The reptile-like ancestors of mammals evolved to be warm-blooded — but the timing of this transition is hotly contested. Now scientists have used fossilized inner ear canals to suggest that the adaptation occurred around 230 million to 200 million years ago1. But other researchers say this evidence is unlikely to settle the debate.
Warm-blooded animals, or endotherms, can maintain a constant high body temperature because of their fast metabolisms, whereas cold-blooded ectotherms have a low metabolic rate and rely on the environment to keep warm. But it is difficult to measure these traits in fossils, so researchers have used skeletal features such as height and bone structure to infer metabolic rate. Some past studies have suggested that endothermy emerged as recently as 145 million to 66 million years ago2; other research has put it as long ago as 300 million to 250 million years3.
In a study published in Nature on 20 July1, Ricardo Araújo, a palaeontologist at the University of Lisbon, and his colleagues propose that the shape and size of the bony canals of the inner ear could be used as a proxy for body temperature. The movement of fluid through the canals helps the body to monitor head position and motion, which is essential for vision and balance. And the fluid’s viscosity changes with body temperature. The research team hypothesized that as body temperature increased and the animals became more active, the shape of the ear canals would have evolved to less viscous fluid to preserve balance and movement.
To track this adaption, the team compared the inner ear structures and physiology of 50 living vertebrates, including reptiles, fish, birds and mammals. They developed a thermo-motility index based on inner ear shape, which, when adjusted for body size, enabled them to predict an animal’s body temperature.
When they analysed the inner ear canals of 56 extinct synapsids — the reptile-like ancestors of mammals — and fitted them into the index, the authors found that the shape of the canals had changed abruptly in the Late Triassic period, which lasted from 237 million to 201 million years ago. The authors suggest that this is when synapsids became warm-blooded. According to the index, this transition would have represented an increase in body temperature of 5–9 °C, and an increased metabolism.
The authors conclude that this adaptation gave early endotherms an advantage in dealing with the Triassic climate, which was cooler than that of the preceding Permian period.
José Eduardo Bicudo, a comparative physiologist at the University of São Paulo in Brazil, says the team’s approach was novel and offered a new way of looking at the emergence of endothermy.
Biologist Roger Seymour at the University of Adelaide in Australia says that the origin of endothermy has been debated intensely for the past 60 years, and there are multiple theories — each with evidence to support it. His and Bicudo’s own research suggests that endothermy evolved in the Permian, between 300 million and 250 million years ago3.
But he questions whether it is feasible that endothermy emerged during the Late Triassic. He points out that the body temperature proposed for the early endothermic synapsids — around 34 °C — is still relatively low, similar to that of living monotreme mammals such as the platypus and echidna.
Platypuses and echidnas are sluggish compared with other mammals. Seymour suspects that animals with similar body temperatures would have had difficulty surviving alongside the other reptiles of the time, which included archosaurs — the ancestors of modern birds — that walked on two legs and could move quickly. “If you have a bipedal archosaur running around and you have a physiology that is like a modern echidna or platypus, you’re going to be dinner.”