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Home , Ctenocystoidea, Gordia, House Range, Kaili Formation, Morania, Peronopsis

Dynamic palaeoredox and exceptional preservation in the Spence of

DANIEL E. GARSON, ROBERT R. GAINES, MARY L. DROSER, W. DAVID LIDDELL AND AARON SAPPEN- FIELD

Garson, D.E., Gaines, R.R., Droser, M.L., Liddell, W.D. & Sappenfield, A. 2011: Dynamic palaeoredox and exceptional preservation in the Cambrian of Utah. Le- thaia, DOI: 10.1111 ⁄ j.1502-3931.2011.00266.x

Burgess Shale-type faunas provide a unique glimpse into the diversification of metazoan life during the Cambrian. Although anoxia has long been thought to be a pre-requisite for this particular type of soft-bodied preservation, the palaeoenvironmental conditions that regulated extraordinary preservation have not been fully constrained. In particular, the necessity of bottom water anoxia, long considered a pre-requisite, has been the sub- ject of recent debate. In this study, we apply a micro-stratigraphical, ichnological approach to determine bottom water oxygen conditions under, which -type biotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of the Spence Shale are characterized by fine scale (mm-cm) alternation between laminated and bioturbated intervals, suggesting high-frequency fluctuations in bottom water oxygena- tion. Whilst background oxygen levels were not high enough to support continuous infaunal activity, brief intervals of improved bottom water oxygen conditions punctuate the succession. A diverse skeletonized benthic fauna, including various polymerid trilo- bites, hyolithids, and ctenocystoids suggests that complex dysoxic benthic community was established during times when bottom water oxygen conditions were permissive. Burgess Shale-type preservation within the Spence Shale is largely confined to non-bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, where these were preserved. However, some soft-bodied fossils are found within weakly to moderately bioturbated intervals (Ichnofabric Index 2 and 3). This suggests that Bur- gess Shale-type preservation is strongly favoured by bottom water anoxia, but may not require it in all cases. h Anoxia, Burgess Shale, Burgess Shale type-preservation, Langston Formation, Spence Shale Member, Utah.

Daniel E. Garson [[email protected]], Department of Earth Sciences, University of California, Riverside, CA 92521, USA; Robert R. Gaines [[email protected]], Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA; Mary L. Droser [[email protected]], Department of Earth Sciences, University of California, Riverside, CA 92521, USA; W. David Liddel [[email protected]], Department of Geology, Utah State University, Logan, UT 84322-4505, USA; Aaron Sappenfield [aaron.sappenfi[email protected]], Department of Earth Sciences, University of California, Riverside, CA 92521, USA; manuscript received on 14 September 2010; manuscript accepted on 04 February 2011.

Burgess Shale-type (BST) biotas are named after the preservation of BST assemblages may be just as world famous locality from which they were first importantastheexceptionalfossilsthemselvesasit described, but represent a global, if rare, phenomenon. speaks directly to the unique environmental condi- Well-described BST localities are known from North tions that were widespread in the marine realm at the America, , , and and are time. largely confined to Series 2 and Series 3 of the Cam- Anoxia, at least within the sediments, has been con- brian (Conway Morris 1989a; Butterfield 1995). These sidered a necessary pre-requisite for BST preservation deposits provide a unique window on the early diver- (Allison & Brett 1995; Butterfield 1995; Gaines et al. sification of the Metazoa (Conway Morris1989a, 2005). Anoxia may increase the preservation potential 1992). BST assemblages are characterized by a com- of soft tissues in two ways: by preventing the direct mon preservational style, in which soft tissues of scavenging of tissues by , and by potentially organisms were conserved as carbonaceous compres- helping to slow the normal processes of microbial sions in fine-grained marine sediments (Butterfield decay (Allison & Briggs 1991). Oxygen is the most 1995; Gaines et al. 2008). However, the mechanisms energetically favourable oxidant for degradation of controlling this taphonomic pathway, referred to as organic matter (Berner 1981) and soft tissues are BST preservation, and this pathway’s restriction in degraded quickly under oxic conditions. However, time are not fully understood. Insight into the anoxia alone is not sufficient to explain the

