BURGESS -TYPE PRESERVATION AND ITS DISTRIBUTION IN! SPACE AND TIME ROBERT R.! GAINES Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711 USA

ABSTRACT.—Burgess Shale-type assemblages provide a unique record of in the immediate aftermath of the so-called “ explosion.” While most soft-bodied faunas in the rock record were conserved by mineral replication of soft tissues, -type preservation involved the conservation of whole assemblages of soft-bodied 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 . Herein, circumstances that led to the preservation of Burgess Shale-type 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 taphonomic phenomenon that was widespread in ! Cambrian marine environments and largely Burgess Shale-type fossil assemblages provide, by disappeared from the marine rock record far, the best records of the development of thereafter (Allison and Briggs, 1993; Butterfield, complex life on Earth following the “Cambrian 1995). explosion.” Fossils from these deposits are the ! primary basis for understanding phylogenetic BURGESS SHALE-TYPE PRESERVATION patterns of the , as well as AS A TAPHONOMIC MODE patterns of morphological diversity and disparity ! of the Cambrian fauna (Conway Morris, 1989a; Preservation of soft-bodied fossils as Wills et al., 1994; Budd and Jensen, 2000; Briggs carbonaceous remains and Fortey, 2005; Marshall, 2006; Erwin et al., Most instances of soft-tissue preservation in 2011). Exceptionally preserved assemblages occur the fossil record involve the replication of soft- abundantly in early and middle Cambrian strata tissues by minerals precipitated in the early burial found worldwide (Conway Morris, 1989b; Allison environment (Briggs, 2003), by means of and Briggs, 1993) and the Burgess Shale-type reactions under microbial control. Conversely, taphonomic pathway for the conservation of Burgess Shale-type preservation (Butterfield, nearly complete soft-bodied fossil assemblages 2003) represents the conservation of primary may have persisted into the early (Van organic tissues as thin (<1 µm) carbonaceous Roy et al., 2010). Burgess Shale-type preservation films (Fig. 1), a pathway that requires suppression represents a unique and non-analogous 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


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FIGURE 1.—Examples of Burgess Shale-type fossils from the middle Cambrian (Series 3) Burgess Shale (A–D) and early Cambrian (Series 2) (E–I) biotas. A) Bedding-plane assemblage from the , , including the soft-bodied Sydneyia inexpectans (S), Naroia compacta (N), Waptia fieldensis (W), and splendens (M), and serratus (O) with soft parts preserved in addition to mineralized carapace; ROM 57772. B, C) Soft-bodied fossils of the Marble Canyon assemblage (from Caron et al., 2014): B) spinifera (Arthropoda), ROM 62973; C) Burgessochaeta cf. setigera (Polychaeta), ROM 62972. D) Marrella splendens (Arthropoda), ROM 62969. E) illecebrosa (Arthropoda). F) Yunnanozoon lividum (Chordata?). (G) (). H) Luolishania longicruris (Lobopodia). I) Longtancunella chengjiangensis (Brachiopoda). Images A–D courtesy of Jean-Bernard Caron, Royal Ontario Museum, E–I courtesy of Hou Xianguang, University. degradation of soft tissues. This means of facies metamorphism of the Burgess Shale preservation was first conclusively identified by (Powell, 2003), and not during early ; Butterfield (1990, 1995), who isolated organic thus, aluminosilicification was not involved in the elements of Burgess Shale fossils by maceration original preservation of the biota (Butterfield et in HF. Contradictory findings were soon reported al., 2007). This case was supported by by Orr et al. (1998), who used in-situ analysis by observations of late-stage aluminosilicification of electron microprobe to determine the elemental trilobite carapaces and of calcitic veins that composition of two Burgess Shale arthropods. crosscut the samples, as well as by the presence of Elemental maps revealed that the fossils are metamorphically derived aluminosilicates as comprised of templates of aluminosilicate coatings on ferns (Butterfield et al., minerals that, importantly, vary in composition 2007) and Ordovician– graptolites (Page among discrete anatomical aspects of fossils, with et al., 2008) that also were originally preserved as carbonaceous remains also present. On this basis, carbonaceous compressions. Tissue-specific Orr et al. (1998) interpreted that the primary variation in the elemental composition of means of fossil preservation in the Burgess Shale aluminosilicate templates was verified as resulting was replacement of soft tissues by clay minerals from metamorphic, rather than early diagenetic shortly after burial. However, evidence for a late processes (Page et al., 2008). These investigations metamorphic origin of aluminosilicate coatings revealed that aluminosilicification of associated with Burgess Shale fossils was carbonaceous fossil remains is an expected provided by Butterfield et al. (2007). Elemental product of metamorphism of mudstones. More mapping was used in combination with importantly, they demonstrated that the original petrographic and textural (e.g. cross-cutting pathway of extraordinary preservation in the relationships) observations to demonstrate that Burgess Shale was the conservation of replacement of Burgess Shale fossils by carbonaceous organic remains. aluminosilicates occurred during greenschist Burgess Shale-type preservation, originally

"124 GAINES: BURGESS SHALE-TYPE PRESERVATION defined by Butterfield (1995, p. 1) as the or in the diet of these taxa may have been preservation of “carbonaceous compressions in important to phosphate mineralization fully marine ,” is now understood to be (Butterfield, 2002; Lerosey-Aubril et al., 2012). the primary mode of preservation in the great Phosphatization of gut anatomy is common within majority of Cambrian Lagerstätten found some taxa, e.g., Leanchoilia (Butterfield, 2002), worldwide (Gabbott et al., 2004; Hu, 2005; but it is not ubiquitous in any taxon. Gaines et al., 2008; Forchielli et al., 2014). Limited pyritization of aspects of soft-bodied Although carbonaceous preservation occurs fossils is most common in the Chengjiang deposit throughout the fossil record, later (Gabbott et al., 2004), but has been reported in marine examples typically include only selected rare examples from the Burgess Shale and refractory tissues (although see Liu et al., 2006; elsewhere (e.g., García-Bellido and Collins, von Bitter et al., 2007) such as those of algae, 2006). Precipitation of sedimentary pyrite during plants, and graptolite or cuticle early diagenesis occurs in anaerobic sediments as (Butterfield, 1995), and do not compare to a byproduct of microbial reduction of iron and Burgess Shale-type preservation in either sulfate during organic decomposition (Berner, anatomical detail or in faunal diversity that is 1984; Raiswell and Berner, 1986). During iron captured in carbonaceous remains. The prevalence reduction, Fe(III), derived largely from Fe-oxides of this taphonomic pathway in Cambrian strata and oxyhydroxides, is used as a terminal electron (Allison and Briggs, 1993) requires widespread, acceptor and is reduced to Fe(II), which is similar, favorable chemical conditions and liberated to solution where it may react with physical depositional environments in early sulfide compounds generated by microbial sulfate Phanerozoic continental seaways (see below). reduction to form Fe-sulfides (e.g., Canfield et al., ! 1992). Pyritization of soft tissues (see Farrell, Auxiliary mineralization of selected soft tissues 2014) is favored by low organic carbon content of Mineralization of selected tissues may occur sediments, which serves to localize microbial in association with carbonaceous remains of reactions around fossil carcasses (Briggs et al., Burgess Shale-type fossils. While mineral 1991, 1996; Raiswell et al., 2008; Farrell et al., replacement of soft tissues sometimes preserves 2013). While this is consistent with the sediments anatomical detail that would otherwise be lost, of Burgess Shale-type deposits, pyritization in mineralization is auxiliary to the primary these deposits was limited in its extent due to 2- carbonaceous taphonomic mode at the scale of strongly reduced availability of SO4 in the early individual fossils. The most important pathways burial environment, as indicated by δ34S of of selective soft tissue mineralization in Burgess sedimentary pyrite (Gaines et al., 2012c), Shale-type fossils are replacement by calcium described below. As a result, when pyrite is phosphate (see Schiffbauer et al., 2014) and by present in Burgess Shale-type fossils, it is pyrite (see Farrell, 2014). frequently concentrated around soft tissues that Phosphatization is primarily restricted to guts, are especially labile (Gabbott et al., 2004) and in particular those of arthropods, in which served as nuclei for early microbial activity. digestive glands are sometimes preserved Pyritization of aspects of soft-bodied fossils is (Butterfield, 2002). Precipitation of phosphate most common in the Chengjiang deposit (Gabbott minerals in these fossils is problematic. While the et al., 2004). However, the extent of early gut tracts of particular animals provided chemical pyritization in Chengjiang fossils has proven microenvironments that favored the precipitation difficult to assess due to extensive weathering of phosphate (e.g., Butterfield, 2002; Lerosey- under a humid, monsoonal climate, which has Aubril et al., 2012), mass-balance considerations substantially altered the primary composition of imply the necessity of an additional source of P the host rocks and the Chengjiang fossils to a from outside the gut. This source is difficult to depth of ~20 m (Gaines et al., 2012c; Ma et al., account for in the typically organic-poor 2012; Forchielli et al., 2014). A comprehensive sediments in which Burgess Shale-type analysis of the effects of weathering on the preservation occurs (Gaines et al., 2012c). This Chengjiang fossils was undertaken by Forchielli auxiliary pathway, however, exhibits a strong et al. (2014), who characterized the elemental taxonomic selectivity and occurs overall in composition of fossils and the mineralogic and relatively few genera, suggesting that originally chemical composition of the host mudstones high concentrations of phosphate in the gut glands across a weathering gradient. The results of this

