Dolphins are odontocetes. That literally translates to "toothed whales", the broader cetacean family that they belong to. But that's just the view from the top. Dig into the evolutionary history of whales some of these seagoing mammals start bucking the trend. There are ancient baleen whales - or mysticetes - that have teeth, for example, and researchers have just announced a 29 million year old "toothed whale" that doesn't have any teeth at all.

Paleontologists Robert Boessenecker and colleagues have named the ancient cetacean Inermorostrum xenops. Or, roughly, the "strange face with a defenseless snout." This fossil dolphin, which used to swim through seas that once covered what's now South Carolina, doesn't have a single tooth in its mouth. This, combined with its short snout, led Boessenecker and coauthors to propose Inermorostrum was a suction feeder, creating little underwater vacuums to slurp up prey.

Of course, no fossil whale stands - or is that swims? - alone. "The Oligocene represents one of the most important periods of whale evolution," Boessenecker and colleagues write, documenting an underwater evolutionary boom of different whale lineages. Inermorostrum is part of that, showing that ancient odotocetes weren't just sharp-toothed hunters. This time period saw ancient whales evolving a variety of ways to snap and sift and suck up their food, setting the stage for the what we see in their relatives today.

The skull of Inermorostrum xenops. Credit: Boessenecker et al 2017

Fossil Facts

Name: Inermorostrum xenops

Meaning: Inermorostrum means "defenseless snout", while xenops means "strange face."

Age: Oligocene, about 29 million years ago.

Where in the world?: Berkeley County, South Carolina.

What sort of organism?: A dolphin belonging to a group called xenorophids.

How much of the organism’s is known?: A partial skull and fragment of braincase.


Boessenecker, R., Fraser, D., Churchill, M., Geisler, J. 2017. A toothless dwarf dolphin (Odontoceti: Xenorophidae) points to explosive feeding diversification of modern whales (Neoceti). Proceedings of the Royal Society B. doi: 10.1098/rspb.2017.0531

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