A MODEL FOR THE PRESERVATION OF SHELLS AS CARBONACEOUS FOSSILS BASED ON TAPHONOMIC OBSERVATIONS AND EXPERIMENTS

Shells preserved as carbonaceous fossils (e.g. Burgess Shale-type microfossils) are common in the Cambrian, but their taphonomy has not been thoroughly studied. To investigate the processes involved in the preservation of shells as carbonaceous fossils—and to identify factors that may account for their commonness in the Cambrian—we studied conotubular Sphenothallus fossils from the lower Cambrian Shuijingtuo Formation at Heziao and Jijiapo (Hubei Province) and the equivalent Niutitang Formation at Siduping (Hunan Province) in South China. In general, the morphology and allometry of the fossils suggest that they represent a single species, and their biostratinomy and authigenic mineralization indicate that they underwent similar pre-burial and early diagenetic processes, particularly transport and focused degradation via microbial sulfate reduction. However, whereas the Heziao and Siduping fossils consist of phosphatic material, the Jijiapo fossils consist entirely of carbonaceous material. Simple taphonomic experiments show that hydrochloric acid treatment of Heziao specimens produces fossils that compositionally and microstructurally resemble the Jijiapo specimens. Based on these observations, we propose a taphonomic model for the preservation of Sphenothallus and other shelly taxa as carbonaceous fossils that involves two sequential (or possibly concomitant) processes: (1) the formation of aliphatic compounds that are stable over geological timescales via in situ diagenetic polymerization (‘kerogenization’) of the shells’ organic matrixes and (2) the loss of the biomineralized microstructures of the shells. This model indicates that shells are preserved as carbonaceous fossils only when their organic matrixes survive early diagenetic microbial degradation. Thus, the model may account for their commonness of shells preserved as carbonaceous fossils in the Cambrian, when low bioturbation intensity and sharp geochemical and redox gradients across the sediment-water interface may have favored the survival of organic matrixes.