In Prehistoric Fossils, a Surprising Find on Aging

Two sets of fossils found 40-something years apart suggest that small mammals who lived during the mid-Jurassic period had a longer "childhood" and a more stretched-out life span than their contemporary counterparts. The new research published Wednesday in the journal Nature examined the fossilized remains of Krusatodon kirtlingtonensis, a long-vanished mouselike creature that lived on Scotland's Isle of Skye about 166 million years ago. The New York Times notes the prehistoric animal "resembled a pint-size possum and weighed less than a hockey puck."

What made the fossils especially interesting was that one set, unearthed in the 1970s, was of an adult version of the animal, while the second set, discovered in 2016, was of a juvenile. "It meant that we could start asking questions we couldn't have dreamt of with just one specimen," paleontologist and study co-author Elsa Panciroli tells New Scientist. After examining the specimens' teeth using X-ray imaging, the researchers discovered that the adult had been about 7 years old at the time of its death—"a ripe old age compared with living mammals of similar size," per the Times—while the youngster was between 7 months and 2 years old. The latter's age was a surprise to the researchers, who, based on the presence of baby teeth in the specimen, thought it was much younger.

"We would expect it to be replacing its teeth much earlier—within weeks or months," says Panciroli, per New Scientist. "So we knew immediately that it must have grown very differently [than modern species]." Researchers figure it took K. kirtlingtonensis about two years to wean off its mother, in comparison with the mere weeks it typically takes today's small mammals. Bone analysis also indicated that this species keeps growing throughout its life, as opposed to small mammals like today's mice, whose growth halts once they reach adult status. Panciroli and her team aren't sure what may have happened in the evolutionary timeline to account for this shift, though they speculate that environmental changes or altered metabolisms may have something to do with it. (More discoveries stories.)