Finding a creature curled inside the belly of an ancient sea beast might not seem surprising at first. After all, the huge fossil belonged to a water reptile with a ridiculously long neck and a knack for swallowing its prey whole.

But when paleontologists took a closer look, they found that the extra fossil inside its body was no meal: Instead, the giant animal was a mother.

In a new paper published in the journal Nature Communications, an international team describes a 245-million-year-old Dinocephalosaurus fossil with the preserved remains of a fetus inside. (Also read "Jurassic ‘Sea Monster’ Emerges From Scottish Loch.")

Found at a fossil site in southern China, several pieces of evidence made it clear that the smaller animal was not food. Like its prehistoric parent, the fetus has the unmistakably long neck joints of Dinocephalosaurus, indicating they belong to the same species.

In addition, the fetus was facing forward. By contrast, getting swallowed head-first was the usual fate of a Dinocephalosaurus snack, and this position is usually preserved during digestion. Then there is the fetus’s distinctive shape.

“The embryo was curled,” says study co-author Jun Liu, a paleontologist from the Hefei University of Technology in China. “If an animal is ingested by something else, there’s no way to preserve that shape.”

Sign of the Time?

Dinocephalosaurus is a member of archosauromorpha, the group of ancient reptiles that included dinosaurs, crocodiles, and birds. Unlike those relatives, Dinocephalosaurus appears to have given birth to live babies rather than laying eggs.

Liu and his team say this sea creature is the first known example of a member of the archosauromorphs giving birth to live young.

That may not be entirely correct, says Michael Caldwell, an expert in extinct marine reptiles and chair of the biological sciences department at the University of Alberta.

Choristodera, a group of semi-aquatic reptiles, gave birth to live young, too. Part of the problem, says Caldwell, is that the archosauromorph category is “a giant grab bag” of seemingly disparate animals. But many paleontologists think choristoderes belong to the archosauromorph clan, which would give them the actual claim to fame.

Either way, Caldwell says, the newfound Dinocephalosaurus baby is worth studying, in part because it may offer some insight into how animals evolve to cope with environmental stresses.

Live birth is linked with animals that determine the sex of their young via genetics rather than environmental factors, such as nest temperature. These traits in turn are associated with animals making the move from land to sea, a transition that has happened multiple times during history—usually during periods of mass extinction.