/ Modified sep 5, 2018 3:20 p.m.

Baby Reptile Fossils in Arizona Fill Gap in Evolution of Mammal Childbirth

The unique fossils can help understand the links between mammals and their non-mammal ancestors.

reptile-kayentatherium-and-mammal-babies Skull CT scans of, left to right, a modern tuatara hatchling, Kayentatherium offspring, and a 27-day-old opossum, shown at the same magnification.
Eva Hoffman, UT Austin

Family tree VIEW LARGER The fossils discovered by the researchers belong to Kayentatherium, an extinct mammal relative that lived during the Early Jurassic.

A unique collection of 38 fossilized baby reptiles found in northern Arizona could help fill in the evolutionary gap from reptilian to mammalian childbirth.

The mammalian family tree is rooted in reptiles like Kayentatherium — beagle-sized plant-eaters that lived roughly 185 million years ago, which share certain features with mammals.

But little is known about how mammals went from having large numbers of offspring, like reptiles do, to birthing small litters of large-brained babies requiring parental care.

Eva Hoffman, who is now pursuing a doctoral degree at Harvard University, was lead author on the University of Texas at Austin study, which appeared in the journal Nature. She said the new find fills in a key moment in that transition.

kayentatherium_adult_vertebra VIEW LARGER CT scan of part of the specimen showing a maternal vertebra (gray) with bones of the babies (colors) in their original positions. The sediment is rendered transparent.
Eva Hoffman, UT Austin

"At this point in mammalian evolution, the new specimen shows us that mammal ancestors were still producing very many offspring; there's still a primitive form of reproduction."

Further support for that conclusion came from the shapes of the young skulls, which showed no sign they had contained larger brains.

The fossils, which were found near Tuba City on Navajo lands, preserve the only known baby remains from any mammal precursor.

Fronteras Desk
This story is from the Fronteras Desk, a collaboration of Southwestern public radio stations, including NPR 89.1. Read more from the Fronteras Desk.
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