Isthminia panamensis
An artist's impression of Isthminia panamensis, a new fossil dolphin from Panama, feeding on a flatfishJulia Molnar?Smithsonian Institution

Analysis of fossil fragments from Panama has revealed a new species of dolphin which swam the rivers of the South American country between 5.8 and 6.1 million years ago. A team of scientists from the Smithsonian closely examined the fossilised remains of a lower jaw with an almost entire set of conical teeth, right shoulder blade, two small bones from the dolphin's flipper and half a skull.

The researchers came to the conclusion that the river dolphin, which the team have dubbed Isthminia panamensis, was longer than 2.7m, according to the research published in PeerJ.

There are only three remaining species of river dolphins remaining. However, all share similar traits that have helped them adapt to freshwater habitats such as broad, paddle-esque flippers, longer and narrower snouts and flexible necks which help them to swim in river currents.

However, fossil records, including these, indicate that river dolphins were commonplace around the globe. The study's lead author Nicholas D Pyenson, curator of fossil marine mammals at the Smithsonian's National Museum of Natural History, said: "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.

"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."

The scientists conclude that unlike other river dolphins, the Isthminia was able to live in saltwater habitats also. "Isthminia is actually the closest relative of the living Amazon river dolphin," said co-author Aaron O'Dea, staff scientist at the Smithsonian Tropical Research Institute in Panama. "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."