According to the Daily Mail, researchers have found skeletal remains of a dolphin that fills a gap in how the species moved from primarily inhabiting rivers to the salt water that they now call home, the sea.

Found in Panama, the skeletal remains, classified as Isthminia panamensis, have been identified by researchers at the Smithsonian as an extinct genus and species of river dolphin that, unlike their modern day relatives, inhabited river deltas around 6 million years ago.

The remains consist of the lower jaw with an almost full set of conical teeth, half of a skull, the right shoulder blade, and two small bones from the dolphins flipper.

Although related to modern day dolphins, this species bones suggest that it was nearly 9 feet long.

“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 curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History.

Today, with only four living species of river dolphins, all are classified as being endangered or, in the case of the Yangtze River dolphin, likely extinct.

Each of these species do show traits that would allow them to hunt and navigate the silty rivers, winding as they might be. With broad, almost paddle-like flippers, flexible necks, and heads that have particularly narrow, long snouts, these traits lend to their ability to live in their habitat.

With these traits not fully exhibited in the fossil, as well as the findings of other fossils from creatures near the skeletal remains, researchers speculate that this extinct species most likely hunted near the coast, in salty waters.