Palaeogeography, Palaeoclimatology, Palaeoecology 277 (2009) 57–62

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Preservation of early and Middle Cambrian soft-bodied arthropods from the PiocheShale, Nevada, USA

Rachel A. Moore, Bruce S. Lieberman ⁎Department of Geology and Biodiversity Research Center, University of Kansas, 1475 Jayhawk Blvd, Lindley Hall, Room 120, Lawrence, KS 66045-7613, USA

⁎ Corresponding author.E-mail address: blieber@ku.edu (B.S. Lieberman).

0031-0182/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.palaeo.2009.02.014

a b s t r a c t

a r t i c l e i n f o

Keywords:

TaphonomyFossilizationHematitePyriteArthropodsAnomalocarisBurgess Shale

The Early–Middle Cambrian Pioche Shale of Lincoln County, Nevada preserves a diverse array of soft-bodiedecdysozoans that are well-known from the Burgess Shale soft-bodied fauna of British Columbia, Canada. AtPioche certain genera occur both above and below the Middle Cambrian boundary, allowing a comparison ofthe nature of their preservation across this interval. In order to investigate the mineralization of this fauna,SEM analysis of mineral textures and element mapping of Anomalocaris, Tuzoia and Canadaspis specimensusing energy dispersive X-ray spectroscopy (EDS) was undertaken. Results of these analyses show that EarlyCambrian specimens are generally preserved in botryoidal hematite which is dark red in hand specimenwhile specimens from the Middle Cambrian are preserved as kerogenized carbon films with associatedoxidized pyrite crystals and framboids. Comparison with other Cambrian soft-bodied biotas reveals that thenature of preservation of the Middle Cambrian biota at Pioche is remarkably similar to that described for theChengjiang soft-bodied fauna, China. There does not seem to be an adequate analogue for the Early Cambrianpreservation of soft-bodied organisms at Pioche. The preservation of the fauna is discussed in relation tochanging environmental conditions at the Early–Middle Cambrian boundary.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The exceptional preservation and diversity of the soft-bodiedanimals from the Burgess Shale have made it one of the most well-known and celebrated fossil sites in the world. Since renewed interestin the Burgess Shale biota in the 1980s (Briggs et al., 1994 andreferences therein) a number of new, geographically widespread,Early and Middle Cambrian soft-bodied biotas have been described.These soft-bodied faunas provide important information on thetiming and nature of the apparently rapid development of modernphyla, known as the ‘Cambrian explosion’ (Fortey, 2001). One of themost spectacular sites, the Early Cambrian Chengjiang biota ofYunnan, China (Hou et al., 2004), yields animals that are commonto the Burgess Shale but also various unique taxa (e.g. Fuxianhuia Hou,1987). The Pioche Shale of Nevada, USA, yields a less abundant soft-bodied biota than the Burgess or Chengjiang deposits, but importantlyspans the Early–Middle Cambrian boundary and so has the potentialto offer important insights into changes in the diversity andpreservation of soft-bodied faunas at this time (Lieberman, 2003).

This study contributes to our understanding of the taphonomicpathways that led to the preservation of Burgess Shale-type biotas.The mechanism by which Burgess Shale-type preservation (BST)occurs has yet to reach a consensus, however it is key to under-standing the geographical distribution and temporal limitations of

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these soft-bodied biotas. Butterfield (1995) defined BST as carbonac-eous compressions in marine shales; yet elemental mapping ofBurgess Shale specimens by Orr et al. (1998) showed that alumino-silicate mineralization is also important in preserving various tissues.Both carbon films and pyrite mineralization (later altered to ironoxides) preserve Chengjiang fossils (Gabbott et al., 2004; Zhu et al.,2005), highlighting anothermeans bywhich soft tissue is preserved inthe Cambrian. Further study of coeval soft-bodied biotas is importantto understand the taphonomic conditions that controlled preservationat these other localities.

