Spatial variation in the diversity and composition of the Lower Cambrian (Series 2, Stage 3) Chengjiang Biota, Southwest China

Abstract

Strata hosting the Lower Cambrian (Series 2, Stage 3) Chengjiang Biota (Maotianshan Shale Member, Yu'anshan Formation) occur throughout the eastern part of Yunnan Province, Southwest China. In this study, literature-based faunal inventories from 10 areas (representing 34 localities), together with 22,038 new specimens collected at three localities from three of the 10 areas, were analyzed quantitatively to assess large-scale spatial variation in taxonomic diversity and composition. Our analyses show substantial co-variation between local paleoenvironmental settings and species diversity, and suggest the presence of three general taphofacies in the Maotianshan Shale Member, from west to east: low diversity (26 species) characterizes the Wuding area, where fossiliferous strata have undergone extensive bioturbation; high diversity (215 species) characterizes the Chengjiang–Haikou–Anning areas, where fossiliferous strata consist of stacked couplets of thin event and background mudstone layers; moderate diversity (55 species) characterizes the Malong area, where fossiliferous strata consist of indistinctly bedded shale showing fewer single-event layers but abundant background layers. In spite of variations in sampling effort between sites, spatial patterns in species diversity and community composition were controlled primarily by variation in depositional environments along a proximal offshore to lower shoreface gradient, which influenced both ecological and taphonomic factors. The offshore settings (Haikou–Chengjiang–Anning areas) show relatively high species diversity and exceptional preservation of soft-bodied fossils, reflecting rapid burial in distal tempestites. Relatively high degrees of bioturbation, time-averaging and hydrodynamic disturbance caused a substantial decrease in the quality of preservation and number of soft-bodied species in the Wuding and Malong areas. Arthropods are the most diverse group in most localities, followed by priapulids, poriferans, brachiopods, lobopods and hyoliths.This study shows that the Chengjiang Biota lends itself very well to high resolution characterization of spatial variation in taxonomic diversity, faunal composition and fossil preservation. Furthermore, the Chengjiang Biota may provide a unique opportunity to assess the roles of environmental factors, taphonomy and ecological controls on species diversity at local to regional spatial scales.