• DOI:10.1038/s41586-022-04963-z
• Corpus ID: 236245230

Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy

@article{Arajo2021InnerEB,
  title={Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy},
  author={Ricardo Ara{\'u}jo and Romain David and Julien Benoit and Jacqueline K. Lungmus and Alexander Stoessel and Paul M. Barrett and Jessica A Maisano and Eric G Ekdale and Maeva Judith Orliac and Zhe‐Xi Luo and Agustín G. Martinelli and Eva A. Hoffman and Christian A. Sidor and Rui M. S. Martins and Fred Spoor and Kenneth D. Angielczyk},
  journal={Nature},
  year={2021},
  volume={607},
  pages={726 - 731}
}

• R. Araújo, R. David, +13 authors K. Angielczyk
• Published 2 June 2021
• Biology
• Nature

Endothermy underpins the ecological dominance of mammals and birds in diverse environmental settings1,2. However, it is unclear when this crucial feature emerged during mammalian evolutionary history, as most of the fossil evidence is ambiguous3–17. 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 awareness18–22. Increased body temperatures during the ectotherm–endotherm transition of mammal ancestors would decrease endolymph viscosity, negatively affecting semicircular duct biomechanics23,24, while simultaneously increasing behavioural activity25,26 probably required improved performance27. Morphological changes to the membranous ducts and enclosing bony canals would have been necessary to maintain optimal functionality during this transition. To track these morphofunctional changes in 56 extinct synapsid species, we developed the thermo-motility index, a proxy based on bony canal morphology. The results suggest that endothermy evolved abruptly during the Late Triassic period in Mammaliamorpha, correlated with a sharp increase in body temperature (5–9 °C) and an expansion of aerobic and anaerobic capacities. Contrary to previous suggestions3–14, all stem mammaliamorphs were most probably ectotherms. Endothermy, as a crucial physiological characteristic, joins other distinctive mammalian features that arose during this period of climatic instability28. The functional morphology of the fluid-filled semicircular ducts of the inner ear is adapted to body temperature and behavioural activity and can be used to investigate the evolution of endothermy.