Etymology
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plesiosaurus (n.)

extinct gigantic long-necked marine reptile, 1825, from Modern Latin Pleisiosaurus (1821), coined by English paleontologist William Daniel Conybeare (1787-1857) from Greek plēsios "near" (related to pelas "near, nearby," and probably from PIE *pelh- "to approach") + sauros "lizard" (see -saurus). It was one of the earliest "antediluvian" reptile fossils discovered in the scientific age and was so called for being more like a modern lizard than the ichthyosaur fossils that had been found a few years earlier in the same rock in England.

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Definitions of plesiosaurus

plesiosaurus (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;
Synonyms: plesiosaur
From wordnet.princeton.edu