A previously unknown species of dolphin -- one that has been extinct for millions of years -- has been identified using a fossil jaw fragment that had lain unexamined in the vast collection of the Smithsonian National Museum of Natural History for over a half-century.
The fossil, a partial skull about 9 inches long, was discovered in 1951 in southeastern Alaska by Donald J. Miller, a geologist with the United States Geological Survey. It then spent decades among the more than 40 million such specimens in the Smithsonian collection -- until Nicholas Pyenson, the museum's curator of fossil marine mammals, and Alexndra Boersma, a researcher in his lab, focused their attention on what Boersma calls "this beautiful little skull from Alaska." They were able to use the fragment as the basis of a 3-D model of the whole skull, which you can explore online.
By comparing the skull to those of dolphins both living and extinct, Pyenson and Boersma determined that the species was of a new genus and species -- which they named Arktocara yakataga -- which, they say, is most closely related to the endangered South Asian river dolphin. They believe Arktocara swam in sub-Arctic marine waters at least 25 million years ago.
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The South Asian river dolphin, Platanista gangetica, comprises two sub-species that are found primarily in the Indus and Ganges Rivers and their tributaries. Completely blind, it relies on its echolocation to navigate the murky riverine waters, through which it swims on its side. It's not the only species of dolphin to live partly or primarily in rivers or brackish waters: others include the Amazon river dolphin Inia geoffrensis; the La Plata dolphin Pontoporia blainvillei; and the baiji or Yangtze river dolphin, Lipotes vexillifer, which was declared functionally extinct in 2006.
The South Asian river dolphin is the sole remaining survivor of a once much larger and more diverse group of dolphins known as the Platanistoidea; like its apparently exterminated Chinese cousin, it faces threats from human activities, including fishing nets, pollution and habitat destruction. It now numbers only a few thousand animals.
"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," Boersma said. Based on the age of nearby rocks, the scientists estimate that the Arktocara fossil comes from the late Oligocene epoch, around the time ancient whales diversified into two groups -- baleen whales (mysticetes) and toothed whales (odontocetes).
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"It's the beginning of the lineages that lead toward the whales that we see today," Boersma added. "Knowing more about this fossil means that we know more about how that divergence happened."
The finding comes less than a year after Pyenson and colleagues announced the discovery of a fossil dolphin species from Panama, which they estimated lived in the region at least 5.8 million years ago.
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