Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

plesiosaur
     n : 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: {plesiosaurus}]