Peloneustes

Peloneustes Temporal range: Ma)[1] PreꞒ Ꞓ O S D C P T J K Pg N ↓
Skeletal mount, Museum of Paleontology, Tuebingen
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Family:	†Pliosauridae
Genus:	†Peloneustes Lydekker, 1889
Species:	†P. philarchus
Binomial name
†Peloneustes philarchus Seeley , 1869
Synonyms
Plesiosaurus philarchus (Seeley, 1869) Thaumatosaurus philarchus (Lydekker, 1888) Pliosaurus philarchus (Seeley, 1892) Peloneustes philarchus var. spathyrhynchus (Linder, 1913)

Peloneustes (meaning 'mud swimmer') is a

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

, 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

The

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]

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

Naturalist

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,

British Museum of Natural History

1913 skeletal reconstruction based on the above mount

Another of the species described by Seeley in 1869 was Pliosaurus evansi, based on specimens in the Woodwardian Museum.

In 1998, palaeontologist Frank Robin O'Keefe proposed that a pliosaurid specimen from the Lower Jurassic

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

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.

Characteristically, the

The teeth of Peloneustes have circular cross sections, as seen in other pliosaurids of its age.

In 1913, Andrews reported that Peloneustes had 21 to 22 cervical, 2 to 3 pectoral, and around 20

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

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.

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

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

Neoplesiosauria.^[32]

Within Pliosauridae, the exact phylogenetic position of Peloneustes is uncertain.

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 Ketchum and Benson, 2022.^[37]

	Marmornectes candrewi Thalassophonea Peloneustes philarchus Eardasaurus powelli "Pliosaurus" andrewsi Simolestes vorax Liopleurodon ferox Pliosaurus spp. Brachaucheninae

Palaeobiology

Life restoration

Plesiosaurs were well-adapted to marine life.

vascular canals, which would have aided them while diving.^[39]

Plesiosaurs such as Peloneustes employed a method of swimming known as

Judy Massare proposed in 1988 that plesiosaurs could actively search for and pursue their food instead of having to lie in wait for it.^[44]

Feeding mechanics

In a 2001 dissertation, Noè noted many adaptations in pliosaurid skulls for predation. To avoid damage while feeding, the skulls of pliosaurids like Peloneustes are highly akinetic, where the bones of the cranium and mandible were largely locked in place to prevent movement. The snout contains elongated bones that helped to prevent bending and bears a reinforced junction with the facial region to better resist the stresses of feeding. When viewed from the side, little tapering is visible in the mandible, strengthening it. The mandibular symphysis would have helped deliver an even bite and prevent the mandibles from moving independently. The enlarged coronoid eminence provides a large, strong region for the anchorage of the jaw muscles, although this structure is not as large in Peloneustes as it is in other contemporary pliosaurids. The regions where the jaw muscles were anchored are located further back on the skull to avoid interference with feeding. The kidney-shaped mandibular glenoid would have made the jaw joint steadier and stopped the mandible from dislocating. Pliosaurid teeth are firmly rooted and interlocking, which strengthens the edges of the jaws. This configuration also works well with the simple rotational movements that pliosaurid jaws were limited to and strengthens the teeth against the struggles of prey. The larger front teeth would have been used to impale prey while the smaller rear teeth crushed and guided the prey backwards toward the throat. With their wide gapes, pliosaurids would not have processed their food very much before swallowing.^{[42]: 193, 236–240}

The numerous teeth of Peloneustes rarely are broken, but often show signs of wear at their tips. Their sharp points, slightly curved, gracile shape, and prominent spacing indicate that they were built for piercing. The slender, elongated snout is similar in shape to that of a dolphin. Both the snout and tooth morphologies led Noè to suggest that Peloneustes was a

belemnite remains in its stomach was assigned to Peloneustes by Andrews in 1910,^{[6]: xvi–xvii} it is very incomplete and may actually belong to the contemporaneous pliosaurid Simolestes instead, as suggested by Noè.^{[42]: 233–234, 241–242} Peloneustes has also been suggested to have inflicted bite marks upon a specimen of Cryptoclidus by Bruce Rothschild and colleagues in 2018.^[46]

