[ 'pli:sɪəsɔ: , 'pli:z- ]
■ noun a large fossil marine reptile of the Mesozoic era, with large paddle-like limbs and a long flexible neck.
Origin
C19: from mod. L. Plesiosaurus , from Gk plēsios 'near' (because closely related to the lizards) + sauros 'lizard'.