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Inermorostrum xenops. Photo: College of Charleston

A fossilized skull recovered from a river near Charleston, South Carolina belonged to a newly identified species of ancient, toothless dwarf dolphin, researchers report.

The extinct dolphin, named Inermorostrum xenops, likely used its mouth like a vacuum to suck up small fish, squid, sea cucumbers and other soft prey.

Inermorostrum xenops translates to “weaponless-snouted strange face in Latin.

The skull fossil was discovered by divers who were on the hunt for shark teeth in the Wando River.

A team of researchers from the College of Charleston, led by Robert Boessenecker, analyzed the fossil and the layers of sandy limestone in the same location where the skull was found, and determined the animal lived 28 to 30 million years ago.

They also revealed that the species was just about 4 feet long, and weighed 100 pounds – making it an easy target for predators like sharks, whales and other giant creatures it shared the ocean with.

As a point of reference, the modern bottlenose dolphin ranges in length from 7 to 12 feet.

Inermorostrum xenops belonged to an early group of echolocating dolphins – the xenorophidae – which, ironically, represent the earliest diversification of toothed whales. This group evolved just 4 million years before Inermorostrum xenops appeared, indicating it only took that timespan to evolve a toothless, suction-feeding dolphin from ancestral whales, according to the research team. It also evolved within 5 million years of modern cetaceans (a group of aquatic mammals including whales, dolphins and porpoises).

Boessenecker explained that both short and long snouts evolved numerous times on different parts of the evolutionary tree, demonstrating that the aquatic mammals can rapidly adapt their feeding behaviors and specializations.

The bottlenose dolphin has the optimal snout length, according to Boessenecker. Its snout is twice as long as it is wide, allowing it to suction feed and catch prey as it chooses.

Inermorostrum xenops had the shortest jawbone of any known cetacean, living or extinct, and enlarged holes in its snout suggest the dwarf dolphin may have had whiskers or fleshy lips.

“The discovery of a suction-feeding whale this early in their evolution is forcing us to revise what we know about how quickly new forms appeared, and what may have been driving early whale evolution” said Danielle Fraser, a paleontologist at the Canadian Museum of Nature. “Increased ocean productivity may have been one important factor.”

The paper was published in the journal, Proceedings of the Royal Society B.

The skull fossil is now on display at the Mace Brown Museum of Natural History at the College of Charleston.
 

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