The Yellowstone supervolcano destroyed an ecosystem but saved it for us

Death was everywhere. Animal corpses littered the landscape and were mired in the local waterhole as ash swept around everything in its path. For some, death happened quickly; for others, it was slow and painful.

This was the scene in the aftermath of a supervolcanic eruption in Idaho, approximately 1,600 kilometers (900 miles) away. It was an eruption so powerful that it obliterated the volcano itself, leaving a crater 80 kilometers (50 miles) wide and spewing clouds of ash that the wind carried over long distances, killing almost everything that inhaled it. This was particularly true here, in this location in Nebraska, where animals large and small succumbed to the eruption’s deadly emissions.

Eventually, all traces of this horrific event were buried; life continued, evolved, and changed. That's why, millions of years later in the summer of 1971, Michael Voorhies was able to enjoy another delightful day of exploring.

Finding rhinos

He was, as he had been each summer between academic years, creating a geologic map of his hometown in Nebraska. This meant going from farm to farm and asking if he could walk through the property to survey the rocks and look for fossils. “I’m basically just a kid at heart, and being a paleontologist in the summer was my idea of heaven,” Voorhies, now retired from the University of Nebraska-Lincoln, told Ars.

What caught his eye on one particular farm was a layer of volcanic ash—something treasured by geologists and paleontologists, who use it to get the age of deposits. But as he got closer, he also noticed exposed bone. “Finding what was obviously a lower jaw which was still attached to the skull, now that was really quite interesting!” he said. “Mostly what you find are isolated bones and teeth.”

That skull belonged to a juvenile rhino. Voorhies and some of his students returned to the site to dig further, uncovering the rest of the rhino’s completely articulated remains (meaning the bones of its skeleton were connected as they would be in life). More digging produced the intact skeletons of another five or six rhinos. That was enough to get National Geographic funding for a massive excavation that took place between 1978 and 1979. Crews amassed, among numerous other animals, the remarkable total of 70 complete rhino skeletons.

To put this into perspective, most fossil sites—even spectacular locations preserving multiple animals—are composed primarily of disarticulated skeletons, puzzle pieces that paleontologists painstakingly put back together. Here, however, was something no other site had ever before produced: vast numbers of complete skeletons preserved where they died.

Realizing there was still more yet to uncover, Voorhies and others appealed to the larger Nebraska community to help preserve the area. Thanks to hard work and substantial local donations, the Ashfall Fossil Beds park opened to the public in 1991, staffed by two full-time employees.

Fossils discovered are now left in situ, meaning they remain exposed exactly where they are found, protected by a massive structure called the Hubbard Rhino Barn. Excavations are conducted within the barn at a much slower and steadier pace than those in the '70s due in large part to the small, rotating number of seasonal employees—mostly college students—who excavate further each summer.

The Rhino Barn protects the fossil bed from the elements.

Credit: Photos by Rick E. Otto, University of Nebraska State Museum

A full ecosystem

Almost 50 years of excavation and research have unveiled the story of a catastrophic event and its aftermath, which took place in a Nebraska that nobody would recognize—one where species like rhinoceros, camels, and saber-toothed deer were a common sight.

But to understand that story, we have to set the stage. The area we know today as Ashfall Fossil Beds was actually a waterhole during the Miocene, one frequented by a diversity of animals. We know this because there are fossils of those animals in a layer of sand at the very bottom of the waterhole, a layer that was not impacted by the supervolcanic eruption.

Rick Otto was one of the students who excavated fossils in 1978. He became Ashfall’s superintendent in 1991 and retired in late 2023. “There were animals dying a natural death around the Ashfall waterhole before the volcanic ash storm took place,” Otto told Ars, which explains the fossils found in that sand. After being scavenged, their bodies may have been trampled by some of the megafauna visiting the waterhole, which would have “worked those bones into the sand.”

This includes the fossils of gomphotheres—elephant-like animals that sometimes had four tusks and, at others, had flattened lower tusks resembling shovels. To date, no gomphothere fossils have been found in the ash bed; they have only been found in the sand below the ash.

Voorhies also noted the profuse “evidence of aquatic life in the sand layer just underneath the ash.” That includes ancient lizards, frogs, and turtles. Fossilized ripple marks offer further evidence of water.

“We know that this area had been used as a watering hole by dozens and dozens of species of animals, probably for centuries,” Voorhies continued, “because we find the scattered remains of pretty much the same kinds of animals that you find in the ash bed [in that bottom layer of sand].”

A paucity of bony fish fossils coupled with evidence of water level fluctuation further indicate a seasonal waterhole. The one at Ashfall, noted Sandy Mosel, a Museum Associate, is similar to waterholes in African countries today: a large “bowl-shaped depression in the landscape that would fill up with water during the rainy season, and then it would dry out almost completely during the dry season.” (Mosel started working at Ashfall the same year the park opened and has remained there ever since.)

That waterhole is the key to why so many fossils have been preserved and to understanding what happened to the animals within it. And what we know about the “sequence of deposition” of the site, Otto explained, came out of the excavations in the late '70s, when they “literally dissected the site.”

