Evolution of mammalian auditory ossicles

The evolution of mammalian auditory ossicles, that is, of the bones in the middle ear, from the jaw bones of reptiles is one of the most well-documented and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re-purposing of existing structures during evolution.

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The jaws of early synapsids were similar to those of other tetrapods of the time, with a lower jaw consisting of a tooth-bearing dentary bone and several smaller posterior bones. Of particular interest for this article are the bones which comprised the jaw joint, namely the articular and angular bones in the lower jaw and the quadrate in the upper jaw. At this time, the middle ear consisted of a single bone, the stapes (also called the columella), linking the inner ear to the tympanic membrane, which was likely to be externally visible as it is in modern lizards. The stapes usually was directly connected to the quadrate, making the bones of the middle ear intimately connected to the jaw itself. The hinge of the jaw itself was located at the interface between the quadrate and articular bones.

During the Permian and early Triassic, the dentary of synapsids continually enlarged while other jaw bones were reduced. Eventually, the dentary was able to make contact with the squamosal, a bone in the upper jaw located anterior to the quadrate, allowing two simultaneous jaw joints - an anterior "mammalian" joint between the dentary and squamosal and a posterior "reptilian" joint between the quadrate and articular. This "twin-jointed jaw" can be seen in late cynodonts and early mammaliforms.

As the dentary continued to enlarge during the Triassic, the posterior joint fell out of use. Some of the bones were lost, but the quadrate (which is directly connected to the stapes), the articular (connected to the quadrate) and the angular (connected to the articular) became free-floating and associated with the stapes.

The frequency range and sensitivity of the ear is dependent upon the shape and arrangement of the middle-ear bones. Because of the functional role of the quadrate and articular in the reptilian jaw, these bones could be modified only minimally to alter the frequency range of the ear. But once these bones were no longer involved in the jaw joint, variations which affected hearing would not also affect jaw joint function, allowing unconstrained evolution of the mammalian hearing apparatus.

By the Jurassic, the typical mammalian ear had evolved, in which the angular had become the tympanic annula (a bony support for the tympanic membrane), while the articular and quadrate had become the malleus and incus, respectively, connected in series with the stapes. This series of three bones acts as an amplification system to allow enhanced hearing.

The transition between these two states is one of the most well-documented and supported in all of evolution, and newly discovered fossils from this transitional period have recently enriched our understanding of this transition.