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 In the past, there existed a distinct species of dolphin in the Amazon, and it was enormous.

23 March 2024 - NATURE

By Michelle Starr

Source SCIENCEalert.com

Perhaps the largest freshwater toothed whale ever seen by humans was a dolphin that lived in the Amazon basin 16 million years ago.

The extinct creature reached a length of 3.5 meters (11.5 feet), which is far greater than the present-day 2.7-meter (9-foot) pink Amazon river dolphins that consume piranhas in its natural habitat.

The recently unearthed species does, however, demonstrate the ancient richness present in the waterway's past, although being far smaller than the largest dolphin in today's waters—the orca.

It's interesting to note that the recently discovered Pebanista yacuruna is more closely connected to dolphins found in the Ganges and Indus rivers of India, Pakistan, Nepal, and Bangladesh than it is to the modern Amazon dolphins.

According to Swiss paleontologist Aldo Benites-Palomino of the University of Zurich, "we found that its size is not the only remarkable aspect."

"With this fossil record unearthed in the Amazon, we expected to find close relatives of the living Amazon River dolphin – but instead the closest cousins of Pebanista are the South Asian river dolphins (genus Platanista)."

Marine dolphins that haven't quite baked look a lot like freshwater river dolphins. Though their beaks are longer and they tend to be pinker, they do have a similar appearance. However, separate cetacean lineages are the ancestors of both freshwater and marine dolphin families.

Pebanista was identified based on a single skull that was unearthed from the Pebas Formation, a set of Miocene fossil beds that include the remains of numerous extinct creatures that formerly inhabited the Amazon region. However, enough information may be gleaned from that skull regarding the animal's life to date.

There is enough of the skull's features left, however partial, for Benites-Palomino and his associates to be able to compare it to other extinct and extant creatures. Similar to other members of the Platanistid genus, it possessed prominent forehead crests, which are linked to the capacity for echolocation.

Paleontologist Gabriel Aguirre-Fernández of the University of Zurich states, "For river dolphins, echolocation, or biosonar, is even more critical as the waters they inhabit are extremely muddy, which impedes their vision."

Pebanista also has a very large snout, or rostrum, similar to other river dolphins. It is possible that Pebanista's diet was similar to that of live river dolphins, as its snout aids in the search and snaring of the fish that these animals mostly consume.

In terms of origin, the experts believe that Pebanista began as marine cetaceans entering the Amazon basin, which then filled with a network of lakes and rivers that we now refer to as Pebas. The newcomers settled in permanently, adjusting and making themselves feel completely at home after discovering that it was lush and abundant with food. Things were going well in dolphin country for a few million years.

However, as time passes, landscapes alter, and Pebanista is no exception. New habitats replaced the vanished ones when the Pebas system began to transform into the Amazon basin that it is today. After the dolphin stopped feeding on the creatures, Pebanista finally vanished as well.

The river dolphins who live there now gradually inhabited and took advantage of the ecological niche that this left unfilled. While the discovery is thrilling for us and provides new knowledge about the vulnerability and adaptability of species in a changing prehistoric world, it also sheds light on shifting ecosystems. That's a rather tragic ending for Pebanista.

Pebas might have evolved, but the Amazonian food web's current structure might resemble the Miocene more than previously believed.

"This finding confirms not only an independent marine-freshwater transition of cetaceans in South America," according to the investigators, "but also that this diversity in the vast Pebas mega-wetland system might have greatly benefited from the warmer Middle Miocene climatic conditions in the area."

Science Advances has published the research.

Source sciencealert.com