The Macro Library(beta)
Evolution of the mammalian middle ear
The evolution of the mammal ear is a unique event among tetrapods, and the three-bone structure of the mammalian middle ear is, in fact, a
of Mammalia. It is a common misperception that this particular transition is too subtle to be documented, and that no transitional fossil evidence exists.
amniotes have a jaw joint that has an articulation between the quadrate and articular bones. While many
have heavily modified their skulls, this two-bone articulation point is still readily observable1. However, it can be seen that through a rather gradual process the point of articulation in synapsids grew to accommodate a two-joint articulation which finally became the middle-ear / jaw system in extant mammals. This, in turn, could grant them higher auditory sensitivity than is found in all other tetrapods, which only have the stapes and lack the mammalian incus and malleus.
Looking at Figure 1, it can be seen that early synapsids, sauropsids, and thus the common ancestor of both employed an articulation point comprised of the quadrate and articular bones. However, even in early specimens such as Procynosuchus, the quadrate and articular are both strongly reduced
with respect to extant sauropsids. By the arrival of Probainognathus, the synapsid jaw articulation had been highly modified. Many of the various other jaw elements seen in Procynosuchus had fused into the dentary bone, and the articular and quadrate had been further reduced. For this animal, all four bones were articulating with each other, reducing the reliance on any individual bone (or pair of bones) for the articulation, allowing further modification of the jaw joint. There are other animals, such as Diarthrognathus, for which this articulation is even more evident2.
Upon the arrival of animals such as Morganucodon, the quadrate and articular were barely involved in the articulation, to the point where they become nearly invisible for diagrammatic purposes. This condition is then easily modified into the solely dentary-squamosal articulation present in mammals, represented in Figure 1 by a possum.
In modern mammals, the articular and quadrate reside in the middle ear, connected to the stapes in series and have lost their connection to the jaw, which is composed in mammals of only the dentary bone. This modification to the jaw makes it more robust, but the tradeoff is that mammals have less flexibility in their jaw movements and can never move their jaws in the way that a snake or monitor lizard might.
Simple presence of a change is insufficient, however, and the selective pressures for causing such a change are important to recognize. In dicynodonts, therocephalians, and cynodonts vibrations from the air passed primarily through the
to the quadrate, to the
By loosening the bones
to the dentary, they became more free to vibrate and thus could transmit higher frequencies3. Once these became connected with the stapes, they permitted not only the hearing of higher frequencies but of lower amplitudes, as well, acting as an amplification system. The resulting excellent hearing fits with fossil finds that suggest Mesozoic mammals were primarily burrowing, climbing, and/or insect eating4. A plausible and generally the most well-accepted explanation for this series of adaptations being selectivly advantageous is that it permitted these Mesozoic premammalian ancestors to better hear insects and other small prey, most particularly during the night. This works with the fossil evidence and other hypotheses about
molar shapes, and generally unremarkable mammalian color vision to conclude mammal ancestry is rooted in nocturnal insectivorous niches, because most other terrestrial niches were occupied by dinosaurs.