FOSSILS PREPARED TWO WAYS: ADVANTAGES AND SHORTCOMINGS OF MICRO-CT AND ACID MACERATION IN COMPLEMENT

Acid maceration is a common and inexpensive preparation technique for gently and efficiently extracting non-calcareous fossils from limestone. However, the potential for body size- or taxon-selective breakage during washing makes this method a possible source of bias in paleoecological studies relying on it. X-ray microtomography (μCT), a non-destructive imaging technique, can be used to document fossils still within the matrix; however, μCT has its own limitations. By applying both methods to the same specimens, the biases inherent to each can be described and quantified. In this study, samples from the Edinburg Formation, an Upper Ordovician deposit known for silicified fossils, are analyzed using these paired methods.

Analysis of μCT imagery in comparison to recovered residues from acid maceration reveals that very small (< ~1mm) fossils are poorly resolved in μCT, leading to undercounting of such taxa as ostracods and bryozoans. Acid maceration, meanwhile, fails to recover large poorly mineralized features such as burrow traces. Tests for patterns of breakage indicate no significant size or taxonomic bias in breakage during extraction; however, poorly preserved fossils may be more likely to be lost. Comparisons of μCT and residue imagery of individual fossils reveal taxon-specific patterns of fragmentation and allow biostratinomic and preparational breakage to be differentiated. Paleoecological ordinations and cluster analyses of datasets based on μCT and residues produce generally concordant results but indicate that true variation between taxonomic composition of the samples is outweighed by the biases of μCT, limiting the utility of this method for paleoecological questions. These results depend strongly on the textural and compositional characteristics of the samples in question; applying similar methods to different deposits will help to generalize conclusions drawn on the relative strengths and weaknesses of the methods.