ABSTRACTAlthough Cambrian Burgess Shale–type (BST) biotas are fun-
damental to understanding the radiation of metazoans, the nature of their extraordinary preservation remains controversial. There remains disagreement about the importance of the role of early min-eral replication of soft tissues versus the conservation of primary organic remains. Most prior work focused on soft-bodied fossils from the two most important BST biotas, those of the Burgess Shale (Canada) and Maotianshan Shale (Chengjiang, China). Fossils from these two deposits do not provide ideal candidates for specimen-level taphonomic study because they have been altered: the Burgess Shale by greenschist facies metamorphism and the Maotianshan Shale by intensive subsurface weathering. Elemental mapping of soft-bodied fossils from 11 other BST deposits worldwide demonstrates that BST preservation represents a single major taphonomic pathway that may share a common cause wherever it occurs. The conservation of organic tissues, and not early authigenic mineralization, is the primary mech-anism responsible for the preservation of BST assemblages. Early authigenic mineral replacement preserves certain anatomical fea-tures of some specimens, but the preservation of non-biomineralized BST fossils requires suppression of the processes that normally lead to the degradation of organic remains in marine environments.
INTRODUCTIONBurgess Shale–type (BST) biotas are of critical importance to
understanding the early evolution of the Metazoa (Conway Morris, 1989a; Butterfi eld, 2003; Briggs and Fortey, 2005). The great majority of species preserved in BST deposits lack biomineralized tissues ( Conway Morris, 1986). Preservation of soft-bodied organisms is rare in the geo-logic record (Briggs, 2003) and BST deposits provide a record of early Phanerozoic biodiversity otherwise unknown. The mode of preservation of these exceptional biotas, however, has been much debated and the precise circumstances that facilitated exceptional preservation in BST deposits remain disputed.
It has long been recognized that an anomalously large number of deposits in Lower and Middle Cambrian strata yield exceptionally preserved biotas compared to the rest of the Phanerozoic (Allison and Briggs, 1993). Soft-bodied fossils occur in abundance at nine Cambrian localities worldwide (Conway Morris, 1998), and more rarely in perhaps as many as 40 other deposits of Cambrian age (e.g., Conway Morris , 1989b; Steiner et al., 2005). Although a small number of Cambrian deposits preserve three-dimensional soft-bodied fossils by replacement in calcium phosphate (e.g., Waloszek, 2003), the great majority of Cam-brian exceptional biotas are preserved as two-dimensional compression fossils. Deposits yielding biotas preserved in this characteristic manner are termed Burgess Shale–type deposits after Walcott’s classic locality in the Canadian Rockies (Conway Morris, 1989a, 1989b). It has been suggested that this type of preservation is largely absent from the fossil record after the Middle Cambrian (Butterfi eld, 1995).
The importance of organic preservation versus early authigenic mineral replacement of soft tissues in BST deposits has been disputed. A number of microbial decomposition reactions facilitate the precipita-tion of minerals on decaying tissues, replicating their form (Briggs, 2003). Once soft tissues are replicated by mineral templates, they may survive to enter the fossil record even if the original organic material is lost. On the other hand, the preservation of whole biotas as organic remains requires inhibition of the normal processes of decay.
Most prior work focused on the preservation of two of the most important BST assemblages, those of the Middle Cambrian Burgess Shale and the Lower Cambrian Maotianshan Shale (Yuanshan For-mation, Chengjiang, China). Typically, BST fossils are preserved as dark-colored fi lms, often refl ective, exposed on bedding surfaces of the host mudstones. Burgess Shale fossils were fi rst interpreted to be the result of siliceous replacement (Walcott, 1919). Subsequent analy-ses supported a carbonaceous (Whittington, 1971) or alumino-silicate composition (Conway Morris, 1977, 1986). Organic remains of fea-tures of some Burgess Shale fossils were documented in thin section and isolated by digestion in HF (Butterfi eld, 1990). These fi ndings led to the defi nition of Burgess Shale–type preservation as the conserva-tion of two-dimensional carbonaceous compressions in marine shales (Butterfi eld, 1995). However, elemental mapping of Burgess Shale fossils subsequently demonstrated that the abundance of Al, Si, and K varies in different morphological features and relative to the mud-stone matrix (Orr et al., 1998). This discovery led to the conclusion that more labile tissues of Burgess Shale fossils are replicated in clay minerals, which precipitated onto the organic templates of decom-posing organisms in the early burial environment (Orr et al., 1998). Orr et al. (1998) concluded that while organic remains of the more decay-resistant cuticles are preserved, authigenic mineralization was the principal pathway by which the more labile tissues in fossils of the Burgess Shale were preserved.
