SILICIFICATION AS A COMMON MODE OF PRESERVATION IN NORTH AMERICAN CAMBRIAN LAGERSTÄTTEN

The Burgess Shale and Kinzers formations (British Columbia and Pennsylvania respectively) host prolific fossil beds in which soft-tissues are preserved. Well documented modes of preservation in these fossil localities include carbonaceous compressions, infillings by authigenic phyllosilicates, phosphatization of gut regions, and pyritic crusts. Elemental mapping was undertaken on the abundant, widespread, and anatomically simple organisms Marpolia spissa (filamentous cyanobacteria) and Margaretia dorus (green algae?) from the Burgess Shale formation in British Columbia and the Kinzers Formation in Pennsylvania. A specimen of Marpolia from the Kinzers Formation is composed of a carbonaceous surrounded by a shell of polygonal quartz crystals. The fossil filaments lack Al, K, Ca and Fe, indicating a lack of phyllosilicates, iron oxides, pyrite and carbonates. A sample of Margaretia from the Burgess Shale yielded elemental maps that indicate that the fossil is composed of an intergrowth of quartz and muscovite. Relative intensity of x-ray counts of K, Al, Fe, and C indicate that quartz is highly enriched in the fossil relative to the matrix, whereas muscovite is less abundant in the fossil; disseminated carbonaceous material occurs in the matrix but is absent from the fossil. Similar Si-enrichment of fossil material and a corresponding lack of Al, K and C, a chemical signature consistent with silicification, had been noted in the Burgess Shale arthropod Alalcomenaeus, suggesting that silicification may be a mode of preservation common to a diverse assemblage of fossils in Cambrian lagerstätten. Thus fossils of the Burgess Shale, and those of similar North American deposits, display a wide range of preservation modes (carbonization, phosphatization, pyritization, phyllosilicate molds, and silicification); no simple definition of “Burgess Shale-type” preservation is possible.