Fossils of world's largest dolphin are discovered in Amazon - ancient creature was more than 11 feet long when it swam the oceans over 16 million years ago

Remains of the world's largest dolphin have been uncovered in the Peruvian Amazon, revealing the mammal measured up to 11 feet long.

The remains of an ancient species distantly related to the rare and endangered river dolphin have been discovered in the Peruvian Amazon.

Paleontologists at Switzerland's University of Zurich (UZH) found the fossils suggested the ancient creature was distantly related to the rare and endangered river dolphin that live around South America.

The fossils suggested the newly found Pebanista yacuruna had poor eyesight, an elongated snout and numerous teeth when it roamed the oceans more than 16 million years ago.

The team named the new species in honor of the mythical people known as Tacuruna, who were said to live in underwater cities around the Amazon basin.

The fossilized remains of an ancient dolphin believed to have lived 16 million years ago was found in the Peruvian Amazon

A team of researchers from the University of Zurich found the skull of the largest dolphin ever discovered

Researchers first discovered the skull of the dolphin during a 2018 expedition to Peru when they saw the fossil protruding from the embankment of the Napo River. 

Surviving river dolphins were 'the remnants of what were once greatly diverse marine dolphin groups', Aldo Benites-Palomino told The Guardian, adding that it's believed they left oceans in exchange for freshwater rivers to find food sources.

'Rivers are the escape valve … for the ancient fossil we found, and it is the same for all river dolphins living today,' he said.

At the time when the ancient dolphin populated the oceans, the Peruvian Amazon had a very different landscape - It was covered by large lakes and swamps that covered Colombia, Ecuador, Bolivia, Peru, and Brazil.

Aldo Benites-Palomino first discovered the dolphin's skull in an embankment of the Napo River in Peru in 2018

A changing climate caused the Pebanista to disappear, the researchers said, because their prey started to disappear and the lack of a food source drove the dolphin to extinction.

About 10 million years ago, the Amazonian waters worked through sandstone in a westward direction, forcing the remaining lake water to flow eastward.

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At this time, the large lake started drying up and became a river, turning the area from a humid and diverse ecosystem to a more arid and sparse region. 

'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,' said Marcelo R. Sánchez-Villagra, director of the Department of Paleontology at UZH.

'We were especially intrigued by its peculiar biogeographical deep-time history.'

Benites-Palomino told NewScientist that the region where he and his team found the fossil was once covered by an 'insanely big' lake, 'almost like a little ocean in the middle of the jungle.

The dolphin's small eye sockets led researchers to believe that it had poor eyesight and Benites-Palomino told the outlet: 'We know that it was living in really muddy waters because its eyes started to reduce in size.'

Researchers found the Pebanista had an elongated snout and numerous teeth which indicated that the dolphin fed on fish, like many other species of modern-day river dolphins.

Benites-Palomino and his team expected the dolphin to be closely related to the modern-day Amazon river dolphin but instead found that the raised crests on its head that help with echolocation make it similar to the South Asian river dolphin.

Echolocation is an animal's ability to 'see' by listening to the echoes of their high-frequency sounds which are used for hunting.

'For river dolphins, echolocation, or biosonar, is even more critical as the waters they inhabit are extremely muddy, which impedes their vision,' said Gabriel Aguirre-Fernández, a UZH researcher and co-author of the study.

Finding fossils in the Amazon is increasingly difficult because paleontologists need to wait until the region's 'dry season' when river levels are low enough to expose fossilized remains.

Collecting the fossils is a time-sensitive process because if paleontologists don't extricate them before the dry season ends, the rising river tides could sweep them away and they could be lost forever.