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From water hole to rhino barn: twelve million years ago, a volcanic ashfall entombed prehistoric animals that roamed what is now Nebraska
Natural History, Sept, 2004 by Sandy S. Mosel
If you drive across northeastern Nebraska on U.S. Highway 20, you see about what you'd expect: fields of corn and soybeans, rolling pastures with grazing cattle. But if you could travel in time, back to, say, 12 million years ago, you'd come face to face with a very different scene: an African-style savanna, teeming with herds of camels, elephants, rhinoceroses, and small horses, all grazing in seas of grass or browsing on shrubby trees. As rich as it once was, though, this ancient savanna and its ecosystem have left few traces in soil that has been tilled or pastured for more than a century.
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But in some places the evidence of that distant time is still well preserved. The most spectacular remains occur at Ashfall Fossil Beds State Historical Park, about 140 miles northwest of Omaha. There the articulated skeletons of many extinct mammals have been preserved in a bed of volcanic ash. More than 200 complete or nearly complete skeletons have been exposed through patient paleontological excavations, and more, undoubtedly, are still to come.
The Ashfall site occupies a hill overlooking a tributary of Verdigre Creek, which flows into the Niobrara River--itself a tributary of the Missouri River. Cutting a rugged course through northern Nebraska, the Niobrara has exposed abundant remains of fossilized mammals, which paleontologists have been collecting since the 1850s.
Until 1971, however, the Ashfall site remained hidden. In that year, while exploring the upper slope of a ravine, Michael R. Voorhies, a paleontologist at the University of Nebraska State Museum in Lincoln, noted that a recent rainfall had revealed a thick layer of volcanic ash. Looking at it closely, he found the intact skull and jaws of a baby rhinoceros eroding out of the ash. Subsequent digging yielded the entire skeleton, then several more--a highly unusual phenomenon. (Almost all fossilized mammals occur in ancient stream deposits: logjams of scattered bones and teeth. Fully articulated skeletons are extremely rare.)
Voorhies directed a major excavation of the Ashfall site during the summers of 1978 and 1979, bringing to light dozens of skeletons of birds, camels, horses, and rhinoceroses. That work continues. Geologists think this mass graveyard was one consequence of a great volcanic eruption that took place 12 million years ago in what is now southwestern Idaho, nearly 900 miles due west. Prevailing winds carried the ash eastward, blanketing a huge area of the Great Plains. Exposures of the same ash layer--usually about a foot thick--occur in various places along a 250-mile stretch of northern Nebraska. At Ashfall, though, the layer becomes ten feet thick. The site was a water hole: essentially, a large depression in the landscape that retained water during the wet seasons but could dry out in times of drought.
Made up of minute shards of volcanic glass, the ash clouds that drifted across Nebraska from the west were abrasive and dangerous. Small animals, especially birds, must have succumbed almost immediately. Larger animals could have survived the initial ashfall, but they would then have encountered a landscape covered with powdery, abrasive dust that buried their food supplies and became airborne again at the slightest step or breeze. The camels, horses, and large rhinoceroses suffocated within days or weeks, as their lungs filled with ash. Ill and feverish, they probably converged on the water hole. As they perished, some of their remains were scavenged by animals such as Aelurodon, a bone-crushing predator in the dog family. But little by little, wind blown ash filled the water hole, until it covered the carcasses.
The most abundant skeletons--more than a hundred have been excavated to date--are those of Teleoceras major, a short, stocky rhinoceros with stout legs and a large, barrel-like midsection. With so many skeletons available from one population, investigators can infer a good deal about the species' behavior and ecology. Young adult males are notably absent, and only one mature adult male turns up for every five adult females, suggesting that the rhinoceroses formed harems. Numerous skeletons of young calves have also been unearthed, many of them nestled next to adult females, no doubt their mothers. Because the calves are all generally about the same age, calving must have taken place at a particular season.
Fossilized grass seeds lodged between the teeth of the rhinoceros fossils belong to subtropical species that resemble grasses that now grow in Central America, indicating that the climate was warmer than it is today. Supporting that conclusion are the fossilized giant tortoises that have been discovered at the site: such large land reptiles could not have survived where temperatures dipped below freezing.
Fossil plants, animals, and ancient soils throughout Nebraska show that by 12 million years ago, woodland environments were giving way to grass lands. The horse skeletons from the Ashfall site corroborate that inference: Some of the horse species have three toes on each foot--a large toe in the middle and a smaller one on each side. Other species have just one large toe. Side toes may have provided extra traction on soft forest floors, but they were less useful on the harder ground of the open grassland.
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