river dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water. they are an informal grouping of dolphins, which itself is a paraphyletic group within the infraorder cetacea. the river dolphins comprise the extant families platanistidae (the indian dolphins) (old world family), iniidae (the amazonian dolphins) and pontoporiidae (the brackish dolphins) (new world families). there are five extant species of river dolphins. river dolphins, alongside other cetaceans, belong to the clade cetartiodactyla, with even-toed ungulates, and their closest living relatives the hippopotamuses, from which they diverged about 40 million years ago.