EVIDENCE FOR ABUNDANT SEAFLOOR MICROBIAL MATS AND ASSOCIATED METAZOAN LIFESTYLES IN THE LOWER CAMBRIAN ZHONGYICUN PHOSPORITES OF CHINA

The increase in bioturbation levels through the Proterozoic-Phanerozoic transition changed the substrates on which marine benthos lived from being relatively firm with a sharp sediment-water interface to having a high water content and blurry sediment-water interface. In addition, abundant microbial mats, once dominant on normal marine Proterozoic seafloors, were relegated to stressed settings lacking intense metazoan activity. This change in substrates has been termed the agronomic revolution (Seilacher and Pflüger, 1994), and the impact of this substrate transition on benthic metazoans has been termed the Cambrian substrate revolution (Bottjer et al., 2000). The granular phosphorites of the Lower Cambrian Zhongyicun Member, Yuhucun Formation, Yunnan Province, China were deposited in a shallow marine environment and contain ample evidence indicating the presence of seafloor microbial mats. This evidence includes abundant and distinctive bedding planes that are stained red and enriched in heavy iron minerals and mica, intrepreted as resulting from mat-decay mineralization and mica trapping by microbial mats. The radular grazing trace fossil Radulichnus is also found on these bedding planes, indicating a firm, microbial mat-bound substrate. Along with these radular scratches are preserved circular impressions (8-10 cm in diameter), which are possibly the fossilized remains of large soft-bodied organisms, and may provide additional evidence that Ediacaran-type preservation still occurred in the Early Cambrian. The first relatively intense bioturbation in this region is also preserved in this formation and is dominated by horizontal Thalassinoides burrows in thin beds of ii 3-4. These Thalassinoides could represent significant undermat mining behavior and indicate that the unlithified substrate in which they were emplaced was relatively firm. The variety of evidence for the presence of seafloor microbial mats in the Zhongyicun phosphorites, and for metazoan lifestyles associated with such mat-bound seafloors, reveals that environments dominated by typical Proterozoic-style soft substrates were still common in the Early Cambrian. However, common thin bioturbated beds heralds the change towards increasing bioturbation that is characteristic of the Proterozoic-Phanerozoic transition.