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 fresh water habitats in the northern 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.
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.
Platanistidae is a family of river dolphins containing the extant Ganges river dolphin and Indus river dolphin but also extinct relatives from marine deposits in the Neogene.
The squamous part of the frontal bone is the superior portion when viewed in standard anatomical orientation. There are two surfaces of the squamous part of the frontal bone: the external surface, and the internal surface.
Functional extinction is the extinction of a species or other taxon such that:
- It disappears from the fossil record, or historic reports of its existence cease;
- The reduced population no longer plays a significant role in ecosystem function; or
- The population is no longer viable. There are no individuals able to reproduce, or the small population of breeding individuals will not be able to sustain itself due to inbreeding depression and genetic drift, which leads to a loss of fitness.
The baiji is a possibly extinct species of freshwater dolphin native to the Yangtze river system in China. It is thought to be the first dolphin species driven to extinction due to the impact of humans. This dolphin is listed as “critically endangered: possibly extinct” by the IUCN, has not been seen in 40 years, and several surveys of the Yangtze have failed to find it. In China, the species is also called the Chinese river dolphin, Han river dolphin, Yangtze dolphin and whitefin dolphin. Nicknamed the "Goddess of the Yangtze", it was regarded as the goddess of protection by local fishermen and boatmen. It is not to be confused with the Chinese white dolphin or the finless porpoise.
Piscobalaena is an extinct genus of cetaceans, which lived from the Middle to Late Miocene epochs in Peru and Florida. Its fossils have been found in the Pisco Formation of Peru and the Bone Valley Formation of Florida. At least some individuals of this diminutive whale were preyed on by the shark C. megalodon.
"Balaenoptera" ryani is an extinct species of Balaenopteridae from the late Miocene of California. It was originally considered a species of Balaenoptera, but is now recognized as more primitive than any extant or fossil balaenopterid and thus in need of a new generic name.
The Astoria Formation is a geologic formation in Washington state & Oregon. It preserves fossils dating back to the early to middle Miocene.
Zarhinocetus is an extinct genus of whale from the Early to Middle Miocene of the eastern North Pacific.
Araeodelphis is an extinct genus of river dolphin from the early Miocene of the US Eastern Seaboard.
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.
Kumimanu is an extinct genus of giant penguin, which lived around 60 to 56 million years ago. The type species is K. biceae, which arose after the extinction of the non-avian dinosaurs. Fossils were found in New Zealand, and the discovery was announced in December 2017. A second species, Kumimanu fordycei, was named in February 2023.
Ninjadelphis is an extinct genus of river dolphin from the early Miocene (Burdigalian) of Japan.
Brujadelphis is an extinct genus of river dolphin-like cetaceans of uncertain family placement from the Late Miocene epoch (Serravallian) of present-day Peru. The type species is Brujadelphis ankylorostris, recovered from the Pisco Formation.
Urkudelphis is an extinct genus of cetacean. The type species, Urkudelphis chawpipacha, was described in 2017 based on fossils found in the Dos Bocas Formation of Ecuador. It was a type of river dolphin.
Macrosqualodelphis is an extinct genus of river dolphins from the Early Miocene (Burdigalian) Chilcatay Formation of the Pisco Basin, Peru. The type species is M. ukupachai.
Awadelphis hirayamai is a species of extinct oceanic river dolphin discovered in the Senhata Formation of Japan dating around 6 million years ago (mya). The genus name derives from Awa, an ancient name for the Chiba Prefecture where the holotype specimen was collected, and Ancient Greek delphis meaning "dolphin"; the species name honors the discoverer of the holotype, Ren Hirayama. It may have inhabited a subpolar environment. Awadelphis and other river dolphins of the Late Miocene may have been in abundance due to the decline of competing ancient dolphins and porpoises.
