The remains of a giant sea creature are providing the first proof that these prehistoric reptiles gave birth to their young rather than laying eggs.

Plesiosaurs, which lived at the time of dinosaurs, were large carnivorous sea animals with broad bodies and two pairs of flippers. Researchers have long questioned whether they would have been able to crawl onto land and lay eggs like other reptiles or gave birth in the water like whales.

The question has been answered by the analysis of the fossil of a pregnant plesiosaur in Friday's edition of the journal Science.

"This is the first evidence of live birth in plesiosaurs — an exciting find," said geology professor Judy A. Massare of the State University of New York, Brockport, who was not part of the research team.

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Based on the fossil, it appears that plesiosaurs gave birth to a single, large young, unlike other marine reptiles that gave birth to many, smaller young. (S. Abramowicz/Dinosaur Institute/Natural History Museum)

The newly unveiled fossil was originally discovered in 1987 in the Midwest state of Kansas. But it remained encased in rock, with most of the bones and their details hidden from view, for many years. 

The fossil was stored in the basement of the National History Museum of Los Angeles County until resources were available to separate the fossil from the rock itself for display at the museum.

Only then did F. Robin O'Keefe of Marshall University and the museum's Luis Chiappe uncover the bones of an adult plesiosaur and the remains of a fetus inside her.

The museum dated the fossil, which is more than five metres long, to between 72 million and 78 million years ago.

O'Keefe said he had seen photos of the fossil, but was still surprised when he first saw it.

"I wasn't prepared for the emotional response I had," he said in a telephone interview. "You don't very often walk up to one and say: 'That is a really cool fossil."'

"I walked around it for about a half hour, it excited me the way I used to get excited as a kid," he said.

There had been evidence of live births in an ancestor of plesiosaurs. But the lack of proof for plesiosaur birth has been puzzling, said R. Ewan Fordyce, head of the geology department at the University of Otago, New Zealand, who was not involved in the research.

Fordyce said the researchers did a good job of ruling out the other possible explanation for the extra collection of bones in the plesiosaur's belly — that they had been part of a meal. He said it was the right size for a fetus and in the right place, and there's no sign that it had been eaten and digested.

Anthony Russell of the biological science department at the University of Calgary, Canada, called the find "significant."

"It would be hard to imagine these animals coming out onto land laying eggs somewhere," he added, so arguing that all plesiosaurs were doing this is a reasonable hypothesis.

In their paper, O'Keefe and Chiappe suggest a parallel between plesiosaurs and modern whales, also large animals that give birth to relatively big offspring. Like whales, they said, plesiosaurs may have formed social groups and tended their young.