Credit: Les Howard, Nebraska Conservation and Survey Division

Massive excavation

In an expanse “about half the size of a basketball court,” the team worked in two separate shifts: one during the morning and the other through around 10 pm, Otto said. The process involved digging until finding remains, wrapping them in a field jacket—the plaster coating that protects and binds fossils so that they can be safely transported—and removing it from the area. The teams could then dig further.

Like a layer cake, distinct sets of animals were found grouped in sections of ash. The top layer contained the biggest animals—the rhinos—followed by a layer with horses and camels, then a layer of smaller animals such as saber-toothed deer. The final layer contained the tiniest of creatures, such as rodents and birds. Under all of the ash, they came upon the aforementioned waterhole sand, with an entirely different set of fossils representing over 60 species of vertebrates, none of which were fully articulated or even complete. These were the remains of animals that died long before the eruption.

These excavations uncovered the sequence of events in reverse. “That’s what helped put together the story, the evidence of what happened to the animals at that time,” Otto explained. The top layer exposed the animals that survived the longest; the final layer of sand would have been the bottom of the waterhole millions of years ago.

The animals found in the ash bed include, among many others, the rhinos (Teleoceras major)—bulky, stubby-legged tanks often referred to as “barrel-bodied.” There were also four species of camel, the biggest of which, according to Otto, “were not much bigger than a llama.” Five varieties of horses co-existed at this time, the largest of them “not much bigger than a white-tailed deer.” Twelve million years ago, this area also included a species of musk deer (Longirostromeryx wellsi), also known as saber-toothed deer for the two fangs that protruded from their mouths.

Largely flattened and broken fossils of birds, turtles, snakes, and rodents are found within the very bottom layer of ash, offering evidence of the first chapter in the sequence of devastating events.

Killed by Yellowstone

This was the work of a supervolcano—a name denoting its extra gigantic size—from the Yellowstone Hotspot. The initial eruption is thought to have covered the Great Plains 1,600 kilometers (about 900 miles) away from the volcano with perhaps a foot of ash—material that is essentially tiny glass shards. Wind then lofted that ash into clouds and continued to spread it over many weeks or months.

Breathing the ash was a death sentence, but it was immediate only for the tiniest animals with equally tiny lungs. They would have died soon after the ash began to fall, becoming quickly covered by it. Within days or weeks, saber-toothed deer visiting the waterhole succumbed and were also covered by ash. They were followed by the various species of camel and horses. Last to die were the rhinos. Having survived perhaps four to six weeks, they lay at the very top of the accumulated layers of ash before being buried as well, which is why their bodies were excavated first.

The distinct sets of animals in subsequent layers of ash provided evidence that their deaths were staggered.

Mosel described the scene as animals visiting the waterhole, perhaps to cool off. “They’d lay down; they wouldn’t get back up. That’s where they died.” That ash continued to fall, “drifting in [and] around them and over the top of them.” The deeper parts of the waterhole contain the most skeletons, she said, and “that is probably because there was enough water there that they could find comfort in it, yet keep their noses out of it when they laid down.”

Adding more evidence to a belated death, the larger animals have bone pathologies that couldn’t have developed immediately. The bone lesions resemble those suffering from Marie’s disease or hypertrophic osteopathy today, which can appear as a result of lung diseases like tuberculosis or pneumonia. Scientists link this pathological bone to the impact of breathing in the ash and eventually dying by asphyxiation. It's a death, Otto said, caused by “very, very slow suffocation.”

“The grazing animals—the rhinos, the horses and camels—would have been sucking the volcanic ash into their lungs all day long while they were attempting to feed,” he explained.

In 2013, Kathleen Bundy and Samuel D. Matson analyzed oxygen isotopes from the pathological bones of the animals at Ashfall to see if they could determine whether they suffered from fever, another symptom of Marie’s disease. Consistently lower stable oxygen isotopes indicate a higher body temperature, which is exactly what they found in these bones. They were not able, however, to conclusively determine the cause of that higher temperature.

Credit: Rick E. Otto, University of Nebraska State Museum

A “once-in-a-lifetime” collection

The slower pace of excavation since the park opened has enabled scientists to see things they wouldn’t have if they were merely digging to find and quickly remove body fossils, as was done initially. Such details include rodent and carnivore burrows, evidence that some creatures managed to survive the catastrophe.

Shane Tucker started working at Ashfall as an intern; he’s currently a highway paleontologist with the University of Nebraska State Museum. In 2014, he was the lead author of a book chapter that details more of the remarkable discoveries made at Ashfall. It’s a revealing look into an entire ecosystem; along with the aforementioned species, there are plants, fish, snakes, rabbits, squirrels, dogs, and so much more. Some elements of the ecosystem were discovered because they were preserved meals. For example, “in the rhinos,” he said, “you see grass seeds in their mouths so you know what they were basically eating right up to the end.” Various fossil lizards and a mouse were preserved within bird stomachs.

