A fossil collection from the Smithsonian's National Museum of Natural History has yielded a new genus and species of extinct river dolphin. Arktocara yakataga swam in subarctic waters some 25 million years ago.

The fossil, a 9-inch-long partial skull, was recovered by geologist Donald J. Miller in southeastern Alaska in 1951. Until recently, it sat unstudied in the museum's collection.

The fossil was rediscovered by Nicholas D. Pyenson, curator of fossil marine mammals at the Smithsonian. Pyenson and Alexandra Boersma, a researcher in his lab, believe the new species is an ancient relative of the endangered South Asian river dolphin, Platanista gangetica.

Scientists believe the South Asian river dolphin is the lone survivor of a once large and varied family of dolphins, and is also part of the oldest lineages of modern toothed whales.

Researchers remain fascinated by Platanista gangetica. The species is divided into two subspecies, the Indus dolphin and Ganges dolphin, named for their preferred rivers. The unique creature is blind and relies on echolocation to navigate India's murky freshwater rivers.

"One of the most useful ways we can study Platanista is by studying its evolutionary history, by looking at fossils that are related to it to try to get a better sense of where it's coming from," Boersma explained in a news release. "Exactly how that once diverse and globally widespread group dwindled down to a single species in Southeast Asia is still somewhat a mystery, but every little piece that we can slot into the story helps."

The newly discovered species may also help scientists better understand the evolutionary history of whales and dolphins. Researchers believe Arktocara yakataga's position on the family tree is at the junction dividing the lineage of today's toothed whales and the lineage of dolphins that produced Platanista.

"It's the beginning of the lineages that lead toward the whales that we see today," Boersma said. "Knowing more about this fossil means that we know more about how that divergence happened."

Pyenson says the latest findings -- detailed in the journal PeerJ -- are a reminder of the rich evolutionary heritage at risk of being lost to extinction.

"Some species are literally the last of a very long lineage," he said. "If you care about evolution, that is one basis for saying we ought to care more about the fate of Platanista."