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The mystery of the evolution of warm-blooded mammals may finally be solved

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Approximately 230 million years ago, when the first dinosaurs stood on their feet, the ancestors of modern mammals appeared. Somewhere along the way, they developed a remarkable ability: to generate their own heat.

This decisive evolutionary step towards endothermy the ability to generate heat from within and maintain an almost constant core body temperature even with fluctuations in ambient temperature has since allowed this diverse class of animals to thrive in a variety of environments around the world.

But exactly when warm-bloodedness, or endothermy, first developed in animals has remained a big mystery to evolutionary biologists. until now.

A new study led by an international team of scientists led by University of Lisbon paleontologist Ricardo Araujo has found evidence that endothermy originated some 233 million years ago, during the Late Triassic, the geological epoch that marked the age of the dinosaurs.

This evidence was not found in blood, but in the fossilized inner ears of ancient mammal ancestors.

While the inner ear may seem like an unlikely place to look for clues about body temperature, it was actually a logical step after researchers realized that body temperature affects the viscosity, or fluidity, of the fluid that flows around the tiny semicircular canals of the ear canal. inner ear.

The primary function of these loopy, fluid-filled structures in the inner ear is to help detect head movements, which is essential for balance, vision, and coordinated movement.

“Until now, the semicircular canals have been commonly used to predict the movement of fossil organisms,” explains Romain David, author of the study and a paleontologist specializing in the biomechanics of the auditory canals at the Natural History Museum in London.

“However, after carefully studying their biomechanics, we realized that we could also use them to determine body temperature.”

Several different approaches have been used in the past to determine exactly when endothermy likely developed in ancient mammals and birds. But those studies that have tried to link metabolic rate, oxygen use, and traces of body hair to average body temperature have come up with vague or inconsistent results, the researchers say.

They are confident enough in their new method of analyzing the size and shape of the bony and soft tissues of the inner ear to infer whether animals were hot or cold, confirming it in more than 360 living and extinct vertebrates, before returning to the fossil record. .

These preliminary analyzes indicated that the internal ear canals of animals with persistently high body temperatures, such as mammals, must have changed shape in order to continue to function normally with liquid fluids.

This means that the structure of the inner ear can be used as an accurate indication of when endothermia developed.

Indeed, when the researchers analyzed the fossils of a group of 56 extinct species that gave rise to mammals, they noticed that these ancient animals had smaller and narrower canals than cold-blooded creatures of the same size.

Internal auditory canals of ancient warm-blooded (left) and cold-blooded (right) animals. (David and Araujo)

The researchers found that these changes in the structures of the inner ear were sudden and correlated with a sharp increase in body temperature of about 5-9 degrees Celsius (9-16 degrees Fahrenheit).

Modeling that traces changes in fossilized ears over time suggests that endothermy evolved much later and therefore faster than paleontologists thought — in about less than a million years.

It is possible that these ancestors grew fur at a time when their metabolism was switching to maintaining higher body temperatures at a time when the Triassic climate was rapidly cooling.

“Endotherapy, as an essential physiological characteristic, is combined with other mammalian hallmarks that emerged during this period of climatic instability.” Araujo and his colleagues write in their article.

“This was not a gradual, slow process over tens of millions of years as previously thought, but it may have been achieved quickly when driven by new mammalian-like metabolic pathways and the origin of fur,” adds Araujo in a press release.

While the life we ​​see on Earth today shows how beneficial the evolution of warm-bloodedness has always been for birds and mammals, this is hardly the only reason endotherms have come to dominate ecology.

The study by Araujo, Angelchik and their colleagues echoes another study published in Nature earlier this year, who used some equally inventive methods to conclude that most dinosaurs were not ectotherms like the modern reptiles they look like, but warm-blooded animals like birds and mammals.

What’s interesting about this discovery by Yale University molecular paleobiologist Jasmina Wiemann and colleagues is that it seems to rule out another long-standing hypothesis that the warm-bloodedness of birds and mammals somehow helped their ancestors survive the mass extinction in the end of the Cretaceous period, which crushed most of the dinosaurs.

So, as is often the case in paleontology, when potential answers to one mystery are uncovered, another plot emerges.

A new study has been published in Nature.

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