Hearing in extinct cetaceans as determined by cochlear structure

The bony cochlea is described in representative Odontoceti, Mysticeti and Archaeoceti, with special emphasis on adaptations to high-frequency echolocation. The latter include (a) a small distance between the bony spiral laminae throughout most of the cochlea, (b) a strong secondary spiral lamina reaching nearly up to the apex, (c) a thick separating wall between the turns, (d) a cross sectional area ratio of scala tympani to scala vestibuli which decreases slowly from basal to apical region and which has a point of equality in the apical part of the cochlea, (e) a voluminous spiral canal and (f) a small ratio of height to diameter of the cochlea. Systematic changes of these values from basal to apical region are described. The anatomical findings are correlated with cochlear theory, with sound production, and with structural adaptations of the skull. The material examined indicates that high-frequency echolocation (sonar) was developed first by Squalodontoidea during the Oligocene. In Miocene time, the Odontoceti already had a full-fledged sonar system. There is good evidence that neither the Archaeoceti nor the Mysticeti ever possessed a sonar system. In contrast to the Odontoceti, the Mysticeti adapted their hearing organ to the perception of sound of very low frequencies.