An artist’s rendering of Krusatodon kirtlingtonesis adult and juvenile© Maija Karala
Two new fossils of a mouse-sized animal from the age of dinosaurs indicate that early mammals grew more slowly and lived longer than mammals today.
The international study, led by researchers at National Museums Scotland and published today in the journal Nature, compares one adult and one juvenile fossil from the same mammal species, Krusatodon kirtlingtonensis, that lived 166 million years ago.
The specimens were discovered decades apart in Scotland’s Isle of Skye. The adult was excavated in the 1970s, and the juvenile, the only juvenile Jurassic mammal skeleton known to science, was discovered in 2016 by Roger Benson, the Museum’s Macaulay Curator in the Division of Paleontology, and colleagues.
“When we found the tiny juvenile skull, I didn’t realize what we’d found right away. The part of the fossil that was sticking out of the rock was blasted by erosion, surrounded by barnacles, and looked just like a piece of ash,” Benson said. Using micro-CT scanning, a form of 3D x-ray imaging, Benson was surprised to see a whole skull in the rock.
“Even in the context of the amazing paleontological finds on Skye in recent years, these fossils are remarkable,” said Stig Walsh, senior curator of vertebrate paleobiology at National Museums Scotland and co-author on the study. “Mammal fossils of this age are exceptionally rare worldwide, and most are just single teeth found by sieving sediment. To find two such rare fossil skeletons of the same species at different growth stages has rewritten our understanding of the lives of the very earliest mammals.”
The research team used X-ray imaging at the European Synchrotron Radiation Facility to count the growth rings in the teeth of both specimens, revealing their ages when they died. The adult Krusatodon was about seven years old, and the juvenile was between one to two years old, and still in the process of replacing its baby teeth.
Today, small mammals have much shorter lifespans, with some living as little as 12 months. They also typically mature quickly, losing their baby teeth and weaning within months of birth.
“Adult Krusatodon weighed 100 grams (about 3.5 ounces) or so, similar to a hamster, but it grew much more slowly,” Benson said. “In fact, it grew slower than any small mammal alive today.”
The Krusatodon fossils suggest that the fundamental shift in the growth patterns and life expectancy of mammals must have taken place during or after the Middle Jurassic, between about 174 and 161 million years ago.
“These fossils are among the most complete mammals from this time period in the world,” said Elsa Panciroli, the lead author of the study and an associate researcher of paleobiology at National Museums Scotland. “They give us unprecedented insights into the lives of the first mammals in the time of dinosaurs.”