Ear fossils hint at origin of warm-blooded mammals

Fossilised inner-ear canals suggest that the moment mammals evolved to be warm-blooded — the timing of which is highly contested — occurred around 230 million to 200 million years ago. A team of researchers hypothesised that hotter, more active bodies would have less viscous fluid in their vestibular system, which maintains balance and spatial orientation — and the inner ear’s shape would adapt in turn. After calibrating their understanding on the basis of 50 living vertebrate species, they analysed the inner ear canals of 56 extinct synapsid species — the reptile-like ancestors of mammals — and found that the shape of the canals had changed abruptly in the Late Triassic period.