How jaw bones transformed into the complex middle ears of mammals

Science · Published 11 October 2026 Swipe more like this →

In short: In one of the most dramatic structural transformations in vertebrate history, bones that once served the jaw were repurposed for hearing. This evolutionary shift allowed mammals to decouple chewing from sensing sound, paving the way for the massive ecological diversification seen throughout the Age of Mammals.

For most vertebrates, the jaw and the ear function as integrated systems. However, the transition to crown mammals involved a radical reorganization of the skull. In non-mammal vertebrates, the jaw consists of multiple elements, while the ear typically relies on a single element. Mammals reversed this pattern, evolving a single-element jaw—the dentary—and a multi-element middle ear.

This transformation involved the detachment of postdentary bones from the lower jaw. These bones, which are homologous to the articular and quadrate bones found in non-mammal vertebrates, were repurposed as two new middle-ear ossicles. When added to the stapes—the single element common to all tetrapods—they created the complex auditory system found in mammals today.

This decoupling of hearing and mastication (chewing) came with evolutionary trade-offs. While the new mammalian jaw structure allowed for more precise dental occlusion and specialized diets, it resulted in decreased mechanical efficiency and speed compared to non-mammals. Despite this, the shift was successful, enabling mammals to radiate into diverse terrestrial, marine, and aerial environments during the Cenozoic Era, which began approximately 66 million years ago.

This structural shift is considered a key driver for the ecological success of mammals. By separating the mechanical demands of eating from the sensitive requirements of hearing, mammals were able to occupy various adaptive zones left vacant by the extinction of non-avian dinosaurs.

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