Bones show ancient marine reptile was a big baby

a photo of a fossil of developing baby plesiosaur skeleton inside the mother's skeleton
The bones of a developing baby (encircled) can be seen inside the skeleton of this plesiosaur.

Sometimes it’s good to be a big baby. That’s what scientists are learning from a skeleton at the Natural History Museum of Los Angeles, in California. It belongs to a marine reptile known as Polycotylus (Pahl-ee-KOAT-eh-luss). Inside the fossil skeleton, scientists found the bones of a developing baby. By comparing that fossil to other young plesiosaurs, researchers are figuring out how these animals grew.

Polycotylus is a type of plesiosaur (PLEES-ee-oh-sawr). These reptiles swam the seas between 203 million and 66 million years ago. They weren’t dinosaurs, although dinos lived at the same time. Plesiosaurs were enormous — growing to 5 meters (16 feet) in length. We’ve known about they since 1821. That’s when English paleontologist William Conybeare first described one. Back then, no one was sure how these enormous swimmers reproduced.

Instead of laying eggs like other reptiles, a mother Polycotylus gave birth to a single, large offspring. Here, an artist imagines what that looked like.

“We really had no idea,” says Robin O’Keefe. He’s a paleontologist at Marshall University in Huntington, W.V. Today’s sea turtles, also ocean reptiles, lay their eggs on land. No clues existed that plesiosaurs might have had live births. But they seemed too big to venture ashore to lay eggs.

The mystery remained unsolved until 2011. That’s when O’Keefe and his colleagues first studied the Polycotylus skeleton in Los Angeles. As luck would have it, this plesiosaur had been pregnant. That showed scientists that Polycotylus gave birth to live young, instead…

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