Plesiosauria are an order of extinct Mesozoic marine reptiles. They first appeared in the Middle Triassic Period and became especially common during the Jurassic Period, thriving until the K-T extinction at the end of the Cretaceous Period. The name "plesiosaur" is variously used to refer both to the Order Plesiosauria as a whole, and to the long-necked forms (Suborder Plesiosauroidea) only. These latter constitute the plesiosaurs in the popular imagination ("Nessie"). In the current page, "Plesiosaur" is used to refer to the Plesiosauria in general.