Cimoliasauridae
Cryptoclididae
Elasmosauridae
Plesiosauridae
Polycotylidae
Plesiosaurs (IPA /ˈplisɪəˌsɔɹ/) (Greek: plesios meaning 'near' or 'close to' and sauros meaning 'lizard') were carnivorous aquatic (mostly marine) reptiles. After their discovery, they 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 (Suborder Plesiosauroidea) and to the larger taxonomic rank of Plesiosauria, which includes both long-necked (elasmosaurs) and short-necked (polycotylid) forms. Short-necked, large-headed plesiosaurs are more properly called pliosaurs. There were many species of plesiosaurs and not all of them were as large as Liopleurodon, Kronosaurus or Elasmosaurus.
Plesiosaurs (sensu Plesiosauroidea) first appeared at the very start of the Jurassic Period and thrived until the K-T extinction, at the end of the Cretaceous Period. While they were Mesozoic reptiles that lived at the same time as dinosaurs, they were not dinosaurs.