Hearing Organ Evolution and Specialization: Early and Later Mammals

An enormous amount of data is available on the structure and function of the mammalian cochlea and mammalian hearing characteristics. Most of this knowledge is derived from studies of placental mammals, which is only one of the three groups of mammals that exist today. There are only a few studies on the ears and hearing characteristics of the other two living mammalian groups, the monotremes and the marsupials, and only recently have fossil specimens with sufficiently preserved inner ears become available that document important steps in evolution of the mammalian ear from the early Cretaceous up to the present. Mammals are an ancient group among terrestrial vertebrates that arose from mammal-like reptiles probably during the Triassic. The diagnostic features that clearly separate a mammal from its reptilian ancestors and other tetrapods include a property of the hearing apparatus, namely the presence of the three-ossicle middle ear. This key innovation appears to be one prerequisite for the capability of high-frequency hearing that is unique to mammals among tetrapods. The fossil record demonstrates that a middle ear with three ear ossicles precedes the evolution of an elongated and, in the more derived condition, coiled cochlea capable of hearing high frequencies well above 10kHz.