River dolphins are a widely distributed group of fully aquatic mammals that reside exclusively in freshwater or brackish water. They are an informal grouping within the group dolphin, which is a paraphyletic group within the infraorder Cetacea. The river dolphins comprise the extant families Platanistidae (the Indian dolphins), Iniidae (the Amazonian dolphins), and Pontoporiidae (the brackish dolphins). There are five extant species of dolphins, and two subspecies. River dolphins, alongside other cetaceans, belong to the clade Cetartiodactyla, with even-toed ungulates, and their closest living relatives the hippopotamuses, having diverged about 40 million years ago.