First evidence of live birth in dinosaur relative discovered

Previously believed to reproduce exclusively through the laying of eggs, the group of animals that include dinosaurs and birds actually gave birth to live young at some point, according to the authors of a study published Tuesday in the journal Nature Communications.

While live birth “has evolved many times independently in vertebrates,” it had been “unknown” in the group known as the Archosauromorpha, a group currently represented by modern birds, crocodiles and turtles, lead author Jun Liu of the Hefei University of Technology in China and colleagues from the US, UK and Australia wrote in their study.

Now, however, the researchers reveal that they have discovered a pregnant, long-necked marine reptile known as Dinocephalosaurus that dated back roughly 245 million years ago to the Middle Triassic. The specimen, discovered in a fossil deposit in the Yunnan Province of southern China, represents the first-known evidence of live birth in archosauromorphs, BBC News noted.

The discovery should quell speculation that the biology of dinosaurs and their relatives prevented them from giving birth to live young, as the discovery in the embryo of this marine reptile shows that no fundamental barrier preventing members of this clade from carrying an embryo, study co-author and University of Bristol professor Mike Benton added.

Findings also reveal how the offspring’s sex was determined

According to Liu, Benton and their colleagues, the adult Dinocephalosaurus would have been between three and four meters long with an approximately 1.7 meter long neck, while its embryo was about one-half meter long and was positioned inside the rib cage of the would-be mother.

Initially, the researchers thought that the fossil, which BBC News reported was first discovered in 2008, represented an adult archosauromorph and the fossilized contents of its last meal, but a closer look revealed that this was not likely the case, as the smaller animal was facing forward.

In most cases, the study authors explained to the UK news outlet, consumed prey typically face backward because predators find it easier to consume meals head-first. Furthermore, the smaller specimen was found to be the same species as the adult, suggesting that it was offspring that was being carried by a parent – the first and thus far only evidence of live birth in such creatures.

In a statement, Professor Liu said that the team’s research “pushes back evidence of reproductive biology in the group by 50 million years” and added that information on the reproductive biology of pre-Jurassic Period archosauromorphs “was not available until our discovery,” despite the 260 million year history of these creatures.

Co-author and Montana State University professor Chris Organ added that their analysis revealed how the offspring’s sex would have been determined. “Some reptiles today, such as crocodiles, determine the sex of their offspring by the temperature inside the nest,” he noted. “We identified that Dinocephalosaurus, a distant ancestor of crocodiles, determined the sex of its babies genetically, like mammals and birds.”

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Image credit: Dinghua Yang

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