Dissecting a Cambrian lagerstatte: Insights from the Wheeler Formation, Utah

Cambrian fossil lagerstätten are unequivocally the most significant deposits to our understanding of the initial Phanerozoic radiation of the Metazoa. As these deposits contain preservation of soft-bodied organisms, in addition to the skeletonized fossils which comprise the bulk of the “normal” fossil record, they provide an unparalleled view into the nature of this critical interval in the history of life. Despite the great significance of these deposits and the fossils contained within them, very little is known about the specific conditions which promoted exceptional preservation, or the relationship of the living environments to the preservational environments. This study has developed new integrative methods for the examination of one such lagerstätte, the Middle Cambrian Wheeler Formation of Utah, in unprecedented microstratigraphic detail. This study has modified existing ichnological models to interpret relative benthic paleo-oxygen levels at a millimeter-scale for Cambrian mudstones. Application of this technique to targeted continuously-sample intervals (0.1–5.0 m thick) reveals that soft-tissue preservation, which occurs in the mode of 2-dimensional carbon films, occurs in exclusively anoxic environments. Burgess Shale-type preservation in the Wheeler Formation is proposed to have occurred through early diagenetic occlusion of porosity, which would have led to premature cessation of microbial decomposition. Several factors are considered important in promoting this type of preservation; all are related to primary physical features of the depositional environment. Soft-tissue preservation may rarely occur in situ, where pronounced fluctuations in bottom water oxygen exist, but more commonly requires transport of fossils from the living environment to one which favors their preservation. In the Wheeler Formation, a gradient from proximal to distal preservational environments is present and is accompanied by a decrease in articulation, size and diversity of soft-bodied fossils. The well-known trilobite Elrathia kingii was found to have inhabited the “exaerobic zone” at the margin of anoxic and dysoxic benthic environments. This finding constitutes the earliest known example of this lifestyle, and suggests that the trilobite utilized chemoautotrophic bacteria as a primary food source, rather than a phototrophic-based food web. This study has developed a successful methodology for examining these critical, yet cryptic deposits in a new level of detail.