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A pioneering 200-million-year-old analysis of teeth reveals that early mammals lived more like reptiles

Early mammalian teeth

Scientists count fossilized growth rings in teeth like tree rings to find out how long the first mammals lived. From left to right: Morganucodon reconstruction; Morganucodon tooth with cement, the structure that blocks the roots of the teeth to the gum, highlighted in green; growing continuously throughout life, the concrete settles every year like tree rings, highlighted with colored arrows; These were transformed into 3D models to count 1

4 years of life in the shrew-sized Morganucodon. Brief: Scientists count fossilized growth rings in teeth like tree rings to find out how long the first mammals lived. Credit: Nuria Melisa Morales Garcia. Morganucodon based on the model Bob Nicholls / Palaeocreations 2018

A pioneering analysis of 200-million-year-old teeth belonging to early mammals suggests that they functioned like their cold-blooded counterparts – reptiles, leading less active but much longer lives.

The research, led by Bristol University, United Kingdom and University of Helsinki, Finland, published today (12 October 2020) in Nature Communications, is the first time that paleontologists have been able to directly study the physiologies of early fossil mammals and reverses what was previously believed about our early ancestors.

Tooth fossils, about the size of a pinhead, from two of the earliest mammals, Morganucodon is Kuehneotherium, They were first scanned using powerful X-rays, shedding new light on the lifespan and evolution of these small mammals, which roamed the earth alongside the first dinosaurs and were believed to be warm-blooded by many scientists. This allowed the team to study the growth rings in dental orbits, deposited each year as tree rings, which could be counted to tell us how long these animals lived. The results indicated a maximum lifespan of up to 14 years, much older than their similarly sized furry successors like mice and shrews, which tend to only survive a year or two in the wild.

“We have made surprising and surprising discoveries. Key features of mammals, including their warm blood, were thought to have evolved around the same time, “said lead author Dr Elis Newham, research associate at the University of Bristol and earlier. PhD student at the University of Southampton at the time this study was conducted.

“On the contrary, our results clearly show that although they had larger brains and more advanced behavior, they did not live fast and died young, but led slower and longer lives, similar to that of small reptiles, such as lizards. . “

Using advanced imaging technology in this way was the brainchild of Dr. Newham’s supervisor, Dr. Pam Gill, Senior Research Associate at the University of Bristol and Scientific Associate at the Natural History Museum. of London, who was determined to get to the root of his potential.

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

By scanning the fossilized cement, the material that locks the roots of teeth into their socket in the gum and continues to grow throughout life, Dr. Gill hoped the conservation was clear enough to determine the mammal’s lifespan.

To test the theory, an ancient tooth specimen belonging to 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 cement is only a fraction of a millimeter thick, the scan image was so clear that the rings could literally be counted,” said Dr Corfe.

It marked the beginning of a six-year international study focusing on these early mammals, Morganucodon is Kuehneotherium, known from Jurassic rocks in South Wales, UK, dating back nearly 200 million years.

“The small 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 have been able to examine hundreds of individuals of one species, giving more confidence in the results than one might expect from such ancient fossils, ”added Dr Corfe.

The trip saw researchers take around 200 specimens of teeth, provided by the Natural History Museum in London and the University Museum of Zoology in Cambridge, to be scanned at the European Synchrotron (ESRF), the world’s brightest X-ray light source. and to Swiss Light Source, in France and Switzerland respectively.

Looking for an exciting project, Dr Newham took it for his Masters in Paleobiology at the University of Bristol, and then for a PhD from the University of Southampton.

“I was looking for something big to get my teeth into and this more than fits the bill. The scan alone took over a week and we ran 24-hour shifts to get it all done. It was an amazing experience and when the images started coming in, we knew we understood something, ”said Dr Newham.

Dr Newham was the first to analyze the concrete layers and grasp their enormous significance.

“We digitally reconstructed the roots of the teeth in 3-D and they proved it Morganucodon lived up to 14 years e Kuehneotherium up to nine years. I was amazed that these lifespans were much longer than we expected from one to three years for small mammals of the same size, “said Dr Newham.

“They were otherwise quite mammal-like in their skeletons, skulls and teeth. They had specialized chewing teeth, relatively large brains, and likely had hair, but their long lifespan shows that they lived life at a more reptilian pace than that of mammals. There is good evidence that mammalian ancestors began to become increasingly warm-blooded by the late Permian, more than 270 million years ago, but, even 70 million years later, our ancestors still functioned more as modern reptiles than mammals. “

While their pace of life remained reptilian, evidence of an intermediate ability for prolonged 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, which is critical for activities such as foraging and hunting.

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

Reference: 12 October 2020, Nature Communications.
DOI: 10.1038 / s41467-020-18898-4

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