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. Extant river dolphins are placed in two superfamilies, Platanistoidea and Inioidea. They comprise the families Platanistidae, the recently extinct Lipotidae, Iniidae and Pontoporiidae. There are five extant species of river dolphins. River dolphins, alongside other cetaceans, belong to the clade Artiodactyla, with even-toed ungulates, and their closest living relatives the hippopotamuses, from which they diverged about 40 million years ago. Specific types of dolphins can be pink.
South Asian river dolphins are toothed whales in the genus Platanista, which inhabit the waterways of the Indian subcontinent. They were historically considered to be one species with the Ganges river dolphin and the Indus river dolphin being subspecies. Genetic and morphological evidence led to their being described as separate species in 2021. The Ganges and Indus river dolphins are estimated to have diverged 550,000 years ago. They are the only living members of the family Platanistidae and the superfamily Platanistoidea. Fossils of ancient relatives date to the late Oligocene.
Iniidae is a family of river dolphins containing one living genus, Inia, and four extinct genera. The extant genus inhabits the river basins of South America, but the family formerly had a wider presence across the Atlantic Ocean.
Lipotidae is a family of river dolphins containing the possibly extinct baiji of China and the fossil genus Parapontoporia from the Late Miocene and Pliocene of the Pacific coast of North America. The genus Prolipotes, which is based on a mandible fragment from Neogene coastal deposits in Guangxi, China, has been classified as an extinct relative of the baiji, but is dubious. The oldest known member of the family is Eolipotes from the Late Miocene of Japan.
Squalodon is an extinct genus of whales of the Oligocene and Miocene epochs, belonging to the family Squalodontidae. Named by Jean-Pierre Sylvestre de Grateloup in 1840, it was originally believed to be an iguanodontid dinosaur but has since been reclassified. The name Squalodon comes from Squalus, a genus of shark. As a result, its name means "shark tooth". Its closest modern relative is the South Asian river dolphin.
Parapontoporia is an extinct genus of dolphin that lived off the Pacific coast of North America from the Late Miocene until the genus' extinction during the Pliocene. It is related to the baiji. Fossils have been found in California and Mexico. The Tulare Formation is predominately freshwater, which suggests Parapontoporia may have been tolerant of both salt and fresh water.
Kentriodontidae is an extinct family of odontocete whales related to modern dolphins. The Kentriodontidae lived from the Oligocene to the Pliocene before going extinct.
Meherrinia is an extinct genus of inioid river dolphin from the Meherrin River, North Carolina, in the United States. First described in 2012, the dolphin is, in most respects, intermediate in form between the living Amazon river dolphin and the La Plata dolphin, although it is probably more closely related to the former. However, the fossil was discovered in what are believed to be marine deposits, dating from the late Miocene, whereas the Amazon river dolphin is an exclusively freshwater species. Meherrinia therefore was, as of 2012, the only known marine genus of the family Iniidae, although others have been discovered since. Only one species is known.
Otekaikea is an extinct genus of toothed whale closely related to Waipatia. It is known from the late Oligocene (Chattian) of New Zealand.
Atocetus is an extinct genus of pontoporiid dolphin found in Miocene-age marine deposits in Peru and California.
Zarhinocetus is an extinct genus of whale from the Early to Middle Miocene of the eastern North Pacific.
Allodelphinidae is a family of primitive platanistoid river dolphins found in marine deposits in the eastern North Pacific region, Alaska, and Japan.
Araeodelphis is an extinct genus of river dolphin from the early Miocene of the East Coast of the United States.
Lophocetus is an extinct genus of dolphin belonging to the clade Delphinida that is known from late Miocene (Tortonian) marine deposits in California and Maryland. Although usually placed in Kentriodontidae, recent studies have found it only distantly related to Kentriodon.
Xenorophus is a genus of primitive odontocete from late Oligocene (Chattian) marine deposits in South Carolina. It belongs to the Xenorophidae.
Arktocara is an extinct genus of river dolphin from the Oligocene epoch of Alaska, containing one species, A. yakataga. Having been discovered in 25-million-year-old strata near the 60th parallel north, it is perhaps the oldest-known crown toothed whale and the northmost river dolphin discovered. It was a member of the now-extinct family Allodelphinidae, along with the genera Allodelphis, Goedertius, Ninjadelphis, and Zarhinocetus. It measured approximately 2.26 or 2.28 meters, comparable to its closest living relative, the South Asian river dolphin, which measures 2.4 meters (7.9 ft). However, the animal probably had an elongated beak and neck, so it may have been longer. The animal is known only from a partially preserved skull. Its ecology may have been similar to the modern-day Dall's porpoise, and it may have competed with contemporaneous delphinoids. Its remains were found in the Poul Creek Formation, which has also yielded several mollusk species.
Phoberodon is a genus of archaic odontocete cetacean from the Early Miocene (Burdigalian) of Patagonia, Argentina.
Dilophodelphis is an extinct genus of river dolphin from Early Miocene (Burdigalian) marine deposits in Oregon. The type species, Dilophodelphis fordycei, was named in 2017.
Pomatodelphis is an extinct genus of river dolphin from Middle Miocene marine deposits in Alabama, Florida, Brazil, Germany and France.
Delphinida is a clade of cetaceans in the parvorder Odontoceti, the toothed whales. It includes all modern oceanic dolphins, porpoises, and their relatives, such as Lipotidae and Iniidae.