"125 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 study confirmed earlier suggestions (Hu, 2005; generally been used in the literature to refer to the Forchielli et al., 2012) that pyritization in global soft-bodied fauna of the Cambrian, Chengjiang fossils was limited to discrete including members of which that are represented anatomical aspects, and verified the dominance of in younger Lagerstätten (e.g., Conway Morris, carbonaceous preservation in unweathered 1989b). While Burgess Shale-type faunas are specimens. With weathering, Chengjiang fossils overwhelmingly represented by Burgess Shale- were shown to progressively lose carbon and to type preservation, a specific mode of preservation acquire coatings of Fe-oxides derived from the is not implied by this term, which is herein oxidation of pyrite in the enclosing mudstones (as avoided for clarity. evidenced by the loss of S) and resulting from the ! redistribution of Fe along voids and cracks, BURGESS SHALE-TYPE DEPOSITS: A including those associated with Burgess Shale- GLOBAL EARLY PHANEROZOIC type fossils (Forchielli et al., 2014). Thus, the PHENOMENON characteristic reddish aspect of Chengjiang fossils ! (Fig. 1E–I) resulted from alteration accompanying Burgess Shale-type deposits: definition weathering rather than from a different In this paper, the term ‘Burgess Shale-type taphonomic pathway, such as the growth of thin deposit’ is used to refer to bodies of sedimentary pyrite envelopes around carcasses. In the rock that contain exceptional biotas preserved via unweathered condition, Chengjiang fossils closely Burgess Shale-type preservation as defined above. resemble the appearance of those from the While the Burgess Shale biota occurs globally Burgess Shale and elsewhere (Forchielli et al., (Conway Morris, 1989b), the original taphonomic 2014). These findings underscore the importance mode of exceptional biotas at a few localities is of fresh, unaltered fossil material for the accurate p r e s e n t l y u n c e r t a i n d u e t o e x t e n s i v e assessment of taphonomic mode in exceptionally metamorphism and/or weathering, which has preserved fossils. substantially altered the primary composition of ! soft-bodied fossils. These include most Burgess Shale-type preservation: definition prominently the early Cambrian Although other auxiliary modes of (Briggs and Nedin, 1997; Lee et al., 2011) and preservation may occur in association within (Budd, 2011) deposits. While the fossils primarily preserved via Burgess Shale-type overall mode of preservation as two-dimensional preservation, these modes occur rarely, and compression fossils is superficially similar, early predominantly in specific anatomical attributes of and/or late diagenetic mineralization is also a minority of taxa (Butterfield, 2002; Gabbott et particularly prominent in these two deposits al., 2004; García-Bellido and Collins, 2006; (Briggs and Nedin, 1997; Budd, 2011; Paterson et Lerosey-Aubril et al., 2012). A small number of al., 2011), and the degree to which Burgess Shale- deposits dominated by Burgess Shale-type type preservation was involved in fossilization, if preservation also contain examples of soft-bodied at all, is unclear. Therefore, the Emu Bay and preservation within concretions (Schwimmer and Sirius Passet deposits are explicitly excluded from Montante, 2007; Van Roy and Briggs, 2011; considerations of the circumstances of Burgess Gaines et al., 2012a), a wholly different means of Shale-type preservation that follow here, as are all preservation (see McCoy, 2014). However, other deposits for which the mode of original concretions and other modes of exceptional preservation remains ambiguous, such as the late preservation do not occur in the same beds in Cambrian (Lerosey-Aubril et which Burgess Shale-type preservation occurs. al., 2013). Here, the term “Burgess Shale-type preservation” A few prominent deposits of age is used to define occurrences of exceptional also contain algal and problematic fossils preservation of soft-bodied fossils as primary preserved in whole (Zhu et al., 2008; Xiao et al., carbonaceous remains as originally defined by 2013; Wang et al., 2014) or in part (Cai et al., Butterfield (1995, 2003). This mode of 2012) as carbonaceous compressions, and older preservation was widespread in early and middle examples also occur (Butterfield, Cambrian epicratonic seaways, and often occurs 1995; Gaines et al., 2008). At present, the pervasively across large stratigraphic intervals depositional and early diagenetic settings of these (10s of meters) in many of the deposits in which it deposits have not yet been comprehensively occurs. The term “Burgess Shale-type fauna” has investigated, and therefore, these

"126 GAINES: BURGESS SHALE-TYPE PRESERVATION examples of carbonaceous preservation are also et al., 2010, 2014). excluded from discussion herein. It should be The most prominent feature of the Allison and noted, however, that investigation of these Briggs (1993) compilation was the sharp decline deposits represents a promising avenue for further in exceptional preservation following the middle research. Cambrian, inferred to signify the closure of the early Phanerozoic taphonomic window. However, Distribution of Burgess Shale-type deposits in the discovery of Burgess Shale-type fossils within the Early Phanerozoic a small number deposits of late Cambrian (Series It has long been recognized that Cambrian 4) age (Vaccari et al., 2004; García-Bellido and strata are unusually enriched in soft-bodied biotas, Aceñolaza, 2011) suggests that the Burgess Shale- even when rock volume is taken into account type window remained open while facies (Allison and Briggs, 1993). This apparent favorable to preservation exhibit a sharp decline “taphonomic window” has been the subject of i n t h e s e d i m e n t a r y r e c o r d s o f m o s t much interest over the last two decades (Aronson, paleocontinents (Gaines et al., 2013). 1993; Allison and Briggs, 1993, 1994; Butterfield, The early Ordovician Fezouata biota of 1995, 2003; Brasier and Lindsay, 2001; Orr et al., Morocco (Van Roy et al., 2010) is perhaps the 2003; Gaines et al., 2005, 2012b,c). Although most important new soft-bodied assemblage to be several modes of exceptional preservation occur reported from early Phanerozoic strata in recent in early Phanerozoic strata, including most decades. Both Burgess Shale taxa and those prominently Orsten-type preservation (Walossek typical of the later Ordovician are represented in and Müller, 1998; Zhang et al., 2007b) as well as its abundant and diverse faunas. Fezouata soft- Ediacaran-like preservation (Hagadorn et al., bodied fossils are characteristically strongly 2002; Alessandrello and Bracchi, 2003), the weathered (Van Roy et al., 2010), and although signal of the taphonomic window is, by far, the primary taphonomic pathway remains unclear, dominated by Burgess Shale-type deposits the mode of preservation of Fezouata fossils (Conway Morris, 1989b; Allison and Briggs, superficially appears to be consistent with 1993). The underlying causes of this greatly Burgess Shale-type preservation. If the Fezouata enhanced Cambrian fossil record and its deposit indeed is a Burgess Shale-type deposit, limitation in time and space may be determined then this pathway must have persisted across the only through a process-based understanding of Cambrian and into the early Ordovician. This Burgess Shale-type preservation. possibility is taken into account in the discussion The number of Burgess Shale-type deposits that follows. presently known from Cambrian strata is difficult ! to estimate, but certainly exceeds 50 deposits, Were Burgess Shale-type biotas ‘burrowed primarily of early and middle Cambrian (Series away?’ 2–3) age. Conway Morris (1989b) provided a In their 1993 analysis of trends in exceptional compilation of Burgess Shale-type deposits, but preservation across the Phanerozoic, Allison and intensive research in recent decades has resulted Briggs advanced the hypothesis that exceptionally in the discovery of dozens of new localities preserved biotas were ‘burrowed away’ from the (Lieberman, 2003; Steiner et al., 2005; Caron et post-Cambrian sedimentary record (Allison and al., 2010, 2014; Kimmig and Pratt, 2013), and a Briggs, 1993; Orr et al., 2003) as a result of more recent compilation has not yet been increasing mixing by infaunal organisms provided. In such a future compilation, it would (Droser and Bottjer, 1988, 1989). Aronson (1993) be useful to distinguish Burgess Shale-type modeled the increases in depth and extent of preservation from other modes of preservation that would be required to account for using data on taphonomic mode now available for the decline in exceptional preservation, and many of the deposits. It would also be useful to considered such increases to be far larger than tabulate by formation as well as by individual those observed across the early . This locality within each formation (Conway Morris, finding is in agreement with detailed ichnologic 1989b), owing to geographically widespread observations from Cambrian strata (Tarhan and exceptional preservation that occurs at many Droser, 2014). sampled localities within many of the individual Other work has suggested that the settings of formations (e.g., Fletcher and Collins, 1998, Burgess Shale-type deposits likely rendered large 2003; Hu, 2005; Gaines and Droser, 2010; Caron portions of the deposits invulnerable to