DOI 10.1111/j.1502-3931.2011.00266.x 2011 The Authors, Lethaia 2011 The Lethaia Foundation 2 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x preservation of soft tissues as oxidation of organic Hou 2007), which again would have required preser- matter by sulphate-reducing bacteria can occur at vation in close temporal and spatial association with comparable rates to oxic degradation (Foree & at least marginally oxygenated bottom waters. McCarty 1970; Henrichs & Reeburgh 1987; Allison Gaines & Droser (2005) described three distinct 1988; Lee 1992). Actualistic experiments using shrimp oxygen related microfacies within superficially monot- and other non-mineralizing taxa have also shown that onous claystones of the Middle Cambrian Wheeler degradation of soft tissue occurs rapidly under anoxic Formation, a BST deposit, which occurs in the House conditions (Allison 1988; Briggs & Kear 1994). Range and Drum Mountains of Utah, based on ich- Although anoxia is generally considered as a necessary nofabric index (i.i.), a semi-quantitative method to pre-requisite, anoxia alone is not sufficient to account distinguish relative amounts of bioturbation (Droser for BST preservation. Some additional mechanism is & Bottjer 1986) and diagnostic body assem- required for the early diagenetic stabilization of labile blages. This model was subsequently applied to the tissues. This preservational mechanism remains con- , which also occurs in the House troversial (Butterfield 1995; Petrovich 2001; Gaines Range (Gaines & Droser 2010). These studies demon- et al. 2005). strated that nearly all occurrences of BST preservation The necessity of anoxic conditions as a pre-requisite in both formations occur within laminated intervals, for BST preservation is not universally accepted. On entirely lacking bioturbation (i.i.1), consistent with the basis of trace metal ratios interpreted as palaeore- benthic anoxia. In addition, this anaerobic microfacies dox indicators, Powell et al. (2003) argued for mini- includes some skeletal body fossils, dominantly agnos- mally oxic conditions during deposition of much of tid (Gaines & Droser 2005). A dysoxic the Burgess Shale, including intervals containing BST microfacies was defined by the presence of weakly preservation. In addition, Caron & Jackson (2006, developed, shallow ichnofabrics (i.i.2 and i.i.3) and it 2008) have argued for in situ preservation of Burgess includes a low-diversity shelly benthic fauna. At the Shale assemblages; obviously, consistently oxic or dys- margin of the dysoxic environment, the Elra- oxic bottom water conditions would be required to thia kingii occurs in dense (up to 500 individuals ⁄m2) sustain a benthic fauna. This interpretation was based monospecific assemblages that represent an exaerobic on taphonomic evidence suggesting ‘minimal’ trans- microfacies (Savrda & Bottjer 1987) at the transition port of the Burgess Shale biota (Caron & Jackson between laminated (i.i.1) and weakly bioturbated 2006). (i.i.2) sediments (Gaines & Droser 2003). These three Allison & Brett (1995) reported that bioturbation microfacies, recognized at the millimetre scale, may be and BST preservation occur in mutually exclusive closely interbedded (mm-cm scale), and represent horizons in the Burgess Shale and argued for oscilla- oxygen levels that alternated between anoxic environ- tions between anoxic and dysoxic bottom waters dur- ments conducive to the preservation and low-oxygen ing its deposition. Anoxic benthic conditions were environments conducive to the establishment of ben- interpreted to have prevailed with periodic and strati- thic metazoan communities. Applied more broadly, graphically brief (cm) episodes of bottom water oxy- this model suggests that BST preservation occurred genation that allowed colonization by an in situ primarily through transport from the living environ- benthic fauna. ment to the anoxic preservational trap. Gaines & Dro- Evidence from the Lower Cambrian Chengjiang ser (2005, 2010) recognized a proximal–distal gradient BST deposit of South China suggests a complex palae- in composition and articulation of BST assemblages, oredox history. Dornbos et al. (2005) found that sedi- andalsodocumentedexamplesofin situ preservation, ments of the Maotianshan shale in the Yu’anshan although such occurrences are most rare. Formation, which contain BST preservation, are Here, we apply a micro-stratigraphical approach to almost entirely unbioturbated whilst underlying sedi- the Spence Shale Member of the Langston Formation, ments of the Shiyantou Formation, which is not a a BST deposit of early ‘Middle’ Cambrian age, occur- BST deposit, are consistently moderately bioturbated. ring in the lower part of the yet unnamed Cambrian However, Zhang et al. (2007) found examples of trace Series 3 in the Wellsville Mountains of Utah (Fig. 1), fossils in direct contact with soft-bodied fossils in the to determine the palaeoredox context of diverse BST Yu’anshan Formation, demonstrating that it is possi- assemblages known from that deposit (Robison 1991; ble for BST preservation to occur in close association Briggs et al. 2008). The Spence Shale is similar in pal- with sediments deposited under bottom water with aeogeography and depositional environments (Liddell oxygen high enough to support a benthic fauna, at et al. 1997) to the Wheeler Formation, and it contains least periodically. The orientations of Chengjiang fos- a similar fossil biota within a fine-grained claystone sils indicate transport of most assemblages; only rarely lithofacies, providing an opportunity for comparison fossils appear to have been preserved in situ (Zhang & with previous findings. LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 3

