Scientists have found a 250-million-year-old fossil with an embryo inside the mother, evidence of live birth by a group of animals that were thought to reproduce only by laying eggs.

The Dinocephalosaurus belongs to a group called Archosauromorpha, which includes dinosaurs as well as birds and crocodiles alive today. The long-necked marine animal lived in shallow waters near south China during the Middle Triassic Period.

It was believed that this group only laid eggs. However, this new discovery, announced Tuesday in the journal Nature Communications, "pushes back evidence for reproductive biology by about 50 million years," Jun Liu from the University of Technology in China said in a statement.

It was unclear at first whether the paleontologists had found an embryo or the reptile's last meal. Further study revealed the valuable fossil could change our thinking about dinosaurs and their distant descendants.

Researchers ruled out the embryo as a meal because it was found in the rib cage facing forward, whereas prey would be eaten head first. As well the animal and embryo were the same species.

The gender of a crocodile's offspring is influenced by environmental factors. However, genetics was the prime determinant of the Dinocephalosaurus, as is found in birds and mammals.