A skull fossil found in South Carolina has revealed a new ancient dolphin species that slurped its food instead of chewing on it.
The fossilized species had a short snout and was toothless, according to a paper published in Proceedings of the Royal Society B, and the fossil shows holes in the creature’s bones that could indicate enlarged lips or whiskers. Instead of catching prey by tearing and grasping it with their teeth before swallowing the pieces whole—the way modern dolphins typically prefer to eat—this ancient creature sucked up prey from the sea floor in a method known as suction feeding.
Scientists think the dolphin (named Inermorostrum xenops) may be the earliest toothless offshoot from the toothed whale suborder Odontoceti, ultimately leading to many different kinds of modern feeding behavior among the group.
This evolutionary divergence happened within the Oligocene Epoch, one of the most important periods of whale evolution, during the Paleogene Period. The fossil is about 30 million years old, dating to a time when snout shapes and tooth presence were becoming diverse among the toothed whale suborder.
As the order’s snout shape continued to evolve, it eventually settled into the optimum design found on a modern bottlenose dolphin.
The skull came from the Wando River, which now runs past Charleston to the Atlantic Ocean, according to an article in the Atlantic. Divers who were searching for megalodon teeth found the fossil, which was loose at the bottom of the river. The animal is thought to have been roughly the size of today's harbor porpoises, which are about five feet long and weigh up to 120 pounds.
Robert Boessenecker, a paleontologist at the College of Charleston who authored the paper on the fossil, says many similar fossils are being found at the Oligocene Ashley Formation of South Carolina.
“Stuff like this is falling out of ditches and construction sites all over the place,” he says. “It is astonishingly fossiliferous.”
He says very few other basins around the world were actively depositing sediment during that period of time, so there are only three other major places to find good fossils of echolocating dolphins and baleen whales: the South Island of New Zealand, Japan, and the Pacific Northwest.
The fossil is the earliest example of a suction-feeding specialist, or a species that has no choice but to suction-feed, because it was toothless. Boessenecker says some mammals, like bottlenose dolphins, can choose to suction-feed if they want to, though it is harder to do so with a longer snout. But the new species was reliant on suction-feeding, so that made its food options less flexible.
“It was totally, unilaterally adapted toward that feeding mode,” he says.
Though this new species is the first example of this evolutionary divergence, shorter snouts have evolved in species several times since then. The snout of the new species was about twice as short as that of a modern-day bottlenose dolphin, more closely resembling the snouts of pygmy sperm whales and harbor porpoises.
Because the dolphin was dwarf-size, it probably spent time in shallow water because it wouldn’t have the deep diving capabilities of a larger animal.
“We suspect that this animal probably hung out very close to shore because of its size,” he says.
Its snout was also turned down, so Boessenecker says it likely spent a fair amount of time grubbing up prey off of the sea floor.