114˚ 113˚ 112˚ 111˚ carbonatebelt.TheSpenceiswithinthePeronopsis 42˚ N bonnerensis agnostid and is older than the Burgess Shale, Wheeler and Marjum Formations.

Ogden The Spence Shale Member is comprised of a series 41˚ 41˚ of metre-scale shallowing-upward cycles, which repre- Salt 80 sent parasequences (Liddell et al. 1997). Each cycle is Lake City expressed as shale passing upwards into lime mud- Provo stones, grainstones or nodular recording 40˚ 40˚ the influence of the impinging carbonate platform. An onshore-offshore trend is apparent within this 15 Spence Shale of the Wellsville Mountains with locali-

39˚ 39˚ ties in the southwest lying proximal to a carbonate 70 platform, and becoming deeper and more distal towards the northeast (Liddell et al. 1997). The num- ber of metre-scale shallowing-upward cycles expressed 38˚ Utah 38˚ and the type of carbonate capping the cycles varies from locality to locality based on proximity to the car- 100 km bonate platform; sections interpreted to be deeper

37˚ 37˚ express fewer, thicker, cycles. Miners Hollow repre- 114˚ 113˚ 112˚ 111˚ 110˚ 109˚ sents the most distal deposition within the Wellsville Fig. 1. Geographic map of Utah, USA, showing location of the Mountain localities and consists of six metre-scale Spence Shale localities in the Wellsville Mountains (star). cycles, the lowest two of which were capped by lime mudstone, and all higher cycles capped by nodular lime mudstone and fossiliferous wackestone interbed- Geological setting ded with shale (Liddell et al. 1997; Fig. 2). The Spence Shale contains a diversity of biominer- Burgess Shale-type preservation occurs worldwide in alizing fossils including multiple polymerid trilobites, Lower and Middle Cambrian (–3) agnostid trilobites, hyolithids, brachiopods, echino- strata with older occurrences known from Proterozoic derms including eocrinoids and ctenocystoids and strata of several palaeocontinents (Butterfield 1995). (Walcott 1908; Resser 1939; Robison & Sprin- Middle Cambrian occurrences are largely confined to kle 1969; Gunther & Gunther 1981; Sprinkle 1985). and represent the deposition in conti- In addition, the Spence Shale also preserves a number nental slope or basin settings on what was at the time of non-mineralizing organisms preserved as carbona- the northern margin of Laurentia (Conway Morris ceous compressions, including a diverse array of 1989b; Butterfield 1995). Three Middle Cambrian , eldoniids, annelids, priapulids, algae and BST deposits occur in Utah: the Spence Shale Member (Resser 1939; Robison 1969; Conway of the Langston Formation, the Wheeler Formation Morris & Robison 1986, 1988; Robison & Wiley 1995; and the Marjum Formation. During the Middle Cam- Briggs et al. 2008). brian, the present-day western margin of Laurentia was a passive margin with three distinct marine facies belts (Robison 1976). A near-shore inner detrital belt Using trace fossils as a proxy for was separated from a more distal fine-grained outer bottom water oxygenation detrital belt by a broad carbonate belt. BST deposits in Utah and elsewhere are characterized by deposition A relationship between bottom water oxygenation and in the outer detrital belt, near the distal margin of the extent and depth of bioturbation was first described carbonate belt (Robison 1991). by Rhoads & Morse (1971), who developed an oxy- The Spence Shale is exposed in Northern Utah in gen-related biofacies model in modern environments. the Wellsville Mountains and in Southern Idaho in An aerobic biofacies, defined by bottom water dis- the Bear River Range (Liddell et al. 1997). The Spence solved oxygen concentrations above 1.0 ml ⁄ l, is char- ShaleistheMiddleMemberoftheLangstonForma- acterized by great depth and extent of bioturbation tion, and is dominated by shale with minor intervals and a diverse benthic fauna. Dissolved oxygen concen- of at most localities. The Upper and Lower trations between 1.0 ml ⁄ land0.1ml⁄ l are consid- Members of the Langston Formation are both domi- ered dysoxic and are characterized by a low-diversity nantly limestone representing deposition within the benthic fauna and reduced depth and extent of 4 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x

