Palaeoecology of microconchids from microbialites near the Permian-Triassic boundary in South China

Abundant isolated specimens of microconchid tubes have been extracted from a microbialite deposit near the Permian-Triassic boundary (PTB) in the Dajiang section, southern Guizhou Province, South China. They are assignable to Microconchus aff. utahensis, M. aff. aberrans and Helicoconchus aff. elongatus, all of which possess micro-lamellar tube walls. Quantitative analysis of bulk samples indicates that most microconchids occur in the upper part of the PTB microbialite deposit and show substrate selectivity for bioclastic grainstone-packstones. In contrast, very few microconchids were found in the rocks bearing well-developed microbialite structures. Both stratigraphical and substrate preferences indicate proliferation of microconchids coincided with an ebb of microbialite development. Microconchids therefore only proliferated in local niches in which microbial activities were not very active within the PTB microbialite ecosystem. The presence of abundant microconchids further strengthens the impression that PTB microbialite metazoans are much more diverse than previously thought. The end-Permian mass extinction is calibrated to the base of microbialite deposit in South China. Thus, abundant microbialite metazoans, such as ostracods, lingulid brachiopods, microgastropods and microconchids, together with the considerable, temporarily surviving faunas reported from non-microbialite PTB sections in South China, indicate that metazoans diversified immediately after the first episode of the end-Permian mass extinction, supporting the scenario that marine ecosystems underwent episodic collapses during the devastating biocrisis over the Permian-Triassic transition.