River dolphins are an informal grouping of dolphins, associated with the Cetacea infraorder. This is a group of aquatic mammals that consists of nearly 89 living species. These species include various types of whales, dolphins, narwhals, etc. Taxonomists categorize river dolphins together based on similar characteristics. However, river dolphins do not share an immediate common ancestor. In fact, hippopotamuses are the closest living relative to the river dolphin species. It seems bizarre, but it’s true. The two species diverged nearly 40 million years ago.
There’s still a lot to learn about river dolphins, but we know enough to understand that the ancestors of modern river dolphins were displaced from their original marine environments. By whom? More river dolphins –their modern descendants!
Because of convergent evolution, river dolphins share many similarities and adaptations to freshwater environments. This helps explain how Amazon river dolphins are more closely related to dolphins residing in the ocean than South Asian river dolphins.
There are four families of river dolphins:
These four families are placed within three superfamilies, Inioidea, Lipotoidea, and Platanistoidea. A superfamily is a taxonomic category that is ranked above family and below order. Let’s explore these family structures of river dolphins!
Under the Platanistoidea superfamily falls the Platanisdae family. The only species of river dolphin within the Platanistoidea superfamily still living today is the South Asian river dolphin. This family makes up two subspecies of South Asian river dolphins:
- Ganges river dolphin
- Indus river dolphin
These dolphins fall under the Genus Platanista taxonomic category.
Taxonomists once considered the Ganges and Indus river dolphins a separate species while all other river dolphins were categorized under the Platanistidae family. This is no longer the case. Taxonomists now group the two species under the Platanistidae family.
Other extinct families within the Genus Platanista include the Allodelphinidae, Squalodelphinidae, Squalodontidae, and Waipatiidae families.
Under the superfamily Inioidea falls the Iniidae family. Within this family, there is one living genus, known as Inia. The Inia inhabits various river basins in South America. Before environmental threats threatened the river dolphin species, the family once had a wider presence across the Atlantic Ocean.
Several notable species fall under the Genus Inia:
- Amazon river dolphin
- Araguaian river dolphin
- Bolivian river dolphin
Several subspecies of the Amazon river dolphin include the Inia geoffrensis geoffrensis and Inia geoffrensis humbotiana.
Extinct river dolphin species within this superfamily include the Genus Meherrinia and Genus Isthminia.
The Family Pontoporiidae also falls under the superfamily Inioidea. It is a species of river dolphin that lives in the coastal Atlantic waters of southeastern South America. This family differs from the typical river dolphin because it is not fresh water; it actually lives in the ocean and saltwater estuaries. Most river dolphins live exclusively within freshwater systems.
La Plata dolphin falls under the Family Pontoporiidae umbrella. It also includes the extinct species Auroracetus bakerae.
Under the Superfamily Lipotoidea and the family Lipotidae is the Genus Lipotes. This superfamily includes the Baiji, also known as the Chinese river dolphin. We aren’t sure whether or not this species of river dolphin is still in existence today. But it has been categorized as possibly extinct due to the limited data for this particular subspecies.
Unfortunately, the Baiji tells the tale of negative human impact on the environment and ecosystems. The Chinese river dolphin is considered the first river dolphin species to be driven to extinction. That cause? Solely human impact. Although conservationists made an effort to save the species, a 2006 expedition failed to locate any Baiji in the Yangtze River. As a result, organizers declared the species to be fundamentally extinct.
The river dolphin has a highly complex ancestral lineage that makes the species incredibly unique. It is so unique it is polyphyletic, a word used to explain the species’ emergence due to its multiple ancestral sources. Because it doesn’t share a common ancestor, these dolphins are categorized based on shared characteristics. As a result, we now have three superfamilies of this incredible species.
As we saw with the Chinese river dolphin, humans can single-handedly destroy a species. Today, river dolphins are highly susceptible to habitat disruption. All species are endangered, with some critically endangered.
(Featured image by Auch – Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=12401496)