ABOVE: A mammaliamorph’s hot breath suggests it is warm-blooded. Luzia Soares

Changes in inner ear canal morphology over time suggest that mammalian ancestors called mammaliamorphs evolved warm-bloodedness around 233 million years ago during the late Triassic period, according to research published yesterday (July 20) in Nature. The analysis provides new clues to the outstanding question of when mammals switched from relying on external heat to regulate their body temperatures (ectothermy) to generating their own heat (endothermy).

Inside ears, semicircular ducts filled with a fluid called endolymph help animals perceive head motion to improve motor coordination, balance, and spatial awareness, the study authors write. But not all inner ears are created equal.

“Mammals have very unique inner ears,” study coauthor Ricardo Araújo, a vertebrate paleontologist at the University of Lisbon, tells Science News. This led Araújo and his team to hypothesize that because endolymph is more viscous in warm-blooded animals, their canal structures may have evolved differently than in ectotherms, the outlet reports. 

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To find out, Araújo and his colleagues used an X-ray scanning technique called microtomography to analyze the ear canal morphologies of hundreds of vertebrates—both modern and extinct species—and found that mammals have smaller, thinner, and more circular ear canals than do cold-blooded animals, reports New Scientist. From this analysis, according to Science News, the team created an index that maps ear canal shape to body temperature. 

When the researchers applied this indexing methodology to 56 fossils of extinct animals, they found that morphology consistent with warm-bloodedness emerged abruptly in a group of animals called mammaliamorphs around 233 million years ago, during the late Triassic period. Previous research suggests endothermy in mammals evolved gradually over 120 million years, reports New Scientist, but the new evidence tells a different tale. “The fact that it is a sharp break in the data [suggests] the transition happened rapidly—within about a million years,” coauthor Kenneth Angielczyk, a paleontologist at the Field Museum in Chicago, tells Science News.

These findings don’t apply to birds, which evolved warm-bloodedness independently from mammals, reports New Scientist.

Stephen Brusatte, a paleontologist at the University of Edinburgh, tells Science News that the researchers were clever to relate inner ear anatomy to body temperature, but he cautions that fossils don’t reveal the presence of hair cells, soft tissue, or the actual viscosity of endolymph, all of which could affect their conclusions. “Shape alone may not always be sufficient to predict something as complex as body temperature or metabolic style.”