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Any of various large, extinct marine reptiles of the genus Plesiosaurus and related genera of the Mesozoic Era. Most plesiosaurs had a small head on a long neck and a broad body with paddlelike limbs; one group had a large head on a short neck. The exact relationship between plesiosaurs and other reptiles is not known.
any of a group of long-necked marine reptiles found as fossils from the Late Triassic Period into the Late Cretaceous Period (215 million to 80 million years ago). Plesiosaurs had a wide distribution in European seas and around the Pacific Ocean, including Australia, North America, and Asia. Some forms known from North America and elsewhere persisted until near the end of the Cretaceous Period (65 million years ago).