Fig. 7 in The origins of the cochlea and impedance matching hearing in synapsids

Fig. 7. Dual function of the auditory and masticatory apparatus of the non-mammalian synapsid Pristerodon mackayi Huxley, 1868 (MB.R.985) from the Late Permian of Biesjespoort, South Africa. A. Auditory region in posteroventral view showing the mechanics of the jaw articulations during mastication. B. Masticatory forces at the quadrate-quadratojugal complex (in posterior view). C. Auditory region in posteroventral view showing the mechanics of sound conduction. D. Mechanics of sound conduction of the quadrate-quadratojugal complex in posterior view. When the jaw muscles were relaxed, the reflected lamina and its putative tympanic membrane would receive the airborne sound, which would cause medio-lateral vibrations of the postdentary bones (although in small amplitude). The sound vibration was transmitted via the jaw hinge and the horizontal stapes to the inner ear. During the vibration, the quadrate is mobile at its ball and socket joint to the cranium, as permitted by a very thin bony connection with the quadratojugal. However, when the jaw musculature contracted during mastication, the quadrate was immobilized, it was strained by masticatory force and oppressed against the quadratojugal and the cleft between the quadrate and the quadratojugal is closed. During the active mastication, hearing was likely suspended, or reduced to a very low sensitivity if feasible at all. Not to scale.