TAPHONOMY AND DEPOSITIONAL SETTING OF THE BURGESS SHALE TULIP BEDS, MOUNT STEPHEN, BRITISH COLUMBIA

PALAIOS, 2014, v. 29, 309 324 Research Article DOI: http://dx.doi.org/10.2110/palo.2013.095 TAPHONOMY AND DEPOSITIONAL SETTING OF THE BURGESS SHALE TULIP BEDS, MOUNT STEPHEN, BRITISH COLUMBIA LORNA J. O BRIEN, 1,2,3 JEAN-BERNARD CARON, 1,2,4 AND ROBERT R. GAINES 5 1 University of Toronto, Department of Ecology and Evolutionary Biology, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada 2 Department of Natural History Palaeobiology, Royal Ontario Museum, 100 Queen s Park, Toronto, Ontario M5S 2C6, Canada 3 Current address: Royal Tyrrell Museum of Palaeontology, P.O. Box 7500Drumheller, Alberta T0J 0Y0, Canada 4 University of Toronto, Department of Earth Sciences, 25 Russell Street, Toronto, Ontario, M5S 3B1, Canada 5 Pomona College, Geology Department, 185 E. Sixth Street, Claremont, California 91711, USA e-mail: lorna.obrien@gov.ab.ca ABSTRACT: Burgess Shale type deposits represent exceptional preservational windows for examining the biodiversity and ecological structure of some of the earliest metazoan communities that evolved during the Cambrian Explosion. While much attention has been paid to the original Burgess Shale locality, the Walcott Quarry on Fossil Ridge, temporal and regional variations of the depositional environment of the Burgess Shale biota as a whole are still poorly understood. Here we present the first comprehensive taphonomic and sedimentological study of the Tulip Beds on Mount Stephen (Campsite Cliff Shale Member, Burgess Shale Formation), based on a time-averaged assemblage of nearly 10,000 specimens. The taphonomic characteristics size sorting, resistance to decay, and potential flow alignment and mode of deposition of this assemblage are compared specifically to those of the nearby and stratigraphically younger Walcott Quarry assemblage. Like other Burgess Shale type deposits, the Tulip Beds consist of millimeter-laminated, event-derived claystone, but lack the thicker claystone layers and prominent carbonate interbeds that occur in the Walcott Quarry. These differences suggest a depositional environment lower in energy and possibly more distal to the Cathedral Escarpment. Overall, taphonomic analyses suggest no significant decay biases, transport, or sorting of the assemblage, and most specimens, benthic taxa in particular, appear to have been buried close to their living environments. Single bedding planes with large accumulations dominated by a single taxon, e.g., isolated claws of Anomalocaris, suggest short time-averaged assemblages with limited background sedimentation. Overall the Tulip Beds locality is environmentally and taphonomically comparable to the Walcott Quarry and biotic variations between the two sites are likely to be primary in nature, thus paving the way for more detailed paleoecological investigations in the future. INTRODUCTION Burgess Shale type deposits greatly contribute to our understanding of life in the immediate aftermath of the Cambrian Explosion (Briggs and Fortey 2005). Because they preserve the remains of soft-bodied organisms, these deposits provide critical evidence about the origin, early evolution, and ecology of Metazoa that are not otherwise available from the fossil record (e.g., Conway Morris 1986; Chen and Erdtmann 1991; Briggs and Fortey 2005; Smith and Caron 2010; Conway Morris and Caron 2012). One of the most important Konservat-Lagerstätte is the middle Cambrian (Series 3, Stage 5) Burgess Shale in Yoho National Park, British Columbia, Canada. Discovered by Charles Walcott in 1909 between Wapta Mountain and Mount Field, the original site (Walcott s phyllopod bed ) has yielded one of the largest collections of Burgess Shale type fossils anywhere in the world from a limited stratigraphic interval. In addition to its contribution to understanding Cambrian biodiversity, the Burgess Shale has lent itself to addressing questions regarding the composition of some of the earliest complex communities (Conway Morris 1986; Caron and Jackson 2008), as well as the structure of Cambrian food webs (Dunne et al. 2008; Vannier 2012). While the Burgess Shale has been the focus of considerable attention, its status as a typical middle Cambrian community based on the