Burgess Shale - Type Preservation and Its Distribution in Space and Time

—Burgess Shale-type fossil assemblages provide a unique record of animal life in the immediate aftermath of the so-called “Cambrian explosion.” While most soft-bodied faunas in the rock record were conserved by mineral replication of soft tissues, Burgess Shale-type preservation involved the conservation of whole assemblages of soft-bodied animals as primary carbonaceous remains, often preserved in extraordinary anatomical detail. Burgess Shale-type preservation resulted from a combination of influences operating at both local and global scales that acted to drastically slow microbial degradation in the early burial environment, resulting in incomplete decomposition and the conservation of soft-bodied animals, many of which are otherwise unknown from the fossil record. While Burgess Shale-type fossil assemblages are primarily restricted to early and middle Cambrian strata (Series 2–3), their anomalous preservation is a pervasive phenomenon that occurs widely in mudstone successions deposited on multiple paleocontinents. Herein, circumstances that led to the preservation of Burgess Shale-type fossils in Cambrian strata worldwide are reviewed. A three-tiered rank classification of the more than 50 Burgess Shale-type deposits now known is proposed and is used to consider the hierarchy of controls that regulated the operation of Burgess Shale-type preservation in space and time, ultimately determining the total number of preserved taxa and the fidelity of preservation in each deposit. While Burgess Shale-type preservation is a unique taphonomic mode that ultimately was regulated by the influence of global seawater chemistry upon the early diagenetic environment, physical depositional (biostratinomic) controls are shown to have been critical in determining the total number of taxa preserved in fossil assemblages, and hence, in regulating many of the important differences among Burgess Shale-type deposits. 
 INTRODUCTION ! Burgess Shale-type fossil assemblages provide, by far, the best records of the development of complex life on Earth following the “Cambrian explosion.” Fossils from these deposits are the primary basis for understanding phylogenetic patterns of the Cambrian explosion, as well as patterns of morphological diversity and disparity of the Cambrian fauna (Conway Morris, 1989a; Wills et al., 1994; Budd and Jensen, 2000; Briggs and Fortey, 2005; Marshall, 2006; Erwin et al., 2011). Exceptionally preserved assemblages occur abundantly in early and middle Cambrian strata found worldwide (Conway Morris, 1989b; Allison and Briggs, 1993) and the Burgess Shale-type taphonomic pathway for the conservation of nearly complete soft-bodied fossil assemblages may have persisted into the early Ordovician (Van Roy et al., 2010). Burgess Shale-type preservation represents a unique and non-analogous taphonomic phenomenon that was widespread in Cambrian marine environments and largely disappeared from the marine rock record thereafter (Allison and Briggs, 1993; Butterfield, 1995). ! BURGESS SHALE-TYPE PRESERVATION AS A TAPHONOMIC MODE ! Preservation of soft-bodied fossils as carbonaceous remains Most instances of soft-tissue preservation in the fossil record involve the replication of softtissues by minerals precipitated in the early burial environment (Briggs, 2003), by means of reactions under microbial control. Conversely, Burgess Shale-type preservation (Butterfield, 2003) represents the conservation of primary organic tissues as thin (<1 μm) carbonaceous films (Fig. 1), a pathway that requires suppression of the processes that typically lead to the In: Reading and Writing of the Fossil Record: Preservational Pathways to Exceptional Fossilization. The Paleontological Society Papers, Volume 20, Marc Laflamme, James D. Schiffbauer, and Simon A. F. Darroch (eds.). The Paleontological Society Short Course, October 18, 2014. Copyright © 2014 The Paleontological Society. THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 degradation of soft tissues. This means of preservation was first conclusively identified by Butterfield (1990, 1995), who isolated organic elements of Burgess Shale fossils by maceration in HF. Contradictory findings were soon reported by Orr et al. (1998), who used in-situ analysis by electron microprobe to determine the elemental composition of two Burgess Shale arthropods. Elemental maps revealed that the fossils are comprised of templates of aluminosilicate minerals that, importantly, vary in composition among discrete anatomical aspects of fossils, with carbonaceous remains also present. On this basis, Orr et al. (1998) interpreted that the primary means of fossil preservation in the Burgess Shale was replacement of soft tissues by clay minerals shortly after burial. However, evidence for a late metamorphic origin of aluminosilicate coatings associated with Burgess Shale fossils was provided by Butterfield et al. (2007). Elemental mapping was used in combination with petrographic and textural (e.g. cross-cutting relationships) observations to demonstrate that replacement of Burgess Shale fossils by aluminosilicates occurred during greenschist facies metamorphism of the Burgess Shale (Powell, 2003), and not during early diagenesis; thus, aluminosilicification was not involved in the original preservation of the biota (Butterfield et al., 2007). This case was supported by observations of late-stage aluminosilicification of trilobite carapaces and of calcitic veins that crosscut the samples, as well as by the presence of metamorphically derived aluminosilicates as coatings on Carboniferous ferns (Butterfield et al., 2007) and Ordovician–Silurian graptolites (Page et al., 2008) that also were originally preserved as carbonaceous compressions. Tissue-specific variation in the elemental composition of aluminosilicate templates was verified as resulting from metamorphic, rather than early diagenetic processes (Page et al., 2008). These investigations r evea l ed tha t a luminos i l i c i f i ca t i on o f carbonaceous fossil remains is an expected product of metamorphism of mudstones. More importantly, they demonstrated that the original pathway of extraordinary preservation in the Burgess Shale was the conservation of carbonaceous organic remains. Burgess Shale-type preservation, originally