According to the researchers, the new species has features that suggest a close relation to modern river dolphins -- four species that, no surprise, live in freshwater rivers around the world. Unfortunately, all four modern species are endangered, with one (the Chinese river dolphin) thought to be extinct.
But though they now struggle to survive, the species represent an impressive evolutionary path: To get away from crowded oceans and fill a new ecological niche, they developed wide, paddle-like flippers, flexible necks and long snouts that allowed them to thrive in rivers instead.
That's kind of a backwards move. Whales and dolphins evolved from terrestrial mammals, first becoming amphibious around 50 million years ago or so and gradually moving out to the open ocean. Millions of years later, river dolphins made their way back inland.
Isthminia panamensis could provide the most compelling link between river dolphins and their ocean-bound ancestors. While some of the new creature's features tie it to the modern species, the area it was found in suggests it lived in the ocean. Scientists can't be sure that the new species is an ancestor of modern river dolphins, but it could be a close cousin -- one that took a similar evolutionary trek.
"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," lead author Nicholas D. Pyenson, curator of fossil marine mammals at the Smithsonian's National Museum of Natural History, said in a statement. "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."
Based on the find, Pyenson and his team believe that the river dolphins made their way inland around 6 million years ago -- just as the oceans were rising. If that's the case, they say, it's important that we understand exactly how the transition occurred. It could give us clues as to how today's changing planet might influence new migrations and adaptations in modern animals.