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Freshwater dolphins of the Amazon and South Asia have captured hearts around the world with their unusual pink skin and status as one of the world's most endangered mammals. But how did these unusual creatures appear there? New fossils of a 16,5-million-year-old freshwater river dolphin helped answer these questions.
The skull and other fossils of an ancient river dolphin were found in Rio Napo, Loreto, Peru in 2018 and date back more than 16,5 million years to the early to middle Miocene. The dolphin named Pebanista yacuruna gen. et sp. nov., was about 3 meters (9,8 ft) long, making it the largest known freshwater dolphin.
P. yacuruna was believed to be fat, with a long snout and eyes that were extremely small and almost completely blind. Ancient dolphins are thought to have navigated the murky waters of the Amazon using their small, echolocating frontal lobes. After looking closely at the skull, which has a long snout, large teeth and a well-developed forehead, the team of scientists believes that this ancient dolphin was fully adapted to freshwater life.
The Amazon river dolphins known today are descended from a lineage known as the Iniidae. However, the river dolphins of South Asia, known as Platanistidae, have a relatively unknown evolution, as fossils of these ancient mammals are very rare. These include such living species as the Gangetic dolphin (Platanista gangetica gangetica), which survives in the waterways of the Ganges River in India.
After conducting a phylogenetic analysis, the team believes that the skull fossil is the closest relative of the Ganges dolphin. This, combined with the characteristics of the dolphin that allowed it to be completely adapted to fresh water, led the team to believe that this group of planarians began living in the freshwater systems of South Africa and South America earlier than the ancestors of the clade Iniidae, which gave rise to modern river dolphins Amazons.
"The discovery of Pebanista in the proto-Amazonian strata indicates that plananistids entered freshwater ecosystems not only in South Asia, but also in South America, even before the appearance of the modern Amazon river dolphin, at a crucial moment for the evolution of the Amazon," the authors of the study write.
When P. yacuruna lived, the Amazon consisted of vast areas of mega-wetland and freshwater habitats with abundant food and a wide variety of species, which led to the evolution of its body to a larger size.