TABLE 1.—A proposed rank classification of Burgess Shale-type deposits.

Total Number of Fidelity of Soft-Bodied Abundance of Soft- Examples Soft-Bodied taxa Preservation Bodied Fossils

> 100 taxa High: eyes, guts, limbs, etc. Burgess, Tier 1 High Large % Endemic common Chengjiang

Intermediate: carapace and < 100 taxa Intermediate: Guanshan, Kaili, Tier 2 algal/bacterial fossils Small % Endemic (Range: Low–High) Marjum, Spence dominant Low: carapace, cuticle, Latham, Indian Tier 3 < 10 taxa Low algal/bacterial fossils only Springs, Zawiszyn bioturbation. Paleoenvironmental data from bodied taxa, or taxonomic richness, is the critical Burgess Shale-type deposits has confirmed that feature that determines each deposit’s contribution Burgess Shale-type preservation is strongly to understanding phylogenetic patterns and favored under anoxic benthic conditions, which morphological disparity of the Cambrian biota exclude the possibility of bioturbation (Gaines (Conway Morris, 1989a; Briggs et al., 1992; Budd and Droser, 2010). While bioturbation clearly and Jensen, 2000; Marshall, 2006; Erwin et al., limited the extent of Burgess Shale-type 2011), total number of soft-bodied taxa is the preservation in Cambrian strata (Allison and most important criterion for classifying the Brett, 1995), a post-Cambrian increase in deposits. Because the total number of taxa bioturbation would not have affected preservation preserved in Burgess Shale-type fossil potential in the two most important deposits. assemblages is often correlated with the fidelity of Because the fossil-bearing intervals of the preservation, especially of labile (non-cuticle) Chengjiang and the Walcott Quarry member of tissues, classification of the deposits into three the Burgess Shale were deposited in whole or in tiers is straightforward, and provides useful large part under sustained anoxic conditions, insight into important differences among the which are not susceptible to sediment mixing by deposits, as discussed below. It must be bioturbation, preservation potential in comparable emphasized, however, that this classification is settings later in the Phanerozoic would not have based on total sum of soft-bodied taxa presently been affected (Gaines et al., 2012b, references known from each of the deposits and does not therein). These results imply that other controls attempt to account for rock volume, sampling were responsible for limiting Burgess Shale-type intensity, or collection bias. Especially given that preservation and its distribution in time. collection efforts have varied widely among the ! deposits, any deposit has the potential to rise from A proposed rank classification of Burgess one tier to the next with increased collection or Shale-type deposits the discovery of new localities. It is also Burgess Shale-type deposits share a common important to note that within individual deposits, mode of preservation, yet they are not equally fossil density and fidelity of preservation vary informative. As has been long recognized widely by stratigraphic horizon as well as by (Conway Morris 1989a, b; Hagadorn, 2002), the locality, for reasons considered below. This deposits encompass a broad spectrum of simplistic classification scheme is therefore abundance, number of preserved taxa, and quality intended only to provide a framework for of soft-bodied preservation, yet there has been no considering the prominent first- differences formal attempt to rank or classify the deposits. among the deposits. For the purposes of this contribution, I propose a Tier 1 deposits.—These include only the three-tier classification scheme to group Burgess Burgess Shale and Chengjiang, which are Shale-type deposits according to their overall distinguished by an exceptionally high taxonomic paleontological importance (Table 1). richness (>100 taxa) of soft-bodied fossils that Because the total number of known soft- clearly separates them from other Burgess Shale-

"128 GAINES: BURGESS SHALE-TYPE PRESERVATION type deposits (Briggs et al., 1994; Hou et al., 2010; Sun et al., 2013), and the Mount Cap 2008). The high taxonomic richness captured in (Series 2), Pioche (Series 2–3), Spence (Series 3), these two deposits is presumably due in part to the Wheeler (Series 3), and Marjum (Series 3) consistently high fidelity of preservation, found in deposits of Laurentia (Robison, 1991; Butterfield, many intervals, of labile anatomical features (e.g., 1994; Lieberman, 2003; Briggs et al., 2008). eyes, , neural tissue), which are comprised of Preservation of anatomical detail in the tissues most readily lost to decomposition in is typically subsidiary to that found in Tier 1 the early burial environment (Fig. 1; Briggs et al., deposits, and preservation is most often restricted 1994; Hou et al., 2008; Sansom et al., 2010; to relatively robust anatomical tissues such as Caron et al., 2014; Ma et al., 2012, 2014). and cuticle, and algae, although Therefore, the preservation potential for important exceptions to this generalization do organisms lacking robust organic cuticle was occur in Tier 2 deposits (Zhao et al., 2005; Briggs optimized in these two deposits, and the potential et al., 2008). Maximum taxonomic richness of for preservation bias among taxa was minimized. individual beds is low compared to the richest Preservation of soft-tissue in taxa that possessed beds of the Tier 1 deposits (Caron and Jackson, mineralized skeletons, most prominently 2006, 2007; Zhao et al., 2009), and abundance of and , is common in the Tier 1 soft-bodied fossils in individual beds ranges from assemblages, but occurs only rarely in subordinate low to high; possible influences on this pattern are deposits. considered below. Richness of the assemblages is also related to Tier 3 deposits—Tier 3 deposits are the density of fossils in each deposit. Absolute distinguished by low richness (<10 taxa) of soft- abundance of soft-bodied fossils per unit volume bodied faunas, which characteristically are known of rock is far greater in the Walcott Quarry and only from isolated occurrences of individual soft- Marble Canyon assemblages of the Burgess Shale bodied fossils, typically represented by arthropod than at any Chengjiang locality (Caron and and worm cuticle or algae. Examples of Tier 3 Jackson, 2006, 2008; Zhao et al., 2009; Caron et deposits include the Kinzers, Parker, Latham, and al., 2014). However, Burgess Shale fossil Indian Springs deposits of Laurentia (Series 2; assemblages occur most prominently in Conway Morris, 1989b; Gaines and Droser, 2002; concentrations that are localized at the front of the Skinner, 2005; English and Babcock, 2010) and Cathedral escarpment (see below; Aitken, 1971, the Zawiszyn deposit of Baltica (Series 2; 1997; Collins et al., 1983; Fletcher and Collins, Conway Morris, 1989b). Burgess Shale-type 1998, 2003; Caron et al., 2014). By comparison, preservation is quite rare in most Tier 3 deposits, fossil assemblages at Chengjiang are considerably suggesting that conditions favorable to less abundant, with the exception of ‘cluster’ fossilization were present only infrequently assemblages dominated by single taxa (Hu, 2005; during their accumulation, and that the number of Zhao et al., 2009). Furthermore, Chengjiang soft-bodied taxa is unlikely to rise substantially fossils are found across a much larger geographic with future collection (e.g., Latham, Indian area in relatively continuous outcrop (Hu, 2005). Springs). In other deposits, however, the total Fossils occur in soft, weathered rock that is easily number of soft-bodied taxa appears to be limited worked by hand tools, and has been intensively by exposure (e.g., Kinzers, Zawiszyn), and could collected by a number of research groups since its rise with future exploration. discovery in 1984 (Hou et al., 2008), resulting in ! an exceptionally high number of soft-bodied taxa, THE TAPHONOMIC PATHWAY FOR although sampling biases are prominent (Zhao et BURGESS SHALE-TYPE PRESERVATION al., 2012). ! Ti e r 2 d e p o s i t s . — T h e s e d e p o s i t s , History of research characterized by an intermediate richness of soft- Rapid burial and anoxia.—It has long been bodied taxa (10–100 taxa), include the great recognized that rapid burial is an important majority of reported Burgess Shale-type deposits, limiting requirement for exceptional preservation most prominently including the Kaili (Series 2–3), in Burgess Shale-type deposits and elsewhere Guanshan (Series 2), Nuititang (Series 1–2), (e.g., Whittington, 1971; Piper, 1972; Conway Balang (Series 2), and Tsinghsutung (Series 2) Morris, 1986), and event-driven sedimentation of deposits of South (Peng et al., 2005; fossil-bearing horizons has been confirmed for all Steiner et al., 2005; Zhao et al., 2005; Hu et al., deposits that have been investigated to date (Zhu

