Paleontology and Dolphin: Ancient River Dolphin Fossil Identified in Panama

A new river dolphin fossil was recently identified off Panama's Caribbean coast, for an animal that was likely longer than nine feet, researchers said. That fossil, Isthminia panamensis, offers learnings regarding today's freshwater and saltwater dolphins and when their evolutionary paths diverged. 

The fossils were found near the town of Piña and date back 5.8-6.1 million years. Researchers collected a half skull, a lower jaw with almost a full set of conical teeth, a right shoulder blade and two small flipper bones. Other marine species were found around this new river dolphin species, suggesting that it lived in salty oceans before the Panama Isthmus closed, according to the release. 

"We discovered this new fossil in marine rocks, and many of the features of its skull and jaws point to its having been a marine inhabitant, like modern oceanic dolphins," Nicholas D. Pyenson, lead author and curator of fossil marine mammals at the Smithsonian's National Museum of Natural History, said in the release. "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."

There are only four species of river dolphins that exist today, according to the release. All of these species live in freshwater or coastal ecosystems, and are all considered to be endangered. This includes the Chinese river dolphin, which may actually now be extinct. 

Modern dolphin species are equipped to adapt from marine environments to winding freshwater rivers, with new broad, paddle-like flippers, flexible necks and heads and long, narrow snouts. This helps researchers understand when exactly in the species' evolutionary tract they made the transition from oceans to rivers.

"Isthminia is actually the closest relative of the living Amazon river dolphin," Aaron O'Dea, co-author and staff scientist at the Smithsonian Tropical Research Institute in Panama, said in the release. "While whales and dolphins long ago evolved from terrestrial ancestors to fully marine mammals, river dolphins represent a reverse movement by returning inland to freshwater ecosystems. As such, fossil specimens may tell stories not just of the evolution these aquatic animals, but also of the changing geographies and ecosystems of the past."

Their study was recently published in PeerJ. 

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