(Paris) The ancestors of mammals became warm-blooded some twenty million years later than previously estimated, according to a study by researchers who analyzed inner ear fossils to solve “one of the greatest mysteries of paleontology “.
Posted at 12:04 p.m.
The timing of this transition, to an essential feature of mammals, remains a matter of debate. They would have become endothermic, that is to say warm-blooded, and no longer cold as reptiles remained, about 252 million years ago. At the time of one of the great extinctions of species, in the Permian.
“The problem is that you can’t just put a thermometer in a fossil, you can’t know its body temperature,” says Ricardo Araujo, from the University of Lisbon, one of the authors of the study published in Nature.
The researchers got around the problem by examining the semicircular canals of the inner ear fossils of 56 species of extinct mammals.
The inner ear, which is used to maintain balance, is traversed by a fluid, which the researchers found that its temperature rose in concert with that of the animal’s body. With the consequence of making this fluid more liquid.
Ricardo Araujo illustrates this by comparing it to the oil in a fryer, “very viscous and very dense” before being heated, and which afterwards “flows much more easily”, he explains to AFP.
In the case of the inner ear, this fluidity has led to finer semicircular canals, whose size evolution according to that of the body has been modeled. With the conclusion that the transition to a warm-blooded system occurred 233 million years ago.
A model that works for all mammals, including humans, which “allows you to look at your inner ear and tell you how warm-blooded you are”, explains to AFP the main author of the study, Romain David, of the Muséum of natural history in London.
Free yourself from the vagaries of the weather
The model “appears to work well for a wide range of contemporary vertebrates,” says University of Bristol paleontologist Michael Benton, who was not involved in the study.
“It doesn’t simply provide a yes/no answer, but determines the ‘degree’ of endothermy in terms of baseline body temperature,” he told AFP.
Mr Benton, whose research had previously determined the appearance of warm-blooded mammals 252 million years ago, says this transition happened in stages. With “significant advances” preceding the change in architecture of the inner ear.
On the other hand, for Mr. Araujo, the transition to a warm-blooded system took place “very quickly in geological terms, less than a million years”, he argues, excluding “a slow and gradual process over tens of millions of years, as previously thought”.
Another argument against a transition 252 million years ago is that the very high temperatures at that time, around 31 degrees Celsius, would not have provided an advantage to warm-blooded animals. On the other hand, they would have benefited from the drop in temperatures that followed, over millions of years.
“Being an endotherm makes it easier to overcome climatic hazards, to run faster, longer, to explore new habitats, to explore the night and the polar regions, to make long migrations” , according to Mr. Araujo.
So many things that have shaped what a mammal is “and also, ultimately, what a human being would be”, sums up the researcher.