A Prehistoric Toothless Dolphin That Ate by Vacuuming Up Squid

A 30-million-year-old fossil found in South Carolina belonged to a weird, weird creature.

Two dolphins, one of them trying to catch an escaping squid
Artist's drawing of what Inermorostrum xenops may have looked like  (Courtesy of Robert Boessenecker)

The skull came from the Wando River, which today runs past Charleston, South Carolina to the ocean.

Thirty million years ago, it was all under the sea. Ancient dolphins and whales swam over what would become Charleston's cobblestone streets. For millions of years, megalodons also swam in this sea, leaving behind shark teeth bigger than your hand. And it was divers looking for megalodon teeth who initially found the fossilized skull, loose on the bottom of the Wando River.

The skull—unusually wide and unusually squat—made its way to a collector who gave it to the College of Charleston’s Mace Brown Museum of Natural History. By the time the paleontologist Robert Boessenecker came to it, he says “It was already known as something new and very strange.” It was definitely a cetacean, but like none they had ever seen before. Its snout was too short. Its body was likely only a few feet long. And it had no teeth, no tooth sockets even.

It’s hard to overstate just how weird cetaceans are as mammals. Modern whales, dolphins, and porpoises all descended from a wolf-sized creature that returned to the sea, losing its hind legs and most of its fur in the process. Their dental evolution is odd, too. Unlike most mammals, cetaceans do not chew. The ones with teeth grasp and tear like reptiles and then swallow the chunks whole. The ones with baleen filters feed on giant amounts of krill. And about 30 million years ago, there were apparently cetaceans like the one found in South Carolina, that had lost their teeth and ate by suction.

The fossilized skull of Inermorostrum xenops (Proceedings of the Royal Society B)

As Boessenecker and his colleagues measured the partial skull, they realized it is related to modern odontocetes, also known as toothed whales—a name that’s obviously a bit misleading. “It’s definitely a weird, weird toothed whale.” says John Gatesy, who studies cetaceans at the American Museum of Natural History and was not involved in the study. The team studying the South Carolina skull named it Inermorostrum xenops.

There actually are modern odontocetes that don’t really use their teeth either. Male beaked whales, for example, usually have one pair of teeth that is only used to fight for females, whose teeth stay completely hidden in their gums. Beaked whales, along with pilot whales and sperm whales, also catch squid by sucking them into their mouths. But all of these whales evolved recently. Inermorostrum xenops seems to have evolved its toothless suction-feeding independently and much, much earlier than modern suction-feeding whales. “It’s a highly specialized species but it’s essentially a dead end,” says Boessenecker. Evolution, far from being some linear progression, often works this way, hitting dead ends and retrying failed experiments from millions of years earlier.

Inermorostrum xenops’ short snout and toothless suction-feeding is also just one extreme amidst a sudden explosion of whale diversity in this time—at least based on the limited fossil record. Nick Pyenson, curator of fossil marine mammals at the Smithsonian Institution, says that suction feeding as a strategy is not that surprising, since all mammals have the ability to suck for milk. “Every mammal on the planet was a suction feeder since the origin of mammals,” he says. “It’s the basic way a mammal feeds as a baby.” Since all baby mammals must suck, it limits how much their jaws and snouts can change in adulthood to adapt to other feeding strategies. So perhaps more surprising, says Pyenson, is the emergence of other, later cetaceans with long, crocodile-like snouts that they can use to snap while sweeping left and right. Fossils of the ancient cetacean Xiphiacetus, for example, show a snout three feet long with hundreds of teeth.

Boessenecker and his team hypothesize—and it’s only a hypothesis—that the diversity of feeding strategies before and around this period may have to do with changes to the global climate. Antarctica finally broke off as its own separate continent, which allowed the Antarctic Circumpolar Current to swirl around the landmass, bringing nutrients up from the deep sea. Its effects may have rippled out through the world’s oceans, all the way up to to what would one day become the Wando River.