Elemental mapping of fossils from the Maotianshan Shale revealed that two modes of preservation are important in that deposit. In most cases the major morphological features of Chengjiang fos-sils are preserved as carbonaceous compressions; however, features of many of these fossils are preserved in pyrite (Gabbott et al., 2004; Hu, 2005). Pyrite mineralization is interpreted to have occurred soon after burial during limited bacterial sulfate reduction around the most labile tissues, defi ning their morphology; other, more recalcitrant tis-sues are preserved as organic compressions (Gabbott et al., 2004). Thus, analyses of fossils from the Maotianshan Shale (Gabbott et al., 2004) and from the Burgess Shale (Orr et al., 1998) suggested that different diagenetic pathways led to the preservation of fossils in the two deposits . Furthermore, these results implied that no single mecha-nism was responsible for the preservation of BST biotas. Although the Burgess Shale and Maotianshan Shale have yielded the most important Cambrian biotas, their fossils may not provide the most appropriate basis for understanding BST preservation. Burgess Shale fossils from Fossil Ridge have been altered by greenschist facies metamorphism (Powell, 2003) and those of the Maotianshan Shale have been altered by extensive weathering (Zhu et al., 2001).
Cambrian Burgess Shale–type deposits share a common mode of fossilizationRobert R. Gaines1, Derek E.G. Briggs 2, Zhao Yuanlong3
1Geology Department, Pomona College, Claremont, California 91711, USA2Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA3Key Laboratory for Paleobiology, Guizhou University, Guiyang, China
756 GEOLOGY, October 2008
MATERIALS AND METHODSIn this study we analyzed 53 fossils from 11 BST deposits (Appen-
dix 1). All deposits analyzed have undergone less metamorphism than the Burgess Shale, as evidenced by a predominance of illite over chlo-rite and muscovite in the matrix of mudstones, and less weathering than Maotianshan Shale, as evidenced by higher specifi c gravity. A variety of taxa without mineralized skeletons was analyzed, including arthropods, priapulids and other worms, eldoniids, problematic taxa, and algae or alga-like fossils. The least weathered fossils available from each deposit were selected for analysis. Fossils were analyzed uncoated using fi eld emission scanning electron microscope–energy dispersive X-ray analysis (SEM-EDX) at low voltage (5 kV) in order to minimize beam penetration. A subset of samples was also analyzed at higher voltage (15–20 kV). Ele-ment distribution was determined by production of X-ray maps at micron-scale resolution, augmented by spot and line scan transects.
RESULTSElemental mapping revealed that all 53 fossils analyzed are preserved
as carbonaceous fi lms or their degraded remains. Authigenic minerals are responsible for preservation of a morphological feature (the gut) in only one of the samples analyzed. The fossils display carbonaceous preserva-tion that ranges from robust continuous fi lms to what are interpreted as degraded remains of fi lms that are below the threshold of EDX detection.
Elemental mapping of robust and continuous fi lms (Fig. 1) reveals sharp contrasts in composition between the fossils and the matrix. The carbonaceous remains are clearly defi ned by strong enrichment in C and depletion of Al, Si, and O. Where present, Ca and Mg are also depleted relative to the matrix. Under SEM magnifi cation, these fi lms are readily distinguished from the crystalline claystone matrix by enhanced conduc-
tivity and amorphous habit (Fig. 2). Films vary in thickness from several microns to <1 μm, but do not preserve original microstructures.
