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Clams and Brachiopods: Chips That Pass Out of Sight

Society for Sedimentary Geology
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Abstract Data from modern subtidal brachiopod-mollusk dominated death assemblages are used to investigate the effect of sieve size on taphonomic signature, comparing bivalves and brachiopods. The data comprise a total of 4439 specimens, mainly fragments (93%), sorted into five size fractions, and scored for four taphonomic variables: fragmentation, surface alteration, bioerosion, and encrustation. The taphonomic signatures of bivalve and brachiopod shells change as a function of sieve size, primarily from loss of encrusters on small, mobile fragments. Bivalves and brachiopods show significant differences (Wilcoxon rank-sum, α = 0.05) in taphonomic signature that become more pronounced in the fine fractions, mainly as a result of different levels of bioerosion. Bivalves seem to become fragmented after weakening from bioerosion, whereas brachiopods apparently are more prone to mechanical breakage. Thus, brachiopod shells may be shorter-lived in the coarse fractions and enter the fine fractions with less accumulated damage. Due to intrinsic taxonomic differences in the response to taphonomic processes, death assemblages with similar levels of taphonomic modification may have accumulated on different temporal scales. Comparative studies seeking taphonomic control must strive to use material from the same size fractions to avoid spurious results based on sampling procedure alone.

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