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Ancient river dolphin species discovered in Panama

An artist rendering of the newly discovered river dolphin called Isthminia panamensis. Photo: Julia Molnar/Smithsonian Institution
An artist rendering of the newly discovered river dolphin called Isthminia panamensis. Photo: Julia Molnar/Smithsonian Institution

An ancient river dolphin that measured up to nine-feet long and lived around 6 million years ago was discovered in Panama and sheds new light on the evolution of today's freshwater river dolphin species, the Smithsonian announced.

The fossil was uncovered on the Caribbean coast near the town of Pina, Panama, by scientists and colleagues of the Smithsonian Institution, the findings of which were published Tuesday in the scientific journal Peer J.

The Isthminia panamensis fossil, dating from 5.8 million to 6.1 million years ago, consists of half a skull, lower jaw with an almost entire set of conical teeth, right shoulder blade and two small bones from the dolphins flipper. These were compared with fossils and living river dolphins to determine its estimated length.

Today, only four species of river dolphins exists. All live in freshwater or coastal ecosystems and all are endangered. The Isthminia panamensis, which has long been extinct, is the closest relative of the living Amazon river dolphin.

Smithsonian scientists uncover fossil of an ancient river dolphin in Panama. Photo: Jorge Velez-Juarbe/Smithsonian Institution
Smithsonian scientists uncover fossil of an ancient river dolphin in Panama. Photo: Jorge Velez-Juarbe/Smithsonian Institution

From the Smithsonian:

Each of the modern river dolphin species show a common solution to the problem of adapting away from marine to freshwater habitats by converging upon a body plan that includes broad, paddle-like flippers, flexible necks and heads with particularly long, narrow snouts—all the better to navigate and hunt in winding, silty rivers.

But fossil evidence suggests that river dolphins' ancestors were widespread around the globe. I. panamensis was clearly one of them, and its fossil remains have helped the team understand something less clear: When in their evolutionary tract did river dolphins transition from the saltwater of the ocean to the freshwater of rivers? …

Other fossilized animals found at the same site as I. panamensis were marine species, indicating that unlike river dolphins living today, I. panamensis lived in the salty waters of a food-rich Caribbean Sea, before the full closure of the Panama Isthmus.

The skull and jaws of the newly discovered river dolphin. Photo: Nicholas D. Pyenson/NMNH Imaging/Smithsonian Institution
The skull and jaws of the newly discovered river dolphin. Photo: Nicholas D. Pyenson/NMNH Imaging/Smithsonian Institution

"We discovered this new fossil in marine rocks, and many of the features of its skull and jaws point to it having been a marine inhabitant, like modern oceanic dolphins," said Nicholas D. Pyenson, the study's lead author.

"Many other iconic freshwater species in the Amazon, such as manatees, turtles and stingrays have marine ancestors, but until now, the fossil record of river dolphins in this basin has not revealed much about their marine ancestry. Isthminia now gives us a clear boundary in geologic time for understanding when this lineage invaded Amazonia."

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