Evolution of mammalian auditory ossicles

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The evolution of mammalian auditory ossicles was an evolutionary process that resulted in the formation of the mammalian middle ear, where the three middle ear bones or ossicles, namely the incus, malleus and stapes (a.k.a. "the anvil, hammer, and stirrup"), are a defining characteristic of mammals. The event is well-documented^[1] and important^[2][3] academically as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.^[4]

The ossicles evolved from skull bones present in most tetrapods, including amphibians, sauropsids (which include extant reptiles and birds) and early synapsids (which include ancestors of mammals). The reptilian quadrate, articular and columella bones are homologs of the mammalian incus, malleus and stapes, respectively. In reptiles (and early synapsids by association), the eardrum is connected to the inner ear via a single bone, the columella, while the upper and lower jaws contain several bones not found in modern mammals. Over the course of mammalian evolution, one bone from the upper jaw (the quadrate) and one from the lower jaw (the articular) lost their function in the jaw articulation and migrated to form the middle ear. The shortened columella connected to these bones to form a kinematic chain of three ossicles, which serve to amplify air-sourced fine vibrations transmitted from the eardrum and facilitate more acute hearing in terrestrial environments.