presence of common and widespread biomineralized taxa (Conway Morris 1986) remains to be investigated. This requires comparison with other Burgess Shale type deposits using complete fossil inventories, including soft-bodied taxa. Assemblage-wide taphonomic and taxonomic comparisons, like those that have been made across assemblages within the early Cambrian Chengjiang biota (Steiner et al. 2005; Dornbos and Chen 2008; Zhao et al. 2009, 2012, 2014), have not yet been attempted for the Burgess Shale and many important questions remain. In particular, it is not known whether the phyllopod bed, and related fossil horizons discovered subsequently by the Royal Ontario Museum (Caron and Jackson 2008) below it (exposed in today s Walcott Quarry), can be regarded as representative of the range of taxonomic and ecological associations that might have existed during that time in comparable depositional settings. In addition, and despite sharing a common pathway of preservation (Gaines et al. 2012a), variations in preservation of anatomical aspects within and among species and different Burgess Shale type fossil assemblages are still poorly understood, in particular in the vicinity of the Burgess Shale itself. In the Published Online: September 2014 Copyright E 2014, SEPM (Society for Sedimentary Geology) 0883-1351/14/029-309/$03.00

310 L.J. O BRIEN ET AL. PALAIOS FIG. 1. Location and stratigraphy of the Tulip Beds. A) Location map of the Tulip Beds and other main Burgess Shale localities in the Kicking Horse valley, Yoho National Park, British Columbia, Canada. B) Stratigraphy of the Burgess Shale Formation indicating the stratigraphic levels of each of the localities indicated. Redrawn and modified from Fletcher and Collins (2003). Black stars: 1 5 Tulip Beds, 2 5 Trilobite Beds, 3 5 Walcott Quarry. Gray stars 5 other localities. Burgess Shale Formation, soft-bodied fossils occur in more than a dozen stratigraphically related localities above and along the Cathedral Escarpment, from Fossil Ridge to the Monarch, 60 km to the southeast (Collins et al. 1983; Johnston et al. 2009; Caron et al. 2010, 2014), offering a rare opportunity for comparative taphonomic and paleoecological study at a regional scale. Unfortunately, most of these sites are difficult to access, are still poorly known, and few have been sufficiently sampled to warrant more detailed quantitative comparisons. However, significant collections have been made from several other sites. One of the most important localities outside of Fossil Ridge is the Tulip Beds locality on Mount Stephen, discovered in 1983 by the Royal Ontario Museum (ROM), which has so far yielded the most substantial collection with almost 10,000 specimens. This study provides microfacies and taphonomic interpretations of the Tulip Beds for the first time, based on qualitative and quantitative data. This locality differs from the Walcott Quarry, both in terms of position relative to the escarpment and stratigraphic age, thereby allowing the possibility of comparing communities through both space and time. Understanding the taphonomy and geological setting of the Tulip Beds is an important first step in attempting broader-scale paleoecological siteto-site comparisons. Limited material from the Tulip Beds was described in taxonomic studies of sponges (Rigby and Collins 2004), dinocarids (Daley et al. 2009; Daley and Budd 2010), leanchoiliids (García-Bellido and Collins 2007), the enigmatic taxon, Siphusauctum gregarium (O Brien and Caron 2012), and the bivalved arthropod Nereocaris exilis (Legg et al. 2012). Some of these studies have noted potential differences in preservation quality and composition of the Tulip Beds assemblage compared to other Burgess Shale localities (García-Bellido and Collins 2007) but without the benefit of detailed taphonomic and paleoecological comparisons. LOCATION AND GEOLOGIC SETTING The Tulip Beds site (formerly known as S7) is located on the northwest shoulder of Mount Stephen in Yoho National Park, just above the town of Field, British Columbia, Canada. The site belongs to the Campsite Cliff Shale Member and is located, 4 km southwest of the stratigraphically younger Walcott Quarry on Fossil Ridge (Fig. 1), which belongs to the Walcott Quarry Shale Member (Fletcher and Collins 