The Campanian Smoky Hill Member (Niobrara Formation) and the Sharon Springs Member (Pierre Shale) of the Great Plains were deposited in an extensive epicontinental seaway during an interval of enhanced, explosive volcanism on the western continental margin. These stratigraphic units contain organic-rich chalks and shales that are some of the most selenium-enriched stratigraphic intervals in the geologic record. Se concentrations in these members exceed values typical for similar sedimentary rocks by several orders of magnitude. Pathways in the global biogeochemical cycle of Se include volcanic eruption, weathering in terrestrial environments, discharge of dissolved oxyanions to the marine environment, and biological transformations of dissolved oxyanions to insoluble, reduced forms. We used sequential extraction methodologies and hydride generation atomic absorption to quantify the chemical speciation of Se in these Cretaceous rocks among seven operationally defined fractions (water-soluble, ligand-exchangeable, adsorbed, elemental, carbonate, metallic, and organic). Our findings suggest that algal assimilation of dissolved species was the predominant process of sedimentary Se sequestration, and thus represents a likely pathway for the incorporation of excess selenium into the Western Interior Seaway ecosystem. In modern ecosystems affected by Se-toxicity, Se commonly bioaccumulates upward through food-chain trophic levels. A preliminary examination of Se-distribution in fossil bone material of 14 Campanian predatory vertebrates (fish, sharks, and marine reptiles) from the Western Interior has revealed significant, species-specific differences in the concentration and speciation of Se in fossil bone. In addition, Se speciation and concentration are markedly different between bone and adjacent sediment matrix. Total selenium concentrations in these fossils range from 1.6 - 12.2 ppm. If the Se-enrichment in these vertebrate bones is primary, these data suggest that vertebrates in the Western Interior Seaway were bioaccumulating Se at levels significantly higher than those reported for modern vertebrates.