Plesiosaur
Plesiosauria (/ˌpliːsi.ɵˈsɔriə/; Greek: plesios meaning 'near to' and sauros meaning 'lizard') is an order of Mesozoic marine reptiles (sauropsida). Plesiosaurs first appeared in the Early Jurassic (and possibly Rhaetian, latest Triassic) Period and became especially common during the Jurassic Period, thriving until the Cretaceous–Paleogene extinction at the end of the Cretaceous Period. They had a worldwide distribution.
The name "plesiosaur" is used to refer to the order Plesiosauria as a whole, not only to the long-necked forms (suborder Plesiosauroidea). These latter constitute the plesiosaurs in the popular imagination ("Nessie", "Nahuelito").
Read more about Plesiosaur: Description, History of Discovery, Behavior, Classification, Plesiosaurs in Contemporary Culture