Analysis of the environmental impacts affecting Cambrian reef building and carbonate settings during the Miaolingian and Furongian epochs: A hypothesis for consideration

The Miaolingian and Furongian epochs of the Cambrian period have been identified as a time of limited metazoan reef development. The aim of this paper is to improve understanding of the biological and geochemical conditions that affected reefs during this interval, and to propose a hypothesis for understanding why metazoan reef development was inhibited. To address these issues, a global dataset of fossil occurrences (N ¼ 25,307) spanning Cambrian Stage 4 to the early Ordovician (Tremadocian) was extracted from the Paleobiology Database, Paleoreef Database, and a review of the primary literature. Findings show that the proportion of reefs constructed by metazoans fell from 40% in the Wuliuan age to 0% in the Drumian age, with reefs being overwhelmingly dominated by microbial ecosystems through the remainder of the Cambrian. The proportion of skeletal material constructed from carbonate fell from 85% in the Wuliuan age to 63% in the Drumian age across all the fossil occurrence data, before recovering. These findings suggest that environmental conditions may have not been favorable to carbonate organisms, but this does not fully explain the prolonged reduction of metazoans within reefs throughout this interval. A hypothesis proposed here is that Miaolingian to Furongian metazoan reef abundances were low because of two factors: (1) shallow water anoxia – and other factors such as elevated temperatures and ocean acidification – caused the extinction of metazoan reef builders in the late-early Cambrian and (2) deep water anoxia and marine regression, resulted in a loss of habitat. These inhibiting conditions were not necessarily concurrent but are inferred to have collectively suppressed the growth of metazoan reefs until the Early Ordovician when more shelf space for new reef development occurred. This hypothesis provides a first step in exploration of these conditions during the middle and late Cambrian and for reef development in general.