Small skeletal fossils associated with Archaeocyath reefs from the Xiannüdong Formation (Cambrian Series 2), southern Shaanxi, China

Improved understanding of both the Small Skeletal Fossil (SSF) assemblages and archaeocyath reefs of the early Cambrian Period is important to best elucidate the evolution of early marine ecosystems during the Cambrian Radiation. To address this issue, we investigate the well-preserved SSF fauna associated with archaeocyaths of the early Cambrian Xiannüdong Formation (Dayingcun Section, Nanzheng area, southern Shaanxi). The Xiannüdong fossil assemblage includes sphenothallids, archaeocyaths, brachiopods, bradoriids, hyoliths, helcionelloid molluscs, trilobite sclerites, lobopods, tommotiids, chancelloriids, as well as echinoderm plates. Thin-section analysis, field observations and examination of fossil material indicate the Xiannüdong Formation assemblage consists primarily of an archaeocyath-Sphenothallus dominated ecosystem from a well-structured shallow-water mixed shelf featuring oolite shoals, archaeocyath reefs, adjacent fossiliferous allochthonous floatstone and bioclastic grain-packstone. The fossil assemblage reflects a benthic community which has not been previously described from other Cambrian reef strata of the Yangtze Platform. This unique community indicates that complex reef ecosystems were already present in the early Cambrian in this palaeogeographic region.