The colonisation of cryptic surfaces within empty molluscan shells represents an important ecological innovation in Ordovician benthic ecosystems. Empty gastropod shells in the Late Ordovician seas of Baltica provided cryptic hard-substrate microhabitats that were colonised by suspension-feeding encrusting organisms. Analysis of gastropod steinkerns demonstrates that cornulitids, bryozoans, and graptolites settled on internal shell surfaces prior to burial. Compared with contemporaneous bivalve and cephalopod cavities, gastropod interiors hosted sparser and smaller assemblages, reflecting more restricted water circulation and reduced habitat quality. The taxonomic composition nevertheless parallels other Ordovician cryptic communities, indicating that substrate architecture influenced abundance and body size rather than overall community structure. These findings contribute to understanding the expansion of cryptic ecological niches during the Ordovician biodiversification, when increasing predation pressure, coupled with the major diversification of encrusting organisms, may have promoted colonisation of sheltered environments.