An excavation site at an ancient watering hole reveals these secrets of the past and more.
In north-central Nebraska lies a small park with a big name: Ashfall Fossil Beds State Historical Park and National Natural Landmark. Hidden in the rolling hills, hours from the nearest big city, Ashfall is one of the world’s premier paleontology sites. Park superintendent Rick Otto heads down a gently sloping path past two bronze sculptures. One shows two squatty rhinos locked in combat. A startled tortoise looks on from the other.
Signs mark geologic time as the path descends through layers of rock and into the ancient past. Otto pauses at one spot where the soil has been cleared out. It’s now a flat, gray area crisscrossed with shallow trenches. “When I first worked here, this was just a narrow gully,” he recalls. “A few days later is when the bulldozer was brought in for the first time.”
The area is now ringed with small red flags. They mark the first big excavation here, back in 1978. That was the year Mike Voorhies discovered what an extraordinary place this was.
A geologist at the University of Nebraska–Lincoln, Voorhies had already been exploring the area for several years. At one point in 1971, he spotted a jawbone jutting out from the side of the gully. What he dug out — an entire skull — had come from a baby rhino. It was the second rhino skull found in the hillside. Both had been trapped in a thick layer of volcanic ash. Voorhies suspected the ash-filled hillside hid much more.
And he was right. When he returned with a team years later, they uncovered a dozen rhinos and three horse skeletons.
Fossils of the plant-eating rhinos aren’t uncommon in Nebraska. Rhinos roamed the Great Plains of North America for more than 30 million years, until about 5 million years ago. Paleontologists, including Voorhies and Otto, have found rhino fossils all over the state. But those at Ashfall are remarkable. Each is a complete skeleton. Unbroken bones lay in place, oriented exactly as they had been in life.
And that’s what makes Ashfall so unusual. Ongoing excavation has revealed hundreds of skeletons. Layered atop the sandy bottom of an ancient watering hole, they reveal a world that resembled modern-day Africa. Here, predators prowled the area in search of an easy meal. And as new technology has emerged, old questions are finally starting to be answered.
Since his baby-rhino discovery 50 years ago, Voorhies, Otto and a small army of researchers have unearthed a mind-boggling number of fossils here. In 1978, they worked for four solid months. By the end, they had excavated 58 rhinos, 17 horses, six camels, five deer, two dogs, a rodent and dozens of birds and turtles. In all, researchers have unearthed more than 350 animals. The tens of thousands of individual bones and teeth come from animals that range from barrel-bodied rhinos to tiny songbirds, rodents, lizards and snakes.
What preserved so many animals and so well at this site? A supervolcanic eruption. Remains of an ancient volcano lie some 1,600 kilometers (1,000 miles) to the west. Active 12.5 million to 10 million years ago, its eruptions dumped ash across a large swath of North America. One eruption nearly 12 million years ago was especially massive. It left a layer of ash that stretched from its location in southwest Idaho all the way east to the Atlantic Ocean.
In that distant past, Nebraska was a grassy savanna. Trees and shrubs dotted the landscape. It likely resembled today’s Serengeti National Park in East Africa. Years of research show that this Ashfall site had been a watering hole. Rains filled it in the wet season. Afterward, many animals came here to drink. During the dry season, that water would evaporate.
The big volcanic eruption blanketed this part of Nebraska with a foot of ash. It’s easy to imagine the wind blowing it into drifts, like snow after a blizzard. Animals would have inhaled the ash. The tiny glass shards would have sliced up their lungs, making it hard to breathe. Grass-eating grazers would have nosed through it in search of food. They would have sought out the watering hole to drink. Maybe they waded in to escape the pale gray fluff. But it would have brought about their end anyway.
Over days, weeks and months, the animals here died in droves. Birds and turtles died quickly. We know this because their skeletons lie at the bottom of the ash, right on what was the sandy bottom of the watering hole. Other animals occur in layers. Above the birds and turtles lie dog-sized saber-tooth deer. Then five species of pony-sized horses, some with three toes. Above those are camel remains. Atop them all are the biggest, the rhinos, in a single layer. All of this is buried under about 2.5 meters (8 feet) of ash. It must have blown into the water, covering the dead.
Fossils in the ash bed are whole. They haven’t been squashed flat. Their bones are all still in place. They’re also fragile. Most fossils form when groundwater soaks into bones and teeth. Over time, minerals from the water fill in the gaps and even replace some of the original bone. The result is a hard, rock-like fossil that can stand the test of time.
Here, however, the ash eventually locked the skeletons away from water. After the watering hole dried up, the super-fine ash left no room between particles for new water to seep in. The ash protected the bones, preserving them in their original positions. But they didn’t mineralize much. When scientist remove the ash around them, these bones start to crumble.
On the hunt for predators
One curious thing about Ashfall: Only two animals found in the ash bed were predators. Both were small dogs.
But there’s evidence much bigger predators lived in the area at the time of the ash storm. One sign: Some of the rhinos aren’t quite intact. Tooth-scraped ribs are strewn about in one area. Legs are pulled askew in another. Clearly, large predators came through, scavenging the dead. The small dogs couldn’t have done this.
For decades, no one knew what did. Then scientists learned to spot a new type of clue.
Otto heads into the Hubbard Rhino Barn. It sits on the hillside, next to the original dig site. The barn protects the current dig site from the elements. Inside, he follows a wooden walkway around the outer wall. Below, in the ash bed, two barefoot interns are clearing ash away with paintbrushes.
At the far end of the barn, Otto pauses. Here lie the older discoveries. Entire skeletons are jumbled together, held in place by pedestals of ash. He pulls a laser pen from his shirt pocket and shines the green light on a column of ash next to a rhino with scattered ribs.