OREODONTS: MOUNTAIN TEETH & ANCIENT OREGON

Cast of Oreodont, John Day Formation

With a pig-like body, a sheep-ish silhouette and a distant connection to camels, an oreodont is a lovely reminder that ancient life did not always fit neatly into familiar categories. 

These curious mammals were enormously successful, leaving abundant fossils in places as far apart as South Dakota’s Badlands and Oregon’s John Day country. 

The specimen you see here is a cast of an oreodont fossil from the John Day Formation—a chance to meet one of the characters that browsed ancient Oregon long before its present landscape took shape.

Oreodonts lived in North America from roughly 40 million to 5 million years ago, spanning much of the Eocene through the Miocene. They belonged to the artiodactyls, the even-toed mammals whose living members include camels, bison and bighorn sheep. 

Oreodont Reconstruction

Many oreodonts had four hooved toes and sturdy, short-legged bodies. Camels are generally considered their closest living relatives, though the relationship is a distant one. No family reunion invitations have been forthcoming.

Their teeth reveal a plant-eating lifestyle, with cheek teeth suited to processing vegetation such as leaves and shoots. Some oreodonts may have gathered in herds, offering a little collective protection against predators, including the formidable cat-like nimravids. 

Being a herbivore required a balance between finding something delicious and avoiding becoming something delicious yourself.

What’s in a name? What’s in a tooth?

Despite the tempting association, oreodonts have nothing to do with chocolate biscuits dunked in milk. Their name combines the Greek roots for “mountain” and “tooth”, referring to the peaks and ridges of their molars. Think miniature mountain ranges tucked into a mouth. The tooth connection appears in familiar words such as dentist and orthodontist, though I suspect an oreodont orthodontist would have needed a particularly sturdy chair.

Teeth are among palaeontology’s most useful clues. Their size, proportions and arrangement of cusps and ridges can help distinguish one animal from another, sometimes down to species. Yet a single tooth does not always provide a tidy answer. A recent study of John Day oreodonts excluded certain isolated teeth from its analyses because they were too easily confused between groups. A good identification depends on knowing both what a feature reveals and where its limits lie.

There are intriguing clues elsewhere in the skull, too. Some oreodonts, including Leptauchenia, possessed greatly enlarged auditory bullae—the bony capsules enclosing the middle-ear cavities. These structures have been studied for what they might reveal about hearing and habitat. Enlarged middle-ear cavities can improve sensitivity to lower-frequency sounds in living mammals, but they do not, by themselves, tell us the size of an extinct animal’s external ears. Fossils invite us to listen carefully, even when their owners have been silent for millions of years.

The browsers of John Day

In Oregon, the John Day Formation preserves an extraordinary sequence of changing environments. Its middle and upper portions contain major fossil assemblages dating from approximately 30–18 million years ago. Volcanic ash, stream deposits and ancient soils accumulated across landscapes that became cooler and drier over time. The beautiful blue-green claystones of the Turtle Cove Member owe their colour to celadonite, a mineral formed during alteration of the volcanic ash. A lovely palette for a remarkably rich fossil archive.

Eporeodon occidentalis

One of the region’s familiar oreodonts is Eporeodon occidentalis, a stocky browser with four hooved toes and prominent canine teeth. Its imposing tusks accompanied a set of cheek teeth built for processing plants. 

Across the Turtle Cove landscape, oreodonts shared wooded and more open habitats with three-toed horses, mouse-deer and an assortment of predators. These communities changed through the millions of years preserved in the rock; they were never one enormous prehistoric gathering frozen in time.

Then there was Agriochoerus, a wonderfully unusual oreodont relative with curved claws instead of hooves. Its limb anatomy suggests that it could climb, perhaps using trees in a way comparable to living tree kangaroos. That interpretation gives us a delightful possibility: a sheep-sized browser taking its leafy dining arrangements upstairs. With nimravids in the neighbourhood, the option may have had considerable appeal.

Fresh research is still refining this story. A September 2026 study by Meaghan M. Emery-Wetherell, Nicholas A. Famoso and Joshua X. Samuels documented a major change around the Picture Gorge ignimbrite, a volcanic rock layer that helps researchers correlate the fossil-bearing beds. Agriochoerus disappeared locally, the large terrestrial Promerycochoerus arrived, and Eporeodon persisted but became smaller. 

Oreodont Diversity in the Kimberly Member

Regional oreodont diversity peaked in the Kimberly Member, roughly 26.6–25.5 million years ago, with as many as five species represented during that interval.

Each of the fellows you see on the left bear a familial resemblance to their peers but with slight differences.

That is what makes an oreodont fossil—or a carefully made cast—such a compelling introduction to deep time. 

Follow those mountain teeth into the surrounding rocks and an entire world begins to emerge: leaves overhead, ash settling through the air, and unfamiliar animals going about the wonderfully ordinary business of staying alive.

Lead Image: Cast of an oreodont fossil from the John Day Formation, Oregon. Oreodonts were extinct plant-eating mammals whose distinctive cheek teeth inspired their name, “mountain teeth”. This lovely was cast by Rod Bartlett, Vancouver Paleontological Society, back in the mid 2000s.