Species New to Science

In this paper, we describe a new ophthalmosaurid specimen found in the lower Barremian beds of the Paja Formation at Villa de Leiva, Boyacá, Colombia. The specimen represents a new species of Platypterygius, P. elsuntuoso sp. nov. defined by three unique features within the genus: a small ventral peg in the anterior ventral border of the extracondylar area of the basioccipital; a supratemporal groove in the quadrate; and a shallow neck at the base of the acellular cementum ring on the teeth root. In support of our taxonomic determination, in this contribution we also assess the species composition of the genus Platypterygius, comparing the known anatomy of the type species P. platydactylus with that of the other species previously assigned to the genus. We found that a great morphological affinity in the forelimb of the species P. americanus, P. australis, P. hercynicus, and P. sachicarum (lower Barremian to lower Cenomanian) with that of the type species (Aptian) is unquestionable and differentiate them from all other ophthalmosaurids. Three new diagnostic features (synapomorphies of the genus) support their inclusion in Platypterygius. Consequently, the genus name “Kyhytysuka”, recently proposed to differentiate P. sachicarum from the genus Platypterygius is here rejected. A careful comparison of the cranial characteristics of Platypterygius specimens preserving associated cranial and fin remains demonstrate that Platypterygius is a genus distinguishable from other ophthalmosaurids not only by a typical forelimb but also by a particular combination of skull features. In this context, the skull morphology of “Simbirskiasaurus” concurs with that defining Platypterygius and its minor differences are not sufficient to support its generic distinction. The morphological comparisons and our phylogenetic analysis show the new species P. elsuntuoso more closely related to the Colombian species P. sachicarum than to other species of Platypterygius. Given that the new species came from the lower Barremian and P. sachicarum came from the upper Barremian, the minor differences in dentition found between the two species suggest a speciation by adaptation to new food sources in the Cretaceous epicontinental sea of Colombia.

Keywords: Ophthalmosauridae; Platypterygius; Barremian; Colombia

Derivation of name: from “el suntuoso”, Spanish for “the sumptuous”, nickname used by C. B. Padilla, deceased founder of the FCG and CIP, when referring to the specimen.

We document an extensive biostratigraphic investigation of a section of the Phukhaothong Dolomite Member of the marine Triassic Chaiburi Formation (part of the Sibumasu [or Shan-Thai] Block), which is exposed on the north side of an isolated mountain at Khao Thong in the Phatthalung area of southern Thailand. The 104 m+ thick section, consisting of bedded to massive, light grey dolomite, contains ammonoids in the middle to upper parts, and conodonts occur in abundant throughout the section. Seven distinct early Spathian (late Olenekian) ammonoid assemblages, a late Smithian (early Olenekian) conodont zone and three early Spathian conodont zones are recognized in ascending order as follows: ammonoids-Columbites sp. indet. beds, Arctomeekoceras? sp. indet. beds, Tirolites sp. indet. B beds, Tirolites sp. indet. C beds and Tirolites sp. indet. D bed in the Tirolites-Columbites Zone, and the Idahocolumbites cheneyi beds and Idahocolumbites phatthalungensis beds in the Idahocolumbites Zone; conodonts-late Smithian, Hadrodontina aequabilis-Staeschegnathus perrii Zone, early Spathian Icriospathodus crassatus Zone, Triassospathodus symmetricus-Novispathodus anhuiensis Zone and Novispathodus sp. I-Novispathodus sp. J Zone. The age of the primitive ichthyopterygian Thaisaurus chonglakmanii collected from the Idahocolumbites cheneyi beds is constrained to the early Spathian, thus suggesting it is the oldest known ichthyopterygian, because the range of Marcouxia and Idahocolumbites is limited to the Columbites parisianus Subzone of the lower Spathian in the western USA. The Spathian ammonoid faunas exhibit a very strong relationship with other Tethyan as well as eastern Panthalassa faunas in the low paleolatitudes, but bear very little or no relationship with middle and higher latitudinal faunas, suggesting the existence of a strong latitudinal diversity gradient during the Spathian. Late Smithian and early Spathian conodont faunas also exhibit a strong relationship with low paleolatitudinal faunas. Fifty-eight taxa (ammonoids: 26, conodonts: 32) are documented and one new ammonoid species, i.e., Idahocolumbites phatthalungensis, is described.

Keywords: ammonoids, biostratigraphy, Chaiburi Formation, conodonts, Early Triassic, Olenekian,

Phatthalung, Smithian, Spathian, Thailand.

