Study: Postcrania and locomotor function of Mesocyon coryphaeus (Canidae, Carnivora) from the Arikareean of North America (DOI: 10.1017/jpa.2026.10232)
For decades, a 30 million-year-old-fossil sat entombed in rock in an Oregon museum collection. When it was first found, paleontologists thought it might be an early relative of sheep, camels and pigs whose fossilized skeletons are, in the researchers' view, abundant.
Later, they realized it was an early dog fossil, and now, University of Michigan paleontologist Anne Kort has confirmed that it's one of the most complete fossils of a pre-Ice Age dog found in North America, a canid called Mesocyon coryphaeus. In fact, the species, first named in 1879, had only been previously identified by single skulls.
Kort was tasked with describing the fossil as part of a special issue of the Journal of Paleontology, celebrating the 50th anniversary of John Day Fossil Beds National Monument, or JODA. She says the fossil was initially discovered in the late 1980s, near the monument in Oregon, on Bureau of Land Management lands.
At the time, paleontologists carefully extracted the fossil and its surrounding rock and wrapped it for protection in a plaster jacket. In the 1990s, researchers separated the head for display, then determining it was a kind of dog. Park paleontologists including Jennifer Cavin, co-author of the study, uncovered the rest of the skeleton in 2012, realizing how complete the skeleton was.
"They thought at first it was an oreodont, which we like to call sheep-camel-pigs," said Kort, who was a postdoctoral scholar at U-M's Museum of Paleontology. "This was the most magical, fun project to just be handed on a silver platter."
The hip bone is connected to the … baculum
To establish that the fossil was indeed a dog, Kort says researchers typically would compare features to other dog fossils, such as the shape of the fossil's teeth, the shape of the skull and other features. This particular fossil also had a hereditary bone growth on the end of its wrists called an osteochondroma, which is found in about 60% of the earliest dog fossils.
"It's just this really cool direct hereditary link that we wouldn't normally find," Kort said. "This is another line of evidence, and that gives you an idea of how great this fossil is."
The fossil itself is nearly complete, missing only the tail and parts of the hands and feet of the animal. The integrity of the fossil also meant that the researchers did not have to guess whether the specimen was male or female.
"The other fun thing is, it even had the baculum, so, the penis bone," Kort said. "That means we definitely know this was a male."
What the bones tell us
Dogs first originated in North America about 40 million years ago, according to Kort. Their common ancestor is a canid called Hesperocyon. At the time, the North American continent's terrain was composed of dense, rainforest-like forests, which meant that Hesperocyon was likely a "pretty good climber," unlike dogs today, Kort says.
Mesocyon, Kort's fossil, originated about 10 million years later, during a period of global cooling. The dense forests gave way to more open woodland and sometimes shrubland. Kort wanted to see if the Mesocyon fossil would have the same running adaptations as modern dogs.
"The answer is pretty definitely no," said Kort, who studies the interaction between environment and running adaptations in mammals. "Mesocyon also just has more robust bones in general. It's shorter and stockier."
The fossil's elbow joining suggests that it could rotate its forearms, indicating the ability to grapple and climb, she says. The fossil shows that Mesocyon also still had its big toe, or what's called the dew claw in contemporary dogs. This suggests Mesocyan was at least occasionally walking heel-down, instead of on its tip toes, like dogs do today.
Kort says the fossil holds one more lesson for us: When an animal goes extinct, its lineage is lost forever.
"This fossil is not our current dogs' great-great-great uncle, because this whole line went fully extinct. And with that extinction, we did lose a whole kind of dog ecology that we just don't see anymore," she said. "I think sometimes there's an assumption that life finds a way if something goes extinct. It will be replaced, or something will magically bounce back.
"But evolution never finds a perfect fit, it finds good fits. So when we lose things, they're gone forever. They're never going to be perfectly replaced, and I think that's an important lesson to take away when we're thinking about extinctions today."
JODA paleontologist Ted Fremd was the researcher who initially found the specimen, prioritized its collection and completed the initial round of preparation to remove the skull. JODA paleontologist Joshua Samuels directed Cavin to work on removing the fossil from its protective jacket in 2011. Nicholas Famoso, current chief paleontologist for JODA, suggested Kort work on the project. Xiaoming Wang, a researcher at the Natural History Museum of Los Angeles County, is a co-author of the study.
