Sediment samples were collected in northwestern Nebraska from the following lithologic units: (1) the upper Oligocene Monroe Creek Formation, the lower Miocene Harrison Formation and ‘upper Harrison’ beds of the Arikaree Group; and (2) the lower Miocene Runningwater Formation and the Dawes Clay Member (Box Butte Formation) of the Ogallala Group. The samples were processed using heavy liquid extraction and analyzed for siliceous plant microfossils (phytoliths). All samples yielded well preserved and diverse assemblages dominated by grass phytoliths, but included phytoliths produced by woody and herbaceous dicotyledons, palms, and sedges. The abundance of woody dicotyledons and palms decreased up section. The grass assemblages consisted mainly of Festucoid (C₃) phytolith morphotypes. An index assessing the amount of tree cover (d:p, the ratio of dicotyledon phytoliths to grass phytoliths) was employed to interpret the data. According to this analysis, there was open C₃-dominated grassland in the study area between 25 and 17 Ma, at least 7 Ma before supposed adaptations to grasslands in ungulates (hypsodonty) originate.