by a monotypic skeletonized fauna appearing in great High Creek abundance at the micro-stratigraphic boundary 60 Limestone between laminated and weakly bioturbated sediments. Member This environment, lying at the distal margin of oxy- 55 Cycle 7 genated bottom waters promotes the development of extensive populations of sulphur-oxidizing bacteria, 50 Cycle 6 MH6 which may provide a food source for metazoans adapted to low-oxygen concentrations (e.g. Savrda & 45 Cycle 5 Bottjer 1986; Gaines & Droser 2003). The ichnofabric index (Droser & Bottjer 1986) pro- 40 Cycle 4 vides a semi-quantitative means of assessing the rela- tive extent of bioturbation. Completely undisturbed, 35 MH3U laminated sediments are assigned an ichnofabric index (i.i.) of 1, and sediments which have been completely homogenized by bioturbation are assigned to i.i.6,

Meters 30 with intermediate values representing specific ranges of percentage of cross-sectional area of disturbed lami- 25 Cycle 3 nation. Ichnofabric indices have become a standard tool for determining relative bottom water oxygena- 20 MH3L tion for palaeoecological studies (e.g. Twitchett & Wignall 1996; Boyer & Droser 2007; Herringshaw & 15 Davies 2008; Kakuwa 2008). Because ichnofabric indices are determined independently of specific trace 10 fossil morphologies, they are particularly useful for Cycle 2 Cambrian strata in which diversity in 5 deep-water environments was lower than in younger strata (Orr 2001). Furthermore, the resolution of data Cycle 1 0 produced from trace fossil analysis may be much finer Naomi Peak in Cambrian sediments; an overall shallower depth of Limestone bioturbation results in better preservation of thin Member event beds and higher resolution of detail within the shallow tier (Droser et al. 2002). This increased reso- lution due to shallower burrowing is especially benefi- cial to the interpretation of low-oxygen environments with low-sedimentation rates, where even short term oxygenation events have the potential to overprint sedimentary evidence of overall low-oxygen condi- tions. Fig. 2. Stratigraphic section of the Spence Shale at the Miners Hol- low locality (after Liddell et al. 1997). Continuously sampled inter- vals studied herein (MH3L, MH3U and MH6) are highlighted. Materials and methods bioturbation. Oxygen concentrations below 0.1 ml ⁄l Collection of material in the field are considered anoxic and characterized by a lack of bioturbation and the absence of a benthic fauna. Sub- Intervals of two cycles at the Miners Hollow locality in sequently, Savrda et al. (1984) developed a similar the Wellsville Mountains (Figs 1, 2) were targeted for model distinguishing anaerobic from dysaerobic bio- continuous sampling. This locality was chosen as it facies based on trace fossils in modern marine sedi- contains intervals bearing abundant BST preservation ments that could be applied to ancient sediments. of a diverse, soft-bodied biota (Robison 1991; Briggs Savrda & Bottjer (1986) developed a model for creat- et al. 2008). Three continuous intervals ranging in ing relative dissolved oxygen curves based on size and thickness from 61 to 361 cm were collected and type of trace fossil and their cross-cutting relation- returned to the laboratory for micro-stratigraphic ships, allowing further differentiation of oxygen levels analysis. within the dysaerobic zone. Savrda & Bottjer (1987) At each interval, the section was measured perpen- defined the exaerobic biofacies, which is characterized dicular to observed bedding and marked. Additional LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 5 adjacent material was also marked for fossil collection. the position of the specimen within the section to the The interval was then collected by removal of block nearest 0.5 mm and marked on the stratigraphic samples using hand tools and a petrol-powered rock section. Articulation and attitude were described for saw. Additional material taken adjacent to the contin- biomineralized body fossils. uous section was split along bedding planes as finely as possible to identify fossils. Any sample containing a partial or complete body fossil was retained and its Results orientation and approximate position within the sec- tion were recorded. Laboratory analysis of fine lami- The majority (95.7%) of section MH3L is non-biotur- nations within the fossil-bearing shale allowed for the bated (i.i.1); however, a few thin (<2.5 mm) intervals precise position of each fossil to be determined and exhibit in weakly developed ichnofabrics (i.i.2, 3.3%; tied into micro-stratigraphic logs. i.i. 3, 1.0%; Fig. 3). In contrast, the majority of section Two intervals were collected from Cycle 3, which MH3U is weakly bioturbated (Fig. 4). Only 36.6% of exceeds 20 m in thickness and comprises nearly half the thickness of the section is non-bioturbated (i.i.1), of the thickness of the Spence Shale (Fig. 2). A 66-cm with weakly to moderately developed ichnofabrics interval (MH3L) was collected from an exposure ca. present in the rest of the interval (50.1% i.i.2; 13.0% 3 m above the base of Cycle 3 (contact with Cycle 2 i.i.3, 0.4% i.i.4). The sampled interval of Cycle 6 shows covered), which has been interpreted to represent the significant variability in the extent of bioturbation maximum water depth at this locality (Liddell et al. (Fig. 5), with 43.3% of the section assigned to i.i.1, 1997). A second interval, 97.5 cm in thickness, was collected from 62 cm below the top of Cycle 3 (MH3U). This interval was chosen because it contains abundant soft-bodied fossils, including those of meta- zoans, and fresh material is well-exposed in the wall of asmallquarry. A 458 cm interval representing the majority of the shale portion of Cycle 6 was also collected (MH6). Cycle 6 contains at least two intervals bearing abun- dant soft-bodied metazoans, and fresh material from the base of this cycle is well-exposed in a large quarry (Fig. 2). The bottom portion of the interval collected represents the uppermost part of Cycle 5.

