Marine mammals evolved from walking on land to living in the sea. When did river dolphins turn back?

Over the eons, dolphins and other marine mammals moved from the land to the sea. But at one point some dolphins turned back, heading inland and upriver to fresh water. A new fossil discovery sheds light on how and when river dolphins evolved.

Digital reconstructions of Isthminia panamensis, a new fossil dolphin from Panama, bookend a photograph of the actual specimen, which is housed at the Smithsonian’s National Museum of Natural History. The image on the left is based on a normal map of a 3D model; the image on the right is based on a 3D model with enhanced shading for detail. These 3D models can also be visualized, downloaded and printed from the Smithsonian X 3D website at http://3d.si.edu (Credit: Nicholas D. Pyenson / NMNH Imaging)

Smithsonian scientists and colleagues have discovered a previously unknown species of long-extinct river dolphin after examining fossil fragments from Panama. The discovery sheds new light on the evolution of today’s river dolphins.

Today there are only four species of river dolphins―all living in freshwater or coastal ecosystems and all endangered—including the Yangtze river dolphin, which is likely now extinct. Each modern river dolphin species has adapted to freshwater habitats by evolving bodies that include broad, paddle-like flippers, flexible necks and heads with particularly long, narrow snouts—all the better to navigate and hunt in winding, silty rivers.

The fossil discovery, Isthminia panamensis, which dates from 5.8–6.1 million years ago, was found on the Caribbean coast near the town of Piña, Panama. It consists of half a skull, a lower jaw with an almost entire set of conical teeth, a right shoulder blade and two small bones from the dolphin’s flipper. In comparison with other river dolphins—both fossil and living—the shape and size of these parts suggests that the full specimen may have been more than 9 feet long.

Fossil evidence suggests that river dolphins’ ancestors were once widespread around the globe. I. panamensis was clearly one of them, and its fossil remains have helped the team understand something less clear: When did river dolphins transition from the saltwater of the ocean to the freshwater of rivers?

“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,” said the study’s lead author Nicholas D. Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History. “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 Amazonia—the watershed of the 4,000 mile-long Amazon river—has not revealed much about their marine ancestry.Isthminia now gives us a clear boundary in geologic time for understanding when this lineage invaded Amazonia.”

Other fossilized animals found at the same site as I. panamensis were marine species, indicating that unlike today’s river dolphins, I. panamensis lived in the salty waters of a food-rich Caribbean Sea, before the full closure of the Panama Isthmus.

An artistic reconstruction of Isthminia panamensis, a new fossil dolphin from Panama, 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. The fossils of Isthminia panamensis were collected from marine rocks that date to a time (around 6 million years ago) before the Isthmus of Panama formed and a productive Central American Seaway connected the Atlantic and Pacific oceans. (Credit: Julia Molnar)

“Isthminia is actually the closest relative of the living Amazon river dolphin,” said study co-author Aaron O’Dea, staff scientist 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. As such, fossil specimens may tell stories not just of the evolution these aquatic animals, but also of the changing geographies and ecosystems of the past.”

The Smithsonian’s Digitization Program Office collaborated with the scientific team to create a high-resolution 3-D scan of the fossil, allowing the scientists to create 3-D prints of the delicate specimen, whose bones are too fragile to be molded and casted by traditional approaches. A 3-D print of the fossil is on permanent display at Panama’s BioMuseo—the original specimen will remain in the Smithsonian’s collection at the National Museum of Natural History. The public can also explore and download high resolution scans of the dolphin’s skull, jaw and shoulder blade at the Smithsonian X 3-D website, 3D.SI.EDU.

The fossil was discovered by Dioselina Vigil, then a STRI intern and student at the University of Panama. The name of the new genus, Isthminia, recognizes both the Panama Isthmus and the fossil specimen’s living relative, the Amazon river dolphin, Inia geoffrensis. The study’s authors chose the species name, panamensis, to recognize the Republic of Panama, its people, and the many generations of scientists who have studied its geological and biological histories.

The team’s research was published today in the scientific journal Peer J.

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