RECENT BRACHIOPODS AS A TOOL TO UNDERSTAND THE TAPHONOMIC PATHWAYS OF THE EARLIEST CAMBRIAN CHENGJIANG LINGULIFORM BRACHIOPODS

Chengjiang fossil Lagerstätte is one of the critical Burgess Shale –type fossil Lagerstätten to understand the Cambrian bioradiation. To date, the taphonomy of the Chengjiang Fauna is not yet well understood. Here, we propose to use brachiopods as critical model phylum to perform taphonomic analyses. Palaeozoic linguliform brachiopods were supposed to develop an organo-phosphatic shell but, due to the inadequate preservation of unweathered Palaeozoic fossil material, it remained for long time an open question. Our study reveals that during the earliest Cambrian linguliform brachiopods developed an organo-phosphatic shell as in modern times. Analyses performed on fossil material of the earliest Cambrian encompassing the complete spectrum of weathering stages, from the unweathered to weathered grade, show a primary organo-phosphatic biomineralization of the brachiopod shell and a successive dissolution and replacement of the shell during weathering, and the results gained allow discussion of the role of the pyrite minerals in the preservation of Chengjiang Fauna (Cambrian Stage 3, Yunnan Province, China). Systematic elemental mapping and energy dispersive X- ray (EDX) analyses have been carried out for fossil brachiopods and matrix from the Cambrian (Stage 3) and Early Trias (Induan, Guizhou Province, China) and for modern linguliform brachiopods coming from several geographic regions in order to investigate the nature of the organo-phosphatic shell through geological time. This paper aims to use modern linguliform brachiopods as a key to understanding the fossilization potential of the different organs, and to establish whether the environment may have an influence on the shell mineralogy.