2. Locality, age and stratigraphy

The Pioche Shale contains both Early and Middle Cambrian faunas,comprising trilobites, brachiopods, hyolithids, sponges, gastropods,eocrinoids and trace fossils. The soft-bodied bearing units of the PiocheShale outcrop in southern Nevada in the Highland and Chief Ranges nearthe towns of Pioche and Caliente (Fig. 1). The Early-Middle Cambrianfaunal transition occurs at the transition between the Combined Metalsand Comet Shale Members (see Palmer, 1998; Sundberg and McCollum,2000). The LowerCambrian section contains abundantolenellid trilobites;soft-bodied animals in this part of the section occur in one or possiblymore horizons that have not been well localized stratigraphically but liewithin the last one-twometers of the Lower Cambrian part of the section.TheMiddle Cambrianpart of the section contains abundant ptychoparioidtrilobites but no olenellids; in this part of the section soft-bodied animalsoccur in at least one stratigraphic horizon at each of two localities. These

Fig. 1. Location of collection sites within the Pioche Shale, Nevada, USA that yielded thespecimens studied here (indicated by X). Localities are described in detail in Sundbergand McCollum (2000, 2002, 2003) and Lieberman (2003). Figure modified fromSundberg and McCollum (2000), used with permission of the Paleontological Society.

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horizons have not yet beenwell localized stratigraphically, but each likelyoccurs in the lower ten toperhaps 15mof theMiddle Cambrianpart of thesection. The soft-bodied fauna is primarily composed of anomalocarididfrontal appendages (and more rarely whole-body specimens), Tuzoiacarapaces, Ottoia, Canadaspis and Perspicaris. Ottoia is known only fromthe Early Cambrian horizons and Canadaspis and Perspicaris only occur inthe Middle Cambrian horizons (Lieberman, 2003).

3. Materials and methods

Two specimens of Tuzoia and two specimens of anomalocarididappendages housed in the University of Kansas Museum of Inverte-brate Paleontology, Biodiversity Research Center (KUMIP 298507a,298527a, 307020b, 298529a) from the Early and Middle Cambrianwere selected for the taphonomic investigation. A specimen of Cana-daspis (KUMIP 307021a) from the Middle Cambrian horizon was alsoconsidered for comparative purposes. Systematic palaeontology and

detailed locality information for soft-bodied specimens are providedby Lieberman (2003).

Uncoated specimens were studied under low-vacuum conditionsusing a Hitachi 3500N Scanning Electron Microscope (SEM) at theDepartment of Earth Sciences, University of Bristol, England. Bothbackscatter and secondary electron images were taken. The formerprovide an indication of mineralogical differences and the latter provideimages of surface textures. Elemental X-ray maps were obtained usingan EDAX Energy Dispersive Spectrometer (EDS), using an acceleratingvoltage of 15 kV for all elements other than carbon, which was mappedat an accelerating voltage of 5 kV in order tominimize X-ray absorbanceby surrounding material. Similar approaches have been pioneered byOrr et al. (1998) and Gabbott et al. (2004) to study soft-bodiedpreservation in other Cambrian fossils.

4. Preservation

More than 90% of the anomalocarididmaterial from the Pioche Shaleconstitutes disarticulated anterior appendages (Lieberman, 2003). Thepresence of disarticulated body parts indicates that most elements ofthe Pioche biota underwent significant decay before entering the dia-genetic environment.However, threepossiblewhole-body specimensofAnomalocaris (KUMIP 298510, KUMIP 298511, KUMIP 293606) havebeen recovered from the Early Cambrian and a specimen of Canadaspis(KUMIP 307021) with associated appendages from the MiddleCambrian of Pioche (Lieberman, 2003), which illustrates that somecarcasses entered the system at an earlier stage of decay (Briggs, 1994).

The main difference in fossils from the Early Cambrian and thosefrom the Middle Cambrian when seen in hand specimen is the color.Specimens from the Early Cambrian are typically dark red in a pinkish-red to green matrix while those from the Middle Cambrian are darkgray with a distinct reflective film in a lighter gray matrix.

