The use of rocky intertidal assemblages in paleoecology and conservation paleobiology studies is limited because these environments have low preservation potential. Here, we evaluate the fidelity between living intertidal mussel bed communities (life assemblages or LAs) and mollusk shell accumulations (death assemblages or DAs) from the environmentally harsh Patagonian Atlantic Coast. LAs were sampled from rocky mid-intertidal and mussel-dominated habitats while DAs were collected from the high water mark at beaches in close proximity to the living intertidal community to assess live-dead mismatch at regional scales. DAs were restricted to the subset of species in the DAs that inhabit rocky intertidal habitats. A total of 37,193 mollusk specimens from 15 intertidal species were included in the analysis. Ten species were present in LAs, 14 in DAs, and nine were shared by LAs and DAs. DAs showed higher diversity, less dominance, and more rare species than LAs. Despite finding good agreement in species composition between DAs and LAs within the same region, smaller species are underrepresented, as shown by differences in size-frequency distributions. Our findings indicate that the composition of DAs is a result of the combined effects of spatial and temporal averaging, size-related biases, and biases related to low detectability of boring and vagile species in LAs. Thus, DAs do not accurately detect within-provincial latitudinal gradients in composition. However, DAs clearly capture differences between the Argentine–Magellanic Transition Zone and the Magellanic Province, indicating that DAs are informative tools at regional scales despite the environmental harshness to which they are subjected.