Evolution of the craniate middle ear
The evolution of the auditory structures of the middle ear began with the recycling of the second gill arches in primitive fish (Braxeau and Jeffery 2008). While the first set of gill arches became the upper and lower jaws, the upper portion of the second gill arch, the hyoid arch, became a support structure for the cranium as the hyomandibula (Manley 2010). Examinations of Sarcopterygiian fishes such as Rhipidistians and Rhizodonts describes the role of the well developed hyomandibular bones in these fishes as a stabilizer and suspension for the upper and lower jaws (Manley 2010, Thompson 1966), as well as provide support in gill ventilation (Brazeau and Jeffery 2008). While the hyomandibula is an important skull structure in many fishes, its usefulness as an auditory apparatus comes later in evolution with the hearing systems of land dwelling tetrapods. Most fish detect sound as waves through the water that occur from pressure disturbances resulting from an organism or object displacing water as it moves. The sound waves travel through the body of water and are absorbed through the body wall of the animal. An air-filled organ such as a swim bladder-like structure or an enclosed spiracle cavity detects the sound waves as the body’s density changes (Thompson 1966). The spiracle formed from the reduced openings of the first and second gill arches as they migrated dorsally (Manley 2010). Originally used to intake water for respiration, the spiracle in some Sarcopterygiians may have retained air bubbles (Thompson 1966). The functional success from the bubbles that became trapped in the spiracle may have lead to the water intake opening to became sealed off from the aquatic environment to form a sound detecting air pocket (Thompson 1966). The spiracular chamber became the source for the middle ear cavity as the auditory mechanisms evolved from water to terrestrial sound reception (Clack 2002). With the transition from...
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