(CNN) - Imagine a dolphin. Now imagine one that is twice the size of a person.
About 16 million years ago, a dolphin giant traveled the depths of its watery domain. But unlike most modern dolphins, its home was not an ocean; it lived in a freshwater lake in the Peruvian Amazon. And while there are freshwater dolphin species from the Amazon today, they are not closely related to those ancient cetaceans. Its closest relatives are river dolphins that live more than 10,000 kilometers away in South Asia, according to researchers who recently described the previously unknown extinct mammal.
Analysis of the newly identified ancient dolphin's skull told paleontologists that its body would have been at least 11 feet (3.5 meters) long - making it about 20% to 25% larger than modern river dolphins and the largest known freshwater dolphin.
But the skull, which was about 70 centimeters long, was incomplete, so the ancient dolphin may have been even larger, the scientists reported March 20 in the journal Science Advances, published by the American Association for the Advancement of Science.
What makes the find even more exceptional is the insight it provides into the evolutionary history of freshwater dolphins, as these creatures are extremely rare in the fossil record, the study authors wrote. This is because there are typically fewer individual dolphins in freshwater ecosystems, and strong water currents typically prevent fossils from being well preserved.
They named the newly discovered species Pebanista yacuruna; the genus refers to the Pebas Formation in Peru, where the fossil was found, and 'yacuruna' is a term for mythical water people from local legend, in the indigenous Kichua language.
"I think this is a remarkable discovery, especially considering that South America has one species of river dolphins that belongs to a completely different group of odontocetes (toothed whales)," said Jorge Velez-Juarbe, associate curator of marine mammals at the Natural History Museum of Los Angeles County, in an email.
"(It) also makes me wonder how many other records of extinct river dolphins are waiting to be discovered," Velez-Juarbe, who was not involved in the study, told CNN.
'Everyone panicked': Fossil features reveal rare find
Modern freshwater dolphins are known for their very elongated noses, compared to the stubbier snouts of marine dolphins. There is the South Asian river dolphin (genus Platanista) and the Amazon river dolphin (genus Inia), also known as the pink river dolphin, and the two groups include several species and subspecies.
The Chinese Yangtze River dolphin (Lipotes vexillifer) represents a third genus, but the species has not been seen in the wild for four decades and may be extinct, according to the International Union for the Conservation of Nature (IUCN). In fact, all existing species of river dolphins are endangered or critically endangered, the IUCN says.
Researchers discovered the Amazon dolphin fossil in 2018, near the Napo River in Loreto, Peru. Lead study author Aldo Benites-Palomino, a doctoral candidate at the University of Zurich's Department of Paleontology, had stopped to examine some strange-looking rock fragments on the ground, he told CNN. At the same time, co-author John J. Flynn, curator of fossil mammals at the American Museum of Natural History in New York City, pointed out what appeared to be a skull protruding from an embankment.
"I said, 'Hey, John, does this have to do with this piece I have in my hand?'" Benites-Palomino recalled. What he was holding turned out to be a rostrum - the rounded tip of a nose - coming from the embedded skull. As they cleaned it enough to see the shape of the tooth sockets, Benites-Palomino realized they were looking at something unusual.
"We started shouting, 'It's a dolphin!' It's a dolphin!'" Benites-Palomino said.
Initially, they thought it might turn out to be an ancient relative of modern Amazonian river dolphins. But further cleaning revealed that the size and shape of the eye sockets resembled those of South Asian river dolphins, which have much smaller eyes than their South American counterparts.
"That was a moment when everyone panicked because it wasn't an Amazon river dolphin," Benites-Palomino said. This told the scientists that two species of dolphins had moved inland into the region independently and at different times.
Unearthing dolphin diversity
Platanistoids - the group to which P. yacuruna and the modern river dolphins of southern Asia belong - were widespread about 20 million years ago. The ancestors of modern Amazonian river dolphins were common in the oceans about 10 million to 6 million years ago, Benites-Palomino said.
Because both groups of cetaceans were so diverse, some species likely ventured into river and lake ecosystems in search of less competition for food. This freshwater environment in the Amazon was rich in nutrients and teeming with life. It was home to crocodiles, turtles and fish, as well as mammals such as sloths, rodents, ungulates and primates.
"Overall, 'river dolphins' can be considered top predators in these ecosystems," says Velez-Juarbe.
P. yacuruna was among the first wave of dolphins to test the waters in the rivers and lakes of the Amazon; A lack of predators in its new home could explain how the species grew so large, according to the study. But environmental changes such as drought may have later doomed P. yacuruna to extinction, opening freshwater habitat to the ancestors of extant pink river dolphins.
"We now know that this species lived there in the past, but the Amazon is also important for our existing Inia geoffrensis," Benites-Palomino said. "[The discovery] emphasizes that this is a hugely important environment for the evolution of freshwater cetaceans."
Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works.The disappearance of P. yacuruna is a stark reminder that this important environment is all too easily disrupted. Today, modern Amazon river dolphins face an uncertain future, mainly due to mercury pollution from gold mining entering the food chain, according to the World Wildlife Fund. The newly discovered fossil points to the fragility of freshwater ecosystems and the vulnerability of their inhabitants - past and present - to environmental changes, regardless of whether such changes are natural or human-induced, Velez-Juarbe added.
"Pebanista adds another layer to the complicated evolutionary history of cetaceans and 'river dolphins' in particular. The few species that survive to this day are merely the last remnants of groups that were once more diverse."
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