This 236-Million-Year-Old Fossil Just Rewrote the Story of Live Birth

The vast majority of mammals alive today—including us—give birth to live young, but this reproductive mode isn’t unique to mammals nor universal among them. In fact, it evolved independently many times in other groups of vertebrates, and some mammal species actually lay eggs.

For years, scientists believed cynodonts—a major group of mammal-like reptiles that gave rise to all modern mammals—probably laid eggs, too. After all, egg-laying is common among reptiles, as well as the most ancient lineage of living mammals: the monotremes, which include platypuses and echidnas. However, efforts to determine whether cynodonts laid eggs or gave birth have been inconclusive—until now.

A study published today in the journal Frontiers in Mammal Science presents the first fossil evidence to suggest that a cynodont species gave birth. Chiniquodon theotonicus lived roughly 236 million years ago, during the late Triassic period. This discovery pushes the timeline of this evolutionary adaptation back by up to 95 million years, according to the researchers.

Stumbling upon a major discovery

Co-author Maria Miceli Baro, a graduate student at the University of Buenos Aires in Argentina, was examining the bone microstructure of a fully grown C. theotonicus specimen when something surprising appeared.

“She came with her samples, and we were looking at them, and we said, ‘Hey, look, you have embryonic tissue. This is very weird, I have never seen this,'” lead author Leandro Gaetano, a paleontologist at Argentina’s National Scientific and Technical Research Council who is based at the University of Buenos Aires, told Gizmodo. 

Data from living species helped Gaetano and his colleagues identify a neonatal line, a distinct growth ring that can be found in bones or teeth. It results from the strong growth-rate acceleration that occurs just after birth. The researchers realized this discovery could finally offer a way to test the universally accepted hypothesis that cynodonts laid eggs.

“We started trying to figure out how we could use this new information to test the working hypothesis, and we came up with this idea about using body size as a proxy,” Gaetano explained. 

Piecing together a reproductive puzzle

The researchers measured the neonatal line and the specimen’s outer surface to estimate its size and weight at birth and at death. They determined it weighed roughly 3.7 pounds (1.7 kilograms) at birth and 26.5 pounds (12 kg) when it died. That means the newborn C. theotonicus weighed 14% of the mass it reached by the time of its death.

The team then compared this newborn-adult body mass ratio to other groups of living animals to see which group C. theotonicus most resembled. Present-day reptiles such as turtles, snakes, and crocodiles that weigh between 17.6 pounds (8 kg) and 32 pounds (14.5 kg) tend to produce hatchlings that weigh between 9 and 53 grams. This translates to very low newborn-adult body mass ratios of roughly 0.1% and 0.6%. Larger birds like some cranes, pelicans, and vultures also exhibit low newborn-adult body mass ratios of around 1.3% to 4.5%.

Those groups clearly weren’t a good fit, but placental mammals—which make up about 95% of all living mammal species—have newborn-adult body mass ratios that closely align with that of C. theotonicus. Those that weigh between 17.6 pounds (8 kg) and 33.1 pounds (15 kg) can have newborns that weigh between 35.5 grams and 4.1 pounds (1.87 kg), which translates to newborn-adult body mass ratios as high as 18.77%.

Take the bay duiker antelope, for example. This African species gives birth to young that weigh roughly the same as a newborn C. theotonicus. 

“In cynodonts, embryonic tissues were never observed before, let alone a neonatal line,” Miceli Baro said in a statement. “Through its analysis, we found that a trait that is generally linked to evolutionary success was present in animals long before true mammals originated.”

While the researchers only studied a single cynodont species, Gaetano said his team’s findings align with previous studies showing that some features of present-day placental mammals—such as lactation, body hair, and brain size—were already appearing in relatives of C. theotonicus. Such traits are also linked to the genetic basis that underlies live birth, suggesting these changes may have emerged together.

“It is very probable that there was a general shift from egg laying to live birth in these cynodonts,” Gaetano said.

He and his colleagues hope to look for embryonic tissues in other specimens of these animals to try and replicate their findings. Their future work could finally reveal the emergence of mammalian reproduction as we know it today.

Reporting from Gizmodo contributed to this story.