New research has revealed that a fossil of an ancient carnivorous sea creature by the name of Polycotylus latippinus (plesiosaur) contains a fossil of its unborn embryonic baby.

The researchers who made this discovery, Dr. F. Robin O'Keefe of Marshall University and Dr. Luis Chiappe of the Natural History Museum's Dinosaur Institute, have uncovered the mystery of how plesiosaurs gave birth. The fossil of the 78-million-year-old, 15.4-foot-long plesiosaur contained a developing body that included ribs, 20 vertebrae, shoulders, hips, and paddle bones.

Researchers were previously unsure whether or not the aquatic reptile, which lived during the Mesozoic Era during the age of the Dinosaurs, could either birth live young or hatch their offspring from eggs on land. While they previously figured was not the latter, there was no proof until now.

"Scientists have long known that the bodies of plesiosaurs were not well suited to climbing onto land and laying eggs in a nest," Dr. O'Keefe stated. "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."

The discovery of this fossil also opens up a lot of other information on the plesiosaur says O'Keefe. For instance, the large size of the embryo has allowed O'Keefe and his team to compare it to modern species.

"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," O'Keefe said.

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 this time period.

The original specimen was found in 1987 by Charles Bonner on the Bonner Ranch in Kansas. The researchers found the plesiosaur fossil complete minus part of the creature's neck and skull. It is currently on display at Dinosaur Hall at the Natural History Museum.

Findings from the research have been compiled into a paper by O'Keefe and Chiappe. The paper is currently published in the journal Science.

Follow Gabriel Perna on Twitter at @GabrielSPerna