Although intervals of faunal stasis have been identified in the Paleozoic and Cenozoic, here we report the first quantitative documentation of long-term faunal stasis in the Mesozoic, a time of great biological and ecological change. On the basis of an analysis of previously published and newly collected data on Jurassic benthic marine invertebrates from the U.S. Western Interior, we find that significant proportions of bivalve species and associations persisted through major sea-level and environmental changes. In the six Jurassic marine depositional units studied, each spanning 2–6 m.y., 32% to 100% (averaging 56%) of bivalve species were holdovers from the preceding unit, while the proportion of carryover species that persisted into the next unit ranges from 16% to 81% (averaging 46%). Bivalve associations also commonly persisted through depositional units and stratigraphic breaks. The ecological stability documented here does not suggest any coordination of origination or turnover among taxa. We conclude that the pattern of faunal stability is due to stasis within individual species and not to community-level processes. Environmental conditions of the Jurassic Western Interior seaway may have selected for generalist taxa capable of withstanding environmental disturbances and persisting for long intervals of time.