Fossils of the inner ear canal reveal the evolutionary time of thermostatic animals

The ancestors of modern mammals evolved the ability to regulate their own body temperature to adapt to colder climates. Image credit: Luzia Soares

The timing of the evolution of reptile-like ancestors into thermostatic animals has been controversial. Recently, scientists have shown through fossils of the inner ear canal that this adaptation change occurred about 230 million to 200 million years ago. But other researchers argue that this evidence is unlikely to end the debate.

Thermostatic animals (warm-blooded animals) can maintain a constant high body temperature because they have a fast metabolism and can maintain body temperature through internal thermogenicity, while cold-blooded warming animals have a low metabolic rate and rely on the environment to keep warm. But these features are difficult to measure in fossils, so the researchers used skeletal features such as height and bone structure to infer metabolic rate.

Some past studies have shown that the thermostatic animal only appeared between 145 million and 66 million years ago, while others believe that it appeared as early as 300 million to 250 million years ago.

In a study published In Nature on July 20, the team of paleontologist Ricardo Araújo of the University of Lisbon in Portugal proposed that the shape and size of the inner ear bone tubes could be used as representatives of body temperature characteristics. The movement of the liquid through the in-ear duct helps the body monitor head position and movement, which is essential for vision and balance. Because the viscosity of the liquid changes with body temperature, the team hypothesized that as body temperature rises and animals become more active, the shape of the ear canal may evolve into a liquid with less viscosity to maintain balance and movement.

To track this adaptation, the team compared the inner ear structure and physiology of 50 extant vertebrate species, including reptiles, fish, birds and mammals. They developed a thermal movement index based on the shape of the inner ear that, when adjusted to body shape, was able to predict the body temperature of the animals.

The researchers analyzed the inner ear canals of 56 extinct species of inferior foraminifera (reptile-like, ancestors of mammals) and found through indexing that in the late Triassic period, from 237 million years ago to 201 million years ago, the shape of the inner ear canal changed suddenly. The researchers believe that this is the time when the inferior pores become thermostatics. This shift means that their body temperature rises by 5°C to 9°C and their metabolism accelerates.

Because the Triassic climate was colder than the previous Permian climate, the researchers believe that this adaptation gave early thermostatic animals an advantage in coping with the Triassic climate.

José Eduardo Bicudo, a comparative physiologist at the University of São Paulo in Brazil, believes that the team’s method is very novel and provides a new way to study the emergence of thermostatic animals.

The origin of thermostatic animals has been debated for the past 60 years, with multiple theories existing and supported by evidence. Research by biologists Roger Seymour and Bicudo at the University of Adelaide in Australia shows that thermostatic animals evolved during the Permian period, 300 million to 250 million years ago.

But Seymour questioned whether it was feasible for thermostatic animals to appear in the Late Triassic. He noted that the body temperature of early endothermic suborbitals (about 34°C) was still relatively low, similar to that of extant single-hole mammals such as platypus and echidnas.

Platypus and echidnas are slow to move compared to other mammals. Seymour speculates that animals with similar body temperatures may have had difficulty coexisting with other reptiles of the time, including two-legged, fast-moving ancestral dragons (ancestors of modern birds). (Source: China Science Daily Xin Yu)

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