Vertebrae are the backbone of cetacean diversity: How morphological innovations sustained dolphin explosive radiation

With approximately 90 living species, whales, dolphins and porpoises represent the most diverse clade of extant marine tetrapods. This high level of taxonomic diversity has been often related to ocean restructuring that resulted in an explosive radiation of oceanic dolphins within the past 10 Ma. However, the environmental factor hypothesis can be restrictive as it does not entirely explain how organisms have faced environmental constraints suggesting that other factors could also explain this burst of diversification. In marine taxa such as sharks and ichthyosaurs, morphological variations have been linked to several life-styles which have sustained their diversification in different adaptive zones. The aim of our study is to establish the relationship between the morphology of the axial skeleton of cetaceans, their ecology and their diversification. By combining the most extensive morphological dataset describing the axial skeleton of 73 cetacean species with phylogenetic comparative methods, we demonstrate that extant cetaceans have followed two distinct evolutionary pathways in relation to their ecology. Most oceanic species evolved towards an increased body size leading to gigantism in baleen whales. Interestingly, dolphins have invested another way. While riverine and coastal species exhibit a small body size, lengthened vertebrae and a low vertebral count, small oceanic dolphins show an extremely high number of short vertebrae. We discuss how these modifications have operated as key innovations that contributed to the explosive radiation of dolphins.