In the murky waters of the Ganges and Indus rivers, a few thousand blind dolphins swim on their sides, snapping at prey with long, exaggerated beaks and using echolocation to navigate. Because of pollution and habitat destruction, the South Asian river dolphin is on its way to extinction — but a newly-discovered relative may strengthen the case for conserving it. Fossil skull of Arktocara yakataga, an ancestor of the South Asian river dolphin recently discovered in the Smithsonian Museum of Natural History collection. Image: James Di Loreto, Smithsonian
For more than 50 years, an unusual skull from southeastern Alaska sat in the Smithsonian Museum of Natural History's vast collections. By X-ray scanning that skull and carefully analysing a digital model, palaeontologists have now determined that it belongs to an extinct river dolphin that swam in subarctic marine waters some 25 million years ago.
Dubbed Arktocara yakataga, it's a distant relative of the South Asian river dolphin Platanista gangetica, confirming that the geographically-isolated tropical species is the last surviving member of an ancient 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," lead study author Alexander Boersma said in a statement.
"There are rocks preserved in Alaska that have the potential to tell us a lot about the early stages of whale evolution," study co-author Nicholas Pyenson, the Smithsonian Museum's curator of fossil marine mammals, told Gizmodo. "In particular, Alaska has a lot of rocks from the Oligocene."
Artist's reconstruction of Arktocara yakataga swimming offshore of Alaska 25 million years ago. Image: Alexandra Boersma
The Oligocene, a cool chapter in Earth's history that followed the hot, ice-free Eocene, is thought to be a period of rapid cetacean diversification. Although the first whales evolved earlier, fossil evidence suggests that two large groups — the baleen and toothed whales — got their start during the Oligocene some 34 to 23 million year ago. But for palaeontologists interested in reconstructing the evolutionary history of whales, this time period is problematic. "We don't know much about it, because there aren't many rocks available," Pyenson said. "People know a lot about much older or younger whales, but they have ignored the Oligocene."
A rare opportunity to study Oligocene whales presented itself recently, when the Smithsonian got back an unusual cetacean skull that had been on loan to another researcher for years. First deposited in the museum's collection in 1951 by geologist Don Miller, the skull was collected from a 25-million-year-old rock unit in southeastern Alaska.
Boersma and Pyenson's analysis, published today in Peer J, confirms that the fossil belongs to a never-before-seen marine ancestor of the South Asian river dolphin.
There are only four species of river dolphin alive on Earth today. Distantly related and geographically disparate, all of them are considered vulnerable to critically endangered, because of centuries of hunting pressure and habitat loss. But as palaeontologists are learning from the fossil record, these strange, hard-to-find cetaceans are the descendants of a much larger group that went from sea to land multiple times, for reasons that are still unclear.
A 1927 illustration of the Indus river dolphin, one of two subspecies of Platanista gangetica. Image: Wikimedia
In particular, the South Asian river dolphin has dozens of extinct relatives. Now, it can add another to its illustrious heritage. Arktocara yakataga is among the oldest and northernmost river dolphin ancestors have ever found, confirming that Platanista hails from an ancient and possibly globe-spanning group.
"This dolphin is singular today, but when we look at its ancestry, there are dozens of marine fossil whales closer to the South Asian River dolphin than to any other whales," Boersma said. "What this tells us is we should protect [Platanista], because it's the last of a lineage. It is representing evolutionary history that will wink out if we don't preserve it."
Next year, Boersma and Pyenson are planning to return to the site in southeast Alaska where the Arktocara skull was unearthed more than half a century ago. Hopefully, they can find more bones from the extinct creature, and start to flesh out the story of how this lineage evolved and spread to new habitats halfway across the world.
"There are still a lot of questions," Boersma said. "I think it's exciting to find the relative of a tropical river dolphin at the poles. It tells us just how much we don't know about whales."