Pliosaurus

Pliosaurus is the type genus (defining example) of the pliosaurs, one of the major group of the plesiosaurs, an extinct group of aquatic marine reptiles. It lived from the Upper Jurassic to the Lower Cretaceous in what is now Europe and South America. The first known fossil consists of a partial skeleton of an immature specimen collected by William Buckland in Market Rasen, England. Although initially mentioned in a 1824 paper by William Daniel Conybeare, it was not until 1841 that it was first formally described by Richard Owen as belonging to a new species of Plesiosaurus, before being given its own genus by the same author later that year. The genus name Pliosaurus means "more lizard", Owen naming the taxon of because its appearance being more reminiscent of crocodilians than to Plesiosaurus. While many species have been assigned to Pliosaurus in the past, only six are firmly recognised as valid since a 2013 paper. Two additional species from Argentina were named in 2014 and 2018, but their assignment to the genus has not been fully confirmed in subsequent classifications.

The largest species, P. funkei and P. kevani, are among the largest pliosaurs ever discovered, with maximum sizes estimated at around 9–10 metres (30–33 ft) long. Even before both species were described in the scientific literature, their imposing size led some scientists to assign them various nicknames, the most famous being "Predator X", used to refer to the second known specimen of P. funkei. However, a large specimen composed entirely of cervical vertebrae discovered in Abingdon and putatively assigned to the genus could have reached a length exceeding 11 metres (36 ft). The elongated skull of Pliosaurus is robust, and those of the largest known species exceed 2 metres (6.6 ft) in length. The teeth are also robust and are trihedral in cross-section, which is one of the main diagnostic traits of the genus. The neck is one of the most recognizable features of the taxon, being relatively short compared to other plesiosaurs, having only 18 cervical vertebrae. The limbs of Pliosaurus were modified into flippers, with the anterior flippers of P. funkei reaching up to 3 metres (9.8 ft) in length.

Phylogenetic classifications place Pliosaurus in a relatively derived position within the Thalassophonea, with the genus often being recovered as related to Gallardosaurus. Like other plesiosaurs, Pliosaurus was well-adapted to aquatic life, using its flippers for a method of swimming known as subaqueous flight. Pliosaurid skulls are reinforced to better withstand the stresses of their feeding. The long, robust snouts of the various Pliosaurus species were used to capture large prey, piercing them with the numerous trihedral teeth at the back of the jaws. P. kevani would have achieved a bite force ranging from 9,600 to 48,000 newtons (2,160 to 10,790 lbf) depending on the area of the jaws. The rostrum of some species contains internal structures that likely housed a neurovascular system capable of detecting the movements of nearby prey in the water. The fossil record shows that Pliosaurus shared its habitat with a variety of other animals, including invertebrates, fish, thalattosuchians, ichthyosaurs, and other plesiosaurs, which would undoubtedly have constituted prey for the larger species.