HOW DO BURGESS SHALE-TYPE DEPOSITS WORK? PALEOENVIRONMENTAL CONTROL OVER FOSSIL ASSEMBLAGES IN THE THREE UTAH LAGERSTÄTTEN

Burgess Shale-type (BST) deposits are crucial to understanding the early history of the Metazoa, yet the paleoenvironmental settings of these deposits are poorly constrained, obscuring understanding of both the origin of the enigmatic deposits and the paleoecological settings of their exceptional biotas. Here, we document the context of soft-bodied preservation in microstratigraphic detail for the Wheeler and Marjum Formations and the Spence Shale of Utah. All three occur below SWB in basin or distal ramp settings near breaks in broad carbonate platforms. In each, the platform-basin transition coincided with the junction of anoxic and dysoxic bottom water masses, which migrated up and down the slope through time, resulting in the presence of horizons deposited under anoxic benthic conditions as well as those deposited under dysoxic conditions. Two major types of fossil assemblages occur: in situ assemblages containing only skeletonized fossils occurred under dysoxic bottom waters, and transported assemblages of skeletonized and soft-bodied fossils occurred in anoxic settings. In >4000 individual beds (mm-scale) examined, BST fossils occur exclusively under conditions inferred to have been anoxic. In all three deposits, BST assemblages occur in two principal settings, interpreted to reflect distance of transport from the living environment. Those occurring in locations of sustained benthic anoxia contain minute and fragmented fossils, dominantly algae, whereas those occurring near the oxycline show oxygen fluctuations manifest vertically at the cm-scale and contain larger and more complete BST fossils, including common metazoans. These results suggest that assemblage content in the three deposits was regulated by the same controls. Compared to the Burgess Shale, reduced abundance and diversity of metazoan fossils may result in part from paleogeographic settings of the Utah deposits on ramps vs. the escarpment-toe setting of the Burgess: as downslope transport into a restrictive preservational trap is required, a gentler slope allows for fewer organisms to be delivered, and reduced ecological diversity from which they could be drawn, likely sourcing predominantly stressed, dysaerobic habitats. Also differences in taphonomic process may have resulted in the preservation of fewer types of tissues.