1998). The Tulip Beds locality was named after the abundant and enigmatic tulip-shaped animal, Siphusauctum gregarium, which is known only from this locality at the Burgess Shale (O Brien and Caron 2012). The Tulip Beds extend for more than 150 meters along a southwest-facing cliff, which is of variable height (3 12 m). The bottom of the section lies conformably on top of the Yoho River Limestone Member, whereas the top is not present at the locality. Strata are truncated by a modern erosional surface at the top of the exposure. The fossil-bearing horizons are laterally continuous. High consistency of primary sedimentary features reveals no evidence for significant breaks in sedimentation. Fossils come from two areas along the cliff: in situ outcrop (Quarry site also known as the Above Campsite locality) including talus material directly below it, and a talus slope 100 150 m along strike to the south (Talus site) (Fig. 2). At least 3 m of strata (7.7 10.7 m above the base of the Campsite Cliff Shale Member) contain Burgess Shale type fossils at the Quarry site. Greater vertical exposure is present at the Talus site, where the lowermost 3 m of shale represents the lateral equivalent of the beds present at the Quarry site (7.7 m above the base of the section). Although comprehensive in situ sampling at a fine scale (e.g., Caron and Jackson 2006) is generally not possible due to the inaccessibility of most of the cliff face and steepness of terrain, a two-day inspection of the cliffs via rock climbing revealed the presence of exceptionally preserved fossils across a thickness of 12 m of strata, with the top truncated by recent erosion.

PALAIOS TAPHONOMY OF THE BURGESS SHALE TULIP BEDS 311 FIG. 2. Position of the outcrop. A) Close-up of the northwest shoulder of Mount Stephen showing the position of the two sublocalities at the Tulip Beds. The dotted lines mark where the talus material was collected. The dashed line indicates the separation of the Campsite Cliff Shale Member (CCS) and the Yoho River Limestone Member (YRL). B) Inset of the Quarry site, showing the location of the three sections that were cut from the quarry (J.-B. Caron for scale). The dashed lines indicate the overlap between sections two and three; the curvature of the dashed lines is partially due to section two being at the back of the quarry and section three at the front (foreground) and the angle at which this picture was taken. The Campsite Cliff Shale Member consists of mm-laminated, blocky, dark-gray to green calcareous mudstones, with occasional fine-grained limestones and carbonate concretions (Fletcher and Collins 2003). The member lies within the Pagetia bootes Subzone of the Bathyuriscus- Elrathina Zone (Fletcher and Collins 2009). The Trilobite Beds, further southwest on Mount Stephen, also belong to the lower Campsite Cliff Shale Member, and were interpreted to have been deposited in a more proximal location to the Cathedral Escarpment than the Tulip Beds (Fletcher and Collins 2003, 2009). The precise stratigraphic relationship of the Tulip Beds to the Trilobite Beds is uncertain due to the complex structural geological history of the area, the presence of a major fault (the Fossil Gully fault) immediately adjacent to both sites, and the lack of complete sections at both localities (Fig. 1A). The Tulip Beds lie within a fault-bounded block and its exact stratigraphic location is unknown; however, the Tulip Beds locality was interpreted to be slightly older than the Trilobite Beds (Fletcher and Collins 2003). The exact position of the Tulip Beds in relation to the Cathedral Escarpment is also not known, but the nearest contact between the Burgess Shale Formation and the escarpment occurs 1,500 m away on the northeast shoulder of Mount Stephen. The Burgess Shale localities on Fossil Ridge, including the Walcott Quarry, lie in direct contact with the escarpment, as does the newly discovered Marble Canyon assemblage, some 40 km SE (Caron et al. 2014). MATERIALS All fossil material studied herein is held at the Royal Ontario Museum (ROM). A total of 2,017 fossil-bearing slabs were collected by the ROM between 1983 and 2010 across nine field seasons (1983, 1989, 1990, 1996, 1999, 2000, 2002, 2008, and 2010), with the largest collections of specimens