Peloneustes (meaning 'mud swimmer') is a genus of pliosaurid plesiosaur from the Middle Jurassic of

England. Its remains are known from the Peterborough Member of the Oxford Clay Formation, which is
Callovian in age. It was originally described as a species of Plesiosaurus by palaeontologist Harry Govier
Seeley in 1869, before being given its own genus by naturalist Richard Lydekker in 1889. While many
species have been assigned to Peloneustes, P. philarchus is currently the only one still considered valid,
with the others moved to different genera, considered nomina dubia, or synonymised with P.
philarchus. Some of the material formerly assigned to P. evansi have since been reassigned to
"Pliosaurus" andrewsi. Peloneustes is known from many specimens, including some very complete
material.

With a total length of 3.5–4 metres (11–13 ft), Peloneustes is not a large pliosaurid. It had a large,
triangular skull, which occupied about a fifth of its body length. The front of the skull is elongated into a
narrow rostrum (snout). The mandibular symphysis, where the front ends of each side of the mandible
(lower jaw) fuse, is elongated in Peloneustes, and helped strengthen the jaw. An elevated ridge is
located between the tooth rows on the mandibular symphysis. The teeth of Peloneustes are conical and
have circular cross-sections, bearing vertical ridges on all sides. The front teeth are larger than the back
teeth. With only 19 to 21 cervical (neck) vertebrae, Peloneustes had a short neck for a plesiosaur. The
limbs of Peloneustes were modified into flippers, with the back pair larger than the front.

Peloneustes has been interpreted as both a close relative of Pliosaurus or as a more basal (early-
diverging) pliosaurid within Thalassophonea, with the latter interpretation finding more support. Like
other plesiosaurs, Peloneustes was well-adapted to aquatic life, using its flippers for a method of
swimming known as subaqueous flight. Pliosaurid skulls were reinforced to better withstand the stresses
of feeding. The long, narrow snout of Peloneustes could have been swung quickly through the water to
catch fish, which it pierced with its numerous sharp teeth. Peloneustes would have inhabited an
epicontinental (inland) sea that was around 30–50 metres (100–160 ft) deep. It shared its habitat with a
variety of other animals, including invertebrates, fish, thalattosuchians, ichthyosaurs, and other
plesiosaurs. At least five other pliosaurids are known from the Peterborough Member, but they were
quite varied in anatomy, indicating that they would have eaten different food sources, thereby avoiding
competition.

History of research

diagram of the partial upper jaw of the holotype seen from above and below, consisting of the
premaxillae, maxillae, and vomers

Partial upper jaw of the holotype, in top and bottom views

The strata of the Peterborough Member of the Oxford Clay Formation have long been mined for
brickmaking. Ever since the late 19th century, when these operations began, the fossils of many marine
animals have been excavated from the rocks.[2] Among these was the specimen which would become
the holotype of Peloneustes philarchus, discovered by geologist Henry Porter in a clay pit close to
Peterborough, England. The specimen includes a mandible, the front part of the upper jaw, various
vertebrae from throughout the body, elements from the shoulder girdle and pelvis, humeri (upper arm
bones), femora (upper leg bones), and various other limb bones.[3] In 1866, geologist Adam Sedgwick
purchased the specimen for the University of Cambridge's Woodwardian Museum (now the Sedgwick
Museum of Earth Sciences, Cambridge),[2] with the specimen being catalogued as CAMSM J.46913 and
stored in the university's lecture room within cabinet D.[2][3] Palaeontologist Harry Govier Seeley
described the specimen as a new species of the preexisting genus Plesiosaurus, Plesiosaurus philarchus,
in 1869.[3] The specific name means 'power-loving', possibly due to its large, powerful skull.[4] Seeley
did not describe this specimen in detail, mainly just giving a list of the known material.[3] While later
publications would further describe these remains, CAMSM J.46913 remains poorly described.[2]