Laboratory analysis After collection, samples were cut perpendicular to bedding, polished and scanned wet or partially sub- merged in water at ‡600 dpi on an Epson desktop scanner. Once scanned, all samples were arranged in stratigraphic order and digitally assembled in Photo- shop, and any overlaps between samples removed. A micro-stratigraphic log was drawn digitally using the digital images of the continuous sections as a basis. Primary sedimentary structures (bedding), bioturba- tion and authigenic mineral growths were recorded on the logs. i.i. were recorded at the millimetre scale from the digitized continuous section. The approximate position of samples containing body fossils was recorded in the field. The shale sam- ples containing fossil specimens were cut perpendicu- lar to bedding, as close to the fossils as possible. Cut surfaces were then polished and scanned in the way described above for the continuous section samples. Fig. 3. Section MH3L showing ii. Scale in cm. Darker intervals indicate intervals that were darker in appearance after polishing Sedimentary features from the fossil specimens were and indicate higher carbonate content. Intervals missing ii were matched with those of the continuous section to find too weathered for determination. 6 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x

to i.i.4 and 2.9% to i.i.5. The overall pattern within Cycle 6 is increasing extent of bioturbation upsection, consistent with the upward shallowing and increasing benthic oxygen content described by Liddell et al. (1997). Section MH3L is dominated by ,aputative cyanobacterium, and agnostid trilobites. The positions of soft-bodied and skeletonizing fossils within a 10 cm interval from MH3L are shown in Figure 6. Soft- bodied fossils are far more abundant than skeletoniz- ing fossils within this interval and consist almost exclusively of Morania (Fig. 11A). Agnostid trilobites of the Peronopsis are also abundant in this inter- val and commonly occur in the same beds as soft- bodied fossils. The remainder of the skeletonizing fauna consists of polymerid trilobites (mostly pty- choparid trilobites less than 2 cm in length) and bra- chiopods. Almost all fossils occur in non-bioturbated sediments (i.i.1), however, on a single bedding plane BST preservation of Morania occurs with agnostid tri- lobites and small ptychoparid trilobites associated with a thin (1.5 mm) interval of moderate bioturba- tion (i.i.3; Table 1). Both soft-bodied and skeletonizing fossils are com- mon within section MH3U (Fig. 7), but different taxa occur in this section. More than half of the examples of soft-bodied preservation in this interval are repre- sented by the alga Marpolia spissa (Fig. 11B). Two examples of unidentified metazoans with preserved gut traces also occur within this section (Fig. 11C). Other specimens of soft-bodied organisms consisted of indeterminate disarticulated or degraded soft-bod- ied material. Despite an overall higher abundance of skeletonizing fossils, MH3U contains few examples of agnostid trilobites, with only four specimens identified in four different horizons. The remaining skeletoniz- ing fossils represent ptychoparid trilobites in both articulated and disarticulated states, brachiopods including Acrothele (Fig. 11D), as well as the hyolith- ids Haphlophrentis (Fig. 11D) and Hyolithes,bothof which were absent from MH3L. The majority (69%) of occurrences of soft-bodied fossils in this interval Fig. 4. Section MH3U showing ii. Scale in cm. Darker intervals occur within non-bioturbated sediments (i.i.1) and indicate intervals that were darker in appearance after polishing the remainder in weakly bioturbated (i.i.2) sediments and indicate shale with higher carbonate content. Darkest intervals (Table 1). Sixty three percent of benthic (non-agnos- represent carbonate deposition. tid) skeletonizing fossils occur within i.i.2 horizons and the remaining 27% occur in non-bioturbated 20.6% assigned to i.i.2, 28.4% assigned to i.i.3, 4.5% (i.i.1) sediments (Table 1). assigned to i.i.4 and 2.9% assigned to i.i.5. The basal The occurrences of soft-bodied and skeletonized 0.49 m of section MH6 represents the top part of fossils within three intervals of section MH6 are Cycle 5; several intervals (0–13; 19–21; 45–49 cm) are shown in Figures 8–10. Soft-bodied fossils are far less extensively bioturbated (i.i. ‡ 3). The sampled interval common than skeletonized fossils in MH6. Composi- of Cycle 6 shows significant variability in extent of tion of the soft-bodied assemblage in MH6 is similar bioturbation (Fig. 5), with 43.3% of the section to that of MH3U and is dominated by Marpolia spissa, assigned to i.i.1, 20.6% to i.i.2, 28.4% to i.i. 3, 4.5% (Fig. 11B). The skeletonizing fauna is also more LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 7

