Composition and structure of microbialite ecosystems following the end-Permian mass extinction in South China

摘要

After the end-Permian mass extinction, microbialites were widespread in the shallow marine environments in South China. Diverse microbialites are characteristic of the Early Triassic carbonate sequences. Their stratigraphic range is rather short, just across the Permian-Triassic (P-Tr) boundary, and defined by conodonts N. meishanensis, H. changxingensis, and H. parvus Zones. Thus, the P-Tr microbialites grew immediately after the first episode of the end-Permian mass extinction coinciding with the beginning of the conodont N. meishanensis Zone and ceased their growth at the beginning of the conodont I. staeschei Zone, which occurred just before the second episode of the end-Permian crisis at Meishan. They include three main types: stromatolites, thrombolites and dendrites in South China. The presence of abundant microbial remains of coccoid-type and Renalcis-like microbes strengthens the previous view that the P-Tr microbialites are biogenic. Both cyanobacteria and other microbes have participated to construct the microbial ecosystems. The thrombolite communities are dominated by the Renalcis-like microbes and foraminifers and also contain ostracods, microgastropods, and microconchids. The stromatolite communities are dominated by coccoidal microbes with minor constituents of other undetermined microbes. Metazoans are rare. The dendrolite assemblage contains few identifiable microfossils. Most of microbialite communities have a high-abundance, low-diversity community structure, characteristic of marine communities after the end-Permian mass extinction. Some metazoans of either opportunists or disaster forms dwelled in this special ecosystem. The microbialite-builders formed the primary producer communities and constructed the lowest level ecosystem structure. The lower-level ecosystem proliferated due to the collapse of the higher level of ecosystem structure characterized by benthic shelly and predator communities in the end-Permian crisis.