Ears burning? Mammal fossils suggest it could be the Late Triassic calling

A study of the inner ears of mammalian ancestors may pinpoint when they evolved to be warm-blooded.

Why are reptiles cold-blooded, and mammals warm-blooded? And when did these attributes emerge? Ectotherms – which include amphibians, reptiles, most fish and invertebrates – rely on external sources of heat to regulate their internal body temperatures, whilst endotherms (primarily birds and mammals, although there are some fish that have this capability) have biological processes that provide them with warmth and regulate their body temperatures.

Exactly when mammals evolved to produce internal heat and regulate body temperature has been “one of the great mysteries of palaeontology”, says Kenneth Angielczyk, palaeobiologist at the Field Museum, Chicago, US, and one of the authors of a new Nature paper detailing how fossils of the inner ear structure of mammalian ancestors may provide a crucial clue.

There might not be an immediately obvious connection between warm-bloodedness and inner ear structure, but according to Angielczyk: “The canals in our inner ear are sensors that give the brain information about the position and movement of the head. They work by having a fluid [endolymph] in them that sloshes around when we move our heads, which is detected by cells in the canals and the information is transmitted to the brain for interpretation.”

Ear canals of mammal versus reptile ancestors
Size differences between inner ears (in red) of warm-blooded mammaliamorphs (on the left) and cold-blooded ancestors (right). Inner ears are compared for animals of similar body sizes. Credit: Romain David and Ricardo Araújo

Warmer temperatures make for sloshier, or less viscous, endolymph fluid. (A highly viscous fluid is thick and doesn’t flow well – think jelly, for instance.) In modern mammals the ear canals are smaller and rounder – better for this less viscous fluid. In contrast, cold-blooded animals’ ear canals are semi-circular and larger to better suit a less runny, more viscous, endolymph fluid.

In the fossil record there’s a sudden [in geologic terms] evolution of the ear canals of animals from the Mammaliamorpha group. According to Angielczyk, there would “be no evolutionary advantage for changes in canal size and shape if there were no changes in body temperature and endolymph viscosity”

The mammaliamorph group “includes mammals, as well as some fairly close extinct relatives that fall outside of Mammalia proper”, says Angielczyk, and it looks like “mammal-like canals (suggestive of endothermy) appear rather abruptly in the Late Triassic Period”, with an indicated increase in body temperature of the mammalian samples of between 5°C and 9°C.

Inner ear squirrel. Credit romain david
The bony inner ear of a squirrel monkey (in blue) and the soft tissues inside it (in red). Relations between bony and soft tissue structures have been studied to infer how the animal regulated its body temperature. Credit: Romain David

The Late Triassic period is characterised by climatic instability and a time of global environmental changes, including extinction events and climate warming. Although the period marks the extinction of a number of species, it also led to diversification of land and ocean plant and animal life, including the development of many of the characteristic features of mammals today.

Unlike other research in this area – such as studies on bone tissue structure and radioactive isotope ratios – Angielczyk says the shape and sizes of ear canals are a fairly direct indicator of body temperature evolution in mammals, rather than proxies or indirect indicators.

The research team plan to widen the sample of to better pinpoint the exact transition time.

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