"Kayentatherium Skull" by Smokeybjb / CC BY-SA 3.0

How the earliest mammals thrived alongside dinosaurs

Oct 24, 2019

By John Pickrell

Night is falling in the early Jurassic 185 million years ago, and the Kayentatherium is tending to her newly hatched brood. Heavy rains pummel the bank above her den as she looks over her dozens of tiny young. She is about the size of a large cat and could easily pass for a mammal, but her large jawbone, characteristic teeth and lack of external ears give her away: she is a cynodont, a member of the group from which mammals evolved. At some point without warning, the sodden bank collapses, entombing the hatchlings and their mother in mud.

There they remained until the summer of 2000, when a fossil-hunting crew led by Timothy Rowe at the University of Texas at Austin chanced upon their scattered bones among rocks of the Kayenta Formation in northern Arizona.

That initial encounter with the fossils did little to impress the palaeontologists. They dug up the block and shipped it back to the laboratory for safekeeping. It wasn’t until nine years later that a specialist preparing the fossil for study noticed something startling: embedded in the block were tiny teeth, and jawbones just 1 centimetre in length. “Immediately they stopped the preparation and thought about ways of non-destructively examining the babies,” says Eva Hoffman, at Texas with Rowe at the time and now a palaeontologist at the American Museum of Natural History in New York City. Instead of breaking into the rock, Hoffman and Rowe digitally extracted the bones with a microcomputed tomography (microCT) scanner, which uses X-rays to create fine-grained 3D images.

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