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.
