The spatial and temporal distributions of vertebrate fossil assemblages at the terrestrial Cretaceous/Paleogene (K/P) boundary in eastern Montana and western North Dakota are constrained by the meandering fluvial depositional system in which they are preserved. Vertebrate fossils are clasts within that system, and thus their positions within the sedimentary sequence are subject as much to hydrodynamics as to chronostratigraphy or evolution. Laterally discontinuous facies, incomplete stratigraphic sequences, facies-dependent fossil distributions, and reworking distort original patterns of faunal number and diversity through time. The sedimentary effects on vertebrate fossil distributions, however, can be assessed by the adoption of a facies-based biostratigraphy and the utilization of a chronostratigraphy that is concordant with the resolution afforded by the K/P rocks of eastern Montana and western North Dakota.