"129 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 et al., 2001; Caron and Jackson, 2006; Gabbott et insufficient prerequisite for Burgess Shale-type al., 2008; Zhao et al., 2009; Gaines et al., 2011, preservation (Allison, 1988; Butterfield, 1990; 2012c; Caron et al., 2014). Because most Burgess Gaines and Droser, 2010), as is rapid burial. The Shale-type deposits are characterized by preservation of Burgess Shale-type assemblages exclusively fine-grained sediments, event-driven worldwide was facilitated in part by widespread deposition must be established using sediment anaerobic conditions in Cambrian epicontinental microfabric and lateral facies associations (Gaines seas (e.g., et al., 2011), but this condition is et al., 2011, 2012c). Prominent grading in beds not unique to the early Paleozoic rock record, and bearing Burgess Shale-type fossils occurs therefore it cannot account for the taphonomic consistently at Chengjiang, where partial turbidite window. sequences are recognized (Zhu et al., 2001; Hu, Preservation of Burgess Shale-type fossils as 2005), as well as in other subordinate deposits carbonaceous remains necessitates either the (e.g., Gaines and Droser, 2002). Exceptionally special protection of organic material from the preserved fossils typically occur within discrete normal processes of microbial decomposition or event beds, rather than at bed junctions, as would the large-scale suppression of those processes be expected if smothered in situ on the seafloor within sediments of the early burial environment (Gaines and Droser, 2010; Gaines et al., 2012c). (Gaines et al., 2008). Although examples of in-situ burial at bed The clay mineral hypothesis.—The first junctions are present in some deposits (e.g., hypothesis to account for Burgess Shale-type Gaines and Droser, 2005), such cases appear to be preservation was offered by Butterfield (1995), rare. who proposed that preservation occurred via clay Early research on the preservation of the mineral-organic interactions. Recognizing that Burgess Shale biota emphasized the role of some highly reactive clay mineral have benthic anoxia in slowing the decomposition of the capacity to limit microbial activity by direct fossils in the early burial environment (e.g., adsorption of enzymes involved in organic Conway Morris, 1986). This conclusion requires decomposition, Butterfield proposed that that fossil assemblages were in large part unusually reactive clay minerals, such as Fe or transported across a chemocline that separated Mg-rich smectites, were common in Cambrian habitable benthic environments from adjacent sedimentary environments. This possibility was anoxic environments, characterized by Conway discounted by Powell (2003), who used whole- Morris as the ‘pre-slide’ and ‘post-slide’ rock geochemistry to determine that the primary environments (Conway Morris, 1986). This clay mineral assemblage present in the Burgess interpretation is supported by the orientations of Shale was not unusual, as has also been confirmed fossils relative to bedding within event-deposited for the Chengjiang (Forchielli et al., 2014) and beds (Whittington 1971; Zhang et al., 2006; several Tier 2 deposits (Curtin and Gaines, 2011). Zhang and Hou, 2007; Caron et al., 2014). Experimental work (Wilson, 2006) has suggested Experimental work has confirmed that freshly a possible role for the clay mineral kaolinite in killed soft-bodied arthropods can withstand such polymerization of some organic tissues, transport without fragmentation (Allison, 1986), increasing their resistance to decay. However, which is in line with taphonomic evidence (Caron geochemical studies of Burgess Shale-type and Jackson, 2006; Zhang et al., 2006; Zhang and sediments show that kaolinite comprised only a Hou, 2007). small fraction of the primary clay mineral While rapid burial in close proximity to a assemblage, if kaolinite was present at all sharp chemocline has been demonstrated for Tier (Powell, 2003; Curtin and Gaines, 2011, 1 deposits and many of the Tier 2 deposits Forchielli et al., 2014). (Allison and Brett, 1995; Gaines and Droser, The Fe-adsorption hypothesis.—Another 2003, 2005, 2010; Gostlin, 2006; Gabbott et al., hypothesis for protection of organic fossils from 2008; Gaines et al., 2012b), experimental data and the normal processes of microbial decay was observations from modern settings indicate that suggested by Petrovich (2001), who proposed that anoxia alone is insufficient to cause Burgess adsorption of Fe(II) onto and other Shale-type preservation (Henrichs and Reeburgh, biopolymers led to degradation resistance. In this 1987; Allison, 1988; Butterfield, 1990, 1995; Lee, view, fossils were physically armored against the 1992). Therefore, anoxia in the early benthic action of microbial enzymes by coatings of Fe(II), environment is considered a necessary but suggested to have adsorbed selectively onto fossil

"130 GAINES: BURGESS SHALE-TYPE PRESERVATION tissues from solution in the anoxic early burial carbonate ramp settings that accumulated on low- environment. Subsequent elemental mapping of angle slopes (Rees, 1986; Liddell et al., 1997; fossils from a number of deposits, however, has Elrick and Snider, 2002; Brett et al., 2009; excluded a systematic association of Fe with Halgedahl et al., 2009; Gaines et al., 2011). organic remains in unweathered fossil material Notably, both the Walcott Quarry and Marble (Hu, 2005; Gaines et al., 2008; Forchielli et al., Canyon localities of the Burgess Shale and the 2014). While secondary Fe-oxide coatings multiple localities of the Chengjiang deposit resulting from oxidative weathering of represent exceptions to this pattern. The Burgess sedimentary pyrite are commonly associated with Shale was deposited at the edge of a carbonate weathered specimens from a majority of the platform; however, it occurs in direct association deposits, it has been demonstrated that Fe was not with the Cathedral Escarpment, a prominent break involved in Burgess Shale-type preservation in submarine topography that provided a steep except through limited pyritization of discrete local slope (Aitken, 1971, 1997; Fletcher and anatomical aspects of some specimens Collins, 1998, 2003; Caron et al., 2014). A (Butterfield, 2009; Forchielli et al., 2014). minority of Burgess Shale-type deposits, Inhibition of decay by early sealing of the including the Chengjiang, are not associated with burial environment.—Preservation by early a carbonate platform, but instead lie offshore of suppression of microbial activity in the sediments broad clastic shelves (Zhu et al., 2001; Gaines and was hypothesized by Gaines et al. (2005), and Droser, 2002). subsequently supported by geochemical and Most Burgess Shale-type deposits occur >100 petrographic data from a broad survey of Burgess km from the paleo-margin of continental crust, Shale-type deposits, including both Tier 1 and the inland nature of the connection between deposits and six of the most important Tier 2 these environments and the global ocean is deposits (Gaines et al., 2012c). Preservation as unclear. Many prominent deposits, such as the carbonaceous remains was determined to have Chengjiang (Zhu et al., 2001; Hu, 2005) and the resulted from a three-step process that involved: Wheeler and Marjum deposits (Rees, 1986), are 1) limited transportation of fossils and rapid burial interpreted to have accumulated in actively in exclusively fine-grained, clay-rich sediments; subsiding local basins with complex regional 2) low concentrations of sulfate in the global submarine topography. In these settings, as well ocean and low concentrations of oxygen at the as those of epicratonic seas more broadly, sites of burial, which limited the pace of microbial restricted circulation with the open ocean may degradation; and 3) early sealing of the burial have been an important factor in promoting and environment by pervasive carbonate cements at sustaining anoxia in deeper water masses (e.g., bed tops, which further restricted the diffusion of Peters, 2009). Early Paleozoic epicratonic seas o x i d a n t s r e q u i r e d t o s u s t a i n o rg a n i c may have been further vulnerable to restricted decomposition and ultimately resulted in the circulation by means of thermal stratification due conservation of fossils as primary organic remains to the low latitude configuration of landmasses (Gaines et al., 2012c). These steps are described (Brasier and Lindsay, 2001) and the Phanerozoic in detail below. Important differences in maximum in pCO2 that spans this interval (Berner, taxonomic richness and in the fidelity of 2006). preservation among the Burgess Shale-type biotas The frequency and stratigraphic extent of found globally are shown to result primarily from Burgess Shale-type preservation varies widely variations in the conditions surrounding steps 1 among the deposits and among localities within and 2. them. In some cases, Burgess Shale-type ! preservation occurs consistently across meters of Physical depositional settings of Burgess Shale- continuous section, while in others, preservation type deposits is confined to narrow (mm-scale) stratigraphic Burgess Shale-type deposits occur with intervals (Gaines and Droser, 2010). The specific greatest frequency in outer-shelf environments stratigraphic intervals that contain Burgess Shale- that lay near the seaward margins of expansive type preservation worldwide share two carbonate platforms, including the “outer detrital fundamental aspects of the physical depositional belt” of Laurentia (Robison, 1960), and environment: 1) accumulation below (or near) comparable settings of other paleocontinents. maximum Storm Wave Base (SWB); and 2) rapid, These deposits represent mixed siliciclastic- event-driven deposition of a clay-dominated size

