A beautiful skull hiding amidst the 40 million fossils in the collections of the Smithsonian Museum has opened a new window into the earliest history of one of the most ancient families of marine mammals.

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The skull of The skull of Akrtocara yakataga (left). Image: James Di Loreto/Smithsonian Institution

Originally dug up in Alaska in 1951, the small skull lay unnoticed for more than 50 years until it was rediscovered just recently by researchers Nicholas Pyenson and Alexandra Boersma, and finally given a name: Arktocara yakataga. "We are always learning new things about the vast legacy built by our predecessors at the museum," says Pyenson. 

But to understand the find's importance, you need to know a few facts about a creature that's still around today: the rather weird South Asian river dolphin (Platanista gangetica).

As its name suggests, the animal doesn't live in the ocean like nearly all other cetaceans; instead, it's perfectly adapted for life in the rivers of Nepal, India, Bangladesh and Pakistan. On top of that, it can hardly see at all, navigating murky freshwater using mainly echolocation. It's also the only cetacean we know of that's capable of swimming sideways. And finally, it has no close living relatives, not even other river dolphins like those in the Amazon – it's the only living member of a very ancient lineage.

And that ancient family just got a little bit older with the discovery of the new fossil. 

[The 3D model below might take a while to load, but it's worth the wait.]

At around 25 million years old, Arktocara may just be the oldest known relative of Platanista. While its living cousins prefer tropical freshwater, Arktocara was a marine species like most cetaceans, living in the chilly stretches of the north Pacific Ocean.

The simple fact that it was found so far away hints at the long and complex history of this dolphin lineage. "Considering the only living dolphin in this group is restricted to freshwater systems in Southeast Asia, to find a relative that was all the way up in Alaska 25 million years ago was kind of mind-boggling," said Boersma.

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Artist's impression of a pod of Akrtocara yakataga, swimming offshore of Alaska during the Oligocene, about 25 million years ago. Image: Alexandra Boersma.

At that time, during the Oligocene epoch, the oceans were an exciting place, with all modern groups of whales and dolphins just beginning to evolve. Arktocara, too, was one of the first in what would be a long and successful line of cetaceans – fossil finds all over the world tell us that more than a dozen species emerged over the following several million years.

Eventually, these species were lost to extinction. The only one left is the strange, river-dwelling, sideways-swimming South Asian river dolphin, the last in a 25 million-year-old family line. 

And even this straggler may not hold on much longer. Like many freshwater species, the species has suffered years of hunting for its meat and oil, while its habitat has been fragmented by dams and barrages. It's also vulnerable to accidental entanglement in fishing gear. Today, the animals are considered endangered, and without swift conservation action, this unique dolphin species may end up marking the very end of its long evolutionary story. 

For palaeontologists Pyenson and Boersma, a better understanding of the dolphins' past will help us look toward a brighter future. "Some species are literally the last of a very long lineage," said Pyenson. "If you care about evolution, that is one basis for saying we ought to care more about the fate of Platanista."

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