Paleontologists say fossil fragments discovered in Panama are probably those of a previously-unknown, long-extinct species of river dolphin.

The species, dubbed Isthminia panamensis, is a bit of a puzzle, the researchers say; although it has the elongated snout and small eyes of modern Amazon river dolphins and is quite probably a distant ancestor, it clearly lived in the open ocean.

That could make sense given the ancient geology of the region, they say.

"We think it lived in the channel that connected the Atlantic and Pacific oceans before the isthmus of Panama formed," says paleobiologist Nick Pyenson of the Smithsonian Institution in Washington, D.C.

When the fossil fragments were first dug out of a rocky shoreline in Panama, Pyenson recalls, he thought they were from a shark-toothed dolphin, another already-known ancient marine mammal.

"Once we got it back to D.C., and I got a close look at it, I could tell it was like nothing I'd ever seen before," Pyenson says.

It was instead a species new to science, and one that provides some insights into the evolution of freshwater dolphins around the world today.

There are just four species of river dolphins living in freshwater or coastal ecosystems today, and one of them — the Chinese river dolphin — may in fact be extinct since none have been sighted in some time.

All modern river dolphin species show common evolutionary adaptations, allowing them to hunt and navigate in murky, silt-laden rivers or coastal waters, including broad paddle-like flippers and heads with long, narrow snouts located on flexible necks.

The fossil from Panama suggests the ancient dolphin was around nine feet long, slightly shorter than today's Amazon River dolphin.

"Isthminia is actually the closest relative of the living Amazon river dolphin," says study co-author Aaron O'Dea at the Smithsonian Tropical Research Institute in Panama. "While whales and dolphins long ago evolved from terrestrial ancestors to fully marine mammals, river dolphins represent a reverse movement by returning inland to freshwater ecosystems."

A number of freshwater species found in the Amazon River, including turtles, manatees and stingrays have marine ancestors, Pyenson pointed out, but little has been known about the ancestry of the region's river dolphins.

The fossil of Isthminia panamensis has been dated to between 5.8 and 6.1 million years ago, he says, which "gives us a clear boundary in geologic time for understanding when this lineage invaded Amazonia."

The report of the discovery has been published in the journal PeerJ.

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