Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy

Ricardo Araújo (), Romain David (), Julien Benoit, Jacqueline K. Lungmus, Alexander Stoessel, Paul M. Barrett, Jessica A. Maisano, Eric Ekdale, Maëva Orliac, Zhe-Xi Luo, Agustín G. Martinelli, Eva A. Hoffman, Christian A. Sidor, Rui M. S. Martins, Fred Spoor and Kenneth D. Angielczyk ()
Additional contact information
Ricardo Araújo: Universidade de Lisboa
Romain David: Natural History Museum
Julien Benoit: University of Witwatersrand
Jacqueline K. Lungmus: National Museum of Natural History
Alexander Stoessel: Max Plank Institute for Evolutionary Anthropology
Paul M. Barrett: Natural History Museum
Jessica A. Maisano: University of Texas at Austin
Eric Ekdale: San Diego State University
Maëva Orliac: Université de Montpellier
Zhe-Xi Luo: University of Chicago
Agustín G. Martinelli: Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’
Eva A. Hoffman: American Museum of Natural History
Christian A. Sidor: University of Washington
Rui M. S. Martins: Universidade de Lisboa
Fred Spoor: Natural History Museum
Kenneth D. Angielczyk: Field Museum of Natural History

Nature, 2022, vol. 607, issue 7920, 726-731

Abstract: Abstract 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.

Date: 2022
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DOI: 10.1038/s41586-022-04963-z

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