ple·si·o·sau·rus \ˌ ̷ ̷ ̷ ̷ ̷ ̷ˈsȯrəs\noun Etymology: New Latin, from plesi- + -saurus 1.capitalized: a genus of marine reptiles (suborder Plesiosauria) of the Mesozoic of Europe and No. America having a very long neck, a small head, and all four limbs developed as paddles for swimming 2.pluralplesiosau·ri\-ȯˌrī\: any reptile of the genus Plesiosaurus