Ancient Tiny Teeth Reveal First Mammals Lived More Like Reptiles

Scientists count fossilized growth rings in teeth like tree-rings to find out how long the earliest mammals lived. (Credit: Nuria Melisa Morales Garcia)

(CN) — Using an examination of their fossilized teeth, scientists on Monday revealed the world’s earliest mammals were seemingly more reptile-like, upending what paleontologists believed about their lifespan and activity levels.

In a study published in the journal Nature Communications, researchers from the University of Bristol in the United Kingdom and University of Helsinki in Finland examined the 200 million-year-old teeth of the morganucodon and kuehneotherium, two ancient mammal species.

Using X-rays, the research team was able to look at growth rings in the animals’ tooth sockets, allowing them to determine how long each creature lived, much like the rings of a tree. What they discovered was a maximum age of 14 years, a relatively long lifespan for such small mammals and a finding that goes against the conventional thinking that such creatures lived active and short lives.

“We made some amazing and very surprising discoveries. It was thought the key characteristics of mammals, including their warm-bloodedness, evolved at around the same time,” said lead author Elis Newham, research associate at the University of Bristol, in a statement. “By contrast, our findings clearly show that although they had bigger brains and more advanced behavior, they didn’t live fast and die young but led a slower-paced, longer life akin to those of small reptiles like lizards.”

The idea of using powerful X-rays to determine the age of ancient creatures came from Pam Gill, senior research associate at the University of Bristol.

“A colleague, one of the co-authors, had a tooth removed and told me they wanted to get it X-rayed, because it can tell all sorts of things about your life history. That got me wondering whether we could do the same to learn more about ancient mammals,” Gill said.

The team got to work on what would become a six-year international study to uncover the lives of the ancient mammals. They scanned fossilized cementum, which keeps tooth roots in place in the gum. Using X-rays, they soon discovered they could easily count the rings within the cementum.

“To our delight, although the cementum is only a fraction of a millimeter thick, the image from the scan was so clear the rings could literally be counted,” said Ian Corfe of the University of Helsinki.

The researchers gathered the fossils from Jurassic period rocks in South Wales, almost 200 million years ago.

“The little mammals fell into caves and holes in the rock, where their skeletons, including their teeth, fossilized. 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,” Corfe said.

In all, the research team scanned about 200 teeth.

“We digitally reconstructed the tooth roots in 3-D, and these showed that morganucodon lived for up to 14 years, and kuehneotherium for up to nine years. I was dumbfounded as these lifespans were much longer than the one to three years we anticipated for tiny mammals of the same size,” Newham said.

Newham added that the creatures “were otherwise quite mammal-like in their skeletons, skulls and teeth.” 

“They had specialized chewing teeth, relatively large brains and probably had hair, but their long lifespan shows they were living life at more of a reptilian pace than a mammalian one,” he said.

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