Discovering the Largest Fossil River Dolphin in the Amazonian Basin In the vast and enigmatic Amazon rainforest, a team of international researchers has made a remarkable discovery that sheds new light on the ancient aquatic life that once thrived in this region.

The team, led by the University of Zurich (UZH), has uncovered the largest fossil river dolphin ever found, a species that lived 16 million years ago in the proto-Amazonian landscape. The newly discovered species, named “Pebanista yacuruna” in honor of a mythical aquatic people believed to inhabit the Amazon basin, measured between 3 to 3.5 meters in length, making it the largest known river dolphin to have ever existed.

Remarkably, this ancient giant is not directly related to the modern-day Amazon river dolphins, but rather represents the last survivor of a different group of cetaceans that once roamed our planet. The Pebanista dolphin belongs to the Platanistoidea, a group of dolphins that were once common in the world’s oceans between 24 and 16 million years ago.

Researchers believe that their original marine ancestors invaded the prey-rich freshwater ecosystems of the proto-Amazon region and adapted to this new environment. 16 million years ago, the Peruvian Amazon looked very different from what it is today, says lead author Aldo Benites-Palomino from the UZH Department of Paleontology. Much of the Amazon lowland was covered by a vast system of lakes and swamps called Pebas.

This landscape included aquatic, semi-aquatic, and terrestrial ecosystems (such as swamps and floodplains) and stretched across what is now Colombia, Ecuador, Bolivia, Peru, and Brazil. As the Pebas system began to give way to the modern Amazon around 10 million years ago, the new habitats caused the disappearance of Pebanista’s prey, leading to the giant dolphin’s extinction.

This opened an ecological niche that was then exploited by the ancestors of the modern Amazon river dolphins (Inia), which were also facing extinction in the oceans due to the rise of new cetacean species, such as modern ocean dolphins. We discovered that its size is not the only remarkable aspect, says Benites-Palomino. With this fossil record from the Amazon, we expected to find close relatives of the modern Amazon river dolphin, but instead, the closest relatives of Pebanista are the river dolphins of South Asia (genus Platanista).

Pebanista and Platanista share highly developed facial crests, specialized bony structures associated with echolocation, the ability to “see” by emitting high-frequency sounds and listening to their echoes, which they rely heavily on for hunting.

For river dolphins, echolocation or biosonar is even more critical as the waters they inhabit are extremely turbid, which hinders their vision, explains Gabriel Aguirre-Fernández, a researcher at UZH who also participated in this study. The long snout with many teeth suggests that Pebanista fed on fish, as do other river dolphin species today.

After two decades of work in South America, we had found several giant forms from the region, but this is the first dolphin of its kind, adds Marcelo R. Sánchez-Villagra, director of the UZH Department of Paleontology. We were particularly intrigued by its peculiar biogeographic history over deep time.

The Amazon rainforest is one of the most challenging regions for paleontological fieldwork. Fossils are only accessible during the dry season, when river levels are low enough to expose the ancient fossil-bearing rocks. If these fossils are not collected in time, the rising water levels during the rainy season will wash them away, and they will be lost forever.

The holotype, the single physical specimen on which the description and naming of a new species is based, of Pebanista was found in 2018, when the lead author of the study was still a university student.

The expedition, led by Peruvian paleontologist Rodolfo Salas-Gismondi, a former postdoctoral fellow at the UZH Department of Paleontology, traveled more than 300 kilometers along the Napo River. Dozens of fossils were discovered and collected, but the biggest surprise was waiting at the end of the expedition, after nearly three weeks of exploration: the discovery of the massive dolphin skull, cataloged as MUSM 4017, which has been permanently deposited in the Natural History Museum of Lima.


Sources

University of Zurich | Benites-Palomino, A, Aguirre-Fernández, G., Baby, P. et al., (2024). The largest freshwater odontocete: A South Asian River dolphin relative from the Proto-Amazonia. Science Advances. Doi:10.1126/sciadv.adk6320


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