In modern marine environments coelobiontic (cavity-dwelling) organisms are ubiquitous and abundant. However, little is known of modern coelobionts and even less of their geologic history.The oldest known coelobionts are found in archaeocyathid patch reef cavities from the Lower Cambrian of Nevada and Labrador, where the cavity communities are dominated by algae; foraminifera, brachiopods, pelmatazoans, sponges, archaeocyathids, Bija, Archaeotrypa, and an infauna of burrowers in cavity sediments are accessories in upper Lower Cambrian cavities from the Forteau Formation of Labrador. Earlier cavities in patch reefs from the Poleta Formation of Nevada are much less diverse and are dominated by algae and an infauna of burrowers.From the Lower Cambrian to the present day the history of coelobionts has not been one of major innovation but rather has been conservative. Even as far back as the early Cambrian, coelobiontic communities were similar to their modern counterparts if viewed in terms of community organization and trophic structure.