Plesiosaurus plē˝sēəsôr´əs [key], genus of extinct predatory marine reptiles that arose in the Triassic period of geologic time and continued into the Jurassic and Cretaceous periods. Plesiosaurs became extinct at the close of the Mesozoic era. Fossilized skeletons of them have been found in North America, Europe, and Australia. The plesiosaur had a small, short head, a long, snakelike neck, a broad, solid body, and a short tail. It had sharp interlocking teeth, and its four paddlelike legs were similar to those of a marine turtle. In total length, Plesiosaurus ranged from 10 to 60 ft (3–18 m). The term plesiosaur is sometimes applied more generally to all forms in the order Sauropterygia, which produced two lines of marine reptiles. The Plesiosaurus is an example of one trend. The other group, the pliosaurs, evolved into forms possessing short necks and gigantic skulls; the Kronosaurus, for example, had one of the largest known reptile skulls, reaching 9 ft (2.7 m) in length, and a still unclassified pliosaur first discovered on Spitsbergen in 2006 had a 10-ft (3-m) head. Paleontological evidence suggests that plesiosaurs may have given birth to live young instead of laying eggs. Plesiosaurs are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Sauropterygia.

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