Burgess Shale–type (BST) fossilization of carbonaceous remains that are ordinarily lost to decay is critical to our understanding of the early evolution of complex life. Sediment composition, particularly the abundance of certain clay minerals, has been invoked as a significant factor in BST fossilization. X-ray diffraction data for 213 Cambrian shales from 19 sedimentary successions on four continents provide the first comprehensive test of the association of clay mineral assemblages with BST fossils. Samples containing BST fossils yield mineralogical compositions that form a subset within the range represented by samples containing only fossil mineralized skeletons. Logistic regression and classification tree methods reveal that BST fossils are more likely to be found in sediments rich in berthierine/chamosite and poor in celadonite and illite. This characteristic clay mineralogy probably reflects a high kaolinite/smectite ratio in the original sediment and enhanced iron availability during early diagenesis. Models derived from both methods can predict the occurrence of BST fossils in fossiliferous samples based on clay mineralogy with ∼80% accuracy, providing a mineralogical signature that may be useful in refining the search for BST fossils on Earth and beyond.

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