A series of 33 Late Cretaceous (earliest Campanian through Maastrichtian) paleoshoreline maps was developed to document the migrational evolution of the western edge of the North American Western Interior Seaway. The maps represent a geologic span of roughly 18 million years, and portray the estimated positions of the strandline for each standard Western Interior ammonite zone, beginning with the Clioscaphites choteauensis zone and continuing to the end of the Mesozoic. We attempted correlation of all significant mammal-bearing localities known from the Western Interior with the ammonite-based marine zonation. First approximations of correspondence between ammonite zones and North American Land-Mammal “Ages” (NALMAs) include: Lancian ( Sphenodiscus through “Triceratops” zones); “Edmontonian” (a name not yet faunally defined; Didymoceras cheyennense through Baculites clinolobatus zones); Judithian (the smooth, late form of Baculites sp. through Exiteloceras jenneyi zones); and Aquilan ( Scaphites hippocrepis through Baculites asperiformis zones). Correlations emphasize use of provincial biostratigraphic terminology designed specifically for use in the Western Interior. On the basis of temporal constraints suggested herein, known mammalian fossils from the upper Fruitland and/or lower Kirtland Formations of New Mexico probably are of “Edmontonian,” not Judithian age. Although considerable latitudinally based taxonomic diversification of Judithian mammals is now recognizable across the Western Interior, comparative data are inadequate to defend a similar statement for the remaining Late Cretaceous NALMAs. Quantitative evaluation of geographic patterns of shoreline change suggests occurrence of a general, regional regression of the sea during the entire geologic interval represented in the study. We favor explanation by a slow sea-level depression resulting from topographic evolution of the world’s mid-oceanic system of ridges and volcanic plateaus. Local and subregional asynchronous episodes of shoreline transgressions, stillstands, and regressions are superimposed upon the general regressive trend, and probably represent influences of local tectonism, not eustatic changes in sea level. Strandline evolution of the epeiric sea during the last 20 million years of the Cretaceous in the North American Western Interior is inconsistent with: (1) existence of geologically brief (1 to 10 m.y.) global fluctuations in sea level; and (2) the concept that the late Campanian was represented by an unusually high global sea level.