At the opposite end of the spectrum are fossils that cannot be dis-tinguished from the matrix using EDX analysis or SEM imaging, even though they are clearly visible to the naked eye as dark areas. These speci-mens exhibit no compositional difference from the matrix and no textural indication that authigenic minerals were once present, such as crystal pseudomorphs or molds. Such fossils are preserved as degraded carbon fi lms, which defi ne morphological features but retain insuffi cient carbon to be detected; of the elements analyzed, carbon is the most diffi cult to detect because of its low atomic number and tendency to emit low-energy X-ray radiation. This conclusion is supported by analyses of individual fossils that revealed a gradient in the nature of carbonaceous material, from robust through weakly detectable to nondetectable, and is consistent with previous work (Gabbott et al., 2004).
Authigenic pyrite is present in two of the fossils analyzed, as euhedral crystals (~5 μm) and as framboids (~10 μm). In both cases, pyrite occurs in patches at the sediment-fossil interface that do not replicate fossil mor-phology. However, replacement by early authigenic calcium phosphate preserves the gut of the arthropod Dicranocaris guntherorum (Briggs et al., 2008) in three dimensions within a two-dimensional carbonaceous fi lm that defi nes the rest of the morphology of the specimen.
Authigenic mineral phases have been shown to coat organic remains of soft-bodied fossils from the Burgess Shale (Orr et al., 1998; Butter-fi eld et al., 2007). However, there is no evidence that alumino-silicates or other authigenic mineral phases are present on the surface of the fossils analyzed here but are obscured by robust carbonaceous remains. No com-positional difference between fossil and matrix was detected, even where carbon fi lms are so severely degraded that they are below the threshold of
A′
B′
C′
A
B
CD′
D
C O Al Si Fe
Figure 1. Examples of scanning electron microscope–energy dispersive X-ray (SEM-EDX) elemental maps illustrating preservation of Burgess Shale–type (BST) fossils as carbonaceous fi lms (A–D) and photographs showing location on each specimen at which map data were taken (A′–D′). Brightness of color corresponds to abundance of each element. A: Carapace of Tuzoia, Kaili Formation; scale repre-sents 300 μm. A′: Scale represents 5 mm. B: Margin of indeterminate vermiform metazoan (left), Spence Shale (A); scale represents 400 μm. B′: Scale represents 5 mm. C: Indeterminate alga (top), Doushantuo Formation; scale represents 100 μm. C′: Scale represents 2 mm. D: Gut trace of indeterminate metazoan, Pioche Formation; scale represents 400 μm. D′: Scale represents 1 mm.
GEOLOGY, October 2008 757
detection. Analysis of a subset of carbonaceous samples at higher voltages (15 kV and 20 kV) for deeper beam penetration also revealed no evidence for mineral coatings beneath a surfi cial carbonaceous layer.
DISCUSSIONThese results demonstrate that fossils analyzed from the 11 deposits
are preserved as carbonaceous compressions that are sometimes aug-mented by authigenic mineral replication of particular features. Carbo-naceous remains have also been documented in isolated elements of BST fossils extracted by HF digestion from the Mount Cap Formation (Butter-fi eld, 1994), in the gut of the trilobite Olenoides from the Kaili biota (Lin, 2007), and in soft-bodied fossils from the Middle Cambrian portion of the Pioche Formation (Moore and Lieberman, 2005).
Recently published data indicate that conservation of carbonaceous remains was the primary process involved in preserving the fossils of the Burgess Shale. A petrologic and microanalytical study of fossils from the Walcott Quarry interpreted the replication of tissues in alumino-silicate minerals as primarily resulting from late-stage metamorphic replacement or overgrowth of carbonaceous material (Butterfi eld et al., 2007). These fi ndings, which are based on mineral phase relationships in trilobite exo-skeleton, in mineralized arthropod guts, and in veinlets that cut soft-bodied fossils, are supported by the demonstration of a similar phenomenon in graptolites across a metamorphic gradient. Originally carbonaceous grap-tolites were shown to be replaced progressively by alumino-silicate min-eral fi lms with increasing metamorphic grade as a result of late diagenetic processes (Page et al., 2007). These data suggest that the original com-position of Burgess Shale fossils was carbonaceous, consistent with the composition of fossils from other deposits analyzed in this study.