Fig. 5. Section MH6 showing ii. Scale in cm. Darker intervals indicate intervals that were darker in appearance after polishing and indicate shale with higher carbonate content. Darkest intervals represent carbonate deposition. similar to MH3U than MH3L; however, only a single (4%; Table 1). Only 14% of skeletonized fossils in agnostid trilobite was collected from MH6. Hyolithids MH6 occur in non-bioturbated (i.i.1) sediments. occur in the lowermost third of the entire cycle. The Within MH6 between 21 and 41 cm (Fig. 5) is an ctenocystoid, Ctenocystis utahensis occurs at three interval informally known by collectors as the ‘150¢ horizons within MH6, but was not recovered from layer for its approximate position 150 feet above the MH3L or MH3U. The three occurrences of this cteno- base of the Spence Shale section at Miners Hollow. cystoid occur within i.i.1 horizons and, in one hori- Fossil samples collected from this 20-cm interval zon, it co-occurs with soft-bodied fossils. BST could not be placed precisely within the interval preservation of fossils in these intervals preferentially because weathering has obscured primary fabric. occurs in i.i.1 sediments (77% of soft-bodied fossil However, this interval is known by collectors to con- occurrences n = 13; Table 1), although two specimens tain a higher than average density of soft-bodied fos- (15%) occur in i.i.2 intervals, and a single specimen of sils and has produced specimens of the algae Marpolia Acinocricus occurs in an interval of moderate biotur- and Yuknessia, arthropods including a canadaspid and bation (i.i.3). Skeletonizing fossils within these inter- Metitosoma paradoxum, and a soft-bodied vals preferentially occur in i.i.2 and i.i.3 intervals (Skabelund, personal communication 2008). This (82%) with two occurrences identified in i.i.5 intervals interval is characterized by low extent of bioturbation 8 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x

Fig. 6. Ichnofabric indices and occurrence of fossils within studied interval of MH3L. Ichnofabric indices are represented by different shades of grey according to the key in the column header. The remaining columns from left to right represent soft-bodied occurrences, agnostid trilobites, disarticulated polymerid trilobites, articulated polymerid trilobites, brachiopods, hyolithids and ctenocystoids. Scale in cm from base of MH3L.

Table 1. i.i. of fossil occurrences within all studied intervals.