Alfred Leeds and his brother Charles Leeds had been collecting fossils from the Oxford Clay since around
1867, encouraged by geologist John Phillips of the University of Oxford, assembling what became known
as the Leeds Collection. While Charles eventually left, Alfred, who collected the majority of the
specimens, continued to gather fossils until 1917. Eventually, after a visit by Henry Woodward of the
British Museum of Natural History (now the Natural History Museum in London) to Leeds' collection in
Eyebury in 1885, the museum bought around 5 tonnes (5.5 short tons) of fossils in 1890. This brought
Leeds' collection to wider renown, and he would later sell specimens to museums throughout Europe,
and even some in the United States.[5] The carefully prepared material was usually in good condition,
although it quite frequently had been crushed and broken by geological processes. Skulls were
particularly vulnerable to this.[6]: v–vi 

illustration of a partial mandible and two partial vertebrae

Mandible and vertebrae of the specimen described by Lydekker

Naturalist Richard Lydekker was informed of a plesiosaur skeleton in the British Museum of Natural
History by geologist George Charles Crick, who worked there. The specimen, catalogued under NHMUK
R1253,[2] had been discovered in the Oxford Clay Formation in Green End, Kempston, near Bedford.
While Lydekker speculated that the skeleton was once complete, it was damaged during excavation. The
limb girdles had been heavily fragmented when the specimen arrived at the museum, but a worker
named Lingard in the Geology Department managed to restore much of them. In addition to the limb
girdles, the specimen also consists of a partial mandible, teeth, multiple vertebrae (although none from
the neck), and much of the limbs. Lydekker identified this specimen as an individual of Plesiosaurus
philarchus and published a description of it in 1889. After studying this and other specimens in the Leeds
Collection, he concluded that plesiosaurs with shortened necks and large heads could not be classified
as species of Plesiosaurus, meaning that "P." philarchus belonged to a different genus. He initially
assigned it to Thaumatosaurus in 1888,[7] but later decided that it was distinct enough to warrant its
own genus, which he named Peloneustes in his 1889 publication.[8] The name Peloneustes comes from
the Greek words pelos, meaning 'mud' or 'clay', in reference to the Oxford Clay Formation, and neustes,
meaning 'swimmer'.[4] Seeley, however, lumped Peloneustes into Pliosaurus in 1892, claiming that the
two were insufficiently different to warrant separate genera.[9] Seeley and Lydekker could not agree on
which genus to classify P. philarchus in, representing part of a feud between the two scientists.
However, Peloneustes has since become the accepted name.[7]

diagram of the front and hind paddles

Fore (left) and hind (right) paddles of NHMUK R2440, a specimen from the Leeds Collection

The Leeds Collection contained multiple Peloneustes specimens.[10]: 63–70  In 1895, palaeontologist

Charles William Andrews described the anatomy of the skull of Peloneustes based on four partial skulls
in the Leeds Collection.[11] In 1907, geologist Frédéric Jaccard published a description of two
Peloneustes specimens from the Oxford Clay near Peterborough, housed in the Musée Paléontologique
de Lausanne, Switzerland. The more complete of the two specimens includes a complete skull
preserving both jaws; multiple isolated teeth; 13 cervical (neck), 5 pectoral (shoulder), and 7 caudal (tail)
vertebrae; ribs; both scapulae, a coracoid; a partial interclavicale; a complete pelvis save for an ischium;
and all four limbs, which were nearly complete. The other specimen preserved 33 vertebrae and some
associated ribs. Since the specimen Lydekker described was in some need of restoration, and missing
information was filled in with data from other specimens in his publication, Jaccard found it pertinent to
publish a description containing photographs of the more complete specimen in Lausanne to better
illustrate the anatomy of Peloneustes.[12]

In 1913, naturalist Hermann Linder described multiple specimens of Peloneustes philarchus housed in
the Institut für Geowissenschaften, University of Tübingen and State Museum of Natural History
Stuttgart, Germany. These specimens had also come from the Leeds Collection.[2] Among the specimens
he described from the former institution was a nearly complete mounted skeleton, lacking two cervical
vertebrae, some caudal vertebrae from the end of the tail, and some distal phalanges. Only the rear part
of the cranium was in good condition, but the mandible was mostly undamaged. Another of the
specimens Linder described was a well-preserved skull (GPIT RE/3409),[2] also from the University of
Tübingen, preserving a sclerotic ring (the set of small bones that support the eye), only the fourth time
these bones had been reported in a plesiosaur.[13]

