In 1987, Charles Bonner discovered the fossilised bones of a large sea reptile on his family ranch. It was a flipper-limbed plesiosaur, probably Polycotylus, and one of many such fossils recovered from Logan County in Kansas. But this specimen was special – there was a smaller one inside it. This plesiosaur was pregnant.

The Bonners donated the find to the Natural History Museum of LA County, where it languished for years. No one had the resources to prepare and study it until Luis Chiappe decided to include the specimen in the new exhibit halls he was preparing. He brought in Robin O’Keefe from Marshall University to analyse the historic find.

The mass of small bones inside the adult is haphazardly arranged, but O’Keefe and Chiappe think that it was clearly a foetus. The skeleton shows signs of incomplete growth, as it includes the distinctive forearm bone of Polycotylus. It shows no signs of having been chewed or exposed to stomach acid, so it wasn’t a youngster that the adult had cannibalised.

O’Keefe and Chiappe’s discovery suggests that this reptile gave birth to live young. This isn’t that surprising. Most modern ...