A plesiosaur from greek: plesios (near), and sauros (lizard)was a type of carnivorous aquatic (mostly marine) reptile. 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 (elasmosaurs) and short-necked (polycotylid) 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 (sensu Plesiosauroidea) appeared at the end of the Triassic Period and thrived 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 actually not classified as dinosaurs.