A long-extinct, dog-size predator may have been giving birth in a way that changes an important part of the narrative of mammals, reports Live Science. A new analysis of a 236-million-year-old fossil from Argentina suggests Chiniquodon theotonicus, an early mammal relative known as a cynodont, delivered live young—pushing the origins of live birth in our lineage back by as much as 90 million to 95 million years, according to the study in Frontiers in Mammal Science.
Researchers identified a "neonatal line" in one limb bone, a growth marker seen in modern animals right after birth. The discovery came when study co-author Maria Miceli Baro of the University of Buenos Aires was studying an adult C. theotonicus specimen. "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 of the university tells Gizmodo.
Using the neonatal line and bone measurements, they estimated the animal weighed about 3.7 pounds at birth and 26 pounds as an adult, meaning newborns were roughly 14% of adult size. That's in line with similarly sized mammals today, and far larger than typical hatchlings of birds and reptiles, pointing to live birth rather than eggs. It's not definitive, outside experts caution, but it fits with other signs that classic "mammal" traits—from hair and whiskers to lactation—were already emerging in these Triassic ancestors, which thrived after the planet's worst mass extinction.
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