Fossil occurrences

MH3L MH3U MH6 Total

benthic benthic benthic benthic skeletonizing soft-bodied skeletonizing soft-bodied skeletonizing soft-bodied skeletonizing soft-bodied i.i. fauna fossils fauna fossils fauna fossils fauna fossils i.i.1 6 24 7 9 7 10 20 43 i.i.2 – – 12 4 20 2 32 6 i.i.3 1 1 – – 20 1 21 2 i.i.4––– –– –– – i.i.5––– –2 –2 – n = 7 25 19 13 49 13 75 51

Fig. 7. Ichnofabric indices and occurrence of fossils within studied interval of MH3U. See figure 6 for explanation of columns. Scale in cm from base of MH3U. LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 9

Fig. 8. Ichnofabric indices and occurrence of fossils within lowermost studied interval of MH6. See figure 6 for explanation of columns. Scale in cm from base of MH6. relative to the MH6 as a whole and is dominated by Shale Member, like the Wheeler and Marjum Forma- i.i.1. tions (Gaines & Droser 2005, 2010; Brett et al. 2009), was deposited at the edge of a fluctuating oxycline. Soft-bodied fossil occurrences suggest a broadly Discussion similar pattern to those of previous studies from other deposits in which it was found that BST preservation The extent of bioturbation increases up-section in occurs preferentially in the absence of bioturbation both Cycle 3 (Figs 3, 4) and Cycle 6 (Fig. 5). This, in (Allison & Brett 1995; Dornbos et al. 2005; Gaines & combination with sedimentological evidence for shal- Droser 2005, 2010). In all sections, the majority of lowing-upward cycles (Liddell et al. 1997), suggests occurrences of soft-bodied fossils are within the lami- water depth as a primary control on bottom water nated sediments (i.i.1), whereas skeletonized fossils oxygenation during the period of deposition of the occur preferentially in weakly moderately bioturbated Spence Shale. Fluctuations between laminated and intervals. This suggests that anoxic benthic conditions bioturbated intervals occurs on millimetre and centi- favour BST preservation, whereas in situ skeletonized metre scales throughout, however, and appear to rep- fossils tend to occur under oxygenated bottom waters resent oscillations in bottom water content that were within Spence Shale and BST deposits in general. superimposed on the larger cyclic pattern. The Spence Specifically, the oxygen-related biofacies model 10 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x

Fig. 9. Ichnofabric indices and occurrence of fossils within the second studied interval of MH6. See figure 6 for explanation of ii and columns. Scale in cm from base of MH6. developed for the Wheeler Formation (Gaines & Dro- kingii (Gaines & Droser 2003), but no analo- ser 2005) is largely applicable to the Spence Shale. A gousassemblagefromtheSpenceShalecanbedefini- mostly anoxic assemblage consisting of transported tively assigned to the exaerobic zone. Occurrences of soft-bodied fossils and pelagic organisms, such as ag- the ctenocystoid Ctenocystis utahensis in laminated nostid trilobites (for deeper water intervals), which sediments in an interval that also contains soft-bodied occur in i.i.1 and a dysaerobic assemblage consisting fossils suggests tolerance of very low-oxygen levels, of benthic skeletonizing fossils, which occur in i.i.2 buttheirgreatraritysuggeststhatanopportunistic and higher is for the most part consistent with the mode of life was unlikely. Ctenocystoids are thought findings here. to be benthic, based on functional morphology (Robi- The soft-bodied fossils of the anoxic zone within son & Sprinkle 1969) and their preferential occurrence the Spence Shale can be subdivided into two separate in deep-water laminated sediments has been noted assemblages controlled by depth. This is also in accor- before (Parsley & Prokop 2004). dance with the proximal–distal gradient in soft-bodied Although this broad pattern is similar to that docu- fossil assemblages recognized in the Wheeler and Mar- mented from the Wheeler Formation, a significant jum Formations (Gaines & Droser 2005, 2010). The minority of soft-bodied fossils within the Spence Shale deeper water assemblage seen in MH3L is character- occur in weakly bioturbated sediments. This could be ized by abundant Morania. Agnostid trilobites are the suggestive of a higher benthic oxygen threshold for second most common fossil in this assemblage and BST preservation within the Spence Shale; however, although some skeletonizing benthic fossils including some caution must be exercised in reaching this con- brachiopods and small ptychoparid trilobites are clusion, as it is possible for soft-bodied fossils to occur found within the same bedding planes and adjacent in bioturbated sediments without exposure of soft tis- ones, they are rare. A more proximal soft-bodied sues to oxygen after burial. This is possible under two assemblage represented in MH3U and MH6 is charac- different scenarios, each of which predicts a specific terized by abundant Marpolia spissa.Rarebenthic pattern of soft-bodied fossil occurrence relative to the metazoans and Acinocricus also occur in this more location of discrete burrows. In the first hypothetical proximal shallower water anaerobic assemblage. case (Fig. 12A), soft tissues were buried under anoxic The Wheeler Formation contains a monospecific bottom water conditions resulting in burial either at exaerobic assemblage of individuals of the trilobite thebaseofthebedorwithinit.Atsometime LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 11

