AN IMPROVED TECHNIQUE OF ACID DIGESTION FOR EXTRACTING WEAKLY-SILICIFIED BRACHIOPODS FROM LIMESTONE
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Abstract:
Brachiopods are a group of lophotrochozoan animals, which have a long geological range from the Cambrian to Recent and a considerable geographical distribution in Paleozoic marine environments. Most rhynchonelliformean brachiopod shells are composed of calcite, and preserved in different kinds of sedimentary rocks (e.g. limestone, dolomite, siltstone, mudstone, and shale). An improved technique of acid digestion for extracting weakly-silicified brachiopod fossils from limestone is intro-duced here. This method includes a series of processes, i.e. preparing samples (fresh rock samples cut into cuboid pieces of 40 ? 20 ? 20 cm3 approximately), suspending samples (samples suspended in an acid-resistant container by wire), making an ethanoic acid solution (the concentration of the acetic acid within the range of 0.5%–3% in volume), etching samples (a series of procedures involved to keep acid solution effective for etching samples but harmless to brachiopod shells), and brushing mask liquid (cellulose acetate solution involved here to mask shells from acid etching). This method is effective in extracting weakly-silicified brachiopod fossils from limestone, and is applicable for extract-ing other kinds of fossils with similar preservation. However, this method is time-consuming and complicated compared with the classic acid technique. Sample processing may take two to four months, depending on the host-rock lithology, concentra-tion of the acetic acid solution, and the ambient temperature of acid digestion lab. This improved technique yielded many intact, weakly-silicified, and well-preserved brachiopod fossils with delicate car-dinal process and dental plates, extracted from the upper part of the Tiezufeike Formation (Upper Ordovician) of the Wandunshan section, Butuo County, Sichuan Province. Some of them were photographed under an optical microscope without physical modification and presented here as examples.
