The Ear of Mammals: From Monotremes to Humans

Mammals hear across a greater range of auditory frequencies than other vertebrates, which was made possible through major modifications of sound transmission and processing pathways within the middle and inner ears. The first step in the evolution of the ear of mammals is a combination of the prootic and opisthotic bones to form a single petrosal bone that fully encapsulates the organs of the inner ear. Elongation of the cochlea occurred simultaneously with the development of the petrosal. The curvature of the cochlea itself is variable among Mesozoic mammals, and extreme coiling of the cochlea is coincident with acquisition of major morphogenetic genes within the mammalian genome. A cribriform plate that is penetrated by multiple branches of the cochlear nerve in all extant mammals, internal cochlear structures such as the auditory nerve ganglion canal, and primary and secondary bony laminae for supporting the basilar membrane further broadened the bandwidth of audible frequencies in placental and marsupial mammals. Although often considered to be a major diagnostic feature of mammals, the definitive mammalian middle ear, which consists of a complete separation of middle ear elements from the mandible, appears to have evolved multiple times over the course of mammal evolution. As new important fossils are discovered, described, and included in phylogenetic analyses, the accuracy in ascertaining when and how many times the major features of the mammalian ear were acquired will increase.