A cavity-dwelling fauna from the Middle Jurassic of the Western Interior shows a succession controlled by increasing restriction of the cavities, most likely by sediment infilling. The succession proceeds from large and ubiquitous bivalves to smaller serpulids, cyclostome bryozoans, and thecideidine brachiopods; there is a concurrent decrease in biomass and percent cover. This Jurassic fauna demonstrates that marine cavity-dwelling faunas were specifically adapted to their environments at least 160 m.y. ago.