A new exceptionally preserved fauna from a lowest Silurian black shale: Insights into the recovery of deep-water ecosystems after the Late Ordovician mass extinction

After two pulses of mass extinction during the Late Ordovician, a continuous rise in sea level led to the formation of anoxic black shale deposits in deep-water areas in various regions of the world. These deposits usually contain only graptolite fossil assemblages. The discovery of sponge-dominated assemblages in graptolitic shale of the Ordovician−Silurian transition in South China has increased the known biodiversity within these strata. Here we document a new exceptionally preserved fauna, the Huangshi Fauna, in Rhuddanian (lower Silurian) black graptolitic shale of South China. The fauna is composed primarily of sponges, with additional representatives including cephalopods, arthropods, and some enigmatic carbon film fossils. Although this fauna exhibits less diversity compared to other early Paleozoic exceptionally preserved biotas in shallow-water facies, it provides new data on the biological assemblage of deep-water ecosystems after the first mass extinction. This sponge-dominated fauna not only enhances insight into the spatial and temporal distribution of sponges during the Ordovician−Silurian transition in South China but also implies that the depositional environment of the widespread lowest Silurian black shale was not entirely anoxic, indicating periods of intermittent oxygenation at the seafloor.