Phylogeny and evolution of river dolphins

The term ‘river dolphins’ has been applied to four peculiar genera of odontocete cetaceans that inhabit disjunct river systems in South America and Asia. Three are obligately freshwater and have colonized the lowlying reaches of their respective river basins: the Amazon river dolphin Inia geoffrensis, the Chinese river dolphin Lipotes vexillifer , and the Indian river dolphin Platanista gangetica. The fourth species, Pontoporia blainvillei, inhabits the Rio de la Plata estuary and the nearby coast, but does not travel far upriver. Based on their similar ecological habit and superficial morphological similarities, the river dolphins were historically united as ‘platanistid’ dolphins (Family Platanistidae or Superfamily Platanistoidea). Cladistic morphological analyses recognize that river dolphins are paraphyletic, but rarely have any two studies reached similar conclusions regarding their relationships. All are morphologically peculiar; Platanista, in particular, is strikingly disparate.; Here I integrate existing paleontological and geological data with original mitochondrial genetic analyses to investigate the phylogeny and evolution of river dolphins. A molecular phylogeny of 29 cetacean species supports river dolphin paraphyly and suggests the Platanista lineage is part of the basal odontocete radiation. Integrating the molecular phylogeny with the limited but important fossil record and river paleohydrology results in a hypothesis of river dolphin isolation during the globally high sea levels of the Middle Miocene. Genetic divergence estimates for the split between Inia and Pontoporia and existing paleontological and paleohydrological data permit evaluation of three alternative hypotheses for their evolution. These data support the hypothesis of isolation during Middle Miocene high sea levels. Genetic variation in the three subspecies of Inia from the Orinoco, Amazon, and Bolivian river basins supports exisiting morphological data indicating the Bolivian Inia are highly distinct.; All four river dolphin lineages are older than previously recognized, and are as genetically distant from one another as any two families of odontocetes. River dolphins thus represent an underappreciated amount of the phylogenetic diversity of odontocete cetaceans. The poorly known Chinese river dolphin is irreversibly near extinction and the conservation status of Pontoporia and Platanista is tenuous. River dolphins merit greater research and conservation attention for their ecological, morphological and evolutionary uniqueness.