Early recovery of sponge framework reefs after Cambrian archaeocyath extinction: Zhangxia Formation (early Cambrian Series 3), Shandong, North China

Abstract

Reefs dominated by the anthaspidellid sponge Rankenella zhangxianensis, the calcimicrobe Epiphyton and the stem-group cnidarian Cambroctoconus orientalis, together with encrusting microstromatolites occur early in the middle Cambrian (Series 3, late Stage 5) of Shandong, eastern China. In the Zhangxia Formation, these in situ components created a tight framework, with centimeter-scale growth cavities mainly filled by fine-grained matrix. Among them, R. zhangxianensis and C. orientalis mutually attached and locally formed metazoan-dominated frameworks. These metazoan-microbial reefs form thin lenses b2m wide within microbial mounds, and probably developed at least a fewcentimeters of synoptic relief above these surrounding structures. With an age of N505 Ma, these Rankenella reefs indicate recovery of framework-buildingmetazoans within ~5 million years of the archaeocyath reef decline. In structure, they resemble archaeocyath reefs as well as Early Ordovician lithistid sponge-microbial reefs, having conjoined conical macroskeletons thickly veneered by calcimicrobes and microbial carbonate with largely matrix-filled intervening cavities. In combination with other sponge-microbial reefs reported from Australia, Iran, Korea and the USA, they demonstrate that an anthaspidellid sponge-microbial reef consortium was widespread throughout the mid-late Cambrian.