Photograph of a mounted skeleton in side view

Mounted skeleton in the British Museum of Natural History

Diagram of the same skeleton in side view

1913 skeletal reconstruction based on the above mount

Andrews later described the marine reptile specimens of the Leeds Collection that were in the British
Museum of Natural History, publishing two volumes, one in 1910 and the other in 1913. The anatomy of
the Peloneustes specimens was described in the second volume, based primarily on the well-preserved
skulls NHMUK R2679 and NHMUK R3808 and NHMUK R3318, an almost complete skeleton. NHMUK
R3318 was so well preserved that it could be rearticulated and mounted, although the missing parts of
the pelvis and limbs had to be filled in. The mounted skeleton was put on display in the museum's
Gallery of Fossil Reptiles.[6]: ix [10]: 35, 63  Andrews had described this mount in 1910, remarking that it
was the first skeletal mount of a pliosaurid, thus providing important information about the overall
anatomy of the group.[14]

In 1960, palaeontologist Lambert Beverly Tarlo published a review of pliosaurid species that had been
reported from the Upper Jurassic. Many pliosaurids species had been named based on isolated
fragments, creating confusion. Tarlo also found that inaccurate descriptions of the material and
palaeontologists ignoring each other's work only made this confusion worse. Of the 36 species he
reviewed, he found only nine of them to be valid, including Peloneustes philarchus.[7] In 2011,
palaeontologists Hilary Ketchum and Roger Benson described the anatomy of the skull of Peloneustes.
Since the previous anatomical studies of Andrews and Linder, more specimens had been found,
including NHMUK R4058, a skull preserved in three dimensions, providing a better idea of the skull's
shape.[2]

Other assigned species

Many further species have been assigned to Peloneustes throughout its history, but these have all since
been reassigned to different genera or considered invalid.[2] In the same publication in which he named
P. philarchus, Seeley also named another species of Plesiosaurus, P. sterrodeirus based on seven
specimens in the Woodwardian Museum consisting of cranial and vertebral material.[3] When Lydekker
erected the genus Peloneustes for P. philarchus, he also reclassified "Plesiosaurus" sterrodeirus and
"Pleiosaurus" aequalis (a species named by John Phillips in 1871)[15]: 365 [7] as members of this genus.
[8] In his 1960 review of pliosaurid taxonomy, Tarlo considered P. aequalis to be invalid, since it was
based on propodials (upper limb bones), which cannot be used to differentiate different pliosaurid
species. He considered Peloneustes sterrodeirus to instead belong to Pliosaurus, possibly within P.
brachydeirus.[7]

mandible of Peloneustes compared to "Pliosaurus" andrewsi, both seen from above

Mandibles of Peloneustes, specimen NHMUK R3803 (top) and "Pliosaurus" andrewsi, specimen NHMUK
R2443 (bottom)

Another of the species described by Seeley in 1869 was Pliosaurus evansi, based on specimens in the
Woodwardian Museum.[3] These consisted of cervical and dorsal (back) vertebrae, ribs, and a coracoid.
Due to it being a smaller species of Pliosaurus and its similarity to Peloneustes philarchus, Lydekker
reassigned it to Peloneustes in 1890, noting that it was larger than Peloneustes philarchus.[16] He also
thought that a large mandible and paddle attributed to Pleiosaurus ?grandis by Phillips in 1871[15]: 318 
belonged to this species instead.[7] In 1913, Andrews assigned a partial skeleton of another large
pliosaur found by Leeds to Peloneustes evansi, noting that while the mandible and vertebrae were
similar to other Peloneustes evansi specimens, they were quite different from those of Peloneustes
philarchus. Consequently, Andrews considered it possible that P. evansi really belonged to a separate
genus that was morphologically intermediate between Peloneustes and Pliosaurus.[10]: 72  In his 1960
review of pliosaurids, Tarlo synonymised Peloneustes evansi with Peloneustes philarchus due to their
cervical vertebrae being identical (save for a difference in size). He considered the larger specimens of
Peloneustes evansi distinct, and assigned them to a new species of Pliosaurus, P. andrewsi (although this
species is no longer considered to belong in Pliosaurus).[7][17] Hilary F. Ketchum and Roger B. J. Benson
disagreed with this synonymy, and in 2011 considered that since the holotype of Peloneustes evansi is
nondiagnostic (lacking distinguishing features), P. evansi is a nomen dubium and therefore an
indeterminate pliosaurid.[2]

