plesiosaur
Concise Oxford English Dictionary © 2008 Oxford University Press:
plesiosaur /ˈpliːsɪəsɔː, ˈpliːz-/
▶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’.
'plesiosaur' also found in these Oxford entries:

