Evolution of river dolphins

The world's river dolphins (Inia, Pontoporia, Lipotes and Platanista) are a mong the least known and most endangered of all cetaceans. The four extant genera inhabit geographically disjunct river systems and exhibit highly mod ified morphologies, leading many cetologists to regard river dolphins as an unnatural group. Numerous arrangements have been proposed for their phylog enetic relationships to one another and to other odontocete cetaceans. Thes e alternative views strongly affect the biogeographical and evolutionary im plications raised by the important, although limited, fossil record of rive r dolphins. We present a hypothesis of river dolphin relationships based on phylogenetic analysis of three mitochondrial genes for 29 cetacean species , concluding that the four genera represent three separate, ancient branche s in odontocete evolution. Our molecular phylogeny corresponds well with th e first fossil appearances of the primary lineages of modern odontocetes. I ntegrating relevant events in Tertiary palaeoceanography, we develop a scen ario far river dolphin evolution during the globally high sea levels of the Middle Miocene. We suggest that ancestors of the four extant river dolphin lineages colonized the shallow epicontinental seas that inundated the Amaz on, Parana, Yangtze and Indo-Gangetic river basins, subsequently remaining in these extensive waterways during their transition to freshwater with the Late Neogene trend of sea-level lowering.