4.1. Preservation of specimens of Early Cambrian Age

Textural analysis of Early Cambrian specimens using secondaryelectron SEM images revealed that the habit of the fossil-preservingmineral is botryoidal (Fig. 2A and B). Along with the dark red color ofthe mineral in hand specimen this suggests that it is most likely to behematite (Fe2O3). Element maps of Tuzoia and Anomalocaris providefurther evidence for this identification because Fe is elevated in theregion of the fossil (Fig. 3A and B). In Tuzoia elevated Fe is clearlylimited to the characteristic reticulated ornament (Fig. 3A), whichmay have been a positive feature on the surface of the carapace in life.It is possible that the entire surface of the carapace is preserved in thisway but is masked by overlying matrix in-between these ridges.

These findings agreewith the XRD analysis conducted by Lieberman(2003), which showed that hematite was present in Early Cambrianspecimens. Hematite is also the likely source of the red pigmentation inthe shale matrix. Both the nature of shale coloration and the origin ofhematite in red beds have been a source of controversy amongsedimentologists (Van Houten, 1973). Red beds have traditionallybeen thought to indicate oxygenated depositional environments (VanHouten, 1973); however, the influence of secondary pore waters andweathering can contribute to the oxidation of iron and otherminerals togenerate red coloration (Kiipli et al., 2000). Since many aspects of redbed formation are still poorly understood (Bensing et al., 2005) a fullexplanation of themethodormethodsbywhich the red shale originatedat Pioche is beyond the scope of this study.

Element maps of Anomalocaris reveal that Al and Mg in addition toFe are elevated in the region of the fossil (Fig. 3B). Si is not elevated inAnomalocariswhich indicates that the Al andMg are not in the form ofsilicates. Elevated Al and Mg are not evident in the specimen of Tuzoia(Fig. 3A).

A gray dendritic mineral occurs on the surface of fossils from theEarly Cambrian and is visible in hand specimens. In secondary electron

Fig. 2. Secondary electron SEM images of mineralization. A–C Tuzoia (KUMIP 298507a), Early Cambrian. A,B: Botryoidal hematite preserving fossil (Fe2O3). C: Pyrolusite (MnO2) crystalsshowing scaly habit. D–H: Canadaspis (KUMIP 307021a), Middle Cambrian. D: Pyrite mineralization limited to the fossil (top half of image). E: Clay moulds show where pyrite crystalswould have originally occurred. F: Framboid, originally pyrite but now altered to iron oxide. G: Iron-oxide crystals and clay moulds. H: Monazite crystal. Scale bars=20 µm.

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SEM images this is seen as having a scaly habit (Fig. 2C) and elementmaps reveal Mn is elevatedwhere it is present (Fig. 3A). Themineral ismost likely to be pyrolusite (MnO2). Other mapped elements (Ca, C,Na) show no elevation in this mineral or the fossil.

Pyrolusite occurs at other sites of exceptional fossil preservation, suchas the Jurassic SolnhofenLithographic Limestone (Barthel et al.,1990). Thissecondary mineralization is formed when Mn-rich pore waters interactwith oxygenating fluids or the atmosphere (Chopard et al., 1991).

4.2. Preservation of specimens of Middle Cambrian Age

Under the secondary electron SEM beam Middle Cambrian speci-mens of Tuzoia and Anomalocaris show a significantly differentmineralogical texture to the Early Cambrian specimens. Clay moulds,

with discrete patches in which the original disseminated crystals arestill present, define the fossil (Fig. 2D–G). Clusters of framboids arealso present in some regions of the fossil and range from 8–10 µm indiameter (Fig. 2E and F). The framboidal habit implies the originalcomposition was pyrite (FeS2). Element maps confirm that Fe isabundant in the framboids and in the disseminated crystals, but EDSspot analysis on these features did not generate a sulfur peak in theresulting spectra. Element maps also failed to show an elevated signalfor sulfur (Fig. 3D). The most likely explanation for this is that theoriginal pyrite has been secondarily oxidized in a manner similar tothat described by Gabbott et al. (2004) for the fossils of the EarlyCambrian Chengjiang biota. Clay moulds occur over the whole surfaceof the fossil indicating that pyrite was originally present all over thisregion. The crystals that would have originally been present in the clay