Palaeontologist E. Koken described another species of Peloneustes, P. kanzleri, in 1905, from the
Cretaceous of Germany.[2] In 1960, Tarlo reidentified this species as an elasmosaurid.[7] In 1913, Linder
created a subspecies of Peloneustes, P. philarchus var. spathyrhynchus, differentiating it based on its
spatulate mandibular symphysis (where the two sides of the mandible meet and fuse).[13] Tarlo
considered it to be a synonym of Peloneustes philarchus in 1960,[7] and the mandibular symphysis of
Peloneustes is proportionately wider in larger specimens, making this trait more likely to be due to
intraspecific variation (variation within species). Crushing makes accurate measurement of these
proportions difficult.[2] In 1948, palaeontologist Nestor Novozhilov named a new species of
Peloneustes, P. irgisensis, based on PIN 426, a partial skeleton consisting of a large, incomplete skull,
vertebrae, and a partial hind limb, with stomach contents preserved. The specimen was unearthed in
the Lower Volga Basin in Russia.[18][19] In his 1960 review, Tarlo considered this species to be too
different from Peloneustes philarchus to belong to Peloneustes, tentatively placing it in Pliosaurus. He
speculated that Novozhilov had incorrectly thought Peloneustes to be the sole long-snouted pliosaurid,
hence the initial assignment.[7] In 1964 Novozhilov erected a new genus, Strongylokrotaphus, for this
species, but further studies concurred with Tarlo and reassigned the species to Pliosaurus, possibly a
synonym of Pliosaurus rossicus. By then, PIN 426 had suffered from heavy pyrite damage.[19][17]

photo of a skeleton of Hauffiosaurus in a slab of rock

Skeleton of Hauffiosaurus, first thought to be Peloneustes, Urwelt-Museum Hauff

In 1998, palaeontologist Frank Robin O'Keefe proposed that a pliosaurid specimen from the Lower
Jurassic Posidonia Shale of Germany might represent a new species of Peloneustes. In 2001, he
considered it to belong to a separate genus, naming it Hauffiosaurus zanoni.[2][20] Palaeontologists
Zulma Gasparini and Manuel A. Iturralde-Vinent assigned a pliosaurid from the Upper Jurassic Jagua
Formation of Cuba to Peloneustes sp. in 2006.[21] In 2009, Gasparini redescribed it as Gallardosaurus
iturraldei.[22] In 2011, Ketchum and Benson considered Peloneustes to contain only one species, P.
philarchus. They recognised twenty one definite specimens of Peloneustes philarchus, all from the
Peterborough Member of the Oxford Clay Formation. They considered some specimens from the
Peterborough Member and Marquise, France previously assigned to Peloneustes to belong to different,
currently unnamed pliosaurids.[2]

Description

diagram comparing the size of the mounted skeletons at NHMUK and GPIT to a diver

Size comparison of two specimens

Peloneustes is a small-[10]: 34  to medium-sized member of Pliosauridae.[23]: 12  NHMUK R3318, the

mounted skeleton in the Natural History Museum in London, is 3.5 metres (11.5 ft) long,[14] while the
mounted skeleton in the Institut für Geowissenschaften, University of Tübingen measures 4.05 metres
(13.3 ft) in length.[13] Plesiosaurs typically can be described as being of the small-headed, long-necked
"plesiosauromorph" morphotype or the large-headed, short-necked "pliosauromorph" morphotype.[24]
Peloneustes is of the latter morphotype,[24] with its skull making up a little less than a fifth of the
animal's total length.[23]: 13  Peloneustes, like all plesiosaurs, had a short tail, massive torso, and all of
its limbs modified into large flippers.[23]: 3 