"131 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 fraction from bottom-flowing density currents frequency oscillation between anoxic and dysoxic (Zhu et al., 2001; Gabbott et al., 2008; Gaines and conditions (Gaines and Droser, 2010, Gaines et Droser, 2010; Gaines et al., 2012c). Depositional al., 2012b). The use of geochemical proxies to regimes that prevailed at each setting during delineate benthic redox conditions in Burgess deposition of Burgess Shale-type intervals Shale-type deposits is complicated by several permitted nearly 100% accumulation in the near factors. Because most geochemical redox proxies (Chengjiang) or complete absence (most others) are based upon the concentrations of redox- of scour or reworking (Zhu et al., 2001, Hu, 2005; sensitive elements delivered from an anoxic water Gaines and Droser, 2005, 2010; Zhao et al., 2009; column to the sediments (e.g., Poulton and Gaines et al., 2012c). Within this broadly similar Canfield, 2005; Tribovillard et al., 2006), these depositional context, significant variation in the proxies are most effective when sedimentation is physical magnitude of individual depositional slow and relatively continuous, and sediments are events is prominent in the Burgess Shale and in overlain by a thick anoxic water mass. Neither is the Chengjiang. Tier 2 and 3 Burgess Shale-type the case for Burgess Shale-type deposits. Event- deposits are comprised of event-deposited driven sedimentation, which characterizes all claystone laminae that are millimeters in Burgess Shale-type deposits (see above), acted to thickness (Gaines and Droser, 2002, 2010; Gaines dilute any signal of Fe or trace-element et al., 2011, 2012c). By comparison, the thickness enrichment (Hammarlund, 2007), as also of individual event-deposited claystone beds observed in other settings (e.g., Farrell et al., ranges up to 8 cm in thickness in the Burgess 2013). Furthermore, the prevalence of near- Shale (Gabbott et al., 2008; Caron et al., 2014), bottom anoxia in Burgess Shale-type deposits, as and may exceed 10 cm at Chengjiang, where in a interpreted from paleontological, ichnologic, and minority of cases, the D–E portion of the turbidite taphonomic evidence (Gaines and Droser, 2005, succession is present (Zhu et al., 2001; Zhao et 2010; Garson et al., 2012), indicates that the sites al., 2009). As all evidence suggests, if physical of deposition were not overlain by thick anoxic depositional energy may be reliably assumed to water masses. This interpretation suggests that Fe correspond to bed thickness, then the Tier 1 and trace-element enrichments should not be deposits may be clearly distinguished from all expected (Gaines and Droser, 2010). subordinate deposits by the presence of elevated Despite the lack of a clear geochemical proxy depositional energy. Thus, the efficiency of for reconstructing paleoredox environments of transportation of fossil assemblages appears to Burgess Shale-type deposits, the presence, depth have been the critical factor that set Tier 1 and intensity of bioturbation can provide a useful Burgess Shale-type assemblages apart from the means of assessing relative oxygen concentrations other deposits. Similar evidence for enhanced within the deposits (Gaines and Droser, 2005, physical energy is also present in the early 2010; Gaines et al., 2012b). Burgess Shale-type Ordovician Fezouata deposit, although its status deposits all contain intervals that are not as a Burgess Shale-type deposit is unclear. bioturbated as well as those in which shallow It must also be noted that the physical (<10 mm), low-intensity bioturbation is prevalent depositional conditions shared among Burgess (Allison and Brett, 1995; Gaines and Droser, Shale-type intervals of the deposits are 2003, 2005, 2010; Dornbos et al., 2005; Gostlin, widespread in the Phanerozoic rock record. 2006; Gabbott et al., 2008; Gaines et al., 2011, Although Burgess Shale-type preservation is 2012b, Mángano, 2011; Garson et al., 2012; restricted to specific depositional settings that Minter et al., 2012). These two types of settings promoted limited transport and rapid burial of occur in greatly different proportions among and soft-bodied fossils in claystone sediments, other within Burgess Shale-type deposits, and factors were ultimately responsible for frequently occur interbedded in repeated, close exceptional preservation. (mm-scale) association. ! While the co-occurrence of bioturbation and Paleo-redox settings of Burgess Shale-type Burgess Shale-type preservation in individual deposits beds is known from numerous examples (Wang et The benthic settings represented in Burgess al., 2004; Zhang et al., 2007a; Gaines and Droser, Shale-type deposits are characterized by oxygen 2010; Lin et al., 2010; Gaines, 2011; Mángano, deficiency, and range from sustained anoxia to 2011; Gaines et al., 2012b, Garson et al., 2012), it deposition near a fluctuating oxycline with high- is clear that Burgess Shale-type preservation is

