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 also traces back to the Cretaceous period. The evolution of the human cochlea is a major area of scientific interest because of its favourable representation in the fossil record. During the last century, many scientists such as evolutionary biologists and paleontologists strove to develop new methods and techniques to overcome the many obstacles associated with working with ancient, delicate artifacts. In the past, scientists were limited in their ability to fully examine specimens without causing damage to them. In more recent times, technologies such as micro-CT scanning became available. These technologies allow for the visual differentiation between fossilized animal materials