River Dolphins, Evolutionary History and Affinities

Abstract

This chapter describes the evolutionary history and affinities of river dolphins. The term “river dolphins” or Platanistoids has been traditionally used to include the recent odontocetes that live in freshwater and are not members of the other clades of odontocetes: delphinoids, ziphioids, and physeteroids. Their affinities to other groups of odontocetes were unresolved, mainly because they have many plesiomorphic characters. There are four genera of living “river dolphins”: Platanista, Lipotes, Inia, and Pontoporia. Other (partly) freshwater odontocetes include Orcaella (Irrawadi River) and Sotalia (Amazon River) are not included in the Platanistoidea because they are clearly related to the marine dolphins, Delphinidae. There is near consensus that the odontocetes traditionally placed in the “river dolphins” belong to two different groups of dolphins and are polyphyletic: the Platanistoidea and the Delphinida. The Platanistoidea represent the sister group of a clade, which includes the Delphinida and the fossil superfamily Eurhinodelphinoidea. The non-platanistoid “river dolphins” do not represent a monophyletic grouping. Fossil platanistoids are diverse and distributed into several families. Fossil Lipotids and Inioids are still relatively scarce but can be easily related to one of the three non-Platanistoid families of “river dolphins.”