"132 GAINES: BURGESS SHALE-TYPE PRESERVATION ! ! strongly favored in the absence of bioturbation, Shale-type preservation. and that the quality of Burgess Shale-type ! preservation is often diminished in the presence of Preservation of organic fossils by sealing of the bioturbation (e.g., Lin et al., 2010). Because trace early burial environment fossils are well preserved in sedimentary facies Fossil-bearing mudstones of Burgess Shale- that are otherwise identical to those that lack type deposits worldwide are characterized by bioturbation, the absence of bioturbation from prominent early diagenetic carbonate cements that critical intervals of Burgess Shale-type deposits are concentrated at bed tops and penetrate cannot be ascribed to preservational bias. Instead, downwards (Fig. 2; Gaines et al., 2005, 2012c, d). this pattern reflects the general exclusion of Although bed-capping cements are rapidly lost to benthic fauna from the intervals of each deposit weathering, analysis of fresh, unaltered material that lack bioturbation entirely. By analog with the has revealed that these features are pervasive in modern (e.g., Savrda et al., 1984; Savrda and each of the deposits studied to date (Gaines et al., Bottjer, 1986), these intervals, which include the 2012c). Textural data show that carbonate growth critical fossil-bearing portions of the Walcott was early, and resulted in displacement of clay Quarry Member of the Burgess Shale (Gostlin, particles prior to compaction or significant 2006; Gabbott et al., 2008) and the Chengjiang overpressure. The presence of bed-capping (Dornbos et al., 2005; Hu, 2005; Gaines et al., cements in synsedimentary soft-sediment 2012b), are interpreted to have accumulated under deformation features (seafloor slumping) anoxic benthic conditions (but see Powell et al., indicates that cements were emplaced at or near 2003; Caron and Jackson, 2006). the sediment-water interface (Gaines et al., Thus, it is clear that Burgess Shale-type 2012c). Carbon isotope values of the cements lie preservation is most strongly favored under within the range of seawater values, indicating anoxic bottom waters, but may occur in close that cements precipitated directly from seawater association with weakly oxygenated benthic (Gaines et al., 2012c). environments. These findings have validated The precipitation of seafloor cements earlier models of transportation of Burgess Shale- following burial had a profound influence on the type fossil assemblages across the chemocline and processes of microbial decomposition in the burial under anoxic conditions (e.g., Conway sediments. Sulfur isotope data from sedimentary Morris 1986), as also supported by the pyrite reveal that sulfate reduction, the primary orientations of individual fossils within event- pathway for the degradation of organic matter in deposited mudstone beds (Gostlin, 2006; Zhang et marine sediments in the absence of O2 (Jørgensen, al., 2006, 2007a; Gabbott et al., 2008; Zhao et al., 1982), was severely inhibited by early sealing of 2009; Caron et al., 2014). However, they also the burial environment (Gaines et al., 2012c). reveal additional subtleties, including the Tissue loss occurred via limited sulfate reduction, possibility in rare cases of in-situ preservation of as well as by the far less efficient pathways of fossil assemblages lying close to the chemocline methanogenesis and fermentation. Although (Gaines and Droser, 2010; Garson et al., 2012). Burgess Shale-type fossils have lost >99% of their All Burgess Shale-type deposits examined to volume to decay (Gaines et al., 2012d), decay was date have been associated with the close sufficiently restricted that it did not reach juxtaposition of anoxic and oxygenated water completion, and macrofossils were conserved as masses. Transportation of fossil assemblages from organic films. The remarkable anatomical detail the living environment to an anoxic preservational captured in Burgess Shale-type fossils resulted trap was most strongly favored by elevated from the firm and fine-grained nature of physical energy of depositional events, as sediments, which aided in the retention of fine described above. anatomical features—including those of labile Again, this depositional setting is not unlike tissues (Gaines et al., 2012c)—after organic those found in the modern ocean and throughout tissues lost structural integrity and collapsed into the Phanerozoic rock record. While this particular two dimensions as they underwent degradation setting was certainly important in promoting (Briggs and Kear, 1994). preservation, other factors operating in the early ! !burial environment ultimately controlled Burgess !



5 mm 1 mm C D



13.8% 15.9%

19.8% 49.2% 1 cm 1 cm E F G

1 cm 1 cm 1 cm

FIGURE 2.—Carbonate cements in claystones bearing Burgess Shale-type preservation. Thin section micrographs (A, B), polished slab (D) and X-radiograph (C–E). A) Thin section of shown in transmitted light. Carbonate cements at bed tops appear bright, clay-rich bed bases appear gray. B) Cathodoluminescence micrograph of thin section of fossil-bearing interval of , arrow marks top (upper) of a typical claystone lamina, 3 mm in thickness. Carbonates, which exhibit bright orange luminesce, are concentrated at bed tops and become less abundant down bed. The dark, non-luminescent portions of the image are clay-rich with subsidiary carbonate dispersed throughout the claystone fabric. X-radiograph (C) and polished slab (D) of Burgess Shale containing Walcott’s Great Layer (GEL) (arrow), ROM #63065. The distribution of bed-capping authigenic carbonate cements is clearly seen in X-radiograph, where bright areas correspond to high wt.% CaCO3 (data derived from spot-drilled samples). Extensive bed-capping cement is present at the top of the GEL as well as at the tops of the thin, millimeter-scale beds that overlie it. (E–G) X-radiographs of slab samples of claystones bearing Burgess Shale- type preservation. Carbonate-rich portions of event-deposited laminae are less easily penetrated by X-rays and appear bright in X-radiograph, whereas clay-rich portions appear dark in color, revealing concentration of carbonate cements at the tops of individual mm-scale laminae. E) “thin” , locality ROM #59951. F) Wheeler Formation, Drum Mountains locality. G) Marjum Formation, Marjum Pass Locality. "134 GAINES: BURGESS SHALE-TYPE PRESERVATION

BURGESS SHALE-TYPE PRESERVATION concentrations of oxygen and sulfate in the IN TIME AND SPACE Cambrian oceans. Although anoxic and low- ! sulfate conditions were widespread in Cambrian Controls on the temporal distribution of marine environments (Brennan et al., 2004; Dahl Burgess Shale-type preservation et al., 2010; Gill et al., 2011; Gaines et al., 2012c), Widespread Burgess Shale-type preservation these conditions developed repeatedly in marine in the early and middle Cambrian was promoted environments of epicratonic seas throughout the by: 1) transportation of fossil assemblages across Phanerozoic rock record. Therefore, the chemical the chemocline, 2) rapid burial under conditions that favored the development of predominantly anoxic benthic conditions, and 3) carbonate cements are of primary importance in severe inhibition of microbial degradation of soft understanding the temporal distribution of tissues by early cementation and sealing of the Burgess Shale-type preservation. Data from burial environment. While conditions 1 and 2 are multiple sedimentary environments indicates that common throughout the Phanerozoic marine rock conditions favorable to early cementation were record, condition 3 is restricted to early maintained across the early Cambrian–early Phanerozoic strata. The distribution of Burgess Ordovician as reactive basement rocks of the Shale-type deposits in time appears to have continental interiors were progressively reburied resulted from the chemical conditions that underneath sedimentary rocks of the Sauk prevailed in epicratonic seaways during the rise of Sequence, and protected from atmospheric animal life and its subsequent establishment in weathering (Peters and Gaines, 2012). Thus, shallow-marine environments globally. chemical conditions favoring Burgess Shale-type The role of ocean chemistry.—The final and preservation are predicted to have persisted critical step in Burgess Shale-type preservation through the early Ordovician despite the apparent was the emplacement of pervasive bed-capping decline of this taphonomic pathway after the cements at the seafloor (Fig. 2). Early marine middle Cambrian (Allison and Briggs, 1993). carbonate cements in Burgess Shale-type deposits Recently discovered late Cambrian (e.g., and in a range of other marine environments have Vaccari et al., 2004; García-Bellido and been taken as evidence of alkaline conditions in Aceñolaza, 2011) and early Ordovician (Van Roy the Cambrian oceans. These conditions resulted et al., 2010) examples of Burgess Shale-like from a greatly enhanced flux of continental assemblages suggest that the taphonomic window weathering products to the oceans due to exposure for Burgess Shale-type preservation did remain of reactive basement rocks to atmospheric open during this time. The post-middle Cambrian chemical weathering over an area that is decline of Burgess Shale-type preservation may unprecedented in the rock record, and included be correlated to a decline in the area of favorable the interiors of most continental cratons (Peters outer detrital belt environments, which became and Gaines, 2012). A whole-rock chemical displaced from continental seaways due to the database of Cambrian , including Burgess expansion of carbonate platforms during the late Shale-type deposits, demonstrated that Cambrian Cambrian and early Ordovician (Gaines et al., shales are significantly enriched in Ca and 2013). These findings also suggest that additional depleted in Si relative to composite shale Burgess Shale-type assemblages should be standards (Peters and Gaines, 2012), revealing present in late Cambrian and early Ordovician strongly elevated carbonate content as compared strata wherever favorable depositional to average shale. This evidence, as well as that of environments as described above developed. anomalous carbonate cementation in other ! depositional facies (Peters and Gaines, 2012), Burgess Shale-type preservation in space: what indicates that early carbonate cements precipitated factors controlled the differences among broadly in Cambrian epicontinental seas. Under Burgess Shale-type deposits? these favorable conditions, Burgess Shale-type While early carbonate cementation was preservation was promoted wherever fine-grained pervasive in Cambrian marine environments, clastic depositional environments with event- Burgess Shale-type preservation was restricted to dominated sedimentation intersected the environments that promoted transportation of chemocline. fossil assemblages across the chemocline and In addition to high alkalinity, Burgess Shale- rapid burial in fine-grained sediments. The type preservation was also favored by low differences in the number of soft-bodied taxa


FIGURE 3.—Depositional and taphonomic setting of the Burgess Shale at the front of the Cathedral Escarpment, with anoxic and oxygenated water masses separated by a fluctuating oxycline. SWB indicates maximum storm wave base. Physical depositional energy was maximized by the steep slope at the front of the escarpment, which greatly enhanced the potential for transport of fossil assemblages across the oxycline. Potential for preservation of taxonomically diverse and abundant fossil assemblages was maximized in settings lying immediately below the oxycline, where required transport distance was minimized. Preservation potential in the early burial environments of areas of the seafloor lying downslope from this zone was high, however, number of preserved taxa and fossil density were limited by the greater distance of transportation required for fossil benthos to reach these more distal environments (“Supply Limited”). Preservation potential in regions of the seafloor lying upslope from this zone was limited by the prevalence of oxic conditions in the benthic environment, which, when present, resulted in complete decomposition of soft tissues (“Decay Limited”). In these more proximal settings, soft-bodied preservation was favored in intervals in which anoxic conditions developed.

