A fossil of a prehistoric water reptile has an embryo inside, providing the first evidence that plesiosaurs gave birth to live offspring rather than laying eggs, a US study said Thursday.

The 78-million-year-old fossil of the Polycotylus latippinus, a four-flippered swimmer something like a snake-turtle combination, is now on display at the Natural History Museum of Los Angeles County.

Scientists have long suspected that the large creatures, which once were among the top predators in the world’s oceans, were not built for climbing on land and laying eggs, but had no evidence to show otherwise until now.

“This fossil documents live birth in plesiosaurs for the first time, and so finally resolves this mystery,” said co-author Robin O’Keefe of Marshall University in Huntington, West Virginia.

The 4.7-metre-long fossil on display contains an embryonic skeleton, including little ribs, 20 vertebrae, shoulders, hips and paddle bones, said the study in the journal Science.

Several other types of aquatic reptiles from the same Mesozoic period have been known to give birth to live offspring instead of eggs, a behaviour that lends itself to a more social lifestyle, similar to that of dolphins.