Plesiosaur

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Plesiosaurs were any of the various extinct sauropterygian reptiles in the order Plesiosauria. They first appear in the fossil record in the early Triassic, 245 million years ago, and the last species died out 65 million years ago in the Cretaceous-Tertiary extinction, barring tales of lake monsters or misidentified basking shark carcasses.

There are two main body plans among the plesiosaurs, one being the "true" plesiosaur, a broad body with a long thin neck that resembled a "snake threaded through its body," sans shell, and the other being the "pliosaur," a similarly broad body with a short neck and long, crocodile-like head. Originally, Plesiosauria was conveniently divided into two sub-groups, the Plesiosauroidea and the Pliosauroidea. However, anatomical studies on recent fossils show that both plans had evolved several times throughout the Plesiosauria's history. In particular, the Cretaceous plesiosaurs and pliosaurs are now believed to be descended from the Jurassic pliosaurs.

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