made in 1996 and 1999 during limited quarrying operations at the Quarry site. Unfortunately, the precise stratigraphic origin of the specimens collected at the Quarry site was not recorded and the in situ material was treated as an induced time-averaged assemblage. All material collected was examined; 34% comes from the Quarry site and 66% comes from the Talus site. In total, 9,460 fossils were identified, of which 7,913 are included in the final data set after questionable, fragmentary, and indeterminate identifications were removed (Table 1, Supplementary Data File 1, for count details see O Brien 2013). The Quarry site outcrop has a vertical exposure of, 3 m and a width of, 4.6 m. Because of folds and complicated cliff face topology, a complete section could not be sampled; however, three vertical sections measuring, 1.20 m each, were cut from the quarry at various locations, in August 2010 (Fig. 2). The purpose of these sections was, (1) to identify the stratigraphic location of fossiliferous beds and (2), to obtain detailed sedimentological information to help reconstruct the depositional environment. Sections two and three are separated laterally by 3.1 m and have a vertical overlap of, 50 cm, resulting in a cumulative sampled thickness of, 2 m on the quarry face (Fig. 3). Section one was cut but not closely examined, as a connection to the two other sections could not be made due to numerous small faults and associated fragmentation of rock. The remaining parts of the sections, not used for the geological analysis, were finely split and fossils from each fossil-bearing horizon were counted and identified to the lowest taxonomic level possible (Fig. 3, Supplementary Data File 1). METHODS Sedimentology Two partially overlapping continuous sections representing 1.95 m in total were collected from the Quarry site using a concrete saw and hand TABLE 1. Comparison between the number of individuals (n), diversity (genera), size and abundance ranges of taxa (genera) between the Tulip Beds (TB) and the Walcott Quarry. TB - Total TB - Quarry Site TB - Talus Site Walcott Sample (n) 7913 4020 4554 51615 Diversity 110 81 105 132 Length range (mm) 1 250 1 250 1 250 1 360 Abundance range 1 1740 1 972 1 931 1 6263

312 L.J. O BRIEN ET AL. PALAIOS FIG. 3. Stratigraphic column of the Tulip Beds. The inset shows details of Sections 2 and 3 combined from the Quarry site including the horizons from which fossils were recovered.

PALAIOS TAPHONOMY OF THE BURGESS SHALE TULIP BEDS 313 specimens, usually much smaller, are present on the same slabs and were subsequently identified in the lab. Counts were made on both sides of the slabs, using part and counterpart if available. All fossil occurrences from the material associated with the cut and polished sections were grouped into 5 cm bins. Each bin is therefore considered an induced time-averaged assemblage representing multiple millimeter-thick burial events (Fig. 3, Supplementary Data File 2). Although stratigraphic levels were taken for every slab removed, binning is used to compensate for slight variations in bed thicknesses between the two cut sections and because it was not possible to exactly align corresponding laminae of each section. It should be noted that the material used in these bins represents very limited sampling and may not be representative of the whole assemblage; however, the binned data provide evidence for the recurrence of preservational conditions and common taxa. FIG. 4. A rock sample from section three at the Quarry site of the Tulip Beds, showing thin, millimeter-scale beds with slumped intervals. A) Scanned image of polished section, ROM 62298. B) Interpretive drawing of the section. Scale 5 10 mm. The break represents. 