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First report of rodents from the late Hemphillian (late Miocene) Zwiebel Channel and a revised late Neogene biostratigraphy/biochronology of the Sand Draw area of Nebraska

Abstract A new late Hemphillian (late Miocene) rodent assemblage is reported from Zwiebel Channel, a channel cut into underlying Ash Hollow Miocene sediments along Sand Draw, Brown County, Nebraska. This locality extends the temporal range of rodent history in the Sand Draw area. A new biostratigraphic hypothesis proposes that previously described assemblages with Ogmodontomys are older than those with Ophiomys, as is the case in the Meade Basin of southwestern Kansas. Consequently, two Pliocene temporal zones are recognised. Based on a phylogenetic analysis of Ophiomys, rodent biostratigraphy, and paleomagnetic profiles, Sand Draw assemblages with Ogmodontomys are considered to have been deposited about 3.0–2.8 Ma, while those with Ophiomys were laid down between about 2.8–2.5 Ma. The 1.6 Ma date previously suggested for Ophiomys parvus from Froman Ferry, Idaho is probably too young; it is more likely that O. parvus became extinct in Idaho prior to the North American Microtus immigration event at about 2.0 Ma, inhabiting the Snake River basin until around 2.2 Ma.