Amazing stuff! The ectotherm (cold blooded) – endotherm (warm blooded) transition of mammal ancestors.
"The reptile-like ancestors of mammals evolved to be warm-blooded — but the timing of this transition is hotly contested. Now scientists have used fossilized inner ear canals to suggest that the adaptation occurred around 230 million to 200 million years ago. But other researchers say this evidence is unlikely to settle the debate. ...
In a study published in Nature on 20 July, ... propose that the shape and size of the bony canals of the inner ear could be used as a proxy for body temperature. The movement of fluid through the canals helps the body to monitor head position and motion, which is essential for vision and balance. And the fluid’s viscosity changes with body temperature. The research team hypothesized that as body temperature increased and the animals became more active, the shape of the ear canals would have evolved to less viscous fluid to preserve balance and movement.
To track this adaption, the team compared the inner ear structures and physiology of 50 living vertebrates, including reptiles, fish, birds and mammals. They developed a thermo-motility index based on inner ear shape, which, when adjusted for body size, enabled them to predict an animal’s body temperature.
Cooler climate
When they analysed the inner ear canals of 56 extinct synapsids — the reptile-like ancestors of mammals — and fitted them into the index, the authors found that the shape of the canals had changed abruptly in the Late Triassic period, which lasted from 237 million to 201 million years ago. ..."From the abstract:
"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. Increased body temperatures during the ectotherm–endotherm transition of mammal ancestors would decrease endolymph viscosity, negatively affecting semicircular duct biomechanics, while simultaneously increasing behavioural activity probably required improved performance. 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. ..."
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