5 mm. tools. Blocks comprising each section, ranging in width from 8 20 cm, were sliced into, 5-mm-thick pieces and polished to reveal primary and secondary sedimentary features. The slices were imaged by X-radiography and a flatbed scanner for study of mm-scale details (Fig. 4, Supplementary Fig. 1). Thin sections prepared from the material were characterized using a petrographic microscope. Grain size and claystone microfabrics were characterized using a scanning electron microscope (Zeiss Leo982 FE-SEM). Mineralogy was determined using a Rigaku Ultima IV X-ray diffractometer. Major and trace element chemistry of selected samples was determined by X-ray fluorescence (PANalytical Axios XRF) of fused glass beads using the protocol of Johnson et al. (1999), and weight percent carbonate was determined by carbon coulometry. Taxonomic Identification For identification of taphonomically important features, fossils were examined using a stereomicroscope under polarized light with crossed nicols and in some cases immersed in water to enhance details that are not apparent under normal light conditions (see Bengtson 2000 and Crabb 2001 for details). Some specimens were prepared mechanically using an electric vibro-engraver with carbide bits to remove sediment coating on specimen surfaces. In some cases fossils were examined using ammonium chloride to observe fine surficial details and differences in relief of the specimens. Each specimen was identified to the genus level where possible. Although, historically, slabs of various sizes were targeted for collection of particular macro fossil specimens, in most cases many other Collection of Taphonomic Data Qualitative taphonomic observations of the specimens include the level of articulation (i.e., isolated elements, partially articulated, completely articulated), and degree of preservation (e.g., whether internal organs are preserved and whether some parts are pyritized or phosphatized, with some three-dimensionality retained). Direct association of individuals with conspecifics or other species was recorded and, where possible, size measurements were taken. Using these criteria, a range of preservational states among specimens was established for some taxa. For example, there is a clear preservational gradient in S. gregarium (O Brien and Caron 2012), and decay gradients are also evident in leanchoiliids, Wiwaxia, Hallucigenia, and Leptomitella (Fig. 5). Quantitative methods were used to identify potential preservational biases relating to size sorting, resistance to decay, and potential flow alignment. In order to test the hypothesis that larger taxa are more likely to be preserved, and therefore better represented in the assemblage, the maximum size, measured as length of the longest specimen in mm, of each taxon was used to identify any biases toward a particular size range. Size or taxonomic sorting within individuals at the same stratigraphic horizon, as well as preferred orientation, would indicate if specimens had been significantly transported. Although sorting, resistance to decay, and flow alignment of mobile taxa are not an exhaustive list of possible biases in a paleontological data set, they are used to show obvious signs of biases that are comparable across localities and if present, may have a major impact on the interpretations of the original community composition. To examine the effect of size on the preservation of specimens, the correlation between size and abundance, and the frequency distribution of the size metric for each taxon were calculated. This analysis included complete individuals in most cases; however, where individuals were identified from disarticulated elements only, the length of the single largest element was used as a proxy for size (e.g., Hurdia carapaces, Anomalocaris claws). Comparisons with the Walcott Quarry were made using a modified version of the data set used by Caron and Jackson (2008) (Supplementary Data File 3). The data set has been updated to include taxonomic revisions since its original publication, restructured to generic level where possible, and biomineralization (presence or absence) and size values (estimated maximum length in mm) have been added, as they were not part of the original data set (values used are based on published data and examination of new ROM specimens). All abundance and length measurements were log-transformed during analyses to normalize the distribution and minimize the disproportionate effect of outliers. To test for differences in size distribution between the two Burgess Shale localities, the size distribution for specimens collected from the Tulip Beds was compared to that of specimens collected from the Walcott Quarry using a two-sample Kolmogorov-Smirnov test, and both distributions were tested for normality using a Shapiro-Wilk normality test.

