During the Cretaceous Period, the Western Interior Seaway (WIS) was influenced by long-term and short-term environmental changes. Vertebrate bonebeds are particularly abundant within the Cenomanian-aged strata of the Western Interior Sedimentary Basin’s eastern margin. The major control for their accumulation is interpreted to be related to the timing and magnitude of sea-level fluctuations. Bioclasts are interpreted to be concentrated as lags along erosion surfaces during transgressions, while their taphonomic character is interpreted to be linked to the frequency of episodic events. In addition to these deposits, the uppermost Cenomanian to Turonian record along the eastern margin is characterized by rich planktic foraminiferal assemblages. Proximal and distal assemblages record a changing environment with frequent catastrophic perturbations including ashfalls and expansion of the oxygen minimum zone. Looking at both depositional and faunal responses to long- and short-term processes improves our understanding of the dynamics of the WIS during this enigmatic time.