Monday, December 19, 2011

You've got a Lizard in your Ear

The mammalian skull offers a unique opportunity to discuss evolutionary adaptation and and the difference between homologous and analogous features.  When examining the fossil record, the separation of modern "mammals" is demarcated by a change in joint connecting the mandible to the skull.  Prior to this point the connection was formed by two separate bones, the quadrate and the articular.  In birds and lizards and other non-mammalian genera these bone are separate from the body of the mandible and the cranium.  In mammals, the mandible articulates directly with the temporal bone of the cranium, without any intermediary bones.

So, are the quadrate and articticular bones present in mammalian skulls?  In fact these bones are conserved, but they serve a different function in the mammalian skull.  Homologous features are the same physical structure between different taxa.  While the features are morphologically the same, they may preform different functions in different organisms.  On the other hand, analogous features have the same functional uses, but arise from different anatomical structures.  In mammals, the incus and the maleus bones of the ear are homologous to the quadrate and articular, respectively.  Although the bones are the same anatomical structure, in mammals, the bones preform a major role in hearing rather than jaw movement.  Instead, the articulation between the temporal and the mandible is analogous to the quadrate/ articular connection in non-mammals, since it has taken on the functional job of connecting the mandible to the cranium.

Transitional fossils have been found showing organisms with a jaw articulation between the temporal and mandible as well as the quadrate and articular.  The change in the jaw articulation is likely due to several different factors.  First of all, reduction in size of the articular and quadrate made them more sensitive to vibrations and improved the hearing capabilities of these organisms.  Organisms with better hearing were better able to avoid predators and reproduce.  Thus, the continued reduction of the ear bones was adaptively advantageous.  Alternatively, the reduction of the jaw joint limited the size of the jaw and the bite strength of the organism.  In this case, a larger joint was more advantageous because it allowed the organism to consume larger prey and also subdue the prey more efficiently.  Thus, decoupling these two functions, allowed mammals to benefit from both increased hearing and jaw size.

Be Aware, It's a Bear!!!
If life ever gets you down, just remember, you have a tiny little lizard jaw joint working in your ear. :)

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