Evolution of the Auditory System in Synapsida (“Mammal-Like Reptiles” and Primitive Mammals) as Seen in the Fossil Record

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Based on evidence from comparative anatomy, embryology, and paleontology, it is well established that the middle ear of existing mammals is morphologically unique, the tympanic bone, malleus, incus, and tensor tympani muscle all being homologous with components of the feeding apparatus of other vertebrates (Fig. 28.1, see below for details). Extant mammals are also unique in having a very elongate cochlea, and sensitivity to a broader range of sound frequencies than other vertebrates, usually extending beyond 10,000 Hz. In addition, they are distinctive in generally having a protruding pinna (auricle) and a long, tubular external auditory meatus. The present chapter examines the extensive fossil evidence concerning the nature of the auditory machinery of early mammals and their antecedents. Both authors of the present account have written on this subject, arriving at different initial interpretations in certain regards (Hop-son 1966; Allin 1975).