A re-examination of a fossil, which has been exposed since 1951 on the Smithsonian’s National Museum of Natural History collection at Washington DC, helped scientists to settle important pieces on the evolutionary history of whales and dolphins, including the origins of the endangered South Asian river dolphin.

The fossil is a partial skull of a dolphin that swam in sub-arctic marine waters about 25 milion years ago and it was discovered in southeastern Alaska by the geologist Donald J. Miler. The measures approximately 9 inches, being named as “Arktocara yakataga” by Nicholas D. Pyenson, the museum curator of fossil mammals, and Alexandra Boersma, a researcher in his lab.

By analysing the nearby rocks, the scientists estimate that the Arktocara fossil comes from the late Oligocene period. This is also the time ancient whales diversified into two groups within the “Platanistoidea” superfamily – the baleen whales, which comprises blue whales and humpbacks, and the toothed whales, which include sperm whales, porpoises and dolphins.

The partial cranium belongs to a new genus and species and has been compared with living and extinct dolphins, leading the scientists to discover a link between the Arktocara yakataga and the South Asian river dolphin “Platanista Gangetica”, which is the sole surviving species of a once large and diverse group of dolphins. The South Asian river dolphins are unusual creatures that swim on its side, cannot see and uses echolocation to navigate murky rivers in Nepal, India, Bangladesh and Pakistan. The skull is one of the oldest fossils ever found in Platanistoidea superfamily and its discovery confirms that Platanista belongs to one of the oldest lineages of toothed whales still alive today.

This particular river dolphin lives only in fresh water and have been endangered due to human activities, pollution and loose of habitat reducing the number of alive specimens to only a few thousand, making the river dolphins hard to study.

Boesma said “It’s the beginning of the lineages that lead towards the whales that we see today” concluding “Knowing more about this fossil means that we know more about how that divergence happened”.

Platanista’s fossils have been found in marine deposits around the world being the Arktocara the most northernmost find to date. “Arktocara” is derived from the Latin for the “face of the north” while “yakataga” is the indigenous Tlingit people’s name for the region where the fossil was found.

Nicholas D. Pyenson referred “We are always learning new things about the vast legacy built by our predecessors at the museum”.

The findings were reported on August 16 in the journal open access PeerJ and includes a three-dimensional model of the fossil to explore. Pyenson notes that some conservation biologists argue that the South Asian river dolphin should be prioritised for protection to preserve its evolutionary heritage “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”.

– André Raio, Correspondent (Science)

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