BURGESS SHALE FAUNAS WERE NOT COMPLETELY BURROWED AWAY: THE PALEOENVIRONMENTAL CONTEXT OF SOFT-BODIED PRESERVATION IN THE THREE UTAH LAGERSTÄTTEN

The unusual abundance of Burgess Shale-type assemblages in Lower and Middle Cambrian strata and their subsequent disappearance from the fossil record have been the subject of considerable inquiry. The elimination of these exceptionally preserved fossils from the record has popularly been blamed on the increasing depth and intensity of bioturbation across the Early Paleozoic. Previous studies of Cambrian deposits have shown that bioturbation and soft-bodied preservation occur in mutually exclusive horizons, but infer that bioturbation was environmentally regulated, and excluded from beds preserving nonmineralized tissues due to benthic anoxia. We examined the paleoenvironmental context of soft bodied preservation in the three Utah lagerstätten (Spence Shale, Wheeler Formation, Marjum Formation), in order to evaluate the potential effects of a subsequent escalation in bioturbation. Occurrences of soft-tissue preservation were documented in microstratigraphic detail in each of the three units. In each case, soft-tissue preservation was found to occur along a recognizable proximal-distal gradient, inferred to reflect distance of transport from habitable benthic environments into environments favoring preservation. Distal preservational environments (DPE’s) are characterized by sustained bottom water anoxia and abundant soft tissue preservation. DPE’s are dominated by minute and fragmented nonmineralized algae: metazoan diversity is low and pelagic forms dominate. Proximal preservational environments (PPE’s) are characterized by repeated fluctuations in bottom water oxygen content manifest at a cm-scale and interbedding with bioturbated horizons. Soft-tissue preservation in PPE’s is less abundant, but contains a much greater diversity of metazoans as well as larger and unfragmented algae. A post-Middle Cambrian increase in depth of bioturbation would have likely been sufficient to eliminate soft-bodied faunas found in PPE-like environments, yet may not have been sufficient to eliminate preservation DPE’s. Evidence from the three Utah lagerstätten indicates that the diversity of Burgess Shale-type assemblages may have been severely restricted by increasing bioturbation, however, we consider it improbable that Burgess Shale-type assemblages were completely “burrowed away”.