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Sensitivity of Taphonomic Signatures to Sample Size, Sieve Size, Damage Scoring System, and Target Taxa

Susan M. Kidwell, Thomas A. Rothfus and Mairi M. R. Best
PALAIOS
Vol. 16, No. 1 (Feb., 2001), pp. 26-52
DOI: 10.2307/3515551
Stable URL: http://www.jstor.org/stable/3515551
Page Count: 27
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Sensitivity of Taphonomic Signatures to Sample Size, Sieve Size, Damage Scoring System, and Target Taxa
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Abstract

The sensitivity of taphonomic signatures to a battery of common sampling and analytic procedures is tested here using modern bivalve death assemblages from the San Blas Archipelago, Caribbean Panama, to determine (a) the magnitude of methodological artifacts and, thus, the comparability of taphofacies patterns among studies; and (b) the most efficient and robust means for acquiring damage profiles (taphonomic signatures) of death assemblages both ancient and modern. Damage frequency distributions do not stabilize below sample sizes of 120-150 individuals. Using damage to the >8 mm portion of the assemblage as a baseline (interior damage only, fragments included), it is found that qualitative trends among environments (higher damage levels in reefal skeletal gravel versus mud) and the rank-order importance of taphonomic variables per environment (intensity of damage from encrustation, boring, fine-scale alteration, edge-rounding, fragmentation) are robust to most methodological decisions. The exception is the use of target taxa: of three genera tested, only one was sensitive to the same suite of environmental differences as the total-assemblage, and taxa had disparate rank-ordering of variables. In contrast to the general robustness of qualitative trends, quantitative damage levels are affected significantly by methodology. Specifically, the measured frequency of damage is generally lower for finer size fractions and finer sieve sizes, for whole shells versus fragments, for taxonomically well-resolved specimens, for infaunal versus epifaunal species regardless of mineralogy, and for interior surfaces versus exterior or total surface area of shells. Full frequency-distribution data on states of taphonomic damage are most powerful for differentiating samples, but if single-value metrics are desired, the frequency of high-intensity damage is more powerful-and shows less between-operator variance-than presence-absence data or average damage state. To maximize the detection of damage and of between-environment differences in taphonomic signature, and to foster between-study comparisons, the following are recommended: (1) analysis of discrete size-fractions rather than broad spectra and, in particular, the separate treatment of coarse size fractions (>4 mm); (2) examination of complete assemblages (fragments as well as whole specimens; all species or broad subsets of species rather than select taxa); (S) variables scored independently (e.g., encrustation v. boring) rather than grouped into summary grades; and (4) evaluation of rank-ordering of variables in plots of threshold damage profiles as a complement to ternary taphograms.

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