The frigid waters of Alaska's rivers are probably not the first place you'd think to find a river dolphin, but 25 million years ago dolphins would have called Alaska home. At least, that's according to a new analysis of an ancient dolphin skull, originally found in Alaska, that had been hiding in a collection at the Smithsonian Museum for over half a century, reports the BBC.

It's a surprising find, one that becomes even more mysterious when considering that the Alaskan dolphin's closest living relative has been identified as the elusive and endangered South Asian river dolphin, a majestic creature found in the Ganges, Brahmaputra and Indus rivers.

Obviously there aren't any rivers that connect Alaska with South Asia, so the fossil find creates something of an evolutionary jigsaw puzzle. At the very least, it shows that river dolphins may have been more geographically widespread during early cetacean evolution than originally thought. The find also suggests that the South Asian river dolphins alive today represent the last living members of an ancient group.

The species has been named Arktocara yakataga.

"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," said study co-author Alexandra Boersma.

The fossil comes from a pivotal period in cetacean evolution, a time when ancient whales diversified into the two main groups we're familiar with today: the baleen whales, like blue whales and humpbacks, and the toothed whales, such as sperm whales, porpoises and dolphins. So it could provide valuable hints as to which evolutionary factors led to these charismatic beasts populating waters around the world.

The find also highlights the urgency of protecting the South Asian river dolphins alive today that are barely hanging on to existence.