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 overnight.

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.

"Many of the animals alive today that give birth to large, single young are social and have maternal care," added O'Keefe.

"We speculate that plesiosaurs may have exhibited similar behaviours, making their social lives more similar to those of modern dolphins than other reptiles."

The fossil specimen, which is virtually complete except for parts of the adult neck and skull, was discovered in 1987 by Charles Bonner on the Bonner Ranch in Logan County, Kansas.

The mother and child specimen were given extensive conservation by the Natural History Museum and then mounted for display under the supervision of O'Keefe and study co-author Luis Chiappe, director of the museum's Dinosaur Institute.