ชั้นหินบริเวณเขาทองอยู่ในช่วงรอยต่อของต้นยุคไทรแอสซิก อายุย่อย Smithian-Spathian ซึ่งตรงกับเหตุการ Smithian-Spathian boundary (SSB) ที่เป็นช่วงตรวจเจอปริมาณคาร์บอนสะสมเป็นจำนวนมากในทะเลส่วนหนึ่งเป็นผลมาจากการปะทุของลาวา ซึ่งทำให้เกิดการสูญพันธุ์ครั้งใหญ่ช่วงสิ้นยุคเพอร์เมียน (P–T extinction event) ที่กว่าสิ่งมีชีวิตจะฟื้นตัวกลับมาใช้เวลานานมาก ทั้งนี้กลุ่มแอมโมนอยด์และโคโนดอนต์เป็นกลุ่มที่ฟื้นตัวเร็วกว่ากลุ่มอื่น โดยในครั้งนี้สามารถกำหนด Biozone ของหมวดหินนี้จากฟอสซิลได้และเทียบเคียงได้กับชั้นหินเดียวกันทั่วโลก ที่เราพบอยู่ประมาณ 33 แหล่งเท่านั้น

Reptilia Linnaeus, 1758

Diapsida Osborn, 1903

Ichthyosauromorpha Motani et al. 2015

Etymology. “Baise” indicates the provenance of the only known specimen of the genus, which was discovered in Zhebao Township in Baise City; -saurus is a common suffix for genus names of fossil reptiles. 

Etymology. The Latin word robustus means “robust”.

Holotype. CUGW VH107, a partial associated postcranial skeleton.

Locality and horizon. Northwest margin of the Nanpanjiang Basin, Zhebao Township, Longlin County, Baise City, Guangxi, China; upper part of the Luolou Formation, Lower Triassic (upper Spathian).

Diagnosis. A large basal ichthyosauromorph with a unique combination of postcranial characters (* denotes an autapomorphy): anterior dorsal centra deeply amphicoelous, with an average height to length ratio of about 1.2; neural spine long and inclined posteriorly, with the postzygapophyses much more dorsally positioned than the prezygapophyses; parapophyses subrectangular with their anteroventral corners extended to form ventrally directed apices, and situated adjacent to the anterior margins of the centra; diapophyses well developed and semicircular in outline; deep fossae present posterior to the diapophyses*; and radius robust, elongate, and bearing two distal facets, the posterior facet being 80% as wide as the anterior one*.

The largest animals to have ever lived occupied the marine environment. Modern cetaceans evolved their large size over tens of millions of years in response to the increased productivity of cold marine waters. However, whales were not the first marine giants to evolve. Sander et al. describe a 244-million-year-old fossil ichthyosaur that would have rivaled modern cetaceans in size (see the Perspective by Delsett and Pyenson). The animal existed at most 8 million years after the emergence of the first ichthyosaurs, suggesting a much more rapid size expansion that may have been fueled by processes after the Permian mass extinction. —SNV

The iterative evolution of secondarily marine tetrapods since the Paleozoic offers the promise of better understanding how the anatomy and ecology of animals change when returning to the sea. Recurring patterns of convergence in the geological past may suggest predictability of evolution when transitioning from full-time life on land to full-time life in the ocean. Ichthyosaurs (fish-shaped marine reptiles of the Mesozoic) and today’s cetaceans (whales, dolphins, and porpoises) are two of the most informative lineages to exemplify secondary returns to the sea. The notable resemblance in body shape and lifestyle of ichthyosaurs and cetaceans contrasts with their separation in time by nearly 200 million years, providing an often-cited example of convergent evolution. Ichthyosaurs arose 249 million years ago and populated the oceans for the next 150 million years. Cetaceans did not evolve until about 56 million years ago. As tail-propelled swimmers, ichthyosaurs and cetaceans evolved not only convergent body shapes but also large body sizes.

The integration of fossil and extant data can improve understanding of aquatic adaptation and gigantism as patterns of convergent evolution, particularly when interpreted in an ecological context. Our paleontological fieldwork in the Fossil Hill Member (Middle Triassic, Nevada, USA) provided the basis for the marine reptile data and resulted in finds of giant ichthyosaurs as part of the pelagic Fossil Hill Fauna. We compiled data for both fossil and living whales from the extensive literature. Together, these data provide the basis for computational analyses of maximum body size and its evolution over time. Modeling of energy flux in the Fossil Hill Fauna helps in understanding how the Fossil Hill ecosystem could have supported several large to giant tetrapod ocean consumers so early in ichthyosaur evolutionary history.

We describe an ichthyosaur with a 2-m-long skull from the Fossil Hill Fauna as a new species of Cymbospondylus. At present, this is the largest known tetrapod of its time, on land or in the sea, and is the first in a series of ocean giants. The Fossil Hill Fauna includes several other large-bodied ichthyosaurs in the Cymbospondylus radiation. The body-size range in this Triassic fauna rivals the range seen in modern whale faunas, from a total length of about 2 m in Phalarodon to more than 17 m in the new species. As preserved in the fossil record, the Fossil Hill Fauna represents a stable trophic network and could even have supported another large ichthyosaur if it bulk fed on small, but abundant, prey such as ammonoids. In absolute time, the new ocean giant lived 246 million years ago, only about 3 million years after the appearance of the first ichthyosaurs. Our research suggests that ichthyosaurs evolved large body size very early on in the clade’s history, comparatively earlier than whales.

Ichthyosaurs and cetaceans both evolved very large body sizes, yet their respective evolutionary pathways toward gigantism were different. Ichthyosaurs seem to have benefited from the abundance of pelagic conodonts and ammonoids after the recovery from the end-Permian mass extinction, even in the absence of modern primary producers. Cetaceans took different routes, but all appear to be related to trophic specialization, including the loss of teeth in baleen whales (Mysticeti) and the evolution of raptorial feeding and deep diving in toothed whales (Odontoceti).