When mammals were like lounge lizards

The first mammals were rather reptilian in their ways, leading less active but much longer lives than now, a new study suggests.

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

A research team led by the University of Bristol, UK, and Finland’s University of Helsinki used advanced imaging technology to analyse 200-million-year-old fossils of teeth barely the size of a pinhead.

They came from Morganucodon and Kuehneotherium, two early mammals known from Jurassic rocks in South Wales.

Counting the growth rings in their tooth sockets indicated a lifespan of up to 14 years – much longer than for similarly sized mammals such as mice and shrews, which tend to only survive a year or two in the wild.

“It was thought the key characteristics of mammals, including their warm-bloodedness, evolved at around the same time,” says Bristol’s Elis Newham, lead author of a paper in the journal Nature Communications.

“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.”

The research ran for six years and saw the researchers analyse 200 teeth specimens from the Natural History Museum London and University Museum of Zoology Cambridge, using two of the world’s brightest X-ray light sources – the European Synchrotron Radiation Facility in France and the Swiss Light Source.

And it all began with a whim. “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,” says Bristol’s Pam Gill. “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, Gill hoped the preservation would be clear enough to determine the mammal’s lifespan.

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Painting by John Sibbick of Morganucodon (left) and Kuehneotherium hunting in Early Jurassic Wales. Credit: Pam Gill

To test the theory, a specimen from Morganucodon was sent to 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,” Corfe says.

Digital 3D reconstructions show Morganucodon and Kuehneotherium were mammal-like in their skeletons, skulls and teeth, the researchers say, with special chewing teeth, relatively large brains and probably hair. However, they were living life at more of a reptilian pace.

“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,” Newham says.

There was, however, evidence for an intermediate ability for sustained exercise in the bone tissue of these early mammals.

“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. This suggests these early mammals were active for longer than small reptiles but could not live the energetic lifestyles of living mammals.”

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