Morphological diversity of the cetacean mandibular symphysis coincides with novel modes of aquatic feeding

In whales, extreme modifications to the ancestral mammalian feeding
apparatus facilitate novel modes of aquatic feeding. These modifications
manifest in morphological diversity across a suite of characters,
including the mandibular symphysis. Cetaceans span a range of symphyseal
morphologies, with one lineage (crown mysticetes) evolving a highly mobile
condition unique among mammals. Here, we use phylogenetic comparative
methods to examine the evolution of symphyseal fusion and elongation
across 206 extant and fossil cetacean taxa. Ancestral state
reconstructions corroborate observations from the fossil record that
suggest the ancestral condition for Cetacea was a fused, moderately
elongated symphysis. Shifts in symphyseal morphology coincided with ocean
restructuring and diversification of feeding modes. Evolutionary rates
peaked in the middle-late Eocene and at the Eocene-Oligocene boundary as
whales evolved shorter, unfused symphyses. During the Eocene, ankylosed
mandibles became less common with the appearance of increasingly pelagic
whales. Mysticetes evolved decoupled, highly mobile mandibles near the
Eocene-Oligocene boundary. Several odontocete lineages underwent a trait
reversal and converged on fully fused, elongated mandibles in the Miocene.
Analyses evaluating the influence of ecological variables indicate strong
correlations in feeding strategy, dentition, and prey type. The loss of
prey-processing behavior and changes to masticatory loading regimes may
explain concurrent trends in symphyseal morphology and tooth
simplification. We suggest that the functional and morphological diversity
of the symphysis in whales is a consequence of aquatic feeding imposing
different mechanical constraints than those associated with feeding on
land.