Skull

diagram of the skull in side view

Reconstructed skull

While the holotype of Peloneustes lacks the rear portion of its cranium, many additional well-preserved
specimens, including one that has not been crushed from top to bottom, have been assigned to this
genus. These crania vary in size, measuring 60–78.5 centimetres (1.97–2.58 ft) in length. The cranium of
Peloneustes is elongated and slopes upwards towards its back end.[2] Viewed from above, the cranium
is shaped like an isosceles triangle,[10]: 35  with the back of the cranium broad and the front elongated
into a narrow rostrum. The rearmost part of the cranium has roughly parallel sides, unlike the tapering
front regions. The external nares (openings for the nostrils) are small and located about halfway along
the length of the cranium. The kidney-shaped eye sockets face forwards and outwards and are located
on the back half of the cranium. The sclerotic rings are composed of at least 16 individual elements, an
unusually high number for a reptile. The temporal fenestrae (openings in the back of the cranium) are
enlarged, elliptical, and located on the cranium's rearmost quarter.[2]

illustration of Peloneustes skulls seen from above and below

Cranium of Peloneustes figured from below (left, NHMUK 3803) and above (right, NHMUK 2679)

Characteristically, the premaxillae (front upper tooth-bearing bones) of Peloneustes bear six teeth each,
and the diastemata (gaps between teeth) of the upper jaw are narrow. While it has been stated that
Peloneustes had nasals (bones bordering the external nares), well-preserved specimens indicate that
this is not the case. The frontals (bones bordering the eye sockets) of Peloneustes contact both the eye
sockets and the external nares, a distinctive trait of Peloneustes. There has been some contention as to
whether or not Peloneustes had lacrimals (bones bordering the lower front edges of the eye sockets),
due to poor preservation. However, well preserved specimens indicate that the lacrimals are distinct
bones as in other pliosaurids, as opposed to extensions of the jugals (bones bordering the lower rear
edges of the eye sockets). The palate of Peloneustes is flat and bears many openings, including the
internal nares (the opening of the nasal passage into the mouth). These openings are contacted by
palatal bones known as palatines, a configuration used to identify this genus. The parasphenoid (a bone
that forms the lower front part of the braincase) bears a long cultriform process (a frontwards
projection of the braincase) that is visible when the palate is viewed from below, another distinctive
characteristic of Peloneustes. The occiput (rear part of the cranium) of Peloneustes is open, bearing
large fenestrae.[2]

Peloneustes is known from many mandibles, some of which are well-preserved. The longest of these
measures 87.7 centimetres (2.88 ft). The mandibular symphysis is elongated, making up about a third of
the total mandibular length. Behind the symphysis, the two sides of the mandible diverge before gently
curving back inwards near the hind end. Each dentary (the tooth-bearing bone in the mandible) has
between 36 and 44 teeth, 13 to 15 of which are located on the symphysis. The second to seventh tooth
sockets (tooth sockets) are larger than those located further back, and the symphysis is the widest
around the fifth and sixth. In addition to the characteristics of its mandibular teeth, Peloneustes can also
be identified by its coronoids (upper inner mandibular bones), which contribute to the mandibular
symphysis. Between the tooth rows, the mandibular symphysis bears an elevated ridge where the
dentaries meet. This is a unique feature of Peloneustes, not seen in any other plesiosaurs. The
mandibular glenoid (socket of the jaw joint) is broad, kidney-shaped, and angled upwards and inwards.
[2]
illustration of a tooth

Tooth

The teeth of Peloneustes have circular cross sections, as seen in other pliosaurids of its age.[7] The teeth
have the shape of recurved cones. The enamel of the crowns bears regularly-spaced vertical ridges of
varying length on all sides. These ridges are more concentrated on the concave edge of the teeth. Most
of the ridges extend to one half to two-thirds of the total crown height, with few actually reaching the
tooth's apex.[2] The dentition of Peloneustes is heterodont, that is, it has teeth of different shapes. The
larger teeth are caniniform and located at the front of the jaws, while the smaller teeth are more sharply
recurved,[2] stouter, and located further back.[25]

Postcranial skeleton

diagram of an articulated series of cervical vertebrae

Middle cervical vertebrae (NHMUK R3318)