Fig. 3. Photographs and element maps of the arthropod specimens selected for study. Boxed regions in the photographs show the area of the fossil that was mapped. A: Tuzoia nitida,Early Cambrian Pioche Shale, Nevada (KUMIP 298507a). B: Anomalocaris pennsylvanica, Early Cambrian Pioche Shale, Nevada (KUMIP 298527a). C: Tuzoia guntheri, Middle CambrianPioche Shale, Nevada (KUMIP 307020b). D: Anomalocaris cf. saron, Middle Cambrian Pioche Shale, Nevada (KUMIP 298529a). Scale bars: photographs=1 cm, element maps=1mm.

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moulds were presumably lost as the rock was split or remain attachedto the fossil counterpart.

The mineralization of pyrite was clearly key to the preservation of theMiddle Cambrian soft-bodied biota at Pioche. The decay of soft tissues inmarine environments induces the conditions that promote authigenicmineralization and therefore preservation (Briggs, 2003). However, fossilpreservation relies on the rate of mineralization exceeding decay (Allison,1988). In marine environments, such as those that prevailed at Pioche,oxygen is quickly depleted by aerobic bacterial decay of organic materialthus establishing an anoxic environment (Allison,1988). In the absence ofoxygen bacteria use alternative oxidants to metabolise organic material,and in marine environments sulfate reduction dominates due to theavailability of sulfur species (Allison,1988; Grimes et al., 2001, 2002). Thedecaying carcasses of Canadaspis, Anomalocaris and other Pioche animalswould have acted as a reducing substrate for microbial sulfate reduction,forming H2S from the seawater sulfate (Raiswell and Canfield, 1998;Gabbott et al., 2004). They would also have served as a locus forpyritization because of enhanced nucleation of pyrite on organicsubstrates (Grimes et al., 2001). Themost likely source for the Fe requiredfor pyrite formation is from the surrounding Fe-rich sediment, as has beendescribed for Chengjiang fossils by Gabbott et al. (2004). Experimentalpyrite formation in plants has shown that initial pyritization can occurrapidly (within 80 days) and can produce all the mineral texturesidentified in fossilized plant material from the Eocene London Claydeposits, includingmicrocrystalline euhedral forms and framboids (Butlerand Rickard, 2000; Grimes et al., 2000, 2001). Framboids formwhen theenvironment is supersaturatedwith pyrite so that the rate of nucleation isdominant over crystal growth (Butler and Rickard, 2000).

The dissemination of available Fe2+ to nucleation sites is animportant controlling factor in the production of pyrite. Gabbott et al.(2004) suggest that the highest concentration of the constituentsrequired for pyritization occurs where the organic material andsediment meet and framboids appear to be more common at thesediment–fossil junction in our Middle Cambrian specimens; restric-tion of pyrite authigenesis to the carcass indicates that the surround-ing sediment contained little or no carbon (Gabbott et al., 2004).

Element maps of a marginal spine on the Tuzoia carapace clearlyshow that carbon is elevated in the region of the fossil (Fig. 3C). Thisevidence, along with the reflective nature of the fossils in handspecimen, indicates that a carbon film is present. Carbon was notdetected in the Anomalocaris appendage, which may reflect the morerobust, and hence decay-resistant, nature of the Tuzoia cuticle. Thecarbon film in Tuzoia is present around the carapace margin and in thecharacteristic reticulated ornament of the carapace. The presence oforganic carbon film is significant as it is a requirement of Burgess Shale-type preservation as described by Butterfield (1990, 1995, 2003).

4.2.1. Other mineralsThe rare earth element (REE) phosphate mineral monazite was also

found on the surface of both fossils and the surrounding shale; it occursas crystals between 20 and 40 µm in diameter (Fig. 2H). The crystalshave irregular edges and appear brighter under backscatter electronimaging due to charging related to the high atomic weight of theelements present. EDS analysis showed peaks of the rare earth elementsLa, Nd, Ce, Gd in addition to P. No Th peak was detected in the mineral.This agreeswith an identification of low temperature, or gray,monazite,as this mineral typically has less than 1% Th (Burnotte et al., 1989).

