Ancient tiny teeth reveal first mammals lived more like reptiles

Date:
October 12, 2020
Source:
University of Bristol
Summary:
Pioneering analysis of 200 million-year-old teeth belonging to the
earliest mammals suggests they functioned like their cold-blooded
counterparts - reptiles, leading less active but much longer lives.



FULL STORY ========================================================================== Pioneering analysis of 200 million-year-old teeth belonging to the
earliest mammals suggests they functioned like their cold-blooded
counterparts - - reptiles, leading less active but much longer lives.


==========================================================================
The research, led by the University of Bristol, UK and University of
Helsinki, Finland, published today in Nature Communications, is the
first time palaeontologists have been able to study the physiologies of
early fossil mammals directly, and turns on its head what was previously believed about our earliest ancestors.

Fossils of teeth, the size of a pinhead, from two of the earliest mammals, Morganucodon and Kuehneotherium, were scanned for the first time using
powerful X-rays, shedding new light on the lifespan and evolution of
these small mammals, which roamed the earth alongside early dinosaurs
and were believed to be warm-blooded by many scientists. This allowed
the team to study growth rings in their tooth sockets, deposited every
year like 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 such as mice
and shrews, which tend to only survive a year or two in the wild.

"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 Dr Elis Newham,
Research Associate at the University of Bristol, and previously PhD
student at the University of Southampton during the time when this study
was conducted.

"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." 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 London, who was determined to get to the root of
its potential.



==========================================================================
"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," Dr Gill said.

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 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 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, which focused on
these first mammals, Morganucodon and Kuehneotherium, known from Jurassic
rocks in South Wales, UK, dating back nearly 200 million years.



==========================================================================
"The little mammals fell into caves and holes in the rock, where their skeletons, including their teeth, fossilised. 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 added.

The journey saw the researchers take some 200 teeth specimens, provided
by the Natural History Museum London and University Museum of Zoology Cambridge, to be scanned at the European Synchrotron Radiation Facility
and the Swiss Light Source, among the world's brightest X-ray light
sources, in France and Switzerland, respectively.

In search of an exciting project, Dr Newham took this up for the MSc
in Palaeobiology at the University of Bristol, and then a PhD at the
University of Southampton.

"I was looking for something big to get my teeth into and this more than
fitted the bill. The scanning alone took over a week and we ran 24-hour
shifts to get it all done. It was an extraordinary experience, and when
the images started coming through, we knew we were onto something,"
Dr Newham said.

Dr Newham was the first to analyse the cementum layers and pick up on
their huge significance.

"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," Dr
Newham said.

"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.

Dr Newham said: "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."

========================================================================== Story Source: Materials provided by University_of_Bristol. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Elis Newham, Pamela G. Gill, Philippa Brewer, Michael J. Benton,
Vincent
Fernandez, Neil J. Gostling, David Haberthu"r, Jukka Jernvall,
Tuomas Kankaanpa"a", Aki Kallonen, Charles Navarro, Alexandra
Pacureanu, Kelly Richards, Kate Robson Brown, Philipp Schneider,
Heikki Suhonen, Paul Tafforeau, Katherine A. Williams, Berit
Zeller-Plumhoff, Ian J. Corfe.

Reptile-like physiology in Early Jurassic stem-mammals. Nature
Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-18898-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201012103146.htm

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