Early Ordovician lithistid sponge-Calathium reefs on the Yangtze Platform and their paleoceanographic implications

Lithistid sponge-Calathium-microbial reefs were widespread on the Yangtze Platform during the Early Ordovician and are well studied. However, the biological affinity and the role of Calathium in these reefs have remained unclear up to now. We document lithistid sponge-Calathium reefs from the Upper Hunghuayuan Formation (Early Floian) at Huanghuachang in Hubei, South China. These reefs have a three-dimensional skeletal framework that is mostly produced by Calathium and lithistid sponges. Calathium had a critical role in reef construction, as demonstrated by well-developed lateral outgrowths, which connected individuals of the same species and with lithistid sponges. Bryozoans, stromatoporoids and microbial components were secondary reef builders. Morphological, constructional and functional analyses provide evidence that Calathium was a sponge-grade metazoan rather than a receptaculitid alga as previously thought. At the dawn of the Ordovician Radiation, these small-scaled patch reefs thus represent the initial rebound of metazoan-dominated reefs after the Late-Early Cambrian archaeocyath reef crisis. Gradual global cooling through the Early Ordovician may have been a key driver for the return of metazoan reefs.