The fossil of a pregnant reptile that once roamed the seas some 250 million years ago is the first evidence of live birth in an animal group thought to only lay eggs – a group that includes dinosaurs, crocodiles, and birds.

Researchers found the remains of a well-developed embryo inside an ancient reptile known as Dinocephalosaurus, which could alter what we thought we knew about the evolution of live birth.

“We were so excited when we first saw this embryonic specimen several years ago but we were not sure if the embryonic specimen is the last lunch of the mother or its unborn baby,” says Professor Jun Liu, who led the study published in Nature Communications, in a statement. “Upon further preparation and closer inspection, we realized that something unusual has been discovered.”

By looking at the positioning of the younger creature inside the larger, they were able to determine whether or not the remains were truly an embryo or just a hapless juvenile gobbled up by an adult. With most predators that eat their prey whole, the animal is swallowed head first in order to prevent errant limbs getting stuck on the way down, as is seen with owls eating mice, for example. But the remains found in the Dinocephalosaurus fossil clearly show the baby facing the other way.

This, coupled with the fact that the younger one is of the same species as the reptile inside that which it was found, indicates the long-extinct creature did indeed give birth to live young. This would make Dinocephalosaurus the first ever archosauromorph – the group that contains not only dinosaurs, but pterosaurs, crocodiles, and birds – to bear live young rather than lay eggs, as do all other known species to date.

Since all living crocodiles and birds (as well as all dinosaurs to the best of our knowledge) lay eggs, it has long been thought that there is some physiological constraint that makes them unable to have live young. This has always been a bit of a mystery considering live birth has evolved multiple times in different lineages, even in other reptiles, such as some species of lizards and snakes.

Further analysis of the evolutionary tree that gave rise to the Dinocephalosaurus suggests that sex determination in the group may well have been genetic, rather than temperature dependent as with many crocodile species and some testudines. The authors suggest that this shift to genetic sex determination may be vital for the evolution of live birth in animals, and could help explain its distribution within the animal kingdom.