plesiosaur, (clade Plesiosauria), any of a group of long-necked marine reptiles found as fossils from the late Triassic Period into the late Cretaceous Period (215 million to 66 million years ago). Plesiosaurs had a wide distribution in European seas and around the Pacific Ocean, including Australia, North America, and Asia.

Plesiosaurus, an early plesiosaur, was about 4.5 metres (15 feet) long, with a broad, flat body and a relatively short tail. It swam by flapping its fins in the water, much as sea lions do today, in a modified style of underwater “flight.” The nostrils were located far back on the head near the eyes. The neck was long and flexible, and the animal may have fed by swinging its head from side to side through schools of fish, capturing prey by using the long sharp teeth present in the jaws.

Early in their evolutionary history, the plesiosaurs split into two main lineages: the pliosaurs (or pliosauroids, which belong to the suborder Pliosauroidea), in which the neck was short and the head elongated; and the plesiosauroids (which belong to the suborder Plesiosauroidea), in which the head remained relatively small and the neck assumed snakelike proportions and became very flexible. The late evolution of plesiosaurs was marked by a great increase in size. For example, Elasmosaurus, a plesiosaurid, had as many as 76 vertebrae in its neck alone and reached a length of about 13 metres (43 feet), fully half of which consisted of the head and neck. In contrast, Kronosaurus, an early Cretaceous pliosaur whose fossils have been unearthed in Australia and Colombia, grew to about 9–11 metres (29.5–36 feet) long, based on an estimate of the length of K. queenlandicus (known by some researchers as Eiectus longmani) and K. boyacensis (known by some researchers as Monquirasaurus boyacensis); however, the reptile’s fossil skull alone measured between 2.2 and 2.9 metres (7.2 and 9.5 feet) long. An even larger pliosaur from the Jurassic Period, Pliosaurus funkei (known colloquially as “Predator X”), was unearthed in Svalbard in 2009. Its length and weight are estimated at 15 metres (about 50 feet) and 45 tonnes (almost 100,000 pounds), respectively. The jaws of this creature are thought to have produced a bite force of 33,000 psi (pound-force per square inch), perhaps the largest bite force of any known animal.