Petrosal (periotic) and inner ear of a Pliocene kogiine whale (Kogiinae, Odontoceti): implications on relationships and hearing evolution of toothed whales

Abstract

This study describes the petrosal (periotic) bone and the inner ear of a fossil pygmy sperm whale (Kogiinae, Physeteridae, Cetacea) from the Yorktown Formation (Pliocene) of the Lee Creek Mines of North Carolina. A cladistic analysis of 36 petrosal characters among 18 cetacean taxa shows that extant and fossil kogiines are diagnosed by a plate-like and posteriorly oriented posterior process of the petrosal. The Physeteridae is diagnosed by two petrosal apomorphies: presence of an incudal process for the articulation of the incus, and an enlarged accessory ossicle. Petrosal apomorphies suggest that physeterids and ziphiids are sister groups among the odontocetes, and that the Mysticeti and Odontoceti are both monophyletic. The cochlear spiral of the Pliocene kogiine petrosal has two full turns and a cochlear basal/height ratio of 0.6. Within the cochlear canal, the secondary bony lamina for the basilar membrane is present in one and one-quarter turns of the cochlea. The width of the basilar membrane, as inferred from the laminar gap, is much narrower in the fossil kogiine than in an Oligocene squalodontid-like whale. The primary bony lamina contains numerous foramina for the cochlear nerve. The spiral ganglion canal is well developed in one and three-quarter turns of the cochlea. These cochlear structures are associated with high-frequency hearing in modern toothed whales. Comparison of the inner ear between kogiines and other odontocetes suggests that the fossil kogiine could hear about the same frequency ranges as extant delphinoids, and much higher frequencies than the Oligocene squalodontoid. The vestibule of the kogiine petrosal is a small and curved tube; the semicircular canals are very reduced. These specialized features are the most unequivocal synapomorphies in the odontocete inner ears.