Peloneustes (meaning ) 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 this genus, 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. The taxon is known from many specimens, including some very complete material.
With a total length of , 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 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
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. 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. In 1866, geologist Adam Sedgwick purchased the specimen for the University of Cambridge's Woodwardian Museum (now the Sedgwick Museum of Earth Sciences, Cambridge), with the specimen being catalogued as CAMSM J.46913 and stored in the university's lecture room within cabinet D. Palaeontologist Harry Govier Seeley described the specimen as a new species of the preexisting genus Plesiosaurus, Plesiosaurus philarchus, in 1869. 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. While later publications would further describe these remains, CAMSM J.46913 remained poorly described.
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 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. 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.
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 PV R1253, 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, 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 , meaning or , in reference to the Oxford Clay Formation, and , meaning . Seeley, however, lumped Peloneustes into Pliosaurus in 1892, claiming that the two were insufficiently different to warrant separate genera. 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.
The Leeds Collection contained multiple Peloneustes specimens. In 1895, palaeontologist Charles William Andrews described the anatomy of the skull of Peloneustes based on four partial skulls in the Leeds Collection. 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. 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), also from the University of Tübingen, preserving a scleral ring (the set of small bones that support the eye), only the fourth time these bones had been reported in a plesiosaur.
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. 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.
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 often 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. 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.
Formerly assigned species and specimens Many further species have been assigned to Peloneustes throughout its taxonomic history, but these have all since been reassigned to different genera or considered invalid. 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. 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) as members of this genus. 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.