Although pyrite has been reported in association with soft-bodied fossils from the Burgess Shale and the Mount Cap Formation (Butter-fi eld, 2003; Garcia-Bellido and Collins, 2006), extensive preservation of the outline of soft-tissues in Cambrian faunas in pyrite has only been reported in the Maotianshan Shale (Gabbott et al., 2004). This additional
taphonomic pathway in Chengjiang fossils suggests an enrichment of reactive iron in the associated sediment (Briggs et al., 1996). Such enrich-ment would be expected to result from deposition under an anoxic water column (Poulton and Canfi eld, 2005) or near a source of terrigenous clas-tics, or from storm-infl uenced remobilization of sediments in which pyrite had formed previously (Raiswell et al., 2008). The Maotianshan Shale is signifi cantly enriched in reactive iron relative to other BST deposits ( Hammarlund, 2007); although it is possible that reactive iron was con-centrated in the Maotianshan Shale as a result of recent weathering, iron enrichment may represent the only taphonomically signifi cant difference between the Chengjiang and other BST deposits.
Replication of selected soft tissues in authigenic calcium phos-phate is another important auxiliary pathway in BST deposits. Although uncommon, three-dimensional preservation of arthropod guts in calcium phosphate occurs in the Burgess Shale and Maotianshan Shale (Briggs, 1981; Butterfi eld, 2002), in addition to the example from the Wheeler For-mation documented here. Phosphatization of the midgut glands, which sometimes preserves subcellular details, has been linked to the abundance of unordered calcium phosphate present in the living organ (Butterfi eld, 2002). Extensive three-dimensional replication of muscle-tissues in authi-genic minerals occurs in the fossils from Sirius Passet (in silica; Budd, 1998) and the Emu Bay Shale (in calcium phosphate: Briggs and Nedin, 1997); these biotas differ taphonomically from the BST mode of preserva-tion considered here, representing different primary modes of fossiliza-tion, and must be considered separately from other BST deposits.
CONCLUSIONSThe analyses presented here, together with previously published data,
indicate that BST deposits share a common taphonomic pathway that led to the preservation of fossils as carbonaceous fi lms (Butterfi eld, 1995). These results also confi rm that compression fossils of nonmineralized algae in Protero zoic shales followed the same pathway (Butterfi eld, 1995). Auxiliary mineral replacements that occur in association with the carbonaceous fi lms have the potential to preserve more labile structures, which would other-wise be lost (Orr et al., 1998; Butterfi eld, 2002). These associated mineral precipitates may refl ect differences in porewater chemistry and in sediment composition and provide insight into the diagenetic processes under which fossilization occurred. However, in the great majority of cases it is a carbo-naceous fi lm alone that defi nes the overall morphology of the fossils.
These results indicate that BST preservation represents a global phenomenon that required suppression of normal processes of decay in marine sediments. Two types of models have been proposed to explain the preservation of BST fossils as organic remains. The fi rst requires physical and/or chemical protection of carcasses entombed in sediments. Protec-tion of carcasses has been attributed to clay-organic interactions (Butter-fi eld, 1995), or adsorption of Fe3+ onto biopolymers (Petrovich, 2001), both preventing the activity of enzymes involved in decomposition. The second requires an early cessation of normal diagenetic processes, via rapid occlusion of sediment porosity soon after deposition, resulting in reduced fl ux of oxidants into the sediments and the preclusion of micro-bial decomposition (Gaines et al., 2005). Further study of the sedimentol-ogy and geochemistry of these deposits is required to determine which of these possibilities is most likely.
