The mystery of warm-blooded mammal evolution may finally be solved

When the first dinosaurs found their feet about 230 million years ago, the ancestors of modern mammals also appeared. Somewhere along the way they developed a remarkable ability: generate their own warmth.

This is a crucial evolutionary step towards heat absorption The ability to generate heat from within and maintain a near-constant core body temperature even when ambient temperatures fluctuate Since then, it has enabled this diverse class of animals to thrive in many environments around the world.

But exactly when warm-blooded, or endothermic, first evolved in animals, this has remained a formidable mystery to evolutionary biologists. Until now.

A new study from an international team of scientists led by University of Lisbon paleontologist Ricardo Araujo has found evidence that heat absorption arose about 233 million years ago, during the late Triassic, the geological age that heralded the age of the dinosaurs.

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

Although the inner ear may seem 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 flow of fluid that flows around the small semi-circular channels in the aqueduct . inner ear.

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

“Until now, semicircular canals were generally used to predict the movement of fossil organisms,” explains Roman David, study author and paleontologist specializing in the biomechanics of ear canals at the Natural History Museum in London.

“However, by looking carefully at their biomechanics, we concluded that we could also use them to infer body temperatures.”

A few different methods have been used, in the past, to determine when endothermic likely evolved in ancient mammals and birds. But these studies, which attempted to link metabolic rate, oxygen use, and the effects of body hair to average body temperatures, yielded ambiguous or conflicting results, the researchers say.

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

Those preliminary analyzes showed that the inner ear canals of hyperthermic animals, such as mammals, had to change their shape to keep them functioning properly with runoff fluids.

This means that the structure of the inner ear can be used as an accurate guide to the history of endothermic evolution.

In fact, when researchers analyzed fossils from a group of 56 extinct species from which mammals emerged, they noticed that these ancient animals had smaller and narrower ducts than similarly sized, cold-blooded creatures.

Illustration showing differences in the shape of the inner ear canals.The inner ear 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 associated with a sharp increase in body temperature of about 5 to 9 degrees Celsius (9 to 16 degrees Fahrenheit).

Simulations to track changes in fossilized ears over time suggest endotherms evolved much later, and therefore faster, than paleontologists had thought — in roughly less than a million years.

These ancestors were probably growing their fur at the same time their metabolism shifted gears to maintain higher body temperatures at a time when the Triassic climate was rapidly cooling.

“Internal therapy, as an important physiological property, joins with other distinguishing features of mammals that arose during this period of climatic instability,” Araujo and colleagues wrote in their paper.

“It was not a gradual and slow process over tens of millions of years as previously thought, but may have been rapidly achieved when it was triggered by new metabolic pathways similar to mammals and the origin of fur,” Araujo adds in the press release.

While the life we ​​see on Earth today shows how beneficial warm-blooded evolution is to birds and mammals, it is unlikely to be the only cause of warming to ecological dominance.

The research by Araujo, Angelcic and colleagues is similar to another study published in temper nature Earlier this year I used some similar innovative methods to conclude that most dinosaurs were not ectotherms like the modern reptiles they resemble but warm-blooded animals like birds and mammals.

What’s interesting about this discovery by Yale paleontologist Yasmina Wiemann and colleagues is that it seems to rule out another long-standing hypothesis that warm-blooded birds and mammals somehow helped their ancestors survive the late Cretaceous mass extinction, which wiped out on most dinosaurs.

Therefore, as often happens in paleontology, when possible answers to one riddle are revealed, another plot thickens.

The new study was published in temper nature.

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