any of an extinct group (order Sauropterygia) of large water reptiles of the Mesozoic Era, characterized by a small head, long neck, short tail, and four paddlelike limbs
See plesiosaur in American Heritage Dictionary 4
(plēˈsē-ə-sôrˈ, plēˈzē-) also ple·si·o·sau·rus(plēˌsē-ə-sôrˈəs, plēˌzē-)
nounpl.ple·si·o·saurs also ple·si·o·saur·i(-sôrˈī)
A large extinct marine reptile having paddlelike limbs that was common in Europe and North America during the Mesozoic Era.