plesiosaurus(n = noun.animal) plesiosaur - extinct marine reptile with a small head on a long neck a short tail and four paddle-shaped limbs; of the Jurassic and Cretaceous;
A genus of large extinct marine reptiles, having a very long neck, a small head, and paddles for swimming. It lived in the Mesozoic age. [1913 Webster]
Plesiosaurus, n. (also plesiosaur) any of a group of extinct marine reptiles with a broad flat body, short tail, long flexible neck, and large paddle-like limbs.