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(CNN) – A picture of a dolphin. Now imagine one that is twice the size of a person.
About 16 million years ago, a giant dolphin cruised the depths of its aquatic domain. But unlike most modern dolphins, home was not an ocean; he lived in a freshwater lake in the Peruvian Amazon. And while there are Amazonian freshwater dolphin species alive today, they are not related to that ancient mammal. The closest living relatives are river dolphins that live more than 6,000 miles (10,000 kilometers) away in South Asia, according to researchers who recently described the previously extinct mammal.
Analysis of the skull of the newly identified ancient dolphin 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 best known freshwater dolphin.
But the skull, which measured about 27 inches (70 centimeters) long, was incomplete, so the ancient dolphin may have been larger than that, the scientists reported March 20 in the journal Science Advances, published by the American Association for the Advancement of Science.
What is 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 usually fewer individual dolphins in freshwater ecosystems, and strong water currents usually prevent fossils from being preserved.
They called the newly discovered species Pebanista yacuruna; the genus refers to the Pebas Formation of Peru, where the fossil was found, and “yacuruna” is a term for mythical aquatic people according to local legends, in the indigenous Kichua language.
“I think this is an amazing discovery, especially since there is one species of river dolphin in South America that belongs to a completely different group of odontocetes (toothed whales),” said Jorge Velez-Juarbe, associate curator of marine mammals at the Natural. Los Angeles County History Museum, in 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 research, told CNN.
‘Everybody freaked out’: Fossil features reveal rare find
Modern freshwater dolphins are known for their very elongated noses, compared to the sharper snouts of marine dolphins. There is a South Asian river dolphin (Platanista genus) and the Amazon river dolphin (Inia genus), also known as the pink river dolphin, and both groups include several species and subspecies.
China’s Yangtze river dolphin (Lipotes vexillifer) is a third genus, but the species has not been seen in the wild for 40 years and may be extinct, according to the International Union for Conservation of Nature (IUCN) . In fact, all extant river dolphin species are endangered or critically endangered, says the IUCN.
Researchers discovered the Amazonian dolphin fossil in 2018, near the Napo River in Loreto, Peru. Lead study author Aldo Benites-Palomino, a doctoral candidate in the Department of Paleontology at the University of Zurich, stopped to examine some strange rocky fragments on the ground, he told CNN. At the same time, study 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, is this about this piece in my hand?'” Benites-Palomino recalled. It turned out like a rostrum — the tip of a rounded nose — from the embedded skull. When they cleaned it up enough to see the shapes of the tooth sockets, Benites-Palomino realized they were looking at something unusual.
“We started screaming: ‘It’s a dolphin! It’s a dolphin!’” Benites-Palomino said.
At first, they thought it would be an ancient relative of modern Amazonian river dolphins. But further cleaning revealed that the size and shape of the eye socket was similar to that of South Asian river dolphins, which have much smaller eyes than their South American cousins.
“It was that moment when everyone was freaked out, because it was not an Amazonian river dolphin,” said Benites-Palomino. This told the scientists that two types of dolphins had moved inland independently and at different times in the region.
Download the variety of dolphins
Platanistoids – the group that includes P. yacuruna and the modern river dolphins of South Asia – were widespread around 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 the two groups of cetaceans were so diverse, it is likely that some species entered river and lake ecosystems, seeking less competition for food. This Amazonian freshwater environment was rich in nutrients and teeming with life, home to crocodiles, turtles and fish, as well as mammals such as sloths, rodents, crustaceans and primates.
“Overall, in these ecosystems ‘river dolphins’ can be considered apex predators,” said Velez-Juarbe.
P. yacuruna was among the first wave of dolphins to test the waters in Amazonian rivers and lakes; the lack of predators in its new home could explain how the species has changed so much, according to the study. But environmental changes such as drought may have later decimated P. yacuruna and driven it to extinction, opening up the freshwater habitat for the pink river dolphin’s ancestors.
“We now know that this species lived there in the past, but also the Amazonia is important for our Inia geoffrensis,” said Benites-Palomino. “[The discovery] it shows that this is an extremely important environment for the evolution of freshwater whales.”
The disappearance of P. yacuruna is a grim reminder that this important environment is too easily disturbed. Today, modern Amazonian river dolphins face an uncertain future, largely due to mercury pollution from gold mining invading the food chain, according to the World Wildlife Fund. The newly discovered fossil points to the vulnerability of freshwater ecosystems and their inhabitants – past and present – to environmental changes, whether natural or man-made , Velez-Juarbe added.
“Pebanista adds another layer to the complex evolutionary history of cetaceans and especially ‘river dolphins’, the few species that survive to this day are only the last remnants of once more diverse groups.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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