Ubiquitous Burgess Shale–style “clay templates” in low-grade metamorphic mudrocks

Abstract

Despite the Burgess Shale’s (British Columbia, Canada) paleobiological importance, there is little consensus regarding its taphonomy. Its organic fossils are preserved as compressions associated with phyllosilicate fi lms (“clay templates”). Debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism, meaning that it is important to establish the timing of their formation relative to decay. An early diagenetic origin has been proposed based on anatomy-specifi c variations in their composition, purportedly refl ecting contrasts in decay. However, we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low-grade metamorphism. Such phyllosilicates may also occur within voids created by volume loss in maturation, a process that may have aided their formation. In bedding-plane assemblages from graptolitic mudrocks, different taxa are associated with distinct phyllosilicates. This likely refl ects stepwise maturation of their constituent kerogens in an evolving hydrothermal fl uid, with different hyllosilicates forming as each taxon progressively underwent maturation. These observations provide an analogue for the distribution and composition of phyllosilicates on Burgess Shale fossils, which we interpret as refl ecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.