A giant dolphin once resided in the Peruvian Amazon. Its closest living relatives today inhabit the Indus and Ganga

The largest river dolphin that ever inhabited the planet, swam the waters of the Amazon Basin in Peru 16 million years ago, an international team of researchers led by the University of Zurich (UZH) revealed on March 20, 2024.

The closest living relatives of the Pebanista yacuruna are found today in South Asia. The Ganges river dolphin (Platanista gangetica) is found in India, Nepal and Bangladesh. The Indus river dolphin (Platanista minor), on the other hand, is found in the Indus and its tributaries in Pakistan and the Beas (also an Indus tributary) in India.

The team discovered the fossilised skull of the dolphin. The long-gone species would have measured 3.5 metres long, the largest of its kind ever.

“The new dolphin species belongs to the Platanistoidea, a group of dolphins that were common in the world’s oceans between 24 and 16 million years ago. The researchers believe that their originally marine ancestors invaded the prey-rich freshwater ecosystems of proto-Amazonia and adapted to this new environment,” the online science, research and technology news aggregator, Phys.org noted.

Much of the Amazonian Plain 16 million years ago was covered by a large system of lakes and swamps called Pebas.

“This landscape included aquatic, semi-aquatic and terrestrial ecosystems (swamps, floodplains, etc.) and stretched across what is today Colombia, Ecuador, Bolivia, Peru and Brazil,” lead author Aldo Benites-Palomino from the Department of Paleontology at UZH, said.

Then, about 10 million years, the Pebas began to give way to modern Amazonia. The Pebanista’s prey disappeared due to this change in climate, subsequently causing the dolphin itself to vanish.

However, the disappearance of the giant dolphin allowed relatives of the Ina, the extant Amazon river dolphins, to exploit an ecological niche. The Ina’s relatives were also facing extinction in the oceans due to the rise of new cetaceans (marine mammals), such as modern oceanic dolphins, the researchers highlighted.

The Pebanista was not of the same family as the Ina. The latter belongs to the family Iniidae.

The Pebanista, on the other hand, was found to be related to the Indus and Ganges dolphins found in river systems thousands of kilometres away.

“With this fossil record unearthed in the Amazon, we expected to find close relatives of the living Amazon River dolphin—but instead the closest cousins of Pebanista are the South Asian river dolphins (genus Platanista),” Phys.org quoted Benites-Palomino as saying.

Both, Pebanista and Platanista “share highly developed facial crests, which are specialized bony structures associated with echolocation”.

Echolocation is used by river dolphins to find their way in muddy waters. These animals emit high-frequency sounds and listen to echoes as the sound waves reflect off different objects in the environment.

The dolphins use this mechanism not just to find their way in a waterway but also to locate prey.

The second part of the Pebanista’s taxonomic name was derived from the Northern Quechua word yacuruna. It was a mythical aquatic creature believed to inhabit the Amazon basin.

Besides Iniidae and Platanistidae, other modern-day river dolphin families include the Lipotidae and Pontoporiidae.

However, the Yangtze river dolphin (Lipotes vexillifer), which inhabited the Yangtze river basin in China, was declared extinct a few decades ago.

The study titled The largest freshwater odontocete: A South Asian river dolphin relative from the proto-Amazonia have been published in the journal Science Advances.