An international team of scientists has uncovered the exact time of the appearance of mammalian warm-blooded ancestors by studying the anatomy of the inner ear in extinct animals. According to the conclusions of experts, endothermy in synapsids first arose in the late Triassic period, that is, much later than paleontologists thought. The results of the study are published in the journal Nature.

Experts from the University of Lisbon (Portugal), the Natural History Museum in London (UK), the Field Museum of Natural History in the USA and the Brandenburg University of Technology Cottbus in Germany analyzed the evolutionary changes in the structure of the semicircular canals in the inner ear of 56 species of extinct mammalian ancestors.

These cavities, which are part of the vestibular apparatus, are filled with endolymph, a fluid that helps maintain balance and whose viscosity depends on body temperature: the higher the temperature, the lower the viscosity. Thus, in order to ensure the correct functioning of the vestibular apparatus at elevated body temperature, the size of the canals in warm-blooded animals had to decrease.

For each fossil specimen, the scientists calculated the thermo-motility index, which takes into account both the shape of the semicircular canals and the size of the animal. Although the semicircular canals do not leave fossils, their shape can be determined from the bony labyrinth of the inner ear in the skull, with correlations previously calculated on a sample of 50 bony labyrinths of living animals. Paleontologists then scanned the inner ear bones of 362 specimens, including 68 extinct synapsids.

Until now, warm-bloodedness was thought to have originated about 252 million years ago in a group of mammalian ancestors called cynodonts. Traditionally, cynodonts are depicted as furry animals similar to modern small mammals, but in the new work, scientists have found that significant changes in the shape of the channels associated with the development of endothermy occurred much later, 233 million years ago. It was at this time that the climate became unstable, and synapsids developed such characteristic features of mammals as whiskers and fur, which was accompanied by an increase in body temperature by 5-9 degrees Celsius.

Before the advent of warm-bloodedness, synapsids regulated body temperature by developing a wide range of behavioral strategies that are characteristic of modern ectothermic animals. The way of life of synapsids, which lived in the equatorial regions, could resemble geckos active at night. At higher latitudes, under adverse conditions, the ancestors of mammals slowed down their metabolism and fell into a stupor. Warm-bloodedness reduced the dependence of animals on the external environment and gave them the opportunity to maintain high activity for a long time.