In 1913, Andrews reported that Peloneustes had 21 to 22 cervical, 2 to 3 pectoral, and around 20 dorsal
vertebrae, with the exact number of sacral (hip) and caudal vertebrae unknown, based on specimens in
the Leeds Collection.[10]: 47, 52  However, in the same year, Linder reported 19 cervical, 5 pectoral, 20
dorsal, 2 sacral, and at least 17 caudal vertebrae in Peloneustes, based on a specimen in the Institut für
Geowissenschaften, University of Tübingen.[13][2] The first two cervical vertebrae, the atlas and axis,
are fused in adults, but in juveniles they are present as several unfused elements.[10]: 47  The
intercentrum (part of the vertebral body) of the axis is roughly rectangular, extending beneath the
centrum (vertebral body) of the atlas.[2] The cervical vertebrae bear tall neural spines that are
compressed from side to side.[2][10]: 50  The cervical centra are about half as long as wide. They bear
strongly concave articular surfaces, with a prominent rim around the lower edge in the vertebrae
located towards the front of the series. Each cervical centrum has a strong keel along the midline of its
underside.[7] Most of the cervical ribs bear two heads that are separated by a notch.[10]: 53 

The pectoral vertebrae bear articulations for their respective ribs partially on both their centra and
neural arches. Following these vertebrae are the dorsal vertebrae, which are more elongated than the
cervical vertebrae and have shorter neural spines. The sacral and caudal vertebrae both have less
elongated centra that are wider than tall. Many of the ribs from the hip and the base of the tail bear
enlarged outer ends that seem to articulate with each other. Andrews hypothesised in 1913 that this
configuration would have stiffened the tail, possibly to support the large hind limbs. The terminal (last)
caudal vertebrae sharply decrease in size and would have supported proportionately larger chevrons
than the caudal vertebrae located further forwards. In 1913, Andrews speculated that this morphology
may have been present to support a small tail fin-like structure.[10]: 52–53  Other plesiosaurs have also
been hypothesised to have tail fins, with impressions of such a structure possibly known in one species.
[26]

Diagram of the shoulder girdle of Peloneustes, seen from above

Diagram of the pelvis of Peloneustes, seen from above

Shoulder (left) and pelvic (right) girdles of NHMUK R3318

The shoulder girdle of Peloneustes was large, although not as heavily built as in some other plesiosaurs.
The coracoids are the largest bones in the shoulder girdle, and are plate-like in form. The shoulder joint
is formed by both the scapula (shoulder balde) and the coracoid, with the two bones forming a 70° angle
with each other. The scapulae are typical in form for a pliosaurid and triradiate, bearing three prominent
projections, or rami. The dorsal (upper) ramus is directed outwards, upwards, and backwards.[10]: 55 [7]
The underside of each scapula bears a ridge directed towards the front edge of its ventral (lower) ramus.
[7] The ventral rami of the two scapulae were separated from each other by a triangular bone known as
the interclavicle. As seen in other pliosaurs, the pelvis of Peloneustes bears large and flat ischia and
pubic bones. The third pelvic bone, the ilium, is smaller and elongated, articulating with the ischium. The
upper end of the ilium shows a large amount of variation within P. philarchus, with two forms known,
one with a rounded upper edge, the other with a flat upper edge and more angular shape.[10]: 55–56, 
58–60 

The hind limbs of Peloneustes are longer than its forelimbs, with the femur being longer than the
humerus, although the humerus is the more robust of the two elements.[10]: 57, 60  The radius (one of
the lower forelimb bones) is approximately as wide as it is long, unlike the ulna (the other lower
forelimb bone), which is wider than long.[7] The radius is the larger of these two elements.[10]: 58  The
tibia is larger than the fibula (lower hindlimb bones) and longer than wide, while the fibula is wider than
long in some specimens.[7] The metacarpals, metatarsals, and the proximal manual phalanges (some of
the bones making up the outer part of the paddle) are flattened. Most of the phalanges in both limbs
have rounded cross-sections, and all of them have prominent constrictions in their middles. The number
of phalanges in each digit is unknown in both the fore- and hind limbs.[10]: 58, 62 