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PALAIOS TAPHONOMY OF THE BURGESS SHALE TULIP BEDS 315 To test if biomineralization has a clear and significant correlation to the abundance of individual taxa, taxa from each locality were scored for the presence or absence of biomineralized elements (i.e., biomineralized spicules or exoskeletons, irrespective of the type of mineralization) and abundances between these two groups were compared with Kolmogorov- Smirnov and Student s t-tests. To test for significant transport, slabs with abundant specimens of S. gregarium were observed for indications of preferred alignment and direction. Siphusauctum gregarium was chosen because it shows both a clear long axis and a defined anteroposterior axis (O Brien and Caron 2012). As the majority of material at the sites has been collected from talus material, it is not possible to orient specimens from different individual slabs together using a common reference direction or an absolute bearing. However, individual orientations were derived from large slabs using a relative reference point per slab. All statistical tests and plots were run in R Statistical Language (V 1.4) (R Core Development Team 2012), using the packages vegan (Oksanen et al. 2011), gplots (Warnes 2011), and fossil (Vavrek 2011). SEDIMENTOLOGY OF THE TULIP BEDS The Tulip Beds are composed of a single depositional facies made up of millimeter-laminated calcareous claystone (Fig. 4). Analysis of thin sections, polished slabs, X-radiographs, and sediment fabric by SEM reveals that silt and coarser grains are absent from the entire succession, and no evidence of scour or grading is present. While the presence of C-E turbidites in the Burgess Shale type fossil-bearing interval of Chengjiang provides clear evidence for event-driven deposition (e.g., Zhao et al. 2009), younger Burgess Shale type deposits from Laurentia consist exclusively of claystone that does not possess a coarser grain size fraction. Exclusion of the coarse fraction from the outer detrital belt, where Burgess Shale type deposits occur, likely resulted from the trapping of coarse clastics inboard of extensive carbonate platforms that separated the offshore basins from continental sources of clastic input (Gaines et al. 2005). Therefore, interpretation of depositional mode must rely on textural data (O Brien et al. 1980). SEM analysis reveals the presence of randomly oriented clay microfabrics in all samples analyzed, which is indicative of rapid deposition from turbid sediment-gravity flows (O Brien et al. 1980). While such randomly oriented microfabrics may be produced secondarily by extensive bioturbation (e.g., complete sediment mixing), the absence of sediment-mixing bioturbation in the Tulip Beds succession excludes this possibility. Rapid, event-based deposition of a clay-sized fraction is supported not only by the presumed requirements for exceptional preservation (e.g., Conway Morris 1986), but also by the close similarity with event-deposited claystone beds in the Walcott Quarry (Gabbott et al. 2008). The stratigraphic section studied indicates that depositional processes were consistent during accumulation of the Tulip Beds, although periodic firm sediment deformation by slumping at or near the seafloor occurred frequently, indicating a significant slope. Soft sediment deformation in the form of slumping is evident within at least eight different intervals in the 1.95 m section (Fig. 3). Slumped intervals range from 5 to 20 cm in thickness. Polished sections reveal the presence of sedimentary pyrite, which occurs as millimeter-sized clusters of euhedral pyrite crystals within FIG. 6. Thin section showing the thin bedding at the Tulip Beds Quarry site. A) A photograph of the thin section. B) Inset of Part A, captured with transmitted light; the base of the beds are darker due to concentrated organic materials. The beds are capped with carbonates which appear bright in thin section. Scale 5 5mm (A); 2 mm (B). beds and thinner pyrite lenses at bed junctions, as also observed in the Walcott Quarry (Gabbott et al. 2008). As is the case in other Burgess Shale localities (Gabbott et al. 2008; Fletcher and Collins 2009), evidence for background (pelagic) sedimentation between depositional events, as observed at Chengjiang (e.g., Zhao et al. 2009), is lacking. No laminated claystone microfabrics or featureless interbeds are present between event laminae. Assuming a compaction ratio of 8:1, as estimated in the Burgess Shale by Whittington (1975), each of the event-deposited beds was originally no more than a few centimeters thick. The distinctive laminated character of individual beds, apparent as