Ubiquitous Burgess Shale-style "clay templates" in low-grade metamorphic mudrocks

Despite the Burgess Shale's (British Columbia, Canada) paleobiologicalimportance, there is little consensus regarding its taphonomy.Its organic fossils are preserved as compressions associatedwith phyllosilicate films ("clay templates"). Debate focuseson whether these templates were fundamental in exceptional preservationor if they formed in metamorphism, meaning that it is importantto establish the timing of their formation relative to decay.An early diagenetic origin has been proposed based on anatomy-specificvariations in their composition, purportedly reflecting contrastsin decay. However, we demonstrate that these films bear a remarkablesimilarity to those that occur on organic fossils in graptoliticmudrocks and form as a normal product of low-grade metamorphism.Such phyllosilicates may also occur within voids created byvolume loss in maturation, a process that may have aided theirformation. In bedding-plane assemblages from graptolitic mudrocks,different taxa are associated with distinct phyllosilicates.This likely reflects stepwise maturation of their constituentkerogens in an evolving hydrothermal fluid, with different phyllosilicatesforming as each taxon progressively underwent maturation. Theseobservations provide an analogue for the distribution and compositionof phyllosilicates on Burgess Shale fossils, which we interpretas reflecting variations in the maturation of their constituenttissues. Thus, their clay templates seem unremarkable, formingtoo late to account for exceptional preservation.