Taphonomy and community analysis of the Middle Cambrian Greater Phyllopod Bed, Burgess Shale (British Columbia).

摘要

This study focuses on temporal variations in taphonomy and paleoecology of the Middle Cambrian Greater Phyllopod Bed community (Burgess Shale, British Columbia). Fossils come from 36 bedding assemblages, representing discrete obrution deposits, within a seven-metre-thick sequence. I studied over 50,000 specimens belonging to 172 taxa (mostly monospecific and soft-bodied) from 17 major groups and 17 ecological categories. Sixty seven taxa are new.; Qualitative and quantitative variations in preservation between body-plans, species, and individuals within and between single bedding assemblages (regarded herein as census fossil assemblages) demonstrate that decay processes took place prior to burial. Quantitative estimates of preservation biases across the community using Canonical Correspondence Analysis and rarefaction methods (employing the polychaete Burgessochaeta setigera as a taphonomic threshold) suggest no link between degree of preservation and species richness. This supports the view that the original community was not significantly altered by preservation biases. Available evidence suggests that most organisms studied were autochthonous or slightly disturbed during burial and preserved within their habitat.; The community is dominated by epibenthic vagile deposit feeders and sessile suspension feeders, represented mostly by arthropods and sponges. Species richness increased with evenness, based on rarefaction curves, Whittaker plots and the Berger-Parker index. Correspondence Analysis and Minimum Spanning Tree results suggest the existence of four structural and ecological groups. Indicator Species Analysis supports the presence of typical species within each group. These results provide evidence for patterns of species recurrence within groups, and community turnover between groups and bedding assemblages.; Canonical Correspondence Analysis and Mantel tests demonstrate that bedding assemblages are not random and that the community was structured over time. In the community, most species lack temporal persistence. Persistent species are usually dominant in rank abundance. Younger bedding assemblages tend to have more persistent species and a higher degree of evenness, suggesting that the community becomes more stable through time. Comparisons with modern marine benthic communities suggest that the Greater Phyllopod Bed community was probably highly dependent on immigration from the pool of regional species richness after each disturbance event. This could support the view that species interactions (in particular predation) played little importance in structuring the community.