Plesiosauria

Plesiosauria ( /ˌpliːsi.ɵˈsɔriə/; Greek: plesios meaning 'near to' and sauros meaning 'lizard') is an order of Mesozoic marine reptiles. 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.