About 19-20 million years ago a drought occurred in the plains of Western Nebraska. Deprived of food, hundreds of animals died around a few shallow waterholes. Over time the skeletons were buried under silt, fine sand and volcanic ash. A large fossilzed waterhole with hundreds of skeletons is preserved today in the Niobarara River valley at Agate Fossil Beds National Monument. The discovery of this deposit and others nearby in the early 1900s was important to the developing science of paleolentology.
The conditions that caused the drougth and brought the animals together
Millions of years beforte the drought took place, sediments from eroding mountain ranges to the west were laid down to form the bed of a shallow sea during the Age of Dinosaurs. About the time of the extinction of dinosaurs, the Rocky Mountains were developing into the ranges we know today, the ocean receded and tropical lowlands occupied the region tant today is the Great Plains. As time passed, the climate of North America became cooler and drier, and volcanic activity in the western United States produced enormous amounts of volcanic ash that was blown eastward. Ash-mantled plains were home to great heards of plant-eating mammals and their predators. Like the savannas of east Africa today, the rich volcanic soils supported grasses, which, together with small trees and bushes growing along shallow streams, were a ready food source for grass- and leaf-eaters. Many of the animals that thrived and lived here became reliant upon the moderate climate for survival and their numbers expanded to the capacity of the available food supply. As time passed, however, the climate became more arid. To the west, the Rocky Mountains continued to rise, and the flow of moisture-laden air from the west was interupted. With less rain came plants that could survive on less water. Drought became commonplace. Over the years streams dried up and the grasses withered. Water-dependent animals were drawn to waterholes in the stream beds, and they congregated at these places between periods of feeding on the diminishing vegetation. Large animals, such as the rhinoceros and chalicothere Moropus, a distant relative of the horse, were finally unable to travel far enough away to find fresh forage and they died in the shallow water of the few remaining ponds. Hundreds and thousands of some species died, littering the area around and in the waterholes with their remains. In time, the rains returned, the streams filled, and the process of burial began. Silt, sand, and ash covered the remains, burying them under several feet of wind- and stream-transported sediment.