Scientists revealed that 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 and now scientists say a 78 million-year-old fossil of a pregnant plesiosaur suggests they gave birth to single, large young.
"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.
The newly unveiled fossil was originally discovered in 1987 in the Midwest state of Kansas and is the first of a pregnant plesiosaur found. After its discovery, the 5m-long fossil skeleton Polycotylus latippinus lay for two decades in the basement of the Los Angeles museum before being examined.
Scientists started piecing the bones together two year ago but soon realised that they were in fact dealing with two separate animals; an adult plesiosaur and a smaller juvenile.
The study's authors report it was unlikely the juvenile had been eaten by the larger reptile because its small bones showed no evidence of bite marks, and its skeleton suggested an animal only two-thirds of the way through its development.
For more than 200 years palaeontologists have tried to discover how the Plesiosaurs reproduced, so the new findings come as good news.
"[The find] provides the first direct evidence for live birth in plesiosaur," said palaeontologist Adam Smith from the Thinktank Centre, Birmingham Science Museum.
"It's a very interesting find...[and] has been a long time coming."
Meanwhile, Frank O'Keefe, the study's lead author from Marshall University in Huntington, U.S. explained "The lack of fossil evidence of a pregnant plesiosaur was frustrating," explained.
"What is really surprising about this fossil [is] that plesiosaurs [reproduce] differently to other marine reptiles... they give birth to one big baby instead of a lot of little babies," he added.
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, as just like whales, plesiosaurs may have formed social groups and tended their young, they say.