Hearing
Temnospondyls and other early tetrapods have rounded otic notches in the back of the skull that project into the cheek region. In life, the otic notch would have been covered by a membrane called the tympanum, which is seen as a disk-like area in living frogs. The tympanum is involved in hearing, and is similar to the ear drum of more advanced tetrapods. It was traditionally thought that the tympanum developed very early in tetrapod evolution as a hearing organ and progressed to form the ear drum of amniotes. Thus, temnospondyls possessed a hearing system supposedly ancestral to that of living amphibians and reptiles.
Frogs and all other living tetrapods have a rod-like bone called the stapes that aids in hearing by transferring vibrations from the ear drum —or homologous tympanum— to the inner ear. Temnospondyls also have a stapes, which projects into the otic cavity. The stapes likely evolved from the hyomandibula of lobe-finned fishes. The positioning of the stapes and the shape of the otic region suggests that the tympani of temnospondyls and frogs are homologous, but the tympani of these amphibians are no longer considered homologous with the hearing systems of reptiles, birds, and mammals. Therefore, ear structures in temnospondyls were not ancestral to those of all other tetrapods.
The ability of the tympanum and stapes to effectively transmit vibrations is called impedance matching. Early tetrapods like temnospondyls have thick stapes with poor impedance matching, so it is now thought that they were not used for hearing. Instead, these thick stapes may have functioned to support the tissue that covers the otic notch. Early temnospondyls like Dendrerpeton could not hear airborne sound but would have been able to detect vibration in the ground. Later temnospondyls like Doleserpeton had otic regions adapted to hearing. Doleserpeton has a structure in the inner ear called the perilymphatic duct, which is also seen in frogs and is associated with hearing. Its stapes is also a better transmitter of sound. The hearing system of Doleserpeton and related temnospondyls was able to detect airborne sound and may have been ancestral to that of living amphibians.
Read more about this topic: Temnospondyls, Paleobiology
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