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.

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 <italic>Ogmodontomys</italic> are older than those with <italic>Ophiomys</italic>, as is the case in the Meade Basin of southwestern Kansas. Consequently, two Pliocene temporal zones are recognised. Based on a phylogenetic analysis of <italic>Ophiomys</italic>, rodent biostratigraphy, and paleomagnetic profiles, Sand Draw assemblages with <italic>Ogmodontomys</italic> are considered to have been deposited about 3.0-2.8 Ma, while those with <italic>Ophiomys</italic> were laid down between about 2.8-2.5 Ma. The 1.6 Ma date previously suggested for <italic>Ophiomys parvus</italic> from Froman Ferry, Idaho is probably too young; it is more likely that <italic>O. parvus</italic> became extinct in Idaho prior to the North American <italic>Microtus</italic> immigration event at about 2.0 Ma, inhabiting the Snake River basin until around 2.2 Ma. [ABSTRACT FROM AUTHOR]