Developing echolocation: distinctive patterns in the ontogeny of the tympanoperiotic complex in baleen and toothed whales (Cetacea)

Cetaceans (baleen and toothed whales) present a unique set of adaptations
for life in water. Among other abilities, the two living groups can hear
and produce different sound frequencies: baleen whales use low frequencies
primarily for communication, while toothed whales acquired the ability to
echolocate using high frequency sounds. Both groups exhibit modifications
to the morphology of the ear bones (tympanic bulla and periotic) that
closely track their behaviour and ecology. While the evolution of sound
reception in whales is being thoroughly investigated, the changes in
prenatal development (ontogeny) that generate these disparate ear bone
morphologies remain mostly unknown. In this study, we characterize the
ontogeny of the ear bones in Cetacea by looking at the progression of
ossification and associated changes in morphology using a combination of
traditional measurements and an innovative landmark-free method to
quantify shape on a newly assembled 3D dataset spanning the ontogeny and
phylogeny of extant Cetacea. We found that the two groups of Cetacea share
some aspects of inner ear ontogeny, such as a common growth trajectory of
the periotic.  However, differences in ossification, allometry
and growth trajectory, particularly in the tympanic bone, reflect their
divergent inner ear morphology and hearing abilities.