The discovery of a new ancient species of freshwater dolphin in the Peruvian Amazon has been announced by paleontologists from the University of Zurich (UZ) in Switzerland.

The scientists say the creature is the largest river dolphin ever found, and measured 3-3.5m. It lived at least 16 million years ago, and curiously, the closest living relative of the prehistoric cetacean can be found today in South Asia.

River dolphins are among the rarest modern cetaceans, with most species critically endangered. Despite their similar appearance, however, these animals are not directly related, but represent the late survivors of different cetacean groups that once inhabited our planet.

The new species is named Pebanista yacuruna after a mythical aquatic people believed to inhabit the Amazon basin. 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.

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

This landscape included aquatic, semi-aquatic and terrestrial ecosystems such as swamps and floodplains. It stretched across present-day Colombia, Ecuador, Bolivia, Peru and Brazil.

When the Pebas began to give way to modern Amazonia about 10 million years ago, new habitats caused Pebanista’s prey to disappear, driving the giant dolphin to extinction. This opened an ecological niche that was exploited by relatives of today’s Amazon river dolphins (Inia), which were also facing extinction in the oceans due to the rise of new cetaceans, such as modern oceanic dolphins.

“We discovered that its size is not the only remarkable aspect,” says Aldo 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 share highly developed facial crests, specialised bony structures associated with echolocation, which may mean they were able to emit high-frequency sounds and listen for their echoes, which they could rely for hunting.

“For river dolphins, echolocation or biosonar is critical as the waters they inhabit are extremely muddy, which impedes their vision,” explains Gabriel Aguirre-Fernández, a UZ researcher.

The elongated snout with many teeth suggests that Pebanista fed on fish, just as the river dolphins of today do.

“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,” adds Marcelo Sánchez-Villagra, director of the department of paleontology at UZ. “We were especially intrigued by its peculiar biogeographical deep-time history.”

Finding fossils

The Amazon rainforest is one of the harshest regions for paleontological fieldwork. 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, the rising water levels during the rainy season will sweep them away and they will be lost forever.

The holotype of Pebanista - which is a single physical specimen on which the description and name of a new species is based -was found in 2018, when the lead author of the study was still an undergraduate student. The expedition, led by Peruvian paleontologist Rodolfo Salas-Gismondi, former postdoctoral fellow at the department of paleontology at UZ, traversed more than 300km of the Napo River.

Dozens of fossils were discovered and collected, but the biggest surprise waited at the end of the expedition, after almost three weeks of exploration: the discovery of the large dolphin skull, which has been permanently deposited in the Museo de Historia Natural in Lima.