Inner ear biomechanics reveals Late Triassic origin of mammalian endothermy

Abstract Endothermy (“warm-bloodedness”) underpins the ecological dominance of mammals and birds in diverse environmental settings^1-3. However, it is unclear when this crucial feature emerged during mammalian evolutionary history, as most fossil evidence is ambiguous^4-25. Here, we show that new information on this key evolutionary transition can be obtained from the morphology of the endolymph-filled semicircular ducts of the inner ear that monitor head rotations and are essential for motor coordination, navigation, and spatial awareness^26-31. Increased body temperature during the ectotherm–endotherm transition of mammal ancestors would decrease endolymph viscosity, negatively impacting the biomechanics of the semicircular ducts^32,33, while simultaneously increasing activity levels^34,35 required improved performance^36. Specific morphological changes to the membranous ducts and enclosing bony canals were, therefore, necessary to maintain optimal functionality. We track these morphological changes in 341 vertebrates, including 56 extinct synapsids, and show that canals with relatively thin cross-sections and small radii of curvature are indicative of mammalian endothermy. This inner ear morphotype evolved abruptly ~233 million years ago, during the Late Triassic, in Mammaliamorpha. Our conclusion differs from previous suggestions3-17, and we interpret most stem-mammals as ectotherms. Endothermy as a crucial physiological characteristic joins other distinctive mammalian features that arose during this period of climatic instability^37-39.*Ricardo Araújo and Romain David contributed equally to this work.