Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy.

Endothermy underpins the ecological dominance of mammals and birds in diverse environmental settings. However, it is unclear when this crucial feature emerged during mammalian evolutionary history, as most of the fossil evidence is ambiguous. Here we show that this key evolutionary transition can be investigated using the morphology of the endolymph-filled semicircular ducts of the inner ear, which monitor head rotations and are essential for motor coordination, navigation and spatial awareness.