The oldest labechiid stromatoporoids from intraskeletal crypts in lithistid sponge-Calathium reefs

Early Ordovician (early Floian) reefs of South China include lithistid sponge–Calathium reefs with a three-dimensional skeletal framework. These structures areamong the first post-Cambrian skeletal-dominated reef structures and provides an opportunity to test how the novel metazoan builders changed the environments and increased topographic complexity within benthic communities. We document the oldest labechiid stromatoporoid (Cystostroma) in a lithistid sponge–Calathium reef of the Hunghuayuan Formation in southeastern Guizhou, South China. These earliest stromatoporoids may have originated in reefs, and we argue that the complex topography created by the hypercalcified sponge Calathium facilitated the emergence of stromatoporoids. Beyond Cystostroma, keratose sponges, Pulchrilamina (hypercalcified sponge) and bryozoans have also inhabited in the micro-habitats (cavities and hard substrates) provided by Calathium. These findings suggest that ecosystem engineering by Calathium played an important role in the further diversification of reefs during the Ordovician.