APPENDIX: FOSSILS ANALYZED
The 53 fossils analyzed came from 11 Burgess Shale–type (BST) deposits. (1) Five important deposits are from the Lower (LC) and Middle Cambrian (MC): the Kaili Formation [MC: 2 Tuzoia (arthropod), 1 inde-terminate arthropod cuticle, 3 Pararotadiscus guizhouensis (eldoniid), 1 indeterminate alga]; the Kinzers biota of the Ledger Formation (LC: 1 indeterminate cuticle, 1 indeterminate alga); the Marjum Forma-tion [MC: 1 Leanchoilia? sp. cf. protogonia (arthropod), 1 Yuknessia
A
DC
B
Figure 2. Secondary emission scanning electron microscope (SE-SEM) micrographs of carbonaceous residues comprising Burgess Shale–type (BST) fossils. A: Margin of indeterminate vermi-form metazoan defi ned by black (conductive), amorphous carbo-naceous fi lm (top and left) delineated sharply from crystalline clay matrix (lower right), Spence Shale; scale represents 20 μm. B: Beltina (alga?) showing patches of black amorphous carbonaceous material around margins and at center, with weakly detectable carbonaceous remains composing interior of the fossil, Greyson Shale; scale rep-resents 500 μm. C: Yuknessia simplex (alga), Wheeler Formation, showing discrete fl akes of carbonaceous material that compose the fossil. D: Detail of Selkirkia (priapulid) showing thin, discontinuous carbonaceous fi lm that appears dark compared to bright crystalline matrix, Wheeler Formation; scale represents 5 μm.
758 GEOLOGY, October 2008
simplex (alga)]; the Spence Shale Member, Langston Formation [MC: 1 Canadaspis ? (arthropod), 1 eldoniid (metazoan), 2 Wiwaxia (lopho-trocho zoan), 1 indeterminate metazoan cuticle, 2 indeterminate meta-zoans, 3 Marpolia spissa (alga), 1 Margaretia (alga)]; and the Wheeler Formation [MC: 1 Canadaspis ?, 1 Dicranocaris guntherorum (arthropod), 1 Elrathia kingii (arthropod: soft parts analyzed), 1 Selkirkia ( priapulid), 2 Margaretia, 1 Marpolia spissa, 3 Morania (alga?), 2 Yuknessia simplex (alga), 1 indeterminate vermiform metazoan or gut trace]. (2) Four “minor” deposits are from the Lower and Middle Cambrian, and have yielded few soft-bodied fossils: the Balang Formation (LC: 4 Morania), the Latham Shale [LC: 1 Anomalocaris appendage (arthropod)], the Metaline Formation (MC: 1 Margaretia), and the Pioche Formation (MC: 1 indeter-minate metazoan). (3) Two Proterozoic deposits yield compression fossils: the Neoproterozoic Doushantuo Formation (Miahoe Biota) (3 indetermi-nate algae); and the Mesoproterozoic Greyson Shale [5 Beltina (alga), 3 Grypania (alga)].
ACKNOWLEDGMENTSWe thank N. Butterfi eld, J.-B. Caron, P. Orr, and A. Page for comments and
discussion, and D. Haley, D. Tanenbaum, C. Taylor, and Z. Jiang for scanning elec-tron microscope assistance. Some analyses were performed by R. Goossen and B. Markle. We thank S. Halgedahl, P. Jameson, R. Jarrard, M. Kooser, J. Scabelund, and G. Schofi eld for access to collections for analysis. This work was supported by the National Science Foundation for collaborative research between Gaines and Briggs (grants EAR-0518732 and EAR-0518547) and for major equipment acqui-sition by Gaines (DMR-0618417), by a D.L. and S.H. Hirsch Research Initiation grant to Gaines, and by Chinese Natural Science Foundation grant 40672018 to Zhao Yuanlong.
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Manuscript received 19 March 2008Revised manuscript received 6 June 2008Manuscript accepted 10 June 2008