a color variation from medium to dark gray reflects differences in the abundance of calcium carbonate, as is evident in thin section (Fig. 6) and X-radiograph. Here, as in other Burgess Shale type deposits, calcium carbonate cements form prominent caps on individual event-deposited claystone beds (Gaines et al. 2012a), indicating exposure at the seafloor in the absence of detectable background pelagic sediment input. Weight percent CaCO 3 of whole-rock samples that include multiple beds ranges from 5.8% 11.8% (n 5 7). All evidence combined suggests a calm deepwater environment, below storm wave base, with pulsed, event-driven sedimentation separated by episodes of no sediment deposition, as observed elsewhere in the Burgess Shale (Gabbott et al. 2008). The XRD analyses indicate that the Tulip Beds are made up of chlorite, calcite, and quartz. Because quartz silt and sand is absent, quartz is interpreted to have been derived from the clay fraction during metamorphism (Powell 2003; Butterfield et al. 2007). Whole rock geochemical analyses of ten samples that span the quarry interval reveal r FIG. 5. Range of preservation in some common taxa from the Tulip Beds. A) Hallucigenia sp. spines associated with body, ROM 62276. B) Hallucigenia sp. with only spines preserved, ROM 62290. C) Wiwaxia sp. showing associated scleritome, ROM 62280. D) Dissociated sclerites of Wiwaxia sp., ROM 62288. E) An isolated Wiwaxia sp. sclerite, ROM 62289. F) Olenoides serratus exoskeleton with a disarticulated and overturned pygidium, ROM 62282. G) Leptomitella sp. showing no evidence of decay or breakage, ROM 62279. H) Poorly preserved Leptomitella sp. specimen, ROM 62294. I) Partially disarticulated leanchoiliid, ROM 62285. J) Disarticulated leanchoiliid anterior, ROM 62287. K) Disarticulated leanchoiliid tergite with small associated burrow indicated with arrows, ROM 62286. Scale 5 10 mm (A, C, D, G K), 5 mm (B), 1.5 mm (E), 20 mm (F).

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PALAIOS TAPHONOMY OF THE BURGESS SHALE TULIP BEDS 317 Systematic inspection of polished slabs with the aid of magnification and X-radiography reveal no evidence for bioturbation (Supplementary Fig. 1). However, rare millimeter-width horizontal burrows are associated with a small number of fossil carapaces (Fig. 5K). As these burrows have only been identified in talus material, it is unclear how pervasive such examples of bioturbation may be within the 12 m interval from which talus material was sourced at this locality. Like the Tulip Beds, no significant vertical bioturbation is evident at the Walcott Quarry (Gostlin 2006; Gabbott et al. 2008); however, the most conspicuous traces associated with soft-bodied fossils are generally small and of low diversity (Caron et al. 2010; Mángano 2011). Rare trace fossils found at the Tulip Beds are reminiscent of trace fossils associated with body fossils of Banffia constricta (Caron 2005), and with other nonmineralized taxa from other localities such as Chengjiang (Zhang et al. 2007), Kaili (Lin et al. 2010), and Sirus Passet (Mángano et al. 2012). Redox-sensitive trace elements (Mo, U, V) in the succession lack obvious enrichments (Tribovillard et al. 2006). Trace element data, in combination with lack of bioturbation, suggest deposition under an oxic water column with oxygen-deficient bottom waters (Gaines and Droser 2010; Gaines et al. 2012b). TAPHONOMIC MODES FIG. 8. Examples of pyritized fossils from the Tulip Beds. A) Vauxia sp. with pyritized spicules, ROM 62281. B) Diraphora bellicostata with infilling of large cubic pyrite, ROM 62277. C) Marrella sp. with limited three-dimensional preservation of the cephalic region and displaying oxidized pyrite, ROM 62291. Scales 5 5 mm (A C). major element compositions that are consistent with those reported from the Walcott Quarry and other horizons of the Burgess Shale on Fossil Ridge (Powell 2003) (Supplementary Data Table 1). Comparison of these data with the Post-Archean Average Shale composite standard (Taylor and McLennan 1985) reveals depletion of SiO 2 and enrichment of CaO and Al 2 O 3. These differences in Tulip Beds samples versus those of average shale reflect the absence of a detrital quartz fraction that is characteristic of average shale, as well as a commensurate enrichment in clay, and the presence of a substantial carbonate fraction, consistent with the interpretations of Powell (2003) for other Burgess Shale samples, and confirmed by petrographic observations and XRD data from Tulip Beds material. Powell s (2003) interpretation of a primary illite-smectitekaolinite clay mineral assemblage in the Burgess Shale is also supported by the major element data