FIGURE 4.—Depositional and taphonomic setting of the Chengjiang, showing location of the chemocline with respect to the loci of preservation and to maximum storm wave base (SWB). High physical depositional energy facilitated transport of fossil assemblages across the oxycline, in many cases over greater distances than in the Burgess Shale (Figure 3). Preservation of taxonomically diverse and abundant fossil assemblages was maximized in settings lying near the oxycline where required transport distance was minimized. Preservation potential in environments of the seafloor lying downslope from this zone was high, but limited by energy required to transport organisms to more distal environments (“Supply Limited”). Preservation potential in environments lying upslope was limited by frequent oxic conditions in the benthic environment (“Decay Limited”). Soft-bodied preservation was favored in intervals in which anoxic conditions developed.

"136 GAINES: BURGESS SHALE-TYPE PRESERVATION preserved, fossil density, and fidelity of proximity of the basin to a storm-influenced preservation that exist among Burgess Shale-type clastic shelf: in most other cases, the depositional deposits may be shown to have resulted from basins of Burgess Shale-type deposits were subtle but important differences in physical separated from clastic shelfal environments by depositional setting and proximity of the broad carbonate platforms that reached up to chemocline. Preservation potential was optimized hundreds of kilometers in width, and served to under sustained anoxic conditions in the benthic sequester a large fraction of clastic sediments in environment (Gaines and Droser, 2010), a nearshore environments of the inner detrital belt. condition that requires transportation of fossil The direct proximity of the Chengjiang basin, assemblages from the living environments to a which lay below storm wave base, to the storm- downslope preservational trap. The preservation influenced shelf (Zhu et al., 2001; Hu, 2005) of Burgess Shale-type assemblages also required provided a depositional regime dominated by rapid burial in claystone sediments, which, in larger magnitude sediment-gravity flows than are most cases, occurred under relatively low observed in other Burgess Shale-type deposits, depositional energy. even those that occur within the inner detrital belt The Burgess Shale and Chengjiang.—Both of (see below). This setting facilitated the the Tier 1 deposits represent special cases in transportation of soft-bodied fossil assemblages which the physical depositional energy was across greater distances than in the Burgess Shale enhanced relative to the other deposits, promoting or in Tier 2 and 3 deposits, as evidenced by periodic entrainment of whole assemblages, patterns of fossil orientation indicative of transportation across a well-developed gravitational settling of fossils during burial chemocline, and rapid burial in often thick (up to (Zhang and Hou, 2007). 10 cm) claystone beds (see above). In the In both the Burgess Shale and Chengjiang Chengjiang, and in the Walcott Quarry and (Figs. 3, 4), the preservation of abundant and Marble Canyon assemblages of the Burgess diverse assemblages was optimized in a zone that Shale, evidence of transportation in turbid flows lay immediately below the chemocline, where followed by rapid deposition is conspicuous in the potential for delivery to the anoxic environment orientations of individual fossils (Whittington, was greatest, and was accompanied by rapid 1971; Gostlin, 2006; Zhang et al., 2006; Zhang burial. Preservation potential of fossils in and Hou, 2007; Gabbott et al., 2008; Caron et al., environments lying downslope from this zone was 2014). limited by the capacity of bottom-flowing In the Burgess Shale (Fig. 3), physical energy currents to transport fossils from habitable benthic of transportation and deposition was enhanced by environments farther upslope, and are herein the steep slope at the front of the Cathedral termed “supply limited” environments. The Escarpment that lay near the angle of repose preservation potential in environments upslope (Conway Morris, 1986; Fletcher and Collins, from the optimized zone was limited by the 1998, 2003; Caron et al., 2014). Periodic failure presence of oxygen, which promoted more rapid of this slope, presumably under the influence of decay in the days to weeks following burial. Thus, storm wave disturbance, is interpreted to have led the potential for Burgess Shale-type preservation to the development of dense, mud-rich slurries may be considered to have been decay-limited in that promoted greater transport efficiency these environments. (Gabbott et al., 2008). The steep nature of the In the Chengjiang, the optimized zone for slope also greatly reduced the minimum transport Burgess Shale-type preservation and the “decay- distance required to move organisms across the limited” zones tend to occur in >10m thick chemocline. stratigraphic intervals and are not closely At Chengjiang (Fig. 4), depositional energy interbedded (Dornbos et al., 2005; Gaines et al., was enhanced by the descent of storm-generated 2012b). In the Burgess Shale (Raymond Quarry, turbidity currents from the shelf into a fault- M a r b l e C a n y o n , a n d S t a n l e y G l a c i e r bounded basin (Zhu et al., 2001; Hu, 2005), assemblages), however, these zones may be resulting in the deposition of thick distal turbidite interbedded at the cm- to mm-scale (Allison and claystone beds that bear Burgess Shale-type Brett, 1995, Caron et al., 2010, 2014), reflecting fossils (Zhu et al., 2001; Zhao et al., 2009). migration of chemical environments under a Depositional energy was elevated relative to that fluctuating oxycline (Gaines and Droser, 2010). of most other Burgess Shale-type deposits due to The development of decay-limited conditions


FIGURE 5.—Depositional and taphonomic setting of Burgess Shale-type deposits in Outer Detrital Belt environments, showing location of the chemocline with respect to the loci of preservation and to maximum storm wave base (SWB). These environments were characterized by low angle slopes, with reduced physical depositional energy that limited the ability of bottom-flowing currents to transport soft-bodied fossils across chemoclines (“Supply Limited”). As a result, preservation of molts, dead organisms, and hydrodynamically-light (i.e., algal/ bacterial) forms is dominant in the record of soft-bodied fossils. Preservation of labile tissues is infrequent in outer detrital belt environments, but was favored in environments lying near the chemocline, or by the delivery of pelagic organisms to more distal environments followed by rapid burial. Preservation potential in regions of the seafloor lying upslope from this zone was limited by the prevalence of oxic conditions in the benthic environment, which, when present, resulted in complete decomposition of soft tissues (“Decay Limited”). Soft-bodied preservation was favored in intervals in which anoxic conditions developed. over fossil assemblages originally buried under in these deposits represent molts and/or dead and optimized conditions may have had consequences partially decomposed specimens, in addition to a for the fidelity of preservation in these instances. hydrodynamically light fraction dominated by However, processes important to the conservation algal fragments (Gaines and Droser, 2005, 2010; of organic remains of Burgess Shale-type fossils Gaines et al., 2012b; Garson et al., 2012). In the are interpreted to have been operative in the first distal portion of these settings, fossils may occur few weeks following burial (Briggs and Kear, in great density, but exhibit very low overall 1994; Gaines et al., 2012c); thus, the longer-term taxonomic richness, and fossils of animals are migration of chemical environments is not rare (Gaines and Droser, 2010). These include the expected to have influenced the preservation of Kaili (Series 2–3), Balang (Series 2), and fossils buried immediately below the sediment- Tsinghsutung (Series 2) deposits of South China water interface. (Peng et al., 2005; Steiner et al., 2005; Zhao et al., Outer detrital belt settings.—The majority of 2005; Sun et al., 2013), and Pioche (Series 2–3), Burgess Shale-type deposits, including most of Spence (Series 3), Wheeler (Series 3), and the Tier 2 and Tier 3 deposits, occur in outer Marjum (Series 3) deposits of Laurentia (Robison, detrital belt environments (Fig. 5; Robison, 1960), 1991; Lieberman, 2003; Briggs et al., 2008; characterized by relatively low-angle, mixed Webster et al., 2008). In many cases, beds siliciclastic-carbonate slopes that descended from deposited under supply-limited conditions are carbonate platforms towards the seaward margins closely (mm-scale) interbedded with those of the continental crust (Rees, 1986; Liddell et al., deposited under decay-limited conditions (Gaines 1997; Elrick and Snider 2002; Brett et al., 2009; and Droser, 2003, 2005, 2010; Wang et al., 2004; Halgedahl et al., 2009; Gaines et al., 2011). The Webster et al., 2008; Caron et al., 2010; Lin et al., Burgess Shale-type fossil-bearing intervals of 2010; Gaines, 2011; Gaines et al., 2012b; Garson these deposits are comprised of amalgamated et al., 2012) although this is not expected to have mm-scale claystone beds. These beds were affected preservation potential of fossils in the deposited from bottom-flowing currents that are shallow burial environment, as discussed above. interpreted to have typically lacked the transport Examples of soft-bodied fossils bearing energy required to move fossil assemblages en limbs, organs, and other labile tissues are well masse. Most commonly, exceptional fossils found known from these deposits, although they are


