For a long time, most mammals lived in the shadow of the dinosaurs. Fossil teeth suggest that they also maintained a decelerated lifestyle.

Although their great time only began after the extinction of the dinosaurs 66 million years ago, the first real mammals are much older and lived on Earth at least 200 million years ago. According to a study by Elis Newham of the University of Bristol and his team published in Nature Communications, however, their lifestyle was more like reptiles, at least initially. The paleontologist and his colleagues have studied fossil teeth of then living mammals morganucodon and kuehneotherium in order to learn more about the evolution and development of these animals.

With the help of X-rays, for example, they were able to examine annual rings in the dentures of the fossils and derive the lifespan of the animals from it. The results suggest that they lived up to 14 years old, which is much older than their small size might suggest. Today’s rodents or shrews with comparable dimensions of morganucodone and kuehneotherium usually live only one to two years, even if they die of natural death.

“Our study shows that the mammals at the time had a larger brain and more complex behavior than similarly sized reptiles. Nevertheless, they did not live a faster, warm-blooded life and did not die sooner. Instead, they lived a slow and longer life similar to lizards, for example,” says Newham.

For their study, paleontologists were able to study hundreds of specimens that fell and petrified in crevices and fissures 200 million years ago during the Jurassic period in what is now south Wales. Overall, the animals already had many characteristics of modern mammals, such as a specialized denture and a comparatively large brain, and they were most likely already hairy. However, structures in the thigh bones suggest that the blood flow in their vessels was faster than in lizards of the same size, but much slower than in today’s rodents.