Apatite was not found in the specimens studied, although it ispresent in other Cambrian soft-bodied biotas including the Chengjiang(Zhu et al., 2005). This may have been due to the oxidation of pyrite inMiddle Cambrian specimens which would have yielded sulfuric acid(H2SO4) (Kamei and Ohmoto, 2000), an acid that dissolves commonphosphate minerals including apatite (CaPO4). Monazite, by contrast,is resistant to attack by sulfuric acid (Chang et al., 1998).

Burnotte et al. (1989) studied gray monazite in Belgian Cambro-Ordovician black slates and determined that similar abundances of REEs

in the host rock andmonazite implied a desorption of REEs from the clayand re-precipitation in the sedimentduringdiagenesis. A similarprocessmay be responsible for the precipitation of monazite at Pioche; theirregular edges and delicate lace-like structure of the monazite crystalsindicate that they are authigenic rather than detrital.

5. Conclusions

It is tempting to suggest that the differences in preservation betweenthe Early Cambrian and Middle Cambrian soft-bodied biotas of Piochereflect changing environmental conditions across the boundary, asLieberman (2003) argued. However, another possible explanation isthat differences in weathering account for the presence of hematite inthe Early Cambrian soft-bodied fossils and its absence from MiddleCambrian soft-bodied fossils. If this is the case, and pyrite was originallypresent in both Early and Middle Cambrian specimens, it is still unclearwhat process allowedhematite to form in those fromtheEarlyCambrianand not those in theMiddle Cambrian. The extent to whichweathering,and secondary re-mobilization of ions, has influenced fossil preservationis also a complicating factor in resolving the diagenesis of exceptionallypreserved fossils from other localities, such as the Chengjiang.

Thus far, the preservational style of the Early Cambrian soft-bodiedfossils from the Pioche seems the most distinctive. Although otherCambrian biotas show preservation of soft-bodied animals in red-orange minerals that superficially resemble those of the Piocheanalysis has shown that they have different mineralogical composi-tion (e.g., pink calcium carbonate and calcium phosphate preserveslightly sclerotized animals in the Emu Bay Shale; Briggs and Nedin,1997; Nedin, 1997). Therefore, the preservation of the soft-bodiedfauna in the Lower Cambrian part of the Pioche Shale may not have anadequate analogue, and this further highlights the great diversity ofstyles of soft-bodied preservation in the Cambrian. We also note thatin the Lower Cambrian Latham Shale of California soft-bodied animalsappear to be preserved in a gray-green shale that weathers to red(Briggs and Mount, 1982). Fossils from this locality, including Ano-malocaris appendages, are most likely also preserved as kerogenizedcarbon films (Gaines and Droser, 2002). These faunas might usefullybe targeted for future investigation of fossil mineralogy.

By contrast, the preservation of the soft-bodied fauna in theMiddleCambrian part of the Pioche Shale shows similarity to other soft-bodied faunas, in particular, the Early Cambrian Chengjiang biota. Forexample, both have soft tissues similarly replaced by framboidal anddisseminated oxidized pyrite, which are also less frequently preservedas carbon films (Gabbott et al., 2004; Zhu et al., 2005). This type ofpreservation has also been described from the Lower CambrianKinzers Formation of Pennsylvania, although there tissues are alsoreplicated in aluminosilicate films (Skinner, 2005). The preservationof the Burgess Shale animals of British Columbia, Canada also bearssome resemblance to the Middle Cambrian soft-bodied fauna fromPioche. This is particularly evident in the presence of carbon films.Aluminosilicate templating of more labile soft tissues and the outlineof the cuticle has also been shown to be an important preservationmode in the Burgess Shale (Orr et al., 1998), but there is little evidencethat this occurred in the Middle Cambrian Pioche biota. The modes ofpreservation observed in the Middle Cambrian of Pioche mostresemble those described from the Chengjiang biota and thereforethe post-burial taphonomic pathways are likely to have been similar.Therefore, the conditions that promoted the preservation of theChengjiang biota may have a wider taphonomic significance.

