A cold seep is an extreme environment characterized by a specialized group of organisms generally referred to as “chemosynthetic communities”. Until recently, echinoderms were thought to be rare in cold seep environments and had not been treated as a member of chemosynthetic communities, which otherwise are composed of a variety of taxa. One fossil echinoderm assemblage associated with a cold seep comes from the middle Campanian Pierre Shale Formation of the U.S. Western Interior Seaway. In this study, the taxonomy, morphology, and paleoecology of fossil echinoderms (crinoids and echinoids) from the Pierre Shale are discussed. Elemental chemical analyses of the fossil echinoderm skeletons and stable carbon isotopes were used to clarify the influence of seep hydrocarbons chemistry on the formation and diagenesis of echinoderm skeletons. We show that the crinoids Lakotacrinus brezinai from seep carbonates developed highly specialized morphologies and skeletons with low δ¹³C values, suggesting that it was adapted to cold seep environments and might be an obligate member of the chemosynthetic community. The echinoids from the Western Interior Seaway seep carbonates have morphologies and skeletal δ¹³C values not significantly different from those from non-seep environments, suggesting that the echinoids were not obligate members of the chemosynthetic community, but opportunists living in the periphery of the cold seep habitat.