The story
Plesiosaurs are marine reptiles with a broad body, a short tail and four large paddle-shaped flippers, which they flapped much like underwater wings. They descend from Triassic sauropterygians such as the pistosaurs, and the first true plesiosaurs appear around 200 million years ago, just as the end-Triassic extinction removes many marine competitors. Two body forms evolve: long-necked types such as Elasmosaurus, with about 72 neck vertebrae, and short-necked pliosaurs such as Kronosaurus, with massive skulls and teeth for killing large prey.
Plesiosaurs lived in seas from the Early Jurassic to the end of the Cretaceous, and some entered rivers and lakes. A pregnant Polycotylus found in Kansas and described by Robin O'Keefe and Luis Chiappe in 2011 shows that they gave birth to one large baby at a time. Fossils are known from the Lias cliffs of Lyme Regis in England, from the Cretaceous chalk of Kansas, and from the Antarctic and New Zealand. They went extinct with the asteroid at the end of the Cretaceous, when the ocean food chain collapsed.
Why it mattered
- Plesiosaur fossils from Lyme Regis, first collected by Mary Anning, helped shape early paleontology.
- Their live birth of a single large young gives a rare glimpse of reptile parental investment.
- They filled the large predator niche in the Jurassic and Cretaceous seas before mosasaurs and later whales.
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