Abstract The distribution of foraminifera in the Greenhorn Western Interior seaway reflects the influence of water masses from both the north and south. Three distinct benthic biofacies (arenaceous, mixed, calcareous) are recognized. As the seaway deepened a benthic fauna composed of arenaceous species was succeeded in the center of the seaway by a zone barren of benthic foraminifera and in fringing environments shoreward by a mixed biofacies. The northward expansion of the barren zone, which indicates a deoxygenated benthos, coincided with the influx of diverse planktonics of southern affinities. The distributions of the benthic biofacies and of planktonic diversities probably reflect bathymetry. The appearance of a calcareous benthic fauna at the base of the Bridge Creek Limestone Member of the Greenhorn Formation represents oxygenation of the sea floor due to increased vertical circulation. The alternation of limestones and marlstones in the Bridge Creek reflects alternations in the productivity of calcareous plankton. The limestones represent episodes of dry climate at low latitudes when large volumes of warm saline bottom water formed, stratification of the oceans weakened, and widespread upwelling boosted productivity. Marlstones represent wet climates at low latitudes when little WSBW formed, stratification of the oceans intensified, and productivity fell. The Bridge Creek foraminifera do not support the idea that a brackish lid developed periodically on the seaway.