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Stopped in Their Tracks: Ancient mammals were buried by volcanic ash . . . in Nebraska

Ten million years ago Nebraska was a vast savanna, covered with subtropical grasses and patches of jungle. Barrel-bodied rhinoceroses, three-toed horses, camels and other animals roamed about.

I sold this article to Omni magazine in 1995 or 1996, but never received a published copy. Don't know if it was ever published as Omni was in the process of closing down about that time.

Ancient mammals were buried by volcanic ash . . . in Nebraska

By Robert D. Anderson 

Ten million years ago Nebraska was a vast savanna, covered with subtropical grasses and patches of jungle. Barrel-bodied rhinoceroses, three-toed horses, camels and other animals roamed about. Nebraska's Niobrara River valley has beckoned bone hunters for over a hundred years because it contains North America's most complete record of the 20-million-year history of grassland animals.

When Mike Voorhies was a lad growing up in the northeastern Nebraska town of Orchard (population 470), he collected petrified bones and teeth from nearby streams and dry-washes. He never dreamed of the fossil treasures that lay hidden almost beneath his feet.

One day in 1971, three decades later, Dr. Michael R. Voorhies, curator of vertebrate paleontology at the University of Nebraska State Museum, was out prospecting about eight miles from Orchard. Heavy spring rains had eroded a deep gully at the edge of a cornfield and Voorhies spotted the skull of a baby rhinoceros protruding from the side of the gully. It lay near the bottom of a newly-exposed bed of gray volcanic ash. He gingerly brushed away the ash from the little skull, looking for evidence that the rest of the skeleton might be there. It was. And so were the articulated skeletons of three more rhinos, including a full-grown adult, also extended back into the hill. Further testing of the ash bed yielded a dozen more skeletons from an area no larger than an average living room.

With funding from the National Geographic Society, Voorhies and a crew of eight students spent the summers of 1978 and 1979 excavating the ash bed. Dozens of additional rhinos and horse skeletons were found, but a big surprise came in finding the remains of camels, birds, turtles and small saber-toothed deer - a Clarendonian mammal assemblage of the Miocene Epoch. It was clear that a major disaster had occurred at this site.  

Scientists now believe that some ten million years ago, ash from an incredible volcanic eruption in the Rocky Mountains blew eastward, choking and suffocating the animals. They recently found through radiometric dating that the ash in Nebraska matches the ash near the Bruneau-Jarbridge Eruptive Center in southwest Idaho.

Ashfall Fossil Beds State Historical Park opened to the public in 1991 as a joint project of the University of Nebraska State Museum and the Nebraska Game and Parks Commission. Researchers are continuing their excavation under the "Rhino Barn," a 2,000-square-foot shelter where visitors can observe and talk with paleontologists at the site.  

More than 40 species of animals and plants, amazingly well-preserved, have been unearthed and identified. Entire herds died and were buried so quickly that their carcasses remain largely intact. Some rhinos were literally buried in their tracks, with their last footprints clearly visible. Some females have calves next to them while others have unborn young inside.

"If I had been asked if we would ever find fossil evidence of nursing behavior in mammals," says Gregory W. Brown, chief preparator of the State Museum's division of vertebrate paleontology, "I'd have said, ‘never.' Yet, here we are." He points to a young rhino in a nursing pose with his mother. "It's really incredible."

Many skeletons contain the fossilized remains of their last mouthful of grass. Brown says he and fellow paleontologists are trying to discover exactly what happened by studying the arrangement of the bodies of the victims. At the very bottom of the ash bed are small creatures such as pond turtles, birds, and musk deer-which probably died almost immediately. Just above these remains are skeletons of horses and camels that died next. Above these are the rhinos along with occasional giant tortoises. They were the last to die.

Rick Otto, park superintendent, says that nearly 300 complete prehistoric animal skeletons have been discovered and everyone is still uncertain just how extensive this fossil deposit is. "There may well be thousands of complete skeletons buried under this site," he says.

Otto says the glacial ice sheet pushed down within seven miles east of the site and almost obliterated it. "If the earth had been just a little bit cooler at the time, and if the glacier had been a few miles larger in size, it could have pushed across this area and scraped these fossil deposits right along with it."

As excavation of the site continues, Brown says hopes are high that a skeleton of one of the large extinct predators, like a sabertooth cat or bear-dog, will be found. "Bite marks on some of the bones give evidence that big meat eaters were nearby," he says.

One thing is certain: Ashfall will provide a lot of people with research opportunities for a long time. "If you look at the average scope of a master's thesis," Brown says, "there are at least 20 or 30 of them here."