Palaeoenvironment

Peloneustes is known from the Peterborough Member (formerly known as the Lower Oxford Clay) of the Oxford Clay Formation.

bituminous (asphalt-containing)^[47] shale and clay^[49] rich in organic matter.^[50] These rocks are sometimes fissile (splittable into thin, flat slabs).^[48] The member is about 16–25 metres (52–82 ft) thick, stretching from Dorset to Humber.^[47]

The Peterborough Member represents an

faunal interchange to occur between the Tethyan and boreal regions. This sea was approximately 30–50 metres (100–160 ft) deep within 150 kilometres (93 mi) of the shoreline.^[48][2]

Oxford Clay Formation

is located at E1

The surrounding land would have had a

anoxic.^[51][50] Oxygen levels appear to have varied, with some deposits laid down in more aerated conditions than others.^[48]

Contemporaneous biota

There are many kinds of invertebrates preserved in the Peterborough Member. Among these are

dinoflagellates are abundant in the Peterborough Member.^[52]

A wide variety of fish are known from the Peterborough Member. These include the

Actinopterygians were also present, represented by Aspidorhynchus, Asthenocormus, Caturus, Coccolepis, Heterostrophus, Hypsocormus, Leedsichthys, Lepidotes, Leptolepis, Mesturus, Osteorachis, Pachycormus, Pholidophorus, and Sauropsis.^[53] These fish include surface-dwelling, midwater, and benthic varieties of various sizes, some of which could get quite large. They filled a variety of niches, including invertebrate eaters, piscivores, and, in the case of Leedsichthys, giant filter feeders.^[52]

Plesiosaurs are common in the Peterborough Member, and besides pliosaurids, are represented by

metriorhynchids Gracilineustes, Suchodus, Thalattosuchus,^{[10]: 180 [55]} and Tyrannoneustes.^[56] While uncommon, the small piscivorous pterosaur Rhamphorhynchus was also part of this marine ecosystem.^[52]

Liopleurodon ferox

at the University of Tübingen, another pliosaurid from the Peterborough Member

More pliosaurid species are known from the Peterborough Member than any other assemblage.

niche partitioning).^{[42]: 249–251 [43]} The large, powerful pliosaurid Liopleurodon ferox appears to have been adapted to take on large prey, including other marine reptiles and large fish.^{[42]: 242–243, 249–251} The long-snouted Eardasaurus powelli like Liopleurodon also has teeth with cutting edges and may have also taken large prey.^[37] Simolestes vorax, with its wide, deep skull and powerful bite, appears to have been a predator of large cephalopods.^{[42]: 243–244, 249–251} "Pliosaurus" andrewsi, like Peloneustes, possesses an elongated snout, an adaptation for feeding upon small, agile animals.^[52] However, its teeth are suited for cutting, indicating a preference for larger prey, while those of Peloneustes are better adapted for piercing.^[59] "Pliosaurus" andrewsi is also larger than Peloneustes.^[52] Marmornectes candrewi is also similar to Peloneustes, bearing a long snout, and perhaps also fed on fish.^[33][57] Pachycostasaurus dawni is a small, heavily built pliosaur that probably fed on benthic prey. It has a weaker skull than other pliosaurids and was more stable, so it probably used different feeding methods to avoid competition.^[43] Unlike the other pliosaurids of the Oxford Clay, Pachycostasaurus was rather rare, perhaps mainly living outside of the depositional area of the Oxford Clay Formation, possibly inhabiting coastal regions, deep water, or even rivers instead.^[43] While several different types of pliosaurids were present in the Middle Jurassic, the long-snouted piscovorous forms such as Peloneustes died out at the Middle-Upper Jurassic boundary. This seems to have been the first phase of a gradual decline in plesiosaur diversity. While the cause of this is uncertain, it may have been influenced by changing ocean chemistry, and, in later phases, falling sea levels.^[33]

See also

• List of plesiosaur genera
• Timeline of plesiosaur research

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External links

• Media related to Peloneustes at Wikimedia Commons