The fossilised remains of a new species of extinct dolphin that dwelled in the ancient oceans more than 5.8 million years ago are helping scientists solve the mystery of how and when the planet's river dolphins adapted to life away from the sea.

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An artist's recreation of Isthminia panamensis feeding on a flatfish. Many features of this new species appear similar to today’s ocean dolphins, yet the new fossil species is more closely related to the living Amazon River dolphin. Image: Julia Molnar / Smithsonian Institution

Today, river (or freshwater) dolphins are endangered, largely due to human activity. Only five species remain; another, China's Yangtze river dolphin, was declared "functionally extinct" back in 2006.

The animals are truly remarkable and armed with unique adaptations that help them navigate the waters of the murky, winding rivers they inhabit: flexible necks for catching rapidly darting prey; broad, paddle-shaped flippers; and long, narrow snouts – much longer than those of their modern, sea-dwelling cousins.

The new dolphin discovery was made by a team of Smithsonian scientists off the Caribbean coast of Panama, and the species has been aptly named Isthminia panamensis, a nod to the people of the Republic of Panama and the many scientists who’ve studied the biology and geology of the region. 

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Scientists collected the fossils of Isthminia panamensis from the Caribbean coast of Panama. The fossil was encased in a white plaster jacket and recovered as the tide rushed in. Image: Aaron O’Dea / Smithsonian Institution

Smithsonian has even created a 3D scan of the fragile fossil (so you can print out your very own extinct dolphin skull, also available here!).

 

Based on the skull and parts of the right flipper, I. panamensis would have grown to more than nine feet (2.7 metres) in length, just slightly larger than its modern relatives, say the scientists in a new study detailing their findings. “We discovered this new fossil in marine rocks, and many of the features of its skull and jaws point to it having been a marine inhabitant, like modern oceanic dolphins,” says the study’s lead author, Nicholas Pyenson. 

But close comparison of I. panamensis to both fossil and living dolphins shows that despite its ocean-dwelling ways, its closest living relative is likely the modern-day Amazon river dolphin or boto, a freshwater dolphin that inhabits the waterways of South America's Amazon and Orinoco Rivers.

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Isthminia is thought to be the closest relative of the modern-day Amazon river dolphin (pictured). Image: Luciana Christante, Flickr

I. panamensis also had a long, tooth-filled snout that was perfectly adapted for fishing out at sea, a feature that was retained in its modern-day descendants for hunting in rivers. The bones of the shoulder blade are also unusual, with the scapula bone taking on the shape of an “ice cream scoop”, according to the study.

At some point after this ancient dolphin went extinct, its descendants made the evolutionary leap from the salty seas to occupy the rivers of the Amazon basin. “Many other iconic freshwater species in the Amazon, such as manatees, turtles and stingrays have marine ancestors, but until now, the fossil record of river dolphins in this basin has not revealed much about their marine ancestry," explains Pyenson.

The sea-to-land transition means river dolphins headed in the opposite evolutionary direction to their oceanic counterparts and whales, who took to the seas from a land-dwelling ancestor.  

For river dolphins, the move inland may have been triggered by rising sea levels that flooded the continents and forced marine animals to adapt to new environments. “Isthminia now gives us a clear boundary in geologic time for understanding when this lineage invaded Amazonia,” adds Pyenson. 

The new study has been published in the open access journal PeerJ. 

Top header image: Nicholas D. Pyenson / NMNH Imaging / Smithsonian Institution