Fig. 10. Ichnofabric indices and occurrence of fossils within the uppermost interval of MH6. See figure 6 for explanation of ii and columns. Scale in cm from base of MH6. following burial, the return of dysoxic bottom water occurrence of Acinocricus (Fig. 13) in MH6 is within conditions promoted burrowing into the sediment, an interval, where multiple beds are bioturbated. This but by this time-soft tissues had already collapsed into fossil lies near the top of an interval with dense small two dimensions (e.g. Briggs & Kear 1994), and were (0.5 mm in diameter and smaller) burrows and is stabilized by the surrounding sediment. Associations directly overlain by an interval with somewhat larger of this type between trace and soft-bodied fossils have (0.5–1 mm diameter) burrows. The multiple bur- been reported from the Chengjiang (Zhang et al. rowed horizons suggest that at least this example was 2007), the (Yang et al. 2009; Lin likely deposited under or in very close temporal prox- et al. 2010), and the (Caron et al. imity to dysoxic conditions. 2010). In the second hypothetical scenario, dysoxic Average extent of bioturbation is higher in mud- conditions permitted bioturbation of a bed exposed at stone facies of the Spence Shale than the Wheeler the sea floor, but bottom water conditions subse- Formation. The maximum observed extent of biotur- quently became anoxic. Transportation and burial of bation in mudstones of the Wheeler Formation is i.i.3 soft-bodied fossils resulted in preservation of soft tis- (Gaines & Droser 2005), whilst i.i.4 and i.i.5 intervals sues at the top of a burrowed interval directly underly- are present within the Spence Shale. Relatively few ing a laminated interval. trace fossils in the Raymond Quarry and their com- These two possible scenarios, however, are unlikely plete absence from the of the Burgess to explain all occurrences of BST preservation within Shale (Allison & Brett 1995; Gabbott et al. 2008) indi- weakly bioturbated intervals of the Spence Shale. One cate that the Burgess Shale likewise experienced a 12 Garson et al. LETHAIA 10.1111/j.1502-3931.2011.00266.x

Fig. 11. Soft-bodied fossils from the Spence Shale. All scale bars 1 cm. (A) Morania fragmenta with the Acrothele. (UCR10829 ⁄ 1). (B) Marpolia spissa (UCR10832 ⁄ 1). (C) Unknown metazoan with gut trace (UCR10831 ⁄ 1). (D) Hyolithid Haphlophrentis with gut trace, Acrothele and various disarticulated trilobite material. (E) Acinocricus stichus (UCR1083 ⁄ 1). much lesser maximum extent of bioturbation than conditions at the very margins of those, where BST did the Spence Shale. Chengjiang sediments are also preservation was possible, rather than environments dominantly non-bioturbated (Dornbos et al. 2005). where those conditions were maximized for metres or Higher average extent of bioturbation and multiple tens of metres of continuous section. instances of soft-bodied preservation occurring in horizons that also bear trace fossils suggests a dynamic chemocline and with higher amplitude fluctuations Conclusions and periods of greater benthic oxygen availability for Spence Shale than for other BST deposits. Dynamic Occurrences of soft-bodied and skeletonizing fossils palaeoredox conditions may also explain, in part, the within the Spence Shale fit the broad pattern of two rarity and low diversity of soft-bodied fossils in the general oxygen related microfacies: an anoxic micro- Spence Shale compared with Chengjiang and Burgess facies characterized by common occurrences of BST Shale, as the Spence may represent taphonomic fossils in association with pelagic agnostid trilobites LETHAIA 10.1111/j.1502-3931.2011.00266.x Exceptional preservation in the Spence Shale 13

Acknowledgements. – We thank Jake Skabelund for access to col- lecting pits and extensive field assistance, Paul Jamison, Lucas Allen-Williams, Mike Balint, Stephan Hlohowskyj, Paul Hong, Greg Lawson, Brad Markle and Ryan McKenzie for field assistance, and Nigel Hughes for valuable discussion and comments. The manuscript was improved considerably by thoughtful reviews by Carlton Brett and one anonymous reviewer. This research was sup- ported in part by National Science Foundation grants (EAR- 0518732) to RRG and MLD and (DMR-0618417) to RRG.

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