Definition of plesiosaurus
1 capitalized : a genus of marine reptiles (suborder Plesiosauria) of the Mesozoic of Europe and North America having a very long neck, a small head, and all four limbs developed as paddles for swimming
2 plural plesiosauri \-ȯˌrī\ : any reptile of the genus Plesiosaurus
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Origin and Etymology of plesiosaurus
New Latin, from plesi- + -saurus
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