30-Million-Year-Old Fossil Reveals a Short, Stocky Ancient Dog Built for Ambush Hunting
When paleontologists first dug up a 30 million-year-old fossil at John Day Fossil Beds National Monument in Oregon, they thought it belonged to a bygone relative of sheep, camels, and pigs. Only years later would they find out that the fossil was actually the remains of a pre-Ice Age dog.
Most of the dog’s skeleton has been pieced together since then, becoming the most complete skeleton of the ancient species, called Mesocyon coryphaeus. A project to describe this fossil, detailed in a new study published in the Journal of Paleontology, was taken up by University of Michigan paleontologist Anne Kort.
“This was the most magical, fun project to just be handed on a silver platter,” said Kort, who is now an assistant professor at the University of Michigan, in a statement.
Sorting through multiple bones, Kort found that Mesocyon was not quite the same as a modern pooch; the species was short and stocky, and a clumsy, albeit capable climber, but not a long-distance runner like many dogs today. Altogether, its traits tell the story of how a changing environment drove dog evolution millions of years ago.
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An Early Dog’s Identity Crisis
Diagram of Mesocyon coryphaeus fossil.
(Image Courtesy of Jennifer Cavin, John Day Fossil Beds National Monument, and Anne Kort, University of Michigan)
Paleontologists found the Mesocyon coryphaeus fossil in the 1980s, but it wasn’t until 2012 that the near-complete skeleton of this animal was fully uncovered, missing only the tail and parts of the hands and feet. Initially, back when the species was identified only by single skulls, Mesocyon coryphaeus wasn’t thought to be a dog.
Researchers eventually realized that Mesocyon was, in fact, an early dog after comparing its features — like teeth and skull shape — to other dog fossils. But its skeleton, oddly enough, didn’t look like that of a modern dog, due to its shorter foot and robust legs.
“I was most fascinated by how un-dog-like the skeleton of M. coryphaeus was, especially with how distinctively dog-like the skull is,” Kort told Discover.
The fossil of Mesocyon had a hereditary bone growth on the end of its wrists, called an osteochondroma, which is present in around 60 percent of the earliest dog fossils.
And a certain bone made it very clear whether this specimen was a male or a 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.”
From Small to Medium
Illustration of what Mesocyon coryphaeus may have looked like.
(Image Courtesy of Kaori Chambers, University of Michigan)
Before Mesocyon came the earliest known canid, Hesperocyon, which emerged 40 million years ago. According to Kort, Hesperocyon was small and would have gone after small rodents. It was abundant across North America, able to thrive even as the climate cooled. Much later on, dog lineages diversified, evolving larger bodies and adopting hypercarnivorous diets.
Mesocyon, however, occupied a middle ground amidst the transition; it was a mid-sized predator, close to a coyote in size. It originated at a time when the world’s environments were beginning to change from predominantly forests to open woodland and sometimes shrubland.
An Ambushing Predator
Mesocyon hadn’t yet evolved cursoriality, which Kort describes as a broad term for running adaptations, particularly a shorter upper leg (femur and humerus), longer foot, and stabilized joints.
The elbow joint of the Mesocyon fossil suggests that the animal could rotate its forearms, meaning it could likely grapple and climb. Kort also says that it would have stood with its heel on the ground, potentially going up on its toes to run on occasion, and also had a forearm bone that looked like something from the weasel family rather than the dog family.
Mesocyon likely walked or ran in short bursts, lacking the traits suited for long-distance running. And its main method of predation would’ve been ambushing prey, since there was still just enough vegetation around to provide cover during its time.
The Mesocyon fossil shows that early canids didn’t immediately evolve cursoriality; the adaptations would later surface as open grasslands continued to spread after Mesocyon went extinct around 21 million years ago. The extinction of Mesocyon, possibly driven by competition with other carnivores, erased a unique dog lineage.
The dogs that have survived to this day, on the other hand, have thrived in large part because of their cursorial adaptations.
“The ability to cover long distances is probably a big part of what has made the modern canids so successful. Canids were basically restricted to North America until about 8 million years ago, and now they are on every continent but Antarctica,” Kort said to Discover.
Read More: Oldest-Known Fossil of Creature That Seems Part Reptile, Part Mammal Found
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