TAPHONOMY OF THE GREATER PHYLLOPOD BED COMMUNITY, BURGESS SHALE

The degree to which the original community structure of the Middle Cambrian Burgess Shale has been altered by taphonomic biases (in particular through transport and decay) and how taphonomic conditions varied in time and across taxa, are still poorly understood. In this study, variations in preservation are analyzed from a series of 26 Burgess Shale bedding assemblages within a 7-meter stratigraphic unit, the Greater Phyllopod Bed. Over 50,000 specimens belonging to 158 genera (mostly benthic, monospecific and non-mineralized) were identified from this unit.

The presence of articulated moults, attachment parts, direct evidence of interspecies relationships, ichnofossils, intact hemichordates and bacterial mats, and scavenged remains, suggest that most organisms studied were autochthonous or only slightly disturbed during burial, and preserved within their habitat. This conclusion is consistent with results from recent published geochemical analyses. Detailed qualitative comparisons of the degree of preservation of 15 species with non-mineralized parts, representing an array of different body plans, demonstrate that all bedding assemblages contain a mix of complete and in-situ dissociated organisms interpreted respectively as census “life” + “death” components. These findings independently confirm an autochthonous burial of most organisms and support the views that: 1) the time of death was variable within individuals of the same population and between populations; 2) most decay processes took place prior to burial; 3) decay was quickly halted after burial. Canonical Correspondence Analysis summarizes variations in the amount of pre-burial decay across species, individuals, and bedding assemblages and shows that overall the community structure follows the decay gradient of the polychaete Burgessochaeta setigera. However, rarefaction methods show no link between the degree of preservation of this polychaete and species richness, demonstrating limited taxonomic control on preservation after burial. This conclusion is supported by the presence of many putative acuticular organisms in all bedding assemblages. Large bacterial mats in many bedding assemblages could have helped to maintain the redox boundary at the water-sediment interface, allowing the development of a grazing community.