Evolution of Hearing in the Early tetrapods and Emergence of the Tympanic Middle Ear
* Presenting author
During the 120 my from the origin of tetrapods to the origin of tympanic ears in the Triassic, auditory sensitivity must have been based on non-tympanic, bone conduction mechanisms. However, bone conduction is a loose term for several different modes of stimulation of the inner ear. To understand hearing with a non-tympanic ear, I focus on the simplest: that sound translates the head region and that this vibration is transduced by the inner ear. The efficiency of translation of an object by sound is determined by ka (the product of wave number and radius) Comparison of the sensitivity of animals without middle ear (snakes, salamanders, earless frogs, lungfish) to sound and to vibrations of the skull shows that most sensitivity can be explained by translation. Interestingly, simple translation is also the mode of human low-frequency bone conduction sensitivity (for frequencies resulting in a ka>1). Translation by underwater sound is the mode of stimulation of most fishes. It is therefore a straightforward assumption that this was the mechanism of hearing also in the early tetrapods, and assessment of the sensitivity can inform us of the gradual changes leading to the tympanic ear, especially in relation to sensitivity and directionality.