Early Formation And Taphonomic Significance Of Kaolinite Associated With Burgess Shale Fossils

ment to understanding any environmental bias in these fossil assemblages and interpreting the Cambrian fossil record. The role of clay minerals is particularly controversial due to uncertainties over the timing of their formation. Variation in the abundance of clays on anatomical features of fossils from the Burgess Shale Formation may reflect original tissue chemistry, where different clays formed in association with or attached to different decaying tissues, facilitating their preservation (Orr et al., 1998). Alternatively, the clays may be a product of greenschist metamorphism that occurred millions of years after fossilization (Powell, 2003)The role of minerals in Burgess Shale?type fossilization is controversial, particularly that of the clay mineral kaolinite. Kaolinite may have formed on carcasses or attached to them as they decayed, stabilizing organic matter. Alternatively, kaolinite may have formed during metamorphism, playing no role in the preservation of soft tissues. Evaluating the formation and taphonomic role of kaolinite is difficult, because the mineralogy of Burgess Shale?type fossils is incompletely known. We used in situ selected-area X-ray diffraction to constrain the mineralogy of fossils from the classic Burgess Shale Formation in British Columbia, Canada. Fossils can be distinguished from the matrix that surrounds them by the presence of dolomite, kaolinite, and pyrite. Chlorite may be more abundant in the matrix. The preferential survival of kaolinite in association with fossils provides evidence of early diagenetic clay-organic interactions that protected the clay from metamorphic transformation. Kaolinite likely played a crucial role in fossilization, inhibiting the growth of heterotrophic bacteria and aiding polymerization of soft tissue biomolecules. This may result in biases in soft-tissue preservation to areas and times where kaolinite was prevalent.