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Fossil of largest ever river dolphin found in Amazon

Researchers have found the fossilised skull of the largest ever river dolphin. While found in the Peruvian Amazon, the ancient dolphin’s closest living relatives are in South Asia.

Pebanista yacuruna would have measured between 3 and 3.5 m. The skull of the extinct animal was found in 2018 along the Rio Napo near Peru’s border with Colombia, more than 1,000 km northeast of the capital Lima. It is described in a paper published in the journal Science Advances.

Its name is derived from a mythical aquatic people believed to inhabit the Amazon basin called the “yacuruna” in the Quechuan language of the region.

P. yacuruna lived 16 million years ago during the early to mid-Miocene (23–5.3 million years ago).

“Sixteen million years ago, the Peruvian Amazonia looked very different from what it is today,” says lead author Aldo Benites-Palomino from University of Zurich (UZH), Switzerland. “Much of the Amazonian plain was covered by a large system of lakes and swamps called Pebas.”

This landscape covered the region that today stretches across Colombia, Ecuador, Bolivia, Peru and Brazil. When the Pebas gave way to the modern Amazonia about 10 million years ago,  Pebanista’s prey disappeared, causing the ancient river dolphin to become extinct.

The new dolphin is part of a group that was common in the world’s oceans 24–16 million years ago. Palaeontologists believe its marine ancestors invaded prey-rich freshwater ecosystems.

“We discovered that its size is not the only remarkable aspect,” says Benites-Palomino. “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).”

Pebanista and Platanista both have highly developed facial crests associated with echolocation.

“For river dolphins, echolocation, or biosonar, is even more critical as the waters they inhabit are extremely muddy, which impedes their vision,” explains co-author Gabriel Aguirre-Fernández, also at UZH.

Today, river dolphins are among the rarest cetaceans. All modern species are endangered or critically endangered.

Palaeontologist sitting on ground at site with fossils
Paleontologist Aldo Benites-Palomino (Department of Paleontology, University of Zurich) while processing samples during the 2018 expedition to the Napo river. Credit: Rodolfo Salas-Gismondi.

The largest living species is the Amazon River dolphin, also known as the pink river dolphin or boto, which can reach 2.4 m in length.

Pebanista yacuruna joins another ancient Amazonian giant revealed last week: fossils of one of the largest ever freshwater turtles found in Brazil.

“After two decades of work in South America we had found several giant forms from the region, but this is the first dolphin of its kind,” says co-author Marcelo R. Sánchez-Villagra from UZH.

Source: Cosmos