Researchers at the University of Bristol and the Hefei University of Technology in China recently published a paper in Nature Communications, describing their discovery of a fossil of a pregnant sea serpent that they found years ago.

Although this is not the first case of an unearthed pregnant fossil, what they have found changes our understanding of how the reptile's reproductive system evolved.

The marine reptile was identified as Dinocephalosaurus, from a group called the Archosauromorphs, a distant ancestor of modern-day birds and crocodiles.

It was previously thought that this group of living vertebrates only gives birth through eggs, until the recent discovery of the fossil. To their surprise, the embryo found inside the 240-million-year-old fossil was not inside an egg, indicating that Archosauromorphs could also adopt live birth, or viviparity.

The researchers initially thought that the tiny skeleton they found in her belly was her last meal. Upon further analysis, they came to realize that it is her unborn.

Verge notes that Dinocephalosaurus usually gulped their food down head first, facing backward. However, the skeleton, located in the reptile's pelvic region was in the canonical fetal position and facing forward. In addition, the researchers have found a partially digested fish which rules out the theory that the tiny skeleton was a prey. Lastly, the embryo's skeleton is strikingly similar with the reptile, indicating that they are of the same species.

How did the Dinocephalosaurus become viviparous?

Matthew Brandley, a post-doctoral fellow at the University of Sydney, and one of the authors of the study said being an aquatic creature might have something to do with it.

"Just like in sea snakes, if you're aquatic you can't lay eggs underwater, so over evolutionary time, as these terrestrial ancestors moved to an aquatic habitat, they started to develop this viviparous reproductive mode," he told ABC News.

As mentioned by the press release, Dinocephalosaurus has a long neck and is a fish-eater. Their huge body, long necks and small heads had probably made it difficult for them to come to land and lay their eggs.

The instance of live birth pushes back evidence of reproductive biology by 50 million years. But above all, this makes scientists question why despite the Archosauromorphs' ability to give birth to a live young, they evolved away from it and became egg-laying creatures.