The remarkable fossil of a Dinocephalosaurus, discovered in a field in China, shows an embryo inside the ribcage of the mother.
Dinocephalosaurus was a long-necked fish-eating archosauromorph that flourished in the shallow seas of South China in the Middle Triassic period.
In research published in Nature Communications, experts found the embryo, a small reptile of the same species, facing forwards inside the mother.
Swallowed animals generally face backwards because the predator ingests its prey head-first to help it go down its throat.
Professor Jun Liu, from Hefei University of Technology in China, said: “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.
“Upon further preparation and closer inspection, we realised that something unusual has been discovered.
“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.
“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 well known in mammals and is common among lizards and snakes, where the babies sometimes “hatch” inside their mother and emerge without a shelled egg.
Until this discovery, the third major group of living land vertebrates – the crocodiles and birds, part of the wider group Archosauromorpha – was thought to only lay eggs.
Egg laying is the primitive state, seen at the base of reptiles, and in their ancestors such as amphibians and fish.
Evolutionary analysis shows that this instance of live birth was also associated with genetic sex determination.
Professor Chris Organ, from Montana State University, added: “Some reptiles today, such as crocodiles, determine the sex of their offspring by the temperature inside the nest.
“We identified that Dinocephalosaurus, a distant ancestor of crocodiles, determined the sex of its babies genetically, like mammals and birds.
“This new specimen from China rewrites our understanding of the evolution of reproductive systems.”
An international team of scientists carried out the research on the fossil.
The work is part of wider collaborations between palaeontologists in China, the United States, the UK and Australia.
Professor Mike Benton, from the School of Earth Sciences at the University of Bristol, said: “The analysis of the evolutionary position of the new specimens shows there is no fundamental reason why archosauromorphs could not have evolved live birth.”