TAPHONOMIC BIAS OF SELECTIVE SILICIFICATION REVEALED BY PAIRED PETROGRAPHIC AND INSOLUBLE RESIDUE ANALYSIS OF THE LOWER TRIASSIC VIRGIN LIMESTONE MEMBER, WESTERN US

Silicification is a common mode of fossil preservation, but the extent to which silicified material represents an unbiased sampling of the total fossil assemblage within a given rock sample remains poorly quantified. Here, we use paired analyses of thin sections and acid-extracted silicified specimens from the same samples of Lower Triassic Virgin Limestone carbonates preserved in the Muddy Mountains of southern Nevada to constrain the biases introduced during the silicification process. Observations of silicified fossils in thin section also reveal the ways in which preservational biases influence results in residue. We find that abundances measured by point counts correlate positively with abundances measured from acid residues in all major fossil groups. However, there is also substantial bias in the silicified assemblages. Bivalves dominate most thin sections in the point count data, with minor contributions from echinoderms and gastropods, whereas echinoderms, and occasionally gastropods, dominate the residues, which contain more minor contributions from bivalves. These findings suggest that although silicification generally captures relative trends in proportional abundance of higher taxa among samples, the silicification process can be taxonomically biased relative to the fossil grains identified by direct petrographic analysis. We show that residue and point count data show a positive correlation, but one in which consistent and unequal biases operate in certain clades.