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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 films ('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-specific variations in their composition, purportedly reflecting 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 reflects stepwise maturation of their constituent kerogens in an evolving hydrothermal fluid, with different phyllosilicates 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 reflecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

The Jurassic marine biota of La Voulte-sur-Rhone, France, is characterized by three-dimensional preservation of soft-bodied animals and their internal organs. In contrast to other soft-bodied fossils, those from La Voulte are preserved in an unusual suite of minerals, dominated by apatite, calcite, gypsum, barite, and pyrite with accessory Cu, Pb, and Zn sulfides. These are thought to be sedimentary in origin. Fossilization occurred rapidly enough to be influenced by tissue composition and involved a diagenetic sequence: apatite [left arrow] calcite [+ or -] gypsum [left arrow] pyrite [+ or -] chalcopyrite [left arrow] galena. The La Voulte fossils reveal the role of apatite as a 'template' for calcification and pyritization in soft-tissue preservation.

It has been speculated that microbial mats are an important agent in the fossilization of soft tissues, particularly when apatite ([Ca.sub.5][[C[O.sub.3], P[O.sub.4]].sub.3][OH, F]) is involved. This has been tested by chemical analyses of the Jurassic limestones of Cerin, France, where phosphatized soft tissues are abundant and are associated with unequivocal microbial mats. The sedimentary distribution of P, K, and Fe following deposition was controlled by the presence of the mats. P concentrations in the mats may approach 2.5 times those elsewhere in the sediment. The highest P concentrations correlate with the occurrence of phosphatized soft tissues. This is the first analytical evidence to demonstrate a fundamental role for microbial mats in the preservation of soft-bodied fossils.

Correct interpretation of soft-bodied fossils relies on a thorough understanding of their taphonomy. While the focus has often been on the primary roles of decay and early diagenesis, the impacts of deeper burial and metamorphism on fossil preservation are less well understood. We document a sequence of late-stage mineral replacements in panarthropod fossils from the Sirius Passet Lagerstätte (North Greenland), an important early Cambrian Burgess Shale-type (BST) biota. Muscle and gut diverticula were initially stabilized by early diagenetic apatite, prior to being pervasively replaced by quartz and then subordinate chlorite, muscovite, and chloritoid during very low- to low-grade metamorphism. Each new mineral replicates the soft tissues with different precision and occurs in particular anatomical regions, imposing strong biases on the biological information retained. Muscovite and chloritoid largely obliterate the tissues' original detail, suggesting that aluminum-rich protoliths may have least potential for conserving mineralized soft tissues in metamorphism. Overall, the fossils exhibit a marked shift toward mineralogical equilibration with the matrix, obscuring primary taphonomic modes. Sequential replacement of the phosphatized soft tissues released phosphorus to form new accessory monazite (and apatite and xenotime), whose presence in other BST biotas might signal the prior, more widespread, occurrence of this primary mode of preservation. Our results provide critical context for interpreting the Sirius Passet biota and for identifying late-stage overprints in other biotas. [ABSTRACT FROM AUTHOR]