Abstract Two superposed faunas were discovered in southeastern Colorado at the Donnelly Ranch vertebrate site. Sediments containing a late Blancan assemblage are unconformably overlain by sediments containing a Rancho La Brean assemblage. The pre-Nebraskan Donnelly Ranch fauna is slightly older than the Sand Draw, Broadwater, and Blanco faunas. It is younger than the Curtis Ranch fauna. Faunal and paleomagnetic correlation indicate it is about 2.5 m.y. old. Tapirus, Geochelone , and Sigmodon indicate the climate was mild and precipitation was much more effective than at present. Savanna, plains, and marsh communities are represented by the fauna. The Sangamon Mesa De Maya fauna correlates best with the early Sangamon Cragin Quarry fauna. Mammals, molluscs, and pollen indicate that temperatures were more equable than present and precipitation may have been slightly more effective during cooler summers. The fauna consists primarily of grassland inhabitants but a marsh community is represented in artesian spring sediments. The gross topographic and stratigraphic relations, sediment size analysis, fossil orientation, and sedimentary structures indicate stream, viscuous intermediate, and mudflow deposition on a low alluvial fan at the foot of Mesa De Maya. The sediment source was local consisting of basalt from Mesa De Maya, igneous, reworked metamorphic, and sedimentary rock from the Ogallala Formation, and sedimentary rock containing marine fossils from Cretaceous sediments. An artesian spring deposit consisting of predominantly silt size quartz particles was found overlying a gley soil. During the Sangamon, massive caliche formed in channel sands and nodular caliche formed in the mudflow deposits of the alluvial fan. Potassium argon dates, paleomagnetism, and topographic position and correlation indicate that Mesa De Maya is older than Fowlis Mesa and the average age for both of these basalt flows is about 3.3 million years. The Mesa De Maya flows issued from local vents and flowed nearly to the present extent of the mesa. Scarp recession has been minor since the lavas erupted. Flow direction interpreted from air photographs indicate the flows were largely controlled by topography and did not form an extensive sheet of lava from Mesa De Maya to Fowlis Mesa. Erosion has carved deep canyons between the lobes of the flows and has caused a reversal of topography where hills of Ogallala once stood. More than 400 feet of Ogallala and Cretaceous sediments were removed prior to deposition of Pleistocene sediments. Erosion and deposition in the Donnelly Ranch area has been relatively minor since the Sangamon.

Abstract Two superposed faunas were discovered in southeastern Colorado at the Donnelly Ranch vertebrate site. Sediments containing a late Blancan assemblage are unconformably overlain by sediments containing a Rancho La Brean assemblage. The pre-Nebraskan Donnelly Ranch fauna is slightly older than the Sand Draw, Broadwater, and Blanco faunas. It is younger than the Curtis Ranch fauna. Faunal and paleomagnetic correlation indicate it is about 2.5 m.y. old. Tapirus, Geoche/one , and Sigmodon indicate the climate was mild and precipitation was much more effective than at present. Savanna, plains, and marsh communities are represented by the fauna. The Sangamon Mesa De Maya fauna correlates best with the early Sangamon Cragin Quarry fauna. Mammals, molluscs, and pollen indicate that temperatures were more equable than present and precipitation may have been slightly more effective during cooler summers. The fauna consists primarily of grassland inhabitants but a marsh com— munity is represented in artesian spring sediments. The gross topographic and stratigraphic relations, sediment size analysis, fossil orientation, and sedimentary structures indicate stream, viscuous intermediate, and mudflow deposition on a low alluvial fan at the foot of Mesa De Maya. The sediment source was local consisting of basalt from Mesa De Maya, igneous, reworked metamorphic, and sedimentary rock from the Ogallala Formation, and sedimentary rock containing marine fossils from Cretaceous sediments. An artesian spring deposit consisting of predominantly silt size quartz particles was found overlying a gley soil. During the Sangamon, massive caliche formed in channel sands and nodular caliche formed in the mudflow deposits of the alluvial fan. Potassium argon dates, paleomagnetism, and topographic position and correlation indicate that Mesa De Maya is older than Fowlis Mesa and the average age for both of these basalt flows is about 3.3 million years. The Mesa De Maya flows issued from local vents and flowed nearly to the present extent of the mesa. Scarp recession has been minor since the lavas erupted. Flow direction interpreted from air photographs indicate the flows were largely con— trolled by topography and did not form an extensive sheet of lava from Mesa De Maya to Fowlis Mesa. Erosion has carved deep canyons between the lobes of the flows and has caused a reversal of topography where hills of Ogallala once stood. More than 400 feet of Ogallala and Cretaceous sediments were removed prior to deposi— tion of Pleistocene sediments. Erosion and deposition in the Donnelly Ranch area has been relatively minor since the Sangamon.

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.