For years, scientists and researchers have been pondering just how plesiosaur – a giant marine reptile that ruled the oceans 78 million years ago -- gave birth to young ones.

Researchers said on Thursday that they have found the first evidence that plesiosaur gave live births similar to mammals, rather than laying eggs.

The findings were compiled in a paper that was published in the journal Science.

The 4.7-meter-long fossilized plesiosaur was carrying a large fetus when she was unearthed in Kansas in 1987. The fossilized remains resembled two animals, a larger plesiosaur and another much smaller "jumble of bones".

The new study has helped researchers determine that the plesiosaur gave live births in the water, rather than hatching their offspring from eggs on land. The fetuses soft, immature skeleton suggested an animal only two-thirds of the way through its development.

Researchers F. Robin O'Keefe, of Marshall University, and Dr. Luis Chiappe, of the Natural History Museum's Dinosaur Institute, said the fossilized dinosaur contained the embryo of its unborn offspring -- including the baby's ribs, 20 vertebrae, shoulders, hips, and paddle bones.

“Scientists have long known that the bodies of plesiosaurs were not well suited to climbing onto land and laying eggs in a nest," said O'Keefe. "So the lack of evidence of live birth in plesiosaurs has been puzzling. This fossil documents live birth in plesiosaurs for the first time, and so finally resolves this mystery.

"Many of the animals alive today that give birth to large, single young are social and have maternal care. We speculate that plesiosaurs may have exhibited similar behaviors, making their social lives more similar to those of modern dolphins than other reptiles."

The plesiosaur was first discovered in England in the early 19th century and was among the first fossil vertebrates to be found by scientists. Numerous discoveries have been made since then. The plesiosaur discovered with the fetus was found in 1987 by Charles Bonner, on the Bonner Ranch in Kansas.

Plesiosaurs swam the Western Interior Seaway back when modern day North America was split by a body of water. They were among the top predators of their time.

The skeleton of the ancient animal is on display at Natural History Museum of Los Angeles County, where Chiappe is the director.