Classification

Skeleton of Eardasaurus, another pliosaurid from the Oxford Clay, at the Oxford University Museum of
Natural History
Seeley initially described Peloneustes as a species of Plesiosaurus, a rather common practice (at the
time, the scope of genera was similar to what is currently used for families).[23]: 7  In 1874, Seeley
named a new family of plesiosaurs, Pliosauridae, to contain forms similar to Pliosaurus.[27] In 1890,
Lydekker placed Peloneustes in this family,[16] to which it has been consistently assigned since.[9][10]: 
1 [7][2] Exactly how pliosaurids are related to other plesiosaurs is uncertain. In 1940, palaeontologist
Theodore E. White considered pliosaurids to be close relatives of Elasmosauridae based on shoulder
anatomy.[28] Palaeontologist Samuel P. Welles, however, thought that pliosaurids were more similar to
Polycotylidae, as they both had large skulls and short necks, among other characteristics. He grouped
these two families into the superfamily Pliosauroidea, with other plesiosaurs forming the superfamily
Plesiosauroidea.[29] Another plesiosaur family, Rhomaleosauridae, has since been assigned to
Pliosauroidea,[30][20] while Polycotylidae has been reassigned to Plesiosauroidea.[31][32] However, in
2012, Benson and colleagues recovered a different topology, with Pliosauridae being more closely
related to Plesiosauroidea than Rhomaleosauridae. This pliosaurid-plesiosauroid clade was termed
Neoplesiosauria.[32]

Skull, teeth, and vertebrae of Simolestes, a pliosaurid also from the Oxford Clay

Within Pliosauridae, the exact phylogenetic position of Peloneustes is uncertain.[2] In 1889, Lydekker
considered Peloneustes to represent a transitional form between Pliosaurus and earlier plesiosaurs,
although he found it unlikely that Peloneustes was ancestral to Pliosaurus.[8] In 1960, Tarlo considered
Peloneustes to be a close relative of Pliosaurus, since both taxa had elongated mandibular symphyses.
[7] In 2001, O'Keefe and colleagues recovered it as a basal (early-diverging) member of this family,
outside of a group including Liopleurodon, Pliosaurus, and Brachauchenius.[20][2] However, in 2008,
palaeontologists Adam S. Smith and Gareth J. Dyke found Peloneustes to be the sister taxon of
Pliosaurus.[30][2] In 2013, Benson and palaeontologist Patrick S. Druckenmiller named a new clade
within Pliosauridae, Thalassophonea. This clade included the "classic", short-necked pliosaurids while
excluding the earlier, long-necked, more gracile forms. Peloneustes was found to be the most basal
thalassophonean.[33] Subsequent studies have uncovered a similar position for Peloneustes.[34][35]
[36]

The following cladogram follows Valentin Fischer and colleagues, 2017.[36]

Thalassophonea

Peloneustes philarchus Peloneustes Skull Dorsal View - Extracted.png

"Pliosaurus" andrewsi
Simolestes vorax Simolestes Skull Dorsal View.png

Liopleurodon ferox Liopleurodon Skull Dorsal View - Extracted.png

Pliosaurus spp. Pliosaurus Skull Dorsal View - Extracted.png

Brachaucheninae USNM 2361 - Brachauchenius lucasi skull.png

Palaeobiology

illustration showing what Peloneustes may have looked like when alive

Life restoration

Plesiosaurs were well-adapted to marine life.[37][38][39] They grew at rates comparable to those of
birds and had high metabolisms, indicating homeothermy[40] or even endothermy.[39] The bony
labyrinth, a hollow within the skull which held a sensory organ associated with balance and orientation,
of Peloneustes and other plesiosaurs is similar in shape to that of sea turtles. Palaeontologist James
Neenan and colleagues hypothesised in 2017 that this shape probably evolved alongside the flapping
motions used by plesiosaurs to swim. Peloneustes and other short-necked plesiosaurs also had smaller
labyrinths than plesiosaurs with longer necks, a pattern also seen in cetaceans.[37] Additionally,
Peloneustes probably had salt glands in its head to cope with excess amount of salt within its body.
However, Peloneustes appears to have been a predator of vertebrates, which contain less salt than
invertebrates, therefore leading palaeontologist Leslie Noè to suggest in a 2001 dissertation that these
glands would not have had to be especially large.[41]: 257  Peloneustes, like many other pliosaurs,
displayed a reduced level of ossification of its bones. Palaeontologist Arthur Cruickshank and colleagues
in 1966 proposed that this may have helped Peloneustes maintain its buoyancy or improved its
manoeuvrability.[42] A 2019 study by palaeontologist Corinna Fleischle and colleagues found that
plesiosaurs had enlarged red blood cells, based on the morphology of their vascular canals, which would
have aided them while diving.[38]