Indian dolphin

Indian dolphin Temporal range: Template:Fossil range Quaternary – recent
Ganges river dolphin breaking the surface
Scientific classification
:	[[Template:Taxonomy/Platanista]] Wagler, 1830
Type species
Delphinus gangeticus Lebeck, 1801
Species
Platanista gangetica Platanista minor
Current ranges of the Indus river dolphin and Ganges river dolphin

Indian dolphins, belonging to the genus Platanista, are toothed whales found in the waterways of the Indian subcontinent. Historically, they were regarded as a single species (P. gangetica), with the Ganges river dolphin and the Indus river dolphin considered subspecies (P. g. gangetica and P. g. minor respectively). However, due to genetic and morphological studies, they were reclassified as distinct species in 2021. It is estimated that the Ganges and Indus river dolphins diverged around 5,50,000 years ago. These dolphins are the sole surviving members of the family Platanistidae and the superfamily Platanistoidea. Fossils of their ancient ancestors have been traced back to the late Oligocene period.

Taxonomy

Indian river dolphins were historically classified as a single species, known as Platanista gangetica, with the populations in the Ganges and Indus Rivers recognised as subspecies (P. g. gangetica and P. g. minor, respectively). In 1801, Heinrich Julius Lebeck named the Ganges river dolphin Delphinus gangeticus, while Johann Georg Wagler introduced the genus name Platanista in 1830. This Latin term is believed to be derived from the Greek "platanistēs", potentially linked to the Greek words platē (meaning "oar") or platē (meaning "flat" or "broad"). The name was originally attributed to the Ganges dolphin by Pliny the Elder in his work Naturalis Historia around 77 CE. Later, in 1853, Richard Owen examined a specimen from the Indus and identified it as the same species as the Ganges river dolphin, albeit a smaller variant.

In the 1970s, scientists classified Indian river dolphins as separate species based on variations in skull and vertebral structures, as well as differences in blood proteins and lipids. However, these findings faced criticism due to limited sample sizes and a lack of statistical analysis. By the late 1990s, the two populations were once again regarded as subspecies of a single species. A mitochondrial DNA study conducted in 2014 indicated that there were not enough differences to justify their separation as distinct species. Nevertheless, a study in 2021 reassessed the two populations and identified significant genetic divergence along with considerable differences in skull morphology, ultimately confirming that they are indeed separate species.

Evolution

Indian river dolphins represent the sole surviving members of the family Platanistidae and the superfamily Platanistoidea. They do not share a close evolutionary relationship with other river dolphins belonging to the families Lipotidae, Pontoporiidae, and Iniidae, which have all adapted to freshwater environments independently. The accompanying cladogram, derived from the work of Gatesy and colleagues (2012) and McGowen and colleagues (2020), illustrates the evolutionary relationships between Indian river dolphins and other existing families of toothed whales.