A plesiosaur was a type of carnivorous aquatic (mostly marine) reptile that existed during the Cretaceous period of the prehistoric era. After their discovery, plesiosaurs were somewhat fancifully said to have resembled "a snake threaded through the shell of a turtle," although they had no shell. The common name "plesiosaur" is applied both to the "true" plesiosaurs (Superfamily Plesiosauroidea), which include both long-necked and short-necked forms, and to the larger taxonomic rank of Plesiosauria, which includes the pliosaurs. The pliosaurs were the short-necked, large-headed plesiosaurians that were the apex predators for much of the Mesozoic.

Plesiosaurs appeared at the end of the Triassic Period and throve until the K-T extinction, at the end of the Cretaceous Period. While they were Mesozoic diapsid reptiles that lived at the same time as dinosaurs, they were not dinosaurs. Gastroliths are frequently found associated with plesiosaurs.

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