Microfacies controlling the preservation of small shelly fossils from the lower to middle Cambrian of North China

Small shelly fossils have long been recognized as important evidence of both metazoan evolution and biomineralization during the Cambrian radiation. The marked decline in the occurrence, diversity and abundance of small shelly fossils in the middle to late Cambrian, when compared with the early Cambrian, has often been regarded as representative of a closing phosphatization window. Small shelly fossil-like assemblages have, however, been sparsely reported from the entirety of the Phanerozoic, indicating preservation of small shelly fossils is possible outside of the Cambrian. To identify possible factors enhancing small shelly fossil preservation, five microfacies (normal bioclastic limestone, flat-pebble conglomerates with bioclasts, hummocky cross-stratified grainstones with bioclasts, bioclastic grainstone in hardgrounds and glauconite bioclastic wackestone-packstone) from the Houjiashan, Mantou, Maozhuang, Xuzhuang and Zhangxia formations, Cambrian Series 2 to Miaolingian in North China are compared to assess for differences in preservation potential. The majority of extracted small shelly fossils from the studied section come from normal bioclastic limestones, but diversity is generally low and ornament is poorly preserved (except for original organophosphatic shells). The diversity of shelly fossils in flat-pebble conglomerates with bioclasts and hummocky cross-stratified grainstone with bioclasts are similar to normal bioclastic limestone, but with a greater abundance of articulated brachiopods. Bioclastic grainstone from hardgrounds facilitate some soft tissue preservation but the overgrowth of apatite grains on some specimens means shell structures are poorly preserved. Small shelly fossils extracted from glauconite bioclastic wackestone-grainstones are the best preserved, in regard to shell microstructure, and diversity is highest in these layers. These results suggest that limestones with authigenic glauconites could possibly be a good indicator for well-preserved small shelly fossils and that preservation potential of small shelly fossils is likely linked to differences between microfacies in early to middle Cambrian, North China.