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.