Prehistoric Giant Dolphin Discovered in the Amazon Has Foreign Connections

Between 3 and 3.5 meters long and 16 million years old: paleontologists at the University of Zurich have discovered a new species of freshwater dolphin in the Peruvian Amazon. Surprisingly, their closest living relatives are the river dolphins of Southeast Asia.

River dolphins are among the rarest modern whale species, and most of the existing species are critically endangered. Despite their similar appearance, these animals are not directly related to each other, but represent the last survivors of different groups of whales that once populated our planet.

An international research team led by the University of Zurich (UZH) has now discovered the largest river dolphin ever found, which was between 3 and 3.5 meters long. The new species, called Pebanista yacuruna, named after a mythical aquatic people of the Amazon basin, was found in the Peruvian Amazon and is estimated to be 16 million years old.

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Paleontologists (from left to right): Rodolfo Salas-Gismondi (Universidad Peruana Cayetano Heredia, Lima, Peru), Julia Tejada-Lara (California Institute of Technology, California, USA), John Flynn (American Museum of Natura History, New York, USA) & local guide Michel Valles during the 2018 expedition to the Rio Napo excavating a 13-million-year-old fossil outcrop. (Aldo Benites-Palomino/University of Zurich)

Changing Landscape Caused Giant Dolphins to Become Extinct

The new dolphin species belongs to the Platanistoidea, a group of dolphins that were widespread in the world's oceans between 24 and 16 million years ago. The researchers suspect that their original marine ancestors penetrated the prey-rich freshwater ecosystems of early Amazonia and adapted to this new environment.

“16 million years ago, the Peruvian Amazon looked completely different than it does today,” says lead author Aldo Benites-Palomino from the UZH Institute of Paleontology. "A large part of the Amazon lowland was covered by an extensive system of lakes and swamps, the Pebas." This landscape included aquatic, semiaquatic, and terrestrial ecosystems (swamps, floodplains, etc.) and spanned what is now Colombia, Ecuador, Bolivia, Peru, and Brazil.

When the Pebas system began to give way to the modern Amazon region about 10 million years ago, new habitats emerged in which the Pebanista's prey disappeared, and the giant dolphin eventually became extinct. The resulting ecological niche was used by the relatives of today's Amazon river dolphins (Inia), which were displaced from the oceans by new whales and dolphins such as the modern ocean dolphins.

“We had expected a close relative of the living Amazon river dolphin - instead the Pebanista is related to the South Asian river dolphins.”  
Aldo Benites Palomino

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The paleontologist Aldo Benites-Palomino (Paleontological Institute of the University of Zurich) cleaning of the giant Pebanista yacuruna dolphin species skull. (University of Zurich)

Insights into the Evolutionary History of Freshwater Dolphins

“We discovered that it wasn’t just the size of the dolphin we described that was remarkable,” says Aldo Benites-Palomino. "We expected this fossil found in the Amazon to be a close relative of the living Amazon dolphin - instead, the Pebanista is related to the South Asian river dolphins (genus Platanista)."

Both the Pebanista and Platanista have highly developed facial ridges, specialized bone structures associated with echolocation. This gives them the ability to “see” by emitting high-frequency sounds and hearing echoes, which they heavily depend on when hunting.

“Echolocation, also known as biosonar, is even more important for river dolphins. The waters in which they live are extremely muddy, which hinders their visibility,” explains Gabriel Aguirre-Fernández, a postdoc at UZH and also involved in the study. The elongated snout with many teeth suggests that Pebanista fed on fish, as other river dolphin species do today.

“After two decades of work in South America, we have found several giant forms from the region, but this is the first dolphin of its kind,” adds Marcelo R. Sánchez-Villagra, director of the UZH Paleontological Institute. “We were particularly fascinated by its special biogeographical history.”

International team of paleontologists during the 2018 expedition to the Rio Napo. (Aldo Benites-Palomino)

Hunting for Fossils in the Amazon Region

The Amazon rainforest is one of the most difficult areas for paleontological field research. Fossils are only accessible during the dry season, when river levels are low enough to expose the ancient fossil-bearing rocks. If these fossils are not collected in time, they will be washed away by rising floodwaters during the rainy season and will be lost forever.

The holotype - a single specimen on which the description and name of a new species is based - of Pebanista was found in 2018, when the study's lead author was still an undergraduate student. The expedition, led by Peruvian paleontologist Rodolfo Salas-Gismondi, a former postdoc at the UZH Institute of Paleontology, led over 300 kilometers (185 miles) along the Rio Napo.

Dozens of fossils were discovered and collected, but the biggest surprise awaited at the end of the expedition, after almost three weeks of excavation: the discovery of the large dolphin skull, cataloged as MUSM 4017, which was permanently deposited in the Museo de Historia Natural in Lima.

The article ‘Prehistoric giant dolphin discovered in the Amazon’ is a press release by the University of Zurich.

Top image: Pictorial reconstruction of Pebanista yacuruna in the murky waters of the Peruvian proto-Amazon. Source: Jaime Bran/University of Zurich