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.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"). Plesiosaurs have four flippers and long necks. They were considered by some to be at the top of the food chain of the oceans during much of the Mesozoic era. Some appear on land that use their flippers as legs.
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