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Everyone Thought This Ancient Reptile Only Laid Eggs. They Were Wrong.

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An embryo preserved in a 250 million-year-old fossil is evidence that a type of animal previously thought to only lay eggs could actually have a live birth, rewriting scientists’ understanding of the evolution of reproductive systems.

The fossil, which came from China, was of a pregnant long-necked marine reptile called a dinocephalosaurus, a fish-eating creature that would snake its long neck from side to side to snatch its prey. The dinocephalosaurus flourished in the shallow seas of South China in the Middle Triassic – the first period of the Mesozoic Era which occurred between 251 million and 199 million years ago.

The dinocephalosaurus is classified as a type of diapsid reptile called an archosauromorph. Archosauromorpha is a clade that first evolved some 260 million years ago and is represented today by birds and crocodilians. Live birth has been unknown in this group – until now.

By studying the fossil and spotting an embryo inside the mother’s rib cage, a team of researchers from China, the U.S., the U.K. and Australia identified that the Dinocephalosaurus was able to have a live birth. Because the embryo was facing forwards and was the same species as the mother, scientists could determine that it was indeed an embryo, rather than something the mother had eaten.

“We were so excited when we first saw this embryonic specimen several years ago but we were not sure whether the embryonic specimen [was] the last lunch of the mother or its unborn baby,” said the study’s lead author, Professor Jun Liu from Hefei University of Technology in China, in a statement. “Upon further preparation and closer inspection, we realised that something unusual has been discovered.”

The discovery has pushed back evidence of reproductive biology in this group of animals by roughly 50 million years.

The study, called Live Birth in an Archosauromorph Reptile, has been published in the journal Nature Communications.

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Write to Kate Samuelson at kate.samuelson@time.com