Carroll, Robert L. Redpath Museum, McGill University, Montreal, Quebec, Canada.
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An order of extinct aquatic diapsid reptiles within the infraclass Sauropterygia, common throughout the world during the Jurassic and Cretaceous periods. These large carnivores are characterized by long, paddle-shaped limbs and short, dorsoventrally compressed bodies (see illustration). In contrast with nothosaurs (more primitive members of the Sauropterygia), plesiosaurs have greatly expanded ventral portions of both pectoral and pelvic girdles to provide large areas for the attachment of muscles to move the limbs anteriorly and posteriorly. Lateral undulation of the trunk was severely restricted by elaboration of the ventral scales. The great length of the limbs indicates that they must have been used symmetrically, rather than alternatively as in primitive reptiles. The configuration of the skeleton suggests that plesiosaurs swam in the manner of sea lions by horizontal retraction of the limbs, rather than like sea turtles and penguins which use the limbs like wings to fly through the water.
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