Anatomical, ontogenetic, and genomic homologies guide reconstructions of the teeth-to-baleen transition in mysticete whales

The transition in Mysticeti (Cetacea) from capture of individual prey using teeth to bulk filtering batches of small prey using baleen ranks among the most dramatic evolutionary transformations in mammalian history. We review phylogenetic work on the homology of mysticete feeding structures from anatomical, ontogenetic, and genomic perspectives. Six characters with key functional significance for filter-feeding behavior are mapped to cladograms based on 11 morphological datasets to reconstruct evolutionary change across the teeth-to-baleen transition. This comparative summary within a common parsimony framework reveals extensive conflicts among independent systematic efforts but also broad support for the newly named clade Kinetomenta (Aetiocetidae + Chaeomysticeti). Complementary anatomical studies using CTscans and ontogenetic series hint at commonalities between the developmental programs for teeth and baleen, lending further support for a ‘transitional chimaeric feeder’ scenario that best explains current knowledge on the transition to filter feeding. For some extant mysticetes, the ontogenetic sequence in fetal specimens recapitulates the inferred evolutionary transformation: from teeth, to teeth and baleen, to just baleen. Phylogenetic mapping of inactivating mutations reveals mutational decay of ‘dental genes’ related to enamel formation before the emergence of crown Mysticeti, while ‘baleen genes’ that were repurposed or newly derived during the evolutionary elaboration of baleen currently are poorly characterized. Review and meta-analysis of available data suggest that the teeth-to-baleen transition in Mysticeti ranks among the best characterized macroevolutionary shifts due to the diversity of data from the genome, the fossil record, comparative anatomy, and ontogeny that directly bears on this remarkable evolutionary transformation. ### Competing Interest Statement The authors have declared no competing interest.