- Society for Sedimentary Geology
The degree to which the original community composition of the Middle Cambrian Burgess Shale was altered through transport and decay and how taphonomic conditions varied through time and across taxa is poorly understood. To address these issues, variation in fossil preservation was analyzed through a vertical succession of 26 bed assemblages, each representing a single obrution event, within the 7-m-thick Greater Phyllopod Bed of the Walcott Quarry. More than 50,000 specimens belonging to 158 genera—mostly benthic, monospecific and nonbiomineralized—were included in this analysis. The decay gradient of the polychaete Burgessochaeta setigera was used as a taphonomic threshold to estimate how far decay had proceeded in each bed assemblage.
Qualitative comparisons of the degree of preservation of 15 species, representing an array of different body plans, demonstrate that all bed assemblages contain a mix of articulated and in situ dissociated or completely dissociated organisms interpreted respectively as census- and time-averaged assemblages. Furthermore: (1) most organisms studied were preserved within their habitat and only slightly disturbed during burial; (2) most decay processes took place prior to burial and resulted in disarticulation of organisms at the time of burial; (3) the degree of disarticulation was variable within individuals of the same population and between populations; and (4) early mineralization of tissues across all body plans occurred soon after burial.
Canonical correspondence analysis summarizes the apparent variations in the amount of preburial decay, or time averaging, across species, individuals, and bed assemblages. The effect of time averaging, however, must have been limited because rarefaction curves reveal no link between decay and species richness. This suggests that decay is not an important community controlling factor. Overall, our data suggest that transport was trivial and the traditional distinction between a pre- and postslide environment is unnecessary. It is likely that all specimens present at the time of burial would have been preserved independent of their original tissue composition and degree of preburial decay.
The presence of extensive sheets of Morania confluens, a putative benthic cyanobacterium, in most bed assemblages suggests that it: (1) provided a stable substrate and food source for a number of benthic metazoans, and (2) played a possible role in the preservation of nonbiomineralized animals, acting as a barrier in maintaining local anoxic pore-water conditions.