Scientists have always thought that archosauromorpha, an animal group that includes crocodiles, birds and dinosaurs, reproduced only by laying eggs. But now, paleontologists have uncovered a 250-million-year-old fossil of a strange long-necked marine reptile with an embryo inside, demonstrating live birth in archosauromorphs for the first time and forcing a rethink of reproductive evolution.

Researchers at the University of Bristol and the Hefei University of Technology in China actually discovered the embryonic specimen a few years ago. Sitting inside the belly of a marine dinosaur-relative called Dinocephalosaurus that thrived in the South China sea during the Middle Triassic period, the team suspected at first that it might have been the creature's last meal.

Further examination led to the conclusion that it is in fact an embryo. It rests inside the rib cage of the fossilized mother and faces forward, whereas eaten animals will typically face backward after the predator swallows them head-first to help them down the hatch. Another helpful tidbit of evidence backing up the team's theory was the fact that the swallowed reptile is the same species as the mother.

"Further evolutionary analysis reveals the first case of live birth in such a wide group containing birds, crocodilians, dinosaurs and pterosaurs among others, and pushes back evidence of reproductive biology in the group by 50 million years," says Professor Jun Liu from Hefei University of Technology and the study's lead author. "Information on reproductive biology of archosauromorphs before the Jurassic period was not available until our discovery, despite a history of 260 million years."

Live birth is quite common among reptiles like lizards and snakes, where eggs can hatch inside the mother and offspring make their way into the world without a shell. But it was thought that those belonging to the archosauromorpha group reproduced only by laying eggs.

The discovery of this pregnant dino-relative has implications for our understanding of how the ancestors of crocodiles, such as Dinocephalosaurus, determined the sex of their offspring. Where crocodiles and some other reptiles determine the sex of their offspring through the temperature inside their nest, through further analysis the team established that Dinocephalosaurus did so genetically, through sex chromosomes, just like mammals and birds.

"This combination of live birth and genotypic sex determination seems to have been necessary for animals such as Dinocephalosaurus to become aquatic," says Professor Mike Benton from the University of Bristol, and co-author of the study. "It's great to see such an important step forward in our understanding of the evolution of a major group coming from a chance fossil find in a Chinese field."

The research was published in the journal Nature Communications.