Peloneustes

Genus of pliosaurid plesiosaur

Speciesbox
| fossil range = Middle Jurassic, fossilrange|Callovian|PS=~(166.1–163.5 Ma) The specific name means , possibly due to its large, powerful skull. Seeley did not describe this specimen in detail, mainly just giving a list of the known material. 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. but later decided that it was distinct enough to warrant its own genus, which he named Peloneustes in his 1889 publication. The name Peloneustes comes from the Greek words pelos, meaning or , in reference to the Oxford Clay Formation, and neustes, meaning 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. 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. 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. 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. He also thought that a large mandible and paddle attributed to Pleiosaurus ?grandis by Phillips in 1871 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). 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. Palaeontologists Zulma Gasparini and Manuel A. Iturralde Vinent assigned a pliosaurid from the Upper Jurassic Jagua Formation of Cuba to Peloneustes sp. in 2006. In 2009, Gasparini redescribed it as Gallardosaurus iturraldei. 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. NHMUK R3318, the mounted skeleton in the Natural History Museum in London, is 11.5 feet (4 m) long, Peloneustes is of the latter morphotype,

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. 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. In 1890, Lydekker placed Peloneustes in this family, 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. Another plesiosaur family, Rhomaleosauridae, has since been assigned to Pliosauroidea, 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. Subsequent studies have uncovered a similar position for Peloneustes. The following cladogram follows Ketchum and Benson, 2022. or even endothermy. 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. 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. Plesiosaurs such as Peloneustes employed a method of swimming known as subaqueous flight, using their flippers as hydrofoils. Plesiosaurs are unusual among marine reptiles in that they used all four of their limbs, but not movements of the vertebral column, for propulsion. The short tail, while unlikely to have been used to propel the animal, could have helped stabilise or steer the plesiosaur. Due to the reduction in drag by their shorter, deeper bodies, palaeontologist Judy Massare proposed in 1988 that plesiosaurs could actively search for and pursue their food instead of having to lie in wait for it. it is very incomplete and may actually belong to the contemporaneous pliosaurid Simolestes instead, as suggested by Noè. Peloneustes has also been suggested to have inflicted bite marks upon a specimen of Cryptoclidus by Bruce Rothschild and colleagues in 2018.

Peloneustes is known from the Peterborough Member (formerly known as the Lower Oxford Clay) of the Oxford Clay Formation.) of the Upper Jurassic, shale and clay rich in organic matter. When it was deposited, it would have been located at a latitude of 35°N. While traces of green sulphur bacteria indicate euxinic water, with low oxygen and high hydrogen sulfide levels, abundant traces of benthic (bottom dwelling) organisms suggest that the bottom waters were not anoxic. A wide variety of fish are known from the Peterborough Member. These include the chondrichthyans Asteracanthus, Brachymylus, Heterodontus (or Paracestracion), 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. and the mosasaur like and Tyrannoneustes. While uncommon, the small piscivorous pterosaur Rhamphorhynchus was also part of this marine ecosystem. Simolestes vorax, with its wide, deep skull and powerful bite, appears to have been a predator of large cephalopods. "Pliosaurus" andrewsi is also larger than Peloneustes. 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. 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. 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.