ACOUSTIC TRANSFORMER FUNCTION OF THE POSTDENTARY BONES AND QUADRATE
OF A NONMAMMALIAN CYNODONT
Tom S. Kemp, Journal of Vertebrate
Paleontology, 2007, 27(2):431–441
The theory that the reduced postdentary bones and quadrate of non-mammalian 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.