River dolphin cochlea has not evolved convergently.

Convergence is often invoked to explain some common morphological features in the crania and postcrania of river dolphins (Cetacea, Odontoceti). Rivers are a challenging, complex and increasingly threatened habitat, and echolocation is a critical tool for their survival. Cochlear morphology can be indicative of hearing and echolocation abilities in toothed whales. Previous analyses of river dolphin cochleae have not included all extant riverine taxa, potentially obscuring their patterns of evolution. Based on three-dimensional models obtained from micro-CT scans and using a more inclusive definition of river dolphins, we tested for convergent evolution in cochlear morphology, including extant and extinct species in a broader evolutionary framework. Results showed that the cochlea of river dolphins is not significantly disparate from marine forms, and there is more interspecific morphological variability than expected. Statistical analyses also revealed the lack of convergence in the cochlea and that other ecological variables shaped the intricate evolutionary landscape of this structure. We hypothesize that river dolphins solved the challenges imposed by the environment via their sound production apparatus (e.g. skull shape, melon, etc.) rather than the cochlea.