Mystery of the evolution of warm-blooded mammals may finally be solved

(ORDO NEWS) — When the first dinosaurs stood on their feet about 230 million years ago, so did the ancestors of modern mammals. Somewhere along the way, they developed an amazing ability: to generate their own heat.

This crucial evolutionary step towards endothermy – the ability to generate heat from within and maintain a virtually constant body temperature even with fluctuating ambient temperatures – has 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 been a big mystery to evolutionary biologists until now.

A new study 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 petrified inner ears of ancient mammalian ancestors.

While the inner ear may seem like an unlikely place to look for information 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 through the tiny semicircular canals of the inner ear.

The main task of these winding, fluid-filled structures of the inner ear is to help determine the movement of the head, which is necessary for balance, vision, and coordinated movements.

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

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

Many different approaches have been used in the past to determine exactly when endothermy likely developed in ancient mammals and birds. But these studies, which have attempted to link metabolic rate, oxygen consumption and traces of body hair to average body temperature, have produced unclear 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 are hot or cold by testing it on more than 360 living and extinct vertebrates before returning to fossils.

These preliminary analyzes showed that the inner ear canals of animals with sustained high body temperatures, such as mammals, had to change shape in order to function normally in colder fluids.

This means that the structure of the inner ear can be used as an accurate guide to determine the timing of endothermia.

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

Mystery of the evolution of warm blooded mammals may finally be solved 2

These changes in the structures of the inner ear were abrupt, the researchers found, and correlated with a dramatic rise in body temperature of about 5 to 9 degrees Celsius (9 to 16 degrees Fahrenheit).

Modeling of 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 acquired fur at the same time that their metabolism switched to maintaining a higher body temperature at a time when the climate of the Triassic period cooled rapidly.

“The endotherium, as an essential physiological characteristic, joins other mammalian distinctive features that emerged during this period of climatic instability,” Araujo and colleagues write in their paper.

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

Although the life that we see on Earth today shows how beneficial the evolution of warm-bloodedness has been for birds and mammals, it is unlikely that it was the only reason for the ecological dominance of endotherms.

The study by Araujo, Angelcic and colleagues echoes another study published in the journal Nature earlier this year that used equally ingenious methods to conclude that most dinosaurs were not ectotherms, like the modern reptiles they resemble, but warm-blooded animals such as birds and mammals.

What’s interesting about this finding by Yale University molecular paleobiologist Jasmina Wiemann and her 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 at the end of the Cretaceous period, which wiped out most of the dinosaurs.

So, as is often the case in paleontology, as potential answers to one mystery are uncovered, another plot becomes more and more confusing.


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