FIGURE 6.—Depositional and taphonomic setting of Burgess Shale-type deposits in Inner Detrital Belt environments, showing location of the chemocline with respect to the loci of preservation and to maximum storm wave base (SWB). Elevated physical depositional energy favored transportation and rapid burial of fossil assemblages, but preservation was limited by the prevalence of oxic conditions in the benthic environment, which resulted in complete decomposition of soft tissues (“Decay Limited”). Frequent scour and reworking associated with deposition of a silt or fine-sand fraction also acted to limit preservation potential. Burgess Shale-type preservation in these settings occurred very rarely when anoxic benthic conditions developed and when physical depositional energy was also reduced. much more rare than fossil cuticle (typically of 2010) deposits, belonging to Series 2 of arthropods and ) and algal/bacterial forms Laurentia. In each, exceptional preservation of (e.g., Handle and Powell, 2012). The examples of macrofossils is rare, and occurs in infrequent high-fidelity preservation in these deposits intervals that accumulated below the chemocline suggest that the potential for high-fidelity (Gaines and Droser, 2002). Throughout the preservation was present in large portions of majority of each of these deposits, the probability them, but that preservation was limited by the of Burgess Shale-type preservation was precluded delivery of live or freshly killed organisms by oxygen concentrations that accelerated decay (supply limited). Rarely, high-fidelity preservation and promoted colonization by an infauna and the occurred in settings near the chemocline or via the irrigation of sediment pore-waters, despite the delivery of freshly killed pelagic organisms to development of carbonate cements at bed tops. more distal settings, followed by rapid burial. The Individual depositional beds rarely exceed 10 mm total number of taxa preserved in the great in thickness, but each typically contains a silt or majority of Burgess Shale-type deposits appears fine sand fraction at the bed base that contributed to have been supply limited, with overall number to frequent scour and reworking of underlying of taxa and fidelity of preservation constrained beds (e.g., Gaines and Droser, 2002), which is not primarily by biostratinomic rather than diagenetic observed in most other Burgess Shale-type factors. settings. Prevalence of oxic conditions during the Inner detrital belt settings.—A minority of accumulation of these deposits and disturbance of Tier 2 and Tier 3 deposits occur in mudstone- the early burial environment by bioturbation, dominated inner detrital belt settings (Fig. 6) scour, and reworking served to limit the number where conditions promoting decay were more of taxa preserved and the fidelity of exceptional limiting to preservation than transport energy. preservation in these decay-limited settings, from Important among these conditions were greater which only rare examples of Burgess Shale-type availability of oxygen during deposition, and preservation are known. Rarely, Burgess Shale- sediment reworking resulting from elevated type fossils were conserved when infrequent physical energy, both of which are features of anoxic conditions developed, and when reworking shallower water depth and more proximal and bioturbation were minimized. than that of typical ! Burgess Shale-type settings. These distal shelf CONCLUSIONS deposits include the Latham (Gaines and Droser, ! 2002) and Indian Springs (English and Babcock, 1) Burgess Shale-type preservation is a

"139 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 unique taphonomic pathway that is defined by the among Burgess Shale-type deposits resulted from conservation of whole assemblages of soft-bodied biostratinomic factors that controlled the potential fossils as primary carbonaceous remains for transportation of soft-bodied fossils across (Butterfield 1995, 2003; Butterfield et al., 2007; chemical gradients and their rapid burial in fine- Gaines et al., 2008)—a pathway that does not grained sediments. The Chengjiang and Burgess occur in modern marine environments or in the Shale contain the greatest number of soft-bodied great majority of the Phanerozoic rock record. forms because of enhanced transport energy, and, 2) The Burgess Shale-type taphonomic in the Burgess Shale, reduced minimum transport pathway was widespread in early and middle distance required to move organisms across the Cambrian settings, and was permitted wherever chemocline. fine-grained clastic depositional environments 6) Although distal muddy environments characterized by event-based deposition favorable to preservation appear to decline across intersected the redox boundary at the seafloor. the late Cambrian and early Ordovician, 3) Burgess Shale-type preservation declines additional Burgess Shale-type assemblages should sharply after the middle Cambrian (Allison and be present in strata of these ages wherever Briggs, 1993), but this pathway may have favorable physical depositional and local persisted into the late Cambrian (Vaccari et al., chemical conditions developed. 2004; García-Bellido and Aceñolaza, 2011) and ! the early Ordovician (Van Roy et al., 2010). The ACKNOWLEDGMENTS signal of the early Phanerozoic taphonomic ! window (Allison and Briggs, 1993; Orr et al., I am greatly indebted to many mentors, 2003) for exceptional preservation is statistically colleagues, friends and coauthors in this work, dominated by Burgess Shale-type fossil including Terry Abbott, Cedric Aria, Derek assemblages. Briggs, Nick Butterfield, Patrick Burke, Don 4) Burgess Shale-type preservation ultimately Canfield, Jean-Bernard Caron, Lorelei Curtin, resulted from early sealing of the burial Allison Daley, Mary Droser, Maya Elrick, Una environment by carbonate cements emplaced at Farrell, Seth Finnegan, Sarah Gabbott, Daniel the seafloor shortly after deposition. These Garson, Randy Gossen, Val Gunther, Glade cements acted to restrict the diffusion of oxidants Gunther, Emma Hammarlund, Jonathan Harris, into the sediments, thereby retarding the processes Thomas Harvey, Xianguang Hou, Nigel Hughes, of microbial decomposition in the sediments at Paul Johnston, Martin Kennedy, Jade Star Lackey, large, resulting in incomplete decay of organic Paul Jamison, Ganqing Jiang, Gordon Love, Tim remains and the conservation of morphology in Lyons, Xiaoya Ma, Gabriella Mangano, Bradley carbonaceous films (Gaines et al., 2005, 2012c). Markle, Lorna O’Brien, Patrick Orr, Jin Peng, Low concentrations of oxygen and sulfate in the Shanan Peters, Maria Prokopenko, Changshi Qi, Cambrian oceans also contributed to the slowing Jake Skabelund, Michael Streng, Peter Van Roy, of decomposition, but these conditions are not John Vorhies, Mark Webster, Xiguang Zhang, and unique to the early Phanerozoic, and therefore Yuanlong Zhao. Without the conversations, cannot account for Burgess Shale-type guidance, and opportunities provided by these preservation. The non-analogue condition that individuals, this work would not have been was responsible for Burgess Shale-type possible. Burgess Shale work rests on the preservation was the high alkalinity of the immense contributions of the late James Aitken Cambrian oceans, which promoted early towards understanding regional and carbonate cementation at the seafloor in Burgess paleoenvironments. I also wish to thank Parks Shale-type deposits and across multiple marine staff for support of Burgess Shale work, depositional environments during the Cambrian especially Todd Keith and Alex Kolesch. Images (Peters and Gaines, 2012). While this control over in Figure 1 appear courtesy of the Royal Ontario Burgess Shale-type preservation was global in Museum, with thanks to Jean-Bernard Caron and extent, local physical depositional and chemical Peter Fenton for images and access to material, conditions, specifically proximity to the oxycline, and , with thanks to Hou determined where Burgess Shale -type Xianguang. Daniel Garson produced artwork for preservation occurred. Figures 3–6. Funding for this work was provided 5) Prominent differences in total number of b y t h e N a t i o n a l S c i e n c e F o u n d a t i o n preserved taxa and in fidelity of preservation (EAR-1046233, EAR-1053247, EAR-0518732,


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