EARLY RECOVERY OF METAZOAN FRAMEWORK REEFS IN THE AFTERMATH OF THE ARCHAEOCYATH EXTINCTION: MIDDLE CAMBRIAN (LATE STAGE 5) ZHANGXIA FORMATION, SHANDONG PROVINCE, CHINA

During recovery of reef ecosystem after the late Cambrian Series 2 extinction of archaeocyaths (ca. 509 million years ago), microbialites have been considered as dominant reef constituents (Cambrian Series 3–Furongian; over 24 million years). Although some recent studies reported siliceous sponge-microbialite reefs from the middle–late Cambrian Series 3 and Furongian successions, there is still ca. 10 million years gap until the reappearance of metazoan in reef ecosystem. Here we report early Cambrian Series 3 (late Stage 5) reefs of eastern China locally dominated by anthaspidellid sponge Rankenella zhangxianensis, in addition to calcified microbe Epiphyton and some stem-group cnidarian Cambroctoconus orientalis. R. zhangxianensis and Epiphyton constructed main framework of the reefs with centimeter-scale cavities, whereas some frameworks were solely constructed by metazoans attaching each other. These metazoan-microbial reefs formed about 5 million years after the extinction, in contrast to previous suggestion of a much slower recovery of metazoan reef-building communities. Together with the other recent reports of the middle–late Cambrian sponge-microbial reefs, we suggest that the reef-building metazoans, mainly anthaspidellid sponges, appeared shortly after the extinction of archaeocyaths and occurred throughout the middle and late Cambrian. These Cambrian anthaspidellid sponge-microbialite reefs would have been the ancestors of Early Ordovician anthaspidellid sponge-microbialite-calathiid reefs, when they became the dominant reef type.