FUNCTIONAL ROLES OF SILICEOUS SPONGE IN EARLY ORDOVICIAN PATCH REEFS

Numerous late Early Ordovician (early to middle Floian) meter-scale patch reefs in the Dumugol Formation, Korea are analyzed to delineate functional role of early Paleozoic reefal sponges. These reefs consist of laminated to non-laminated microbial micrite and peloids, anthaspidellid Archaeoscyphia, sponge spicule networks, and receptaculitid Calathium, with subordinate burrows and fragments of pelmatozoan, trilobite, ostracod, brachiopod, nautiloid, and intraclasts. The occurrence of well-preserved sponge spicule networks is the key finding of this study which is not widely known from the Early Ordovician reefs. Previous studies of sponge in Early Ordovician reefs have been mostly limited to cone-shaped lithistid sponges and their contribution for reef development. The siliceous sponge spicule networks attach to intraskeletal cavities including spongiocoel of Archaeoscyphia, central cavity of Calathium and inside of gastropod shells. These spicule networks also cover the outer surface of larger constituents and shell fragments. In addition, in rare instance, the spicule networks grow upward and are surrounded by microbial encrustation. The siliceous sponge in the Dumugol reefs are interpreted to provide at least three different ecologic roles in decreasing order of occurrence: 1. cryptobionts as living inside of larger organisms, 2. substrate stabilization by encrusting larger constituents including Archaeoscyphia and Calathium, and 3. limited frame building by providing site for microbial encrustation. The current study provides a new insight of concealed reef formation dynamics of Early Paleozoic reefal siliceous sponges which is similar to the sponges in modern marine ecosystem. The current finding is considered as one of the oldest example of diverse ecologic roles of sponges which were being established as early as 475 m.y. ago.