“VIRTUAL” INNER EARS OF EXTINCT PLATANISTOIDS REVEAL FUNCTIONAL SIGNAL IN THE SEMICIRCULAR CANALS

Platanistoids (Cetacea: Odontoceti: Platanistoidea) are a group of dolphins that currently comprise one endangered riverine-inhabiting species (the Ganges river dolphin – Platanista gangetica). They were once a diverse, cosmopolitan, and mostly marine clade, and have an extensive fossil history extending from the late Oligocene onwards. Despite this, outstanding questions remain regarding the ecology of extinct species, particularly the degree to which they may have inhabited different environments, and/or engaged in niche partitioning, for example through different feeding strategies. Here, we examine the inner ear of five extinct platanistoid species using microCT scanning, generating ‘virtual’ endocasts, and linear measurements, with a focus on the vestibular region. Whereas the cochlea provides information on odontocete ecology in terms of the environment in which the animals are hearing, the orientation and sizes of the semicircular canals provide insights into balance and directional sensitivity. Compared to the other semicircular canals (anterior and posterior), we find overall larger lateral semicircular canal measurements, most strikingly in terms of canal length, suggesting functional signal related to lateral head movements. In comparison with other odontocetes, the platanistoids apparently possessed larger overall semicircular canal sizes, which may indicate more functional importance of the vestibular system in the group. We conclude that semicircular canal sizes should be explored further among odontocetes, as larger lateral canals contrasts with the condition in other mammals and could indicate a functional signal related to angular acceleration and head movement.