The shapes of ruminant inner ears are used to examine millions of years of evolution in herbivorous mammals, described in a paper published in Nature Communications. The findings link inner ear shape variation with the diversification of deer and cows, the climate, and their expansion into new habitats.
The inner ear senses movement and orientation and its shape and size have been used to infer agility in extinct animals. The bone of the inner ear, the bony labyrinth, is unusually dense and preserved well in the fossil record, meaning it can be used to investigate aspects of mammalian evolution over millions of years.
Bastien Mennecart and colleagues studied 306 living and fossilised ruminant inner ear bones — including from giraffes, deer, cows, sheep, and goats — across 35 million years of their evolution. The authors used three-dimensional X-ray data to measure the shape of the bones. They found that small, non-functional variation in inner ear shape corresponded with the evolution of new species in this taxonomic group. For example, accelerating changes in the deer inner ear correspond with the evolution of 19 new species of deer from the Plio-Pleistocene period from about 3 million years ago. In some groups, variation in the inner ear also corresponded with changes in global temperature. The authors conclude that, even in this important sensory system, minor shape differences can reflect climate and evolutionary history.
The findings demonstrate the potential of the inner ear as a tool for evolutionary insight, and supports additional work examining the impact of non-functional shape changes in the inner ear of other groups.
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