A large fossil marine reptile of the Mesozoic era, with a broad flat body, large paddle-like limbs, and typically a long flexible neck and small head.
- Infraorder Plesiosauria, superorder Sauropterygia: several families, including Plesiosauridae (see also elasmosaur, pliosaur)
- While dinosaurs ruled the land, the ichthyosaurs, classified variously in the Ichthyosauria or in the Ichthyopterygia, shared the seas of the world with the other great groups of large marine reptiles, the plesiosaurs and mosasaurs.
- More distantly related to true dinosaurs were the marine plesiosaurs and ichthyosaurs.
- Marine vertebrates included the large dolphin-like ichthyosaurs and the long-necked plesiosaurs, marine crocodiles, turtles, and other reptiles.
Mid 19th century: from modern Latin Plesiosaurus, from Greek plēsios 'near' + sauros 'lizard'.
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