Evidence that proves that giant sea reptiles gave birth to live young rather than laying eggs was found in a 78 million year old fossil of a pregnant Plesiosaur, a U.S. study said on Thursday.

The fossil of the Polycotylus latippinus, a four-flippered swimmer is the first of pregnant plesiosaur found. It is currently at the Los Angeles County Natural History. It was excavated from a ranch in Kansas, U.S. in 1987 and lay for two decades in the basement of the museum.

Scientist finally got around to piece the bones together two years ago and quickly realized they were dealing with two sets of fossils, an adult plesiosaur and a smaller juvenile.

The 15.4 foot (4.7 meter) 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.

The scientists wrote in the report that there was no evidence that the juvenile was eaten by the adult plesiosaur. There were no bite marks on the younger dinosaurs and the skeleton suggested an animal only two-thirds of the way through its development.

Scientists have long wondered how these giant sea dinosaurs reproduced. There were theories that the plesiosaur was too big to lay eggs on a beach like modern turtles and so must have given birth to live young.

"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.

Several other types of aquatic reptiles from the same period have been known to give birth to live offspring instead of eggs. The study's authors have inferred that giving birth to live young made the plesiosaur a more social animal.

"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 behaviors, making their social lives more similar to those of modern dolphins than other reptiles."