X-rays reveal reptilian nature of ancient mammals

By Lauren Davis
Monday, 19 October, 2020

X-rays reveal reptilian nature of ancient mammals

X-ray analysis of 200 million-year-old teeth belonging to some of Earth’s earliest mammals suggests they functioned more like reptiles, leading less active but much longer lives than researchers had previously assumed.

Led by the University of Bristol and the University of Helsinki, a research team used powerful X-rays to scan fossils of teeth the size of a pinhead. This allowed the team to study growth rings in the tooth sockets, deposited every year like tree rings, which could be counted to tell us how long the animals lived. The work is said to be the first time palaeontologists have been able to study the physiologies of early fossil mammals directly, turning on its head what was previously believed about our early ancestors, and has been published in the journal Nature Communications.

Using advanced imaging technology in this way was the brainchild of Dr Pam Gill, a Senior Research Associate at the University of Bristol and Scientific Associate at London‘s Natural History Museum. She explained, “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.”

By scanning the fossilised cementum — the material which locks the tooth roots into their socket in the gum and continues growing throughout life — Dr Gill hoped the preservation would be clear enough to determine the mammal’s lifespan. To test her theory, an ancient tooth specimen belonging to the small mammal Morganucodon was sent to Dr Ian Corfe, from the University of Helsinki and the Geological Survey of Finland, who scanned it using high-powered synchrotron X-ray radiation.

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

It marked the start of a six-year international study focused on Morganucodon and Kuehneotherium, known from Jurassic rocks in South Wales dating back nearly 200 million years. These creatures roamed the Earth alongside early dinosaurs and were believed to be warm-blooded by many scientists.

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

The researchers took some 200 teeth specimens, provided by the Natural History Museum and University Museum of Zoology at the University of Cambridge, to be scanned at the European Synchrotron Radiation Facility (ESRF) in France, said to be the world’s brightest X-ray light source, as well as the Swiss Light Source. Dr Elis Newham, at the time a PhD student at the University of Southampton under Dr Gill’s supervision, was the first to analyse the cementum layers and pick up on their significance.

Synchrotron micro-CT scan of a fossil Morganucodon tooth root from 200 million years ago. Image credit: Elis Newham.

“We made some amazing and very surprising discoveries,” said Dr Newham, now a Research Associate at the University of Bristol. “It was thought the key characteristics of mammals, including their warm-bloodedness, evolved at around the same time.

“By contrast, our findings clearly show that, although they had bigger brains and more advanced behaviour, they didn’t live fast and die young but led a slower-paced, longer life akin to those of small reptiles, like lizards.

“We digitally reconstructed the tooth roots in 3D 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.

“They were otherwise quite mammal-like in their skeletons, skulls and teeth. They had specialised 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. There is good evidence that the ancestors of mammals began to become increasingly warm-blooded from the Late Permian, more than 270 million years ago, but even 70 million years later, our ancestors were still functioning more like modern reptiles than mammals.”

While their pace of life remained reptilian, evidence for an intermediate ability for sustained exercise was found in the bone tissue of these early mammals. As a living tissue, bone contains fat and blood vessels. The diameter of these blood vessels can reveal the maximum possible blood flow available to an animal, critical for activities such as foraging and hunting.

“We found that in the thigh bones of Morganucodon, the blood vessels had flow rates a little higher than in lizards of the same size, but much lower than in modern mammals,” Dr Newham said. “This suggests these early mammals were active for longer than small reptiles but could not live the energetic lifestyles of living mammals.”

Top image: Reconstruction of Morganucodon (left) and Kuehneotherium (right) hunting in Early Jurassic Wales 200 million years ago. Original painting by John Sibbick, 2013. ©Pam Gill.

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