The theory that the reduced postdentary bones and quadrate of non-cyndonts were not only the morphological homologues, but also the functional equivalents of the mammalian tympanic bone and ear ossicles is tested on the basis of detailed new information of a specimen of Chiniquodon. The anatomy is shown to be a compromise between the respective requirements for a persistent, though reduced, stress transmission function of a jaw articulation, and an acoustic transformation function of a middle ear. There was a sound pressure level transformer ratio of about 30, but the mass and compliances of the elements restricted sensitivity to low frequencies, up to perhaps 2 kHz. Neither an air-filled tympanic cavity, nor a dedicated tympanic membrane were present, and snakes and other modern reptiles lacking a tympanic cavity offer a better mechanical analogy than mammals for the ear function of a cynodont. The fully mammalian acoustic transformer system, with tympanic cavity and tympanic membrane, could only have evolved after the origin of the dentary-squamosal jaw articulation, and was correlated with miniaturisation in the lineage leading to basal mammaliaforms.
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