Cambrian Burgess Shale–type deposits share a common mode of fossilization

Abstract

Although Cambrian Burgess Shale–type (BST) biotas are fundamental to understanding the radiation of metazoans, the nature of their extraordinary preservation remains controversial. There remains disagreement about the importance of the role of early mineral replication of soft tissues versus the conservation of primary organic remains. Most prior work focused on soft-bodied fossils from the two most important BST biotas, those of the Burgess Shale (Canada) and Maotianshan Shale (Chengjiang, China). Fossils from these two deposits do not provide ideal candidates for specimen-level taphonomic study because they have been altered: the Burgess Shale by greenschist facies metamorphism and the Maotianshan Shale by intensive subsurface weathering. Elemental mapping of soft-bodied fossils from 11 other BST deposits worldwide demonstrates that BST preservation represents a single major taphonomic pathway that may share a common cause wherever it occurs. The conservation of organic tissues, and not early authigenic mineralization, is the primary mechanism responsible for the preservation of BST assemblages. Early authigenic mineral replacement preserves certain anatomical features of some specimens, but the preservation of non-biomineralized BST fossils requires suppression of the processes that normally lead to the degradation of organic remains in marine environments.