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 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 . The elongated skull of Pliosaurus is robust, and those of the largest known species exceed 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 in length.