Evolution of the cochlea about 7 mm long. Like in lepidosaurs and archosaurs, it contains a lagena, a vestibular sensory epithelium, at its tip. Only in therian mammals (marsupials and placentals) is the cochlea truly coiled 1.5 to 3.5 times. Whereas in monotremes there are many rows of both inner and outer hair cells in the organ of Corti, in therian (marsupial and placental) mammals the number of inner hair-cell rows is one, and there are generally only three rows of outer hair cells. Amphibians have unique inner ear structures. There are two sensory papillae involved in hearing, the basilar (higher frequency) and amphibian
Evolution of the cochlea forces for the larger cochlear labyrinth may have started after the basal primate node. Mammals are the subject of a substantial amount of research not only because of the potential knowledge to be gained regarding humans, but also because of their rich and abundant representation in the fossil record. The spiral shape of the cochlea evolved later on in the evolutionary pathway of mammals than previously believed, just before the therians split into the two lineages marsupials and placentals, about 120 million years ago. Parallel to the evolution of the cochlea, prestins show an increased rate of evolution in therian