A large database recording species of terrestrial vertebrates present in formations above and below the Cretaceous-Tertiary (K-T) boundary in eastern Montana was assembled by J. D. Archibald and L. J. Bryant. Division of the species in this database into freshwater and land-dwelling vertebrate assemblages reveals that the K-T vertebrate extinction was concentrated in land- dwelling forms. In data corrected for the effects of rare taxa, 90% of the species in the freshwater assemblage survived into the Tertiary, but only 12% of the land-dwelling forms survived. The pattern of differential extinction of terrestrial vertebrates in eastern Montana may be in large part the result of the dependence of land-based communities on primary productivity. This is in contrast to the riverine communities, which may derive much of their organic carbon from detritus. The pattern of extinction and survival is compatible with the hypothesis of an asteroid impact after which there was a temporary cessation of primary, photosynthetic productivity.