Hearing abilities of a late-surviving archaeocete (Cetacea: Kekenodontidae), and implications for the evolution of sound in Neoceti.

Kekenodontids are the only known archaeocetes (stem cetaceans) from the late Oligocene. They possess a unique combination of morphological features seen in both more primitive Eocene basilosaurid archaeocetes and more derived Neoceti (mysticetes and odontocetes). However, much remains unknown about the clade, including its acoustic biology. Based on its phylogenetic position crownward to basilosaurids as the latest-diverging archaeocete, we hypothesize that kekenodontids would be specialized for hearing low-frequency sounds. Here, we provide the first report on the cochlear anatomy of a kekenodontid using the holotype of Kekenodon onamata from New Zealand. We compare the cochlear morphology of K. onamata to a sample of extinct and extant cetaceans and quantify shape differences using three-dimensional geometric morphometrics. The analyses show that K. onamata was indeed adapted to hear low frequencies and suggests low-frequency hearing may be a characteristic of raptorial macrophagous fossil cetaceans in contrast to infrasonic bulk filter-feeding mysticetes and ultrasonic echolocating odontocetes.