Evidence for the environmental conditions that prevailed duringthe Early–Middle Cambrian comes from studies of key isotope species.Montañez et al. (2000) created the first high-resolution carbonisotope curve for the Cambrian and discovered a significant shift tonegative ∂13C values at the terminal Early Cambrian. This shift occursjust prior to the Early–Middle Cambrian boundary where a majorfaunal change, including a trilobite mass extinction event, has

62 R.A. Moore, B.S. Lieberman / Palaeogeography, Palaeoclimatology, Palaeoecology 277 (2009) 57–62

previously been documented (Palmer, 1988). Montañez et al. (2000)interpreted the negative shift in ∂13C isotope values to indicate theonset of significant climatic and paleoceanographic changes prior tothe faunal turnover at the Early–Middle Cambrian boundary. Thespread of anoxic bottom waters, depleted in 13C, onto shallowcarbonate platforms and associated decrease in the burial rate oforganic matter at the end of the Early Cambrian, due to major biomassreduction, are believed to have contributed to the negative shift in theC-isotope curve (Moñtanez et al., 2000). Further evidence for thespread of anoxic waters during the Early–Middle Cambrian transitioncomes from independent analysis of sulfur isotopes in francolite(Hough et al., 2006). A sharp shift to high ∂34S values around the Early–Middle Cambrian boundary is believed to be due to increased rates ofbacterial sulphate reduction leading to increased rates of pyrite burialduring the spread of anoxic bottom waters over carbonate platforms(Hough et al., 2006). The development of anoxia is most likely relatedto a marine transgression at the end of the Early Cambrian (Montañezet al., 2000) and an episode of volcanism from the Kalkarindji LargeIgneous Province of northern Australia at c. 510 Ma that released vastquantities of CO2, thus triggering ocean warming during the Early–Middle Cambrian transition (Hough et al., 2006). The preservation ofkey non-mineralized faunas, at such as those that occur at Pioche, islikely to be related to the spread of these anoxic waters onto carbonateplatforms during the Early–Middle Cambrian transgression.

The role of anoxia as a causal mechanism for BST is currently not fullyunderstood. It has traditionally been thought to be a prerequisite for soft-bodied preservation (Allison and Briggs, 1993; Allison, 1988), butgeochemical study of Burgess sediments by Powell et al. (2003) revealedevidence of oxygenated bottom waters at the time of deposition of thefossil-bearing beds. Furthermore, the only evidence for anoxia came frombeds that yielded few, if any, fossils (Powell et al., 2003). However, anoxiadoes play a key role in reducing the degree of scavenging and bioturbation(Briggs, 2003) (the absence of significant bioturbation prior to the LateCambrian has been suggested as a factor in the temporal constraints ofBST; Allison and Briggs, 1993; Droser et al., 2002; Orr et al., 2003).

Further stratigraphic sampling andmineralogical studies of the Early–Middle Cambrian section of the Pioche Formation would yield additionaluseful information on the diagenetic relationship between the faunasdescribed here. This would also help to determine the role of weatheringon the preservation of these faunas and further clarify the degree ofsimilarity to the preservation of the Chengjiang Biota, and hence thesignificance of global environmental changes on fossil preservation.

Acknowledgements

We thank Paddy Orr, Wayne Powell and one anonymous reviewerfor useful reviews of an earlier version of this paper. We are grateful toWayne Powell and Robert Gaines for inviting us to participate in thisvolume. Stuart Kearns is thanked for assistance with the SEM and EDSfacilities at the University of Bristol, UK. We also thank S. Gabbott, P.Orr, and D. Stockli for discussions. Funding was provided by NSF EAR-0518976, NASA Astrobiology NNG4GMYIG and a Self Faculty ScholarAward to BSL. We are grateful to the Gunthers, Whiteleys andMcCollums for contributing specimens used in this study.

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