Triassic Fossil Reveals First Evidence of Live Birth in Archosauromorphs

The first ever evidence of live birth in a group of animals previously thought only to lay eggs has been discovered by an international team of paleontologists from China, the United States, Australia and UK.

Artist’s reconstruction of Dinocephalosaurus showing the rough position of the embryo within the mother. Image credit: Dinghua Yang.

“Live birth (viviparity) is well known in mammals, where the mother has a placenta to nourish the developing embryo,” said University of Queensland Professor Jonathan Aitchison, senior author of a paper on the discovery published Feb. 14 in the journal Nature Communications.

“Live birth is also very common among lizards and snakes, where the babies sometimes hatch inside their mother and emerge without a shelled egg.”

Until recently it was thought the third major group Archosauromorpha, a group that first evolved some 260 million years ago and is represented today by birds and crocodilians, only laid eggs.

“Indeed, egg-laying is the primitive state, seen at the base of reptiles, and in their ancestors such as amphibians and fishes,” Prof. Aitchison noted.

The new fossil is an unusual, long-necked animal called Dinocephalosaurus, an archosauromorph that lived in shallow seas of South China in the Middle Triassic epoch (245 million years ago).

The specimen belongs to the recently discovered Luoping biota and shows an embryo inside the mother — clear evidence for live birth.

“We report the discovery of a pregnant long-necked marine reptile (Dinocephalosaurus) from the Middle Triassic of southwest China showing live birth in archosauromorphs,” Prof. Aitchison and co-authors said.

Photograph of the new Dinocephalosaurus specimen; the three separate blocks are arranged following their original positions in the field; (a) photo showing a close-up of the embryo preserved in the stomach region of the specimen; (b) photo showing a close-up of the perleidid fish preserved in the stomach region of the specimen. Image credit: J. Liu et al, doi: 10.1038/ncomms14445.

The researchers said the embryo was inside the mother’s rib cage, and it faced forward.

Swallowed animals generally face backward because the predator swallows its prey head-first to help it go down its throat.

Furthermore, the small reptile inside the mother is an example of the same species.

“Our discovery pushes back evidence of reproductive biology in the clade by roughly 50 million years, and shows that there is no fundamental reason that archosauromorphs could not achieve live birth,” they said.

Professor Chris Organ from Montana State University, co-author of the study, said evolutionary analysis showed that this instance of live birth was also associated with genetic sex determination.

“Some reptiles today, such as crocodiles, determine the sex of their offspring by the temperature inside the nest,” he explained.

“We identified that Dinocephalosaurus, a distant ancestor of crocodiles, determined the sex of its babies genetically, 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,” said University of Bristol Professor Mike Benton, co-author of the study.

“This new specimen from China rewrites our understanding of the evolution of reproductive systems,” Prof. Organ added.

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J. Liu et al. 2017. Live birth in an archosauromorph reptile. Nature Communications 8, article number: 14445; doi: 10.1038/ncomms14445