Earlier this week, scientists at the Smithsonian announced in the journal PeerJ that they have discovered the fossilized remains of a dolphin from a new species and a new genus. Off the Caribbean coast of Panama they found half a skull, a lower jaw with almost an entire set of conical teeth, a right shoulder blade, and two small flipper bones. These indicate a dolphin specimen over nine feet long that lived around six million years ago. The new species, Isthminia panamensis, resembles modern river dolphins but likely lived in salt water, illuminating what has long been a missing link in evolutionary understanding.

As land mammals moved into the water some 50 million years ago, they evolved from amphibious to marine, eventually moving out into the open ocean as whales. River dolphins—of which there are four endangered, if not extinct, modern species—represent something of an evolutionary step back. They developed wide, paddle-like flippers, flexible necks, and narrow, extra-long snouts that allowed them to return to freshwater rivers. I. panamensis could help bridge the gap in scientists' understanding of how and when dolphins made this move back inland.

"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," the study's lead author Nicholas D. Pyenson said in a statement. "Isthminia now gives us a clear boundary in geologic time for understanding when this lineage invaded Amazonia."

The fossils are too delicate to be molded and cast using traditional methods, so the Smithsonian’s Digitization Program Office has created detailed, 3-D scans of the jaw, skull, and scapula, which you can view online.

[h/t Washington Post]