Around 16 million years ago, a dolphin giant cruised the depths of its watery domain. But unlike most modern dolphins, its home wasn’t an ocean; it lived in a freshwater lake in the Peruvian Amazon. And though there are Amazonian freshwater dolphin species alive today, they aren’t close kin to that ancient cetacean. Its closest living relatives are river dolphins living more than 6,000 miles (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 measured at least 11 feet (3.5 meters) long — making it about 20% to 25% bigger than modern river dolphins and the biggest known freshwater dolphin.
But the skull, which measured about 27 inches (70 centimeters) long, was incomplete, so the ancient dolphin may have been even larger than that, 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 the insight it offers into the evolutionary history freshwater dolphins, as these creatures are extremely rare in the fossil record, the study authors wrote. This is because there tend to be fewer individual dolphins in freshwater ecosystems, and strong water currents typically prevent fossils from preserving well.
They called the newfound species Pebanista yacuruna; the genus references Peru’s Pebas Formation, where the fossil was found, and “yacuruna” is a term for mythical aquatic people of local legend, in the Indigenous Kichua language.
“I think this is a remarkable discovery, particularly given that South America has one species of river dolphin that belongs to a completely different group of odontocetes (toothed whales),” said Jorge Velez-Juarbe, an associate curator of marine mammals at the Natural History Museum of Los Angeles County, in an email.
‘Everybody freaked out’: Fossil features reveal rare find
Modern freshwater dolphins are known for their highly elongated noses, compared with marine dolphins’ stubbier snouts. There’s the South Asian river dolphin (Platanista genus) and the Amazon river dolphin (Inia genus), also known as the pink river dolphin, and the two groups include several species and subspecies.
China’s Yangtze river dolphin (Lipotes vexillifer) represents a third genus, but the species hasn’t been seen in the wild in 40 years and may be extinct, according to the International Union for the Conservation of Nature (IUCN). In fact, all extant river dolphin species are endangered or critically endangered, the IUCN says.
“I said: ‘Hey, John, does this connect to this piece that I have in my hand?’” Benites-Palomino recalled. What he held turned out to be a rostrum — the rounded tip of a nose — from the embedded skull. As they cleaned it up enough to see the shapes of tooth sockets, Benites-Palomino realized that 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 turn out to be an ancient relative of modern Amazonian river dolphins. But further cleaning revealed that the size and shape of the eye socket resembled that of South Asian river dolphins, which have much smaller eyes than their South American cousins.
“That was a moment where everybody freaked out, because it wasn’t an Amazonian river dolphin,” Benites-Palomino said. This told the scientists that two types of dolphins had independently and at different times moved inland in the region.
Digging up dolphin diversity
Platanistoids — the group that includes P. yacuruna and South Asia’s modern river dolphins — were widespread about 20 million years ago. The ancestors of modern Amazonian river dolphins were common in 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, seeking less competition for food. This Amazonian freshwater environment was nutrient-rich and teeming with life, home to crocodilians, turtles and fish, as well as mammals such as sloths, rodents, ungulates and primates.
“Overall, in these ecosystems ‘river dolphins’ can be considered as apex predators,” Velez-Juarbe said.
P. yacuruna was among the first wave of dolphins to test the waters in Amazonian rivers and lakes; a lack of predators in its new home could explain how the species evolved to become so large, according to the study. But environmental changes like drought may have later doomed P. yacuruna and driven it to extinction, opening the freshwater habitat to the ancestors of extant pink river dolphins.
“We now know that this species was living there in the past, but also the Amazonia is important for our extant Inia geoffrensis,” Benites-Palomino said. “[The discovery] highlights that this is a tremendously important environment for the evolution of freshwater cetaceans.”
P. yacuruna’s disappearance is a grim reminder that this important environment is all too easily disrupted. Today, modern Amazonian river dolphins face an uncertain future, mostly due to mercury pollution from gold mining invading the food chain, according to the World Wildlife Fund. The newfound fossil hints at the fragility of freshwater ecosystems and the vulnerability of their inhabitants — past and present — to environmental changes, whether such changes are natural or human-made, Velez-Juarbe added.
“Pebanista adds another layer to the intricate evolutionary history of cetaceans and particularly ‘river dolphins,’ the few species that survive to present day are but the last remnants of groups that were once more diverse.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.