200-Million-Year-Old Teeth Suggest First Mammals Lived Like Reptiles

A research team of European scientists analyzed 200 million-year-old teeth that belonged to some of the earliest mammals, suggesting that these animals lived much like reptiles.

Led by researchers from the University of Bristol in the UK and the University of Helsinki in Finland, the study marks the first direct observation of fossils belonging to early mammals. The research on the physiologies of these animals offer clues about their lifestyle and function.

Studying Tooth Fossils

The new study, published in Nature Communications on Monday, October 12, examines tooth fossils about the size of a pinhead. These teeth are from Morganucodon and Kuehneotherium, two species of mammaliaformes - a clade, or natural group, containing the first mammals. Researchers noted that these early mammals, who lived simultaneously as the first dinosaurs, were widely believed to be warm-blooded like most modern mammals.

Using the high-powered synchrotron X-ray tomographic imaging, researchers examined the growth rings located in their tooth sockets - similar to tree rings - deposited regularly and counted to provide an approximate age up until the animals died and the deposition stops. From this method, it was established that the owner of these fossilized teeth lived up to fourteen years. This is much longer compared to modern mammals of similar build to these mammaliaformes, like shrews and mice, which averages only a year or two in the wild.

Bristol senior research associate Dr. Pam Gill, also a Scientific Associate at the Natural History Museum London, proposed conducting X-ray imaging at the fossilized cementum, a material that holds tooth roots in place and develops throughout life. She hoped that the samples were preserved enough to provide clues about the animal's lifespan.

Leading Slower, Longer Lives

"We made some amazing and very surprising discoveries," said Dr. Elis Newham, lead author of the study and a research associate at the University of Bristol, in a press release from the university. He explained that the most common understanding about the early mammals is that their defining characteristics, like their warm-bloodedness, developed simultaneously.

In contrast, their findings showed that mammals, although equipped with larger brains and more advanced behavior, lived a life different from their modern counterparts - living fast and dying young. Instead, they led longer lives more similar to reptiles.

To further test their theories, another fossil tooth from a Morganucodon was sent to Dr. Ian Corfe of the University of Helsinki and Geological Survey of Finland. He conducted the synchrotron x-ray scan, noting that although the sample's cementum was "only a fraction of a millimeter thick," they were able to recover clear scans that allowed them to count the rings.

The press release noted that it marked a six-year international study, which started with the Morganucodon and Kuehneotherium samples dated back to almost 200 million years. Dr. Corfe explained that these small mammals fell into crevices in rocks where their skeletons fossilized, including their teeth.

"Thanks to the incredible preservation of these tiny fragments, we were able to examine hundreds of individuals of a species, giving greater confidence in the results than might be expected from fossils so old," Dr. Corfe noted.