Abstract The Lagerstätte at Ashfall Fossil Beds—the result of supervolcanic eruption—preserves a mass-death assemblage of articulated skeletons of reptiles, birds, and mammals in a 3-m-thick pure volcanic ash near the base of the Cap Rock Member of the Ash Hollow Formation in Antelope County, Nebraska. The ash originated from the Bruneau-Jarbidge caldera in southwest Idaho, some 1600 km away, and it is geochemically matched with the Ibex Hollow tuff (11.93 Ma). Ashfall is a critical Clarendonian North American Land Mammal Age locality. More than 20 taxa—predominantly medium- and large-sized ungulates preserved in three dimensions—are buried in a late Miocene paleodepression (waterhole) filled with tephra reworked from the landscape by wind and water. Smaller taxa, such as birds, turtles, and moschids, died shortly after the pyroclastic airfall event and their remains are preserved in the basal ash. Remains from the medium-sized ungulates (equids and camelids) are separated from the underlying smaller skeletons by several centimeters of ash, indicating that these animals died at a slightly later time. In turn, more than 100 mostly intact skeletons of the barrel-bodied rhinoceros, Teleoceras major , overlie the remains of the medium-sized taxa. Pathologic bone on the limbs and skulls of the horses, camels, and rhinos suggests short-term survival and slow death several weeks or months after the pyroclastic airfall event. Exquisite preservation in an information-rich context allows aspects of the behavior, social structure, intraspecific variability, and pathology of extinct species to be reconstructed.

Abstract The central Niobrara River valley in northern Nebraska(Fig. 1) has played an important role in the conceptual development of late Tertiary stratigraphy and vertebrate paleontology in the Great Plains. Much of our current understanding of the evolution of such mammals as horses, camels, and proboscideans is based on fossils collected from the Valentine and Ash Hollow Formations exposed in the deep (by Nebraska standards!)canyons of the Niobrara River and its tributaries in Brown, Cherry, and Keya Paha Counties. The Valentine-Ash Hollow sequence in this area represents the northernmost expression of a complex of alluvial deposits collectively known as the Ogallala (variously termed agroup or formation) which blankets much of the Great New Mexico. The only paved road to traverse reasonably good exposures of the classic Ogallala sequence in the Niobrara valley is Nebraska 12 about 3 mi (4.8 km) northeast of Valentine. Two adjacent road cuts at the northwestern margin of Fort Niobrara National Wildlife Refuge (see. 22, T.34N., R.27W., Cornell Dam Quadrangle, Fig. 2) expose nearly the entire thickness of the Valentine Formation and the Cap Rock Member of the Ash Hollow Formation. Examination of the road cuts, combined with a tour of the Refuge, will provide the visiting geologist with an overview of the later Tertiary mantle of the Great Plains near the boundary between the latter physiographic province and the Central Lowlands Province to the east. Geomorphological relation ships are also exceptionally clear along this segment of the Niobrara River; the very late