Exceptional preservation in calcareous nodules

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Abstract
Preservation of soft integument in calcareous nodules seems to be more widespread geographically and stratigraphically than hitherto realized. It cannot be recognized in the field, and to recover such material requires special etching techniques. Such preservation can be of exceptional quality, with fossils preserved three dimensionally either by secondary phosphatization or by silicification. Coating as well as the replacement of integument has been observed even within the same sample. Methodical search for such preservation may be based on the common denominators of depositional, geochemical, and environmental indicators in previously described occurrences. As such exceptionally preserved material may be rare within the samples, large quantities of rock have to be prepared. The examples described here are from anthraconitic limestones (Orsten) of the Upper Cambrian Alum Shale Formation in Sweden. They are now known from many localities and from different trilobite zones. In addition nodules from the Lower Cretaceous Santana Formation in Brazil, the Upper Devonian cephalopod limestone in the Carnic Alps, the Lower Triassic of Spitzbergen and the Miocene Barstow Formation in California have all yielded extremely fine material.

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