The compositional fidelity of dead assemblages—the extent to which subfossil remains reflect the composition, structure, and scale of the original living community—is an indispensable component of studies assessing the quality of the fossil record and the effects of taphonomic biases on species composition and diversity. The aim of the present study is to evaluate (1) how faithfully abundances of subfossil freshwater mollusk shells deposited in dead assemblages correspond to their abundances in the local living assemblages, (2) how reliably diversity of living assemblages is captured by dead assemblages, (3) whether spatial variation in species composition in living assemblages is captured by dead assemblages in the southeastern Pampas, Argentina. Twenty sites corresponding to five lotic and five lentic sites from two geographic areas were analyzed in terms of taxonomic composition and diversity, and homogeneity of multivariate dispersions. We find that (1) living and dead assemblages occupy similar portions of multivariate space and do not significantly differ in composition within an area, (2) living and dead assemblages show similar richness and evenness, and (3) between-environment differences in among-site variation in composition (beta diversity) captured by living assemblages are preserved by dead assemblages. Thus, the results highlight the potential these environments have to enhance the preservation of mollusks and result in fossil assemblages that are suitable for paleoecological and paleoenvironmental studies of freshwater ecosystems.