Ear Canals shed light on the Evolution of Warm-Blooded Animals

New research published in Nature suggests that the size of microscopic structures in the inner ears of fossil animals can be used to estimate their body temperature.

Evolutionarily, mammals evolved from reptiles to warm-blooded creatures but the exact date of this change is a major controversy among scientists. The modification is thought to have taken place between 230 million and 200 million years ago, according to preserved inner ear canals. However, other experts think this information will not be enough to end the issue.

Due to their high metabolic rate and reliance on the environment to keep warm, warm-blooded animals, or endotherms, have the ability to maintain a constant body temperature.

However, it is challenging to measure these characteristics in fossils, thus scientists have utilized skeletal characteristics like height and bone structure to predict metabolic rate. Some studies say endothermy evolved 145 million to 66 million years ago; while other research says 300 million to 250 million years ago.

To determine whether a prehistoric animal was warm-blooded or cold-blooded is a difficult task because a temperature reading on a creature that lived hundreds of millions of years ago cannot be obtained. The fluid in the ear canals of all vertebrate animals helps us keep our balance.

Our inner ears have developed varied diameters so that the fluid can flow properly since the viscosity, or runniness, of that liquid, varies depending on temperature. Animals that are cold-blooded require larger semicircular canals because their ear fluid is colder and thicker than that of warm-blooded.

As animals’ body temperatures rose and they grew more active, the researchers expected that their ear canals would become less viscous to sustain balance and movement. The researchers analyzed the inner ear canal widths of 341 animals, including 243 current species and 64 extinct ones, to chart these evolutionary variations.

They discovered that warm-bloodedness did not evolve until 233 million years ago in mammal ancestors, which is roughly 20 million years later than what scientists had previously hypothesized.

Moreover, based on the timing of the appearance of these semicircular canals in the fossil evidence, it appears that the emergence of warm-bloodedness in mammalian ancestors occurred much more rapidly than previously believed, during the same time that proto-mammals began to grow whiskers, fur, and unique backbones.

Warm-bloodedness and the evolution of fur coincide in time because fur takes in the heat produced by a higher metabolism, allowing the organism to maintain a high temperature necessary for survival.

Dr. Angielczyk one of the authors further adds that “The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology. Many different approaches have been used to try to predict when it first evolved, but they have often given vague or conflicting results”

The scientists draw the conclusion that this modification helped early endotherms survive in the Triassic period, which had a colder environment than the Permian period before it.



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