For decades, Nebraska’s Ashfall Fossil Beds have been famous for a haunting scene frozen in deep time: rhinos, horses, camels and other animals buried almost exactly where they died after a catastrophic volcanic eruption blanketed the Great Plains in ash.
Now researchers are paying closer attention to another set of visitors that moved across the same dying landscape.
Scientists working at Ashfall Fossil Beds State Historical Park in Antelope County, Nebraska, have identified fossil tracks believed to have been left by extinct dog-family carnivores known as borophagines, often nicknamed “bone-crushing dogs.”
The tracks date to roughly 12 million years ago, during the late Miocene Epoch.
They are especially significant because footprints preserve something skeletons often cannot: movement.
Combined with tooth marks on the bones of dead herbivores and fossilised droppings containing fragments of bone, the evidence is helping researchers reconstruct how these carnivores may have moved through the ash-covered watering hole and fed on animals killed by the disaster.
A super-eruption nearly 900 miles away created the Ashfall site
The story begins far from Nebraska.
Geologists have traced the ash at Ashfall to the Bruneau-Jarbidge volcanic field in what is now southwestern Idaho, nearly 900 miles west of the fossil site.
The eruption occurred around 12 million years ago and was immense. The University of Nebraska State Museum describes it as roughly 1,000 times larger than the 1980 eruption of Mount St. Helens. The U.S. Geological Survey links the widespread ash to the approximately 12-million-year-old Ibex Hollow Tuff from the Bruneau-Jarbidge region.
Across much of northern Nebraska, the resulting ash layer is about a foot thick. At Ashfall, however, it accumulated to roughly 8 to 10 feet, largely because the site was an ancient watering hole where wind-blown material collected.
The result was one of North America’s most remarkable fossil deposits.
Rhinos, horses and camels died slowly in the ash
More than 200 complete or nearly complete skeletons have been exposed at Ashfall, including large numbers of rhinos along with horses, camels, birds, deer and other animals.
The animals were not necessarily killed instantly. Researchers believe smaller species died quickly, while larger mammals survived the initial ashfall but continued breathing abrasive volcanic dust as food and water sources became contaminated.
Many appear to have gathered at the watering hole as their condition worsened.
Eventually, their bodies were covered by accumulating ash, preserving complete skeletons in three dimensions.
That extraordinary preservation is why Ashfall is sometimes described as a prehistoric Pompeii.
Then came the scavengers
The new tracks add another layer to that disaster story.
In recent years, researchers have found footprints in and around the ash deposits that appear consistent with caniform predators, including members of Borophaginae.
Borophagines were not domestic dogs, nor were they early versions of modern breeds.
They were an extinct North American branch of the dog family that survived for tens of millions of years before disappearing roughly two million years ago. Some forms were small and omnivorous, while others evolved powerful jaws and teeth adapted for cracking bone.
The animals implicated by the recent Ashfall evidence may have been roughly wolf-sized.
Footprints show movement — bite marks show feeding
Tracks alone can establish that an animal walked through a place.
At Ashfall, however, palaeontologists have additional clues.
Park superintendent Sam Matson says researchers have found tooth marks on the bones of rhinos, horses and camels that appear compatible with the teeth of bone-crushing canids.
They have also found coprolites — fossilised animal droppings — containing chips of bone.
Together, the three clues build a behavioural picture: the tracks show movement, tooth marks show carcass feeding, and bone fragments in coprolites show that at least some carnivores were consuming skeletal material.
Did these ancient canids survive when the herbivores did not?
This is where the Ashfall mystery becomes particularly interesting.
Large numbers of herbivore skeletons were buried in place, yet researchers have struggled to connect the new scavenging evidence with matching skeletons of the animals responsible.
One possibility is that the predators survived long enough to exploit the abundance of dead or dying prey and later left the immediate area before succumbing elsewhere.
Matson has suggested that future excavations could still uncover remains associated with those scavengers, perhaps even in dens.
Ashfall has previously produced fossil remains of Cynarctus, a member of Borophaginae, including an articulated skeleton reported more than a decade ago. So it would be inaccurate to say no borophagine skeleton has ever been found there.
The current mystery is narrower: researchers have not yet linked skeletal remains directly to the scavengers represented by the recent tracks and feeding traces.
‘Bone-crushing dogs’ were very different from today’s pets
For a pet-focused audience, the name can be misleading.
Borophagines belong to the evolutionary history of Canidae, the family that today includes domestic dogs, wolves, foxes, coyotes and related species.
But they were a separate extinct radiation, not simply prehistoric pet dogs and not necessarily direct ancestors of the Labrador, Indie or Shih Tzu living today.
Some borophagines evolved deep skulls, robust jaws and teeth adapted for breaking carcasses apart and consuming more bone than most living canids. The lineage ranged from relatively small forms to giants such as Epicyon, which could exceed 300 pounds.
The Ashfall discoveries therefore reveal a chapter of canine evolution that no longer has a true modern equivalent.
Fossil footprints preserve behaviour that bones cannot
Skeletons reveal anatomy and evolutionary relationships. Footprints can show an animal physically moving through a particular environment, which makes the Ashfall tracks especially valuable.
The fossil bed records not simply which species lived in ancient Nebraska, but how animals responded to an environmental disaster.
Rhinos and other herbivores gathered around the watering hole as ash inhalation worsened. Scavengers moved among the carcasses and apparently consumed bone, while some predators may have remained mobile enough to leave the area.
It is a rare behavioural snapshot from a world that disappeared millions of years before humans existed.
Ashfall is still an active scientific site
The discovery also shows why Ashfall Fossil Beds remains scientifically important more than five decades after palaeontologist Mike Voorhies first recognised the site in 1971.
Unlike many fossil localities, many Ashfall skeletons are excavated and displayed in situ, meaning visitors can see them where they were actually found.
The site became a Nebraska State Historical Park in 1991 through a partnership between the University of Nebraska State Museum and the Nebraska Game and Parks Commission. Research continues every summer, and Matson estimates another 10 to 15 summers of excavation work may remain in the current area.
Somewhere beneath the Nebraska prairie may still be the fossil evidence needed to identify exactly which bone-crushing canids left the tracks, gnawed the carcasses and moved through Ashfall while one of prehistoric North America’s greatest ecological disasters was still unfolding.
Twelve million years later, their footprints are finally allowing scientists to follow them.
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