The Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage, about 203 million years ago. They became especially common during the Jurassic Period, thriving until their disappearance due to the Cretaceous–Paleogene extinction event at the end of the Cretaceous Period, about 66 million years ago. They had a worldwide oceanic distribution. Plesiosaurs were among the first fossil reptiles discovered. In the beginning of the nineteenth century, scientists realised how distinctive their build was and they were named as a separate order in 1835. The first plesiosaurian genus, the eponymous Plesiosaurus, was named in 1821. Since then, more than a hundred valid species have been described.

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Scientists Solve a 120-Year-Old Mystery: How Did the Monstrous Plesiosaurs Swim?

Paleontologists have unearthed several fossilized bones of plesiosaurs in Morocco's Kem Kem beds. Traditionally thought to be marine reptiles, the finding suggests that some plesiosaur species were adapted to tolerate freshwater, possibly even spending their lives there, like today’s river dolphins.

Some Plesiosaurs May Have Lived in Freshwater River Systems

Some Plesiosaurs May Have Lived in Freshwater River Systems