Careful examination of the inner ear canals of hundreds of modern and fossil animals has determined that endothermy arose abruptly in late Triassic mammals.
An abrupt evolution
So far, the analysis of the skeletons of different mammals and their environment suggested that the transition endothermique spanned a period of about 120 million years. In the context of work published in the journal NatureRicardo Araújo of the University of Lisbon and his colleagues decided toexamine the structure of the inner ear of many modern and extinct animal species in order to establish a more precise chronology of events.
The team focused on the fluid-filled semicircular canals, helping the animals maintain balance and orient themselves. Because temperature influences fluid behavior, the shape of the inner ear of warm-blooded creatures was theoretically to be different from that of their cold-blooded ancestors.
Analysis by X-ray microtomography such structures in hundreds of modern vertebrates (mammals, reptiles, birds, amphibians and fish), as well as about 60 extinct species revealed that the inner ear canals of warm-blooded mammals were more circular, and proportionally smaller and thinner relative to their sizecompared to those of reptiles, amphibians and cold-blooded fish.
Armed with this knowledge, the researchers examined the inner ear canals of ancient specimens, spanning several hundred million years, and determined that the mammalian endothermic transition began 233 million years ago and lasted approximately a million years. ” It was a very brutal evolution, and not progressive, as was long believed. “, emphasizes Romain David, of the Natural History Museum of London.
Warm blood would have allowed mammals to venture into colder regions
This period corresponds to the appearance in the fossil record of mammaliamorphes, ancestors of mammals and the first known creatures to possess body hair and whiskers. According to the authors of the new study, warm blood would have allowed these creatures to venture into colder regions, and also to remain active for long periods of time.
With such a feature implying increased energy expenditure in order to maintain a certain body temperature, it would potentially explain why many species, typically living in environments with scarce food and high outside temperatures, have remained cold-blooded.
Warm-blooded descendants of reptiles, birds evolved endothermy separately from mammals, implying differences in the structure of the inner ear. According to the researchers, their channels do not necessarily show the same rates of variation, due to the different chemical composition of the fluids they contain.