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Erosional surfaces are present in middle and upper Coniacian rocks in Montana and Alberta, and probably at the base of the middle Santonian in the Western Interior of Canada. These erosional surfaces are biostratigraphically constrained using inoceramid bivalves and ammonites, which are used to define lower, middle, and upper substages of both the Coniacian and Santonian stages of the Upper Cretaceous in this region. The most detailed biostratigraphy associated with these erosional surfaces concerns the MacGowan Concretionary Bed in the Kevin Member of the Marias River Shale in Montana, where the bed lies disconformably on middle or lowermost upper Coniacian strata, and is overlain by upper Coniacian beds. Surface and subsurface investigations in west-central Alberta reveal that the Bad Heart Formation, bounded by unconformities, is about the age of the MacGowan Concretionary Bed. Coniacian and Santonian strata are present elsewhere in Alberta and adjoining areas, but little has been published concerning the Santonian megafossils.