The trace fossil Thalassinoides bacae in the Cambrian Zhangxia Formation (Miaolingian Series) of North China

Abstract

Branched, complex burrows with thick linings assigned to Thalassinoides bacae are abundant in the Cambrian Zhangxia Formation (Miaolingian Series) in southern North China. These burrows occur in thinly layered micritic limestones, which were formed in a restricted marine environment surrounded by oolitic shoals. The macromorphological characteristics of the burrows observed in outcrop, combined with microscopic burrow characteristics assessed using light microscopy and scanning electron microscopy, were used to analyze the ethology of T. bacae. The complex geometric characters of horizontal branching tubes and subvertical shafts reflect T.bacae is an agrichnical burrow system. The horizontal tubes were used to cultivate microbes for food, while the subvertical shafts provided openings to the sea floor. Organic material, iron minerals and pyrite framboids just present in the margin of the fill indicate a special micro-environment in the burrow system with a reducing environment and a high organic content, and they may provide evidence for microbial garden. The functions of the thick micrite lining may have included maintenancce of environmental conditions within the burrow and provision of suitable conditions for growing microbes. Dark-colored irregular areas around the lining are identified as diagenetic halos. The burrowers of T.bacae built complex burrows system and thick lining for cultivating microbes to survive in a restricted marine environment that was oxygen-poor and with low food availability, reflecting adaptive behavioral evolution of the metazoan in the middle Cambrian.