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

Biology This 236-Million-Year-Old Fossil Just Rewrote the Story of Live Birth Scientists have long thought cynodonts, the ancestors of all modern mammals, laid eggs. For the first time, researchers have found evidence to the contrary. 13, 2026, am ET Reading time 3 minutes The skull of the Chiniquodon theotonicus individual that was used for this study. © Leandro Gaetano Read Later Read Later Comments (0) 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 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.