Plesiosaurswere a diverse and highly successful group of marine reptiles that made up the order Plesiosauria. They were entirely or almost entirely aquatic, and thrived throughout the Jurassic and Cretaceous periods. Plesiosaurs became extinct during the Cretaceous-Paleogene extinction event, which was also responsible for the extinction of non-avian dinosaurs, pterosaurs, and most marine reptiles. The order Plesiosauria was comprised of two superfamilies, Plesiosauroidea and Pliosauroidea.