Bird bones are so fragile that they often don’t survive the fossilization process, which makes the number and preservation of birds at Ashfall exceptional. The most common bird at the site is a crowned crane (Balearica exigua). One specimen preserves both feather impressions and tendons, which rarely fossilize. And a number of extinct bird species have been discovered there, including a new type of vulture and hawk.

But equally exciting are the numerous trace fossils—physical impressions made during the life of an animal—that have been uncovered. “You can see the trackway where the rhinos were walking in and out of this waterhole,” Tucker said. Other traces offer evidence that some life continued after most other species had fallen. Remnants of scavenging were discovered by students, Tucker explained, in the form of pawprints from bone-crushing dogs in the ash bed. “You can see their footprints and measure their pace.” They’ve also discovered about a dozen holes these dogs dug in search of a possible meal.

Otto highlighted other traces of scavenging, noting that some skeletons are not entirely complete. In a few, he said, “the ribs were moved a couple of feet away from the rib cage. Or one of the legs was pulled away and the leg bones were dropped away from the skeleton. And that’s where, if we look [closely] at those displaced bones, sometimes the ends are chewed off, or you see actual bite marks from the teeth of an animal that was strong enough to chomp into bone.”

Skeletons of the victims lie near the burrows dug by scavengers who searched for their bodies.

Credit: Jon Smith, Kansas Geological Survey

Jon Smith is an associate scientist with the Kansas Geological Survey and has been researching, among other things, the ichnofossils at Ashfall. “Some of the layers have dog footprints on them,” Smith said. “I think at Ashfall you can definitely make a strong case that [these] dogs were digging down through the ash and eating really rotten meat.” Tellingly, they left coprolites—fossilized feces—and regurgitates—fossilized vomit. “It was coming out of one end,” he quipped, “maybe both.”

All of this, he added, provides evidence that bone-crushing dogs existed there, despite the lack of body fossils. Smith refers to these trace fossils, or ichnofossils, as the “hidden biodiversity” of the site. Other ichnofossils point to tinier members of the ecosystem: small mammal and arthropod burrows, and, surprisingly, the extensive underground homes of ancient ants, discovered thanks to Smith, who recognized what they were.

“All of these add up to a once-in-a-lifetime deposit,” Tucker said, explaining that other sites tend to be an “accumulation of bones over hundreds or thousands of years.” At Ashfall, it is “an instantaneous capture of what the landscape looked like.”

At least the landscape in the waterhole. The number of animals uncovered in the ash bed so far is phenomenal, but Otto pondered the potential number of animals that didn’t survive fossilization. Consider, he said, what happened when the waterhole was almost completely full of ash. “If an animal lay down and died at that time, its bones would have been close enough to the surface that the bones would have [eventually] deteriorated instead of having been preserved. So there probably were thousands and thousands of rhinos and horses and camels that died out in the open landscape away from the waterhole.”

Given the size of the eruption and the sheer amount and distance the ash fell, one might wonder why another similar site hasn’t been found anywhere else. Otto said that scientists have asked this very question. “This very same thing must have happened across the upper Great Plains—South Dakota, Nebraska, Kansas,” he noted. Volcanic ash has been found in at least a dozen other locations throughout Nebraska, but all of them are only a foot thick. The layer of ash would need to be at least 8 feet or so to indicate the former presence of a waterhole.

Otto feels it’s only a matter of time until we find similar sites, especially “as erosion continues to wear away at the local sedimentary rock. Some day, a thick exposure of ash is going to erode out,” revealing another remarkable fossil site.

Decades of influence

The wealth of information this site provides can’t be overstated, nor can its impact on those who have studied it. Otto went from excavating the site as a student in the '70s to becoming the park’s superintendent for over 30 years. Tucker’s passion for Ashfall sparked a career in paleontology. Mosel, who reluctantly answered an ad for a “Clerk/Typist” at Ashfall 33 years ago, said, “I have loved every minute of it. It has been the best job.” The culture within Ashfall encouraged informal learning, Mosel said, a setting in which she thrived.

Rachel Short, who interned at Ashfall during the summers of 2009 and 2010, said it helped shape her career. Prior to college, she didn’t realize paleontology could be a profession—now she’s an assistant professor in the Department of Natural Resource Management at South Dakota State University. At Ashfall, she not only learned a great deal about fossil excavation but also about interacting with the public and effective science communication.

“I spent a lot of years being asked, ‘Why? Why do we care? Why are you doing this?’” she recalled. These types of questions are one of the reasons she says she works so hard to articulate the meaning of her research. “I think as paleontologists or scientists, we have a responsibility to tell people why we care and what we’re getting out of it.”

Approximately a decade after her work at Ashfall, she discovered another rhino species at the Gray Fossil Site in Tennessee. “I really struggled with ‘how do we make the rhinos mean something?’” she said. To find that meaning, she’s focusing on how the physical differences between the species are a product of their different diets and environments and whether those same mechanisms shape species today.

For all of its phenomenal fossil preservation and the profound impact it has had on those who have worked there, the park is still a bit of a hidden gem. “The one thing that amazes me,” Tucker admitted, “is how many people in Nebraska don’t even know this exists.”