Researchers puzzling together 6-million-year-old fossil fragments from the Caribbean coast near Piña, Panama, have uncovered a new extinct species of river dolphin. While many of its features are similar to seafaring dolphins, the new species is closely related to river dolphins living in the Amazon today, according to findings published in PeerJ this week.
There are only four species of river dolphins these days, and they all live in freshwater or coastal habitats. (The Chinese river dolphin may actually now be extinct.) These modern species all arrived at the same solution for the saltwater-to-freshwater shift challenge: broad, paddle-like flippers, flexible necks, and long, narrow snouts. This body plan helps them navigate windy, sediment-laden rivers. But exactly when river dolphins made the transition from oceans to rivers remains unclear.
"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," Nicholas Pyenson from the Smithsonian's National Museum of Natural History says 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.”
Artistic reconstruction of Isthminia panamensis feeding on a flatfish. Julia Molnar/Smithsonian Institution
They named the new species Isthminia panamensis, and its fossils date back to between 5.8 million and 6.1 million years. Unlike river dolphins today, it lived in the salty waters of a food-rich Caribbean Sea before the full closure of the Panama Isthmus – back when the Central American Seaway connected the Atlantic and Pacific oceans.
Isthminia panamensis is the closest relative of today’s Amazon river dolphin (Inia geoffrensis). The new genus name recognizes the Panama Isthmus and this living relative. "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," Aaron O'Dea of the Smithsonian Tropical Research Institute says. "Isthminia now gives us a clear boundary in geologic time for understanding when this lineage invaded Amazonia," Pyenson adds.
The fossils consist of half a skull, the lower jaw with an almost complete set of conical teeth, the right shoulder blade, and two small bones from the flipper. By comparing these parts with other river dolphins, both living and extinct, the team estimates that the animal would have been more than 2.7 meters (9 feet) in length.