Untangling intricate microbial-sponge frameworks: The contributions of sponges to Early Ordovician reefs

The Early Ordovician witnessed an increased influence of skeletal organisms on the construction of microbial-dominant reefs, which preceded the Middle to Late Ordovician expansion of skeletal-dominant reefs. Skeletal frameworks of Early Ordovician reefs built by stromatoporoids, bryozoans, pelmatozoans and calathiids have recently been reported. In this study, we document the occurrence of localized Archaeoscyphia frameworks and additional unidentified non-anthaspidellid spiculate sponges that played diverse roles in the construction of microbial-siliceous sponge reefs preserved in the Early Ordovician Dumugol Formation, Korea which have not been previously described in coeval reefs elsewhere. Two types of boundstone textures are recognized: microbial-dominated boundstone and sponge-microbial boundstone. Microbialites were primarily responsible for reef construction (making up about one third of reefs), both as framework builders and as encrusters of other reef components. The Archaeoscyphia and spiculate sponges contributed in various ways to reef development. Archaeoscyphia, constituting about one fourth of the Dumugol reefs, mainly constructed sponge-microbial boundstone, together with encrusting microbialites. Clusters of Archaeoscyphia individuals sporadically formed centimeter-sized sponge framestone (3% of reefs) with primary cryptic spaces. Spiculate sponges (4% of reefs), a previously unknown component of Early Ordovician reefs, also played diverse roles, such as encrusting other constituents, dwelling in cryptic spaces, stabilizing reef-flank sediments and even building frameworks. Such framework-building siliceous sponges, together with previously reported skeletal frameworks in other Early Ordovician reefs, reflect an early phase of the transition from microbial- to skeletal-dominated reefs in the early Paleozoic.