Fig. 9 in The origins of the cochlea and impedance matching hearing in synapsids

Fig. 9. Co-evolution of the cochlea and middle ear in synapsids. Hearing in primitive or burrowing non-mammalian synapsids was predominantly by bone conduction from the ground vibration via the mandible. The tympanic hearing started to assume a more important role as the head became lifted from ground due to a more upright forelimb posture. Adaptations to tympanic hearing and modifications of the middle ear for better sensitivity to airborne sound include: an enhanced acoustical isolation of the postdentary bones from the dentary, an enlarged reflected lamina, a smaller stapes footplates and a more horizontal oriented stapes, an elongate medial trochlear condyle of the quadrate. All these coincided with the origin of a distinctive cochlear cavity. Concurrent with middle ear modifications, the inner ear developed a more laterally facing fenestra vestibule and a ventral cochlear cavity. Later, the detachment of the postdentary bones from the mandible and the elongation of the cochlear canal were further steps towards a refined sensitivity to airborne sound, especially in higher frequencies. Partially redrawn from Luo et al. (2011), Cox (1962), and Luo (2011).