The Collaborative International Dictionary of English v.0.48:

Plesiosaur \Ple"si*o*saur\, n. (Paleon.)
   One of the Plesiosauria.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plesiosaur
    n 1: extinct marine reptile with a small head on a long neck a
         short tail and four paddle-shaped limbs; of the Jurassic
         and Cretaceous [syn: plesiosaur, plesiosaurus]