Taphonomic versus ecologic controls on taxonomic relative abundance patterns in tempestites

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Abstract
Evaluation of taphonomic sample bias in tempestites is an important prerequisite for paleoecologic analysis in storm-dominated shelf settings. Data from tempestites in the Upper Cambrian Bison Creek Formation of southern Alberta [Canada] indicate that size-sorting greatly influences trilobite relative abundance patterns. Shifts in relative abundances of taxa both within and between beds are matched by changes in trilobite size. That is, collections fall into two intergrading types of assemblage which are dominated by taxa which reach large and small adult sizes, respectively. These shifts in dominance are interpreted as taphonomic variability within a single trilobite biofacies. Biofacies differentiation is expressed as abrupt changes in relative abundance patterns that are independent of trilobite size. Qualitative general models for size-sorting in tempestites are developed. Size-sorting in graded tempestites produces vertical shifts in relative abundances of taxa that mimic those expected in short-term autogenic ecologic succession. Size-sorting between beds in a single section could lead to an overestimation of the number of biofacies present. Size-sorting between proximal and distal portions of a depth gradient might generate abundance patterns that mimic those expected for ''depth-related communities'', even in the complete absence of any underlying ecologic differentiation. The models represent taphonomic null hypotheses which must be rejected prior to making paleoecologic inferences in storm-dominated shelf environments.