Miniaturized trace fossils in microbialites from the Cambrian Series 2 Qingxudong Formation in the Panshi area, eastern Guizhou, China

Abstract

Trace fossils are especially useful to elucidating metazoan evolution and species extinction events through geologic history. This paper reports results of an investigation of well exposed microbialites in the Cambrian Series 2 (upper Toyonian–lower Amgan) Qingxudong Formation (QF) of the Panshi area, eastern Guizhou Province. Light and scanning electron microscopic examination of these deposits has revealed for the first time small Planolites. Detailed analysis of these trace fossils has helped elucidate their mode of formation mechanism and their relationship with the extinction of Archaeocyathids early in the Toyonian. Our results suggest that the trace-makers had a trophic specialization strategy and selectively fed in a low-energy and Girvanella-rich environment. They were the pioneers of ecosystem recovery following extinction of reef-building Archaeocyathids and had survived the earlier oxygen-depleted environment. Sedimentary facies evolution manifested by the studied deposits and penecontemporaneous siderite indicates that the trace fossils formed in a shallow sea in which a weakly reducing, sulfur-poor sedimentary environment had been established in association with transgression. The recovery and expansion of microbial reefs and the miniaturization of trace fossils suggest that the anoxic environment severely inhibited metazoans and may have contributed significantly to the extinction of reef-building Archaeocyathids during the early Toyonian time and the slow recovery of ecosystems that followed. Our results provide impetus for further research on the extinction event and the recovery process of metazoans in the late period of Cambrian Epoch 2.