انقراض العصر الطباشيري-الثلاثي
Plesiosaurs (sensu Plesiosauroidea) appeared at the start of the Jurassic Period, and thrived until the Cretaceous–Paleogene extinction event, at the end of the Cretaceous Period. While they were Mesozoic diapsid reptiles that lived at the same time as dinosaurs, they were not dinosaurs.