundary in both hemispheres during the
Early Jurassic: the southwards shift in the Southern Hemisphere is matched by an
equivalent northward shift in the Northern Hemisphere. There are not yet detailed
enough data from the Southern Hemisphere to follow this comparison for later
Jurassic times. It is interesting to point out, though, that the greatest expansion of
low-latitude bivalve faunas in the Southern Hemisphere occurred during Middle
Jurassic times (see also Grant-Mackie et al. 2000), and that the northward shift of
the Maorian/Tethyan boundary during Late Jurassic times in western Pacific
regions, albeit somewhat complicated by the tectonic history of the area (Hayami
1984, 1987), roughly corresponds with the widest expansion of the Boreal Realm
in the Northern Hemisphere. These coincidences suggest that the main faunal
shifts are related in their origin.
One of the plausible explanations which can account for both the northern and
southern symmetric migration of the boundaries of the Tethyan Realm toward the
poles during the Pliensbachian and Toarcian is a worldwide warming of the cli-
mate at that time. Based on the study of the distribution of Holocene mollusks in
the Sea of Japan, Lutaenko (1993) concluded that an increase of 0.1–0.2o C in
surface water temperature can cause a shift of about 100 km of the boundary
between warm water mollusks and the Pacific boreal region.
Of course, climatic changes are often invoked to explain such shifts in paleo-
biogeographic boundaries. However, it is far more difficult to discuss what the
cause of such supposed climatic changes was. The main paleogeographic events
which should have influenced the climate of this area during the Jurassic were: (a)
136 6 Global Scale
an inferred latitudinal drift of the region concerned (Iglesia-Llanos et al. 2006),

which could explain per se the observed displacement of climatically controlled
bivalve faunas in the Southern Hemisphere, and (b) the establishment of a per-
manent seaway between Tethys and the eastern Pacific, producing large-scale
alterations on the ocean circulation patterns and climate.
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SpringerBriefs in Earth System Sciences

SpringerBriefs Seaways and Landbridges: Southern

For further volumes:

Javier Echevarría Sonia Ros-Franch

ISSN 2191-589X ISSN 2191-5903 (electronic)

Library of Congress Control Number: 2012944966

Ó The Author(s) 2013

Printed on acid-free paper

Springer is part of Springer Science+Business Media (www.springer.com)

The study of global biodiversity changes is a strong issue these days, as we

bivalve distribution shows latitudinal gradients and distributional patterns, both at

3 A Bivalve Perspective. . . . . . . . . . . . . . . . . . . ..... . . . . . . . . . . 23

4 Regional Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 45

6 Global Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Abstract Bivalves have proven to have a great potential for paleobiogeographic

‘Every naturalist who has directed his attention to the subject

S. E. Damborenea et al., Southern Hemisphere Palaeobiogeography of 1

1.1 Paleobiogeography and Neobiogeography

Although when we refer to the distribution of living organisms we are doing

1.2 Why Bivalves?

Table 1.1 Some practical differences between neobiogeography and paleobiogeography

1.3 Time Frame

Several major geologic events with significant paleogeographic consequences took

1.5 Paleoclimates and Water Temperatures

2.1 The Data

S. E. Damborenea et al., Southern Hemisphere Palaeobiogeography of 13

2.2 Quantification: A Difficult Approach

The application of any analytical technique in biogeography implies the accep-

2.3 Analytic Methods

Phenetic approaches are common techniques in biogeographic analysis; in these

2.3.1 Latitudinal Distributions

2.3.1.1 Cluster Analysis

2.3.1.2 Distribution Limits of Species

2.3.1.3 Generalized Linear Models

2.3.2 Recognition of Biochoremas and Their Characterization

The recognition and characterization of ‘‘conventional’’ biogeographic units, i.e.,

2.3.2.1 Bootstrapped Spanning Network

In order to check biogeographic structures between regions but without assuming

Abstract Mesozoic bivalves have been the subject of many paleobiogeographic

S. E. Damborenea et al., Southern Hemisphere Palaeobiogeography of 23

3.1 Previous Research: A Northern Hemisphere Affair

3.2 Some General Issues

3.2.1 Larval Types and Dispersion

sometimes be recognized in well-preserved specimens with preserved protoconch

information to do so without resorting to large-scale extrapolation. For these

3.2.2 Swimming Habit

In the context of bivalve large-scale geographic dispersion, we are not interested in

3.2.3 Pseudoplanktonic Habit

Exclusive pseudoplanktonic species are expected to be rare faunal elements, but

invasive bivalve species after accidental settlement of a few individuals in suitable

3.2.4 Reef-Forming Bivalves

and brachiopods, they include a variety of thick-shelled bivalves such as Lithiotis,

3.2.5 Low-Oxygen Tolerance

Some Mesozoic bivalves are usually found associated to oxygen-poor sedimentary

3.3 Paleobiogeographic Affinities

in fact bipolar instead (Fig. 6.1), for instance Palmoxytoma, Kolymonectes

the Jurassic, 52 % can be regarded as low latitude, 21 % as high latitude, 6 % as

Damborenea SE (1996) Paleobiogeography of early Jurassic bivalves along the southeastern