There’s a wonderfully undone quality to Earth’s earliest mammals and their relatives. These Jurassic-era critters don’t look like the prototypes for today’s felids or canids so much as the beta versions, still entirely in development. They are primitive both literally and esthetically—snaggle of tooth and small of brain.
About 185 million years ago, or 25 million years after the first mammal birthed its first spawn, a hairy creature resembling a stocky, beagle-sized weasel produced 38 young of her very own. She probably didn’t actually give birth to them: As Live Science reports, Kayentatherium wellesi, as the animal is known, wasn’t quite a mammal. Instead, she was a cynodont—a mammal predecessor who likely reproduced much like a reptile.
In a new study published in the journal Nature, University of Texas researchers Eva Hoffman and Timothy Rowe describe a specific K. wellesi skeleton found in Arizona alongside her prodigious brood of “at least 38 individuals.” It’s not clear how they may have met their demise, or whether the babies had recently hatched or were still in their eggs.
Thirty-eight babies isn’t just a lot—it’s more than twice the average litter size of any living mammal. (There are modern near-equivalents: the tailless tenrec of Madagascar has litters of up to 32, while the average brood size of the naked mole rat is 28.)
Almost as striking as the size of the ancient litter was the relative smallness of their creature’s developing brains. Hoffman and Rowe believe that as mammals developed further from these antecedents, their brains grew as the number of offspring per litter shrank. “Just a few million years later, in mammals, they unquestionably had big brains, and they unquestionably had a small litter size,” Rowe said, in a statement.
Their research tells us a little more about a key turning point in the development of present-day lions, tigers, human beings, and bears. This is the only known instance of mammal precursor babies on record, and it tells us a lot about how the story fits together.
“These babies are from a really important point in the evolutionary tree,” Hoffman said. “They had a lot of features similar to modern mammals, features that are relevant in understanding mammalian evolution.”