Scientists have discovered the fossilised skull of a giant river dolphin, from a species thought to have fled the ocean and sought refuge in Peru’s Amazonian rivers 16m years ago. The extinct species would have measured up to 3.5 metres long, making it the largest river dolphin ever found, Ednews reports via The Guardian.

The discovery of this new species, Pebanista yacuruna, highlights the looming risks to the world’s remaining river dolphins, all of which face similar extinction threats in the next 20 to 40 years, according to the lead author of new research published in Science Advances today. Aldo Benites-Palomino said it belonged to the Platanistoidea family of dolphins commonly found in oceans between 24m and 16m years ago.

Surviving river dolphins were “the remnants of what were once greatly diverse marine dolphin groups”, he said, which were thought to have left the oceans to find new food sources in freshwater rivers.

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

Benites-Palomino discovered the fossil in Peru in 2018 when he was still an undergraduate. He is now working on a doctorate at the University of Zurich’s department of paleontology, and says the research paper was delayed by the pandemic.

He first spotted part of the fossil, a fragment of jaw, while walking with a colleague. “As soon as I recognised it, I saw the teeth sockets. I screamed, ‘this is a dolphin.’ We could not believe it.

“Then we realised it was not related to the Amazon river pink dolphin,” he said. “We [had] found an animal, a giant, whose closest living relative is 10,000km away in south-east Asia.”

Marcelo R Sánchez-Villagra, director of Zurich University’s paleontology department, said the find was intriguing. “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,” he said.

The fossil, said Benites-Palomino, was remarkable both for its size and because it had no links to the river dolphins that now swim in the waters it once inhabited.

A shared problem facing river dolphins – including the fossil’s nearest living relatives, which swim in the Ganges and Indus rivers – is the imminent risk of extinction. Urban development, pollution and mining were the key causes, he said, and had already driven the Yangtze river dolphin to extinction.