Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): testing the roles of organic preservation and diagenetic alteration in exceptional preservation

One important, and poorly understood, issue concerning the taphonomy of nonmineralizing organisms in Cambrian Burgess Shale-type deposits is the diagenetic pathways by which organisms have become exceptionally preserved. Scanning electron microscopy (SEM) analyses of exceptionally preserved fossils from the lower Cambrian Chengjiang deposit of China demonstrate that nonmineralized tissue is preserved in a variety of ways, and further suggest that there was some taxonomic control over the precipitation of authigenic minerals during early diagenesis. Organic preservation is limited to certain decay-resistant structures such as heavily sclerotized spines on the claws of anomalocaridids. Labile organic tissues of nearly all nonmineralizing animals, however, are preserved by thin films of diagenetic minerals. Microstructural studies, energy-dispersive X-ray (EDX) analyses, and elemental mapping of these minerals indicate that mineral films include apatite, pyrite, and Fe-rich aluminosilicates. The results support the assumption that early diagenetic mineralization such as phosphatization and pyritization played a key role in the preservation of nonmineralized organisms. Pyritization seems to be the most important process by which nonmineralizing Chengjiang organisms are preserved in exceptional condition. Precipitation of Fe-rich aluminosilicates, which occurred following pyritization, also played a role in the preservation of some Chengjiang fossils. Phosphatization in the Chengjiang Lagerstätte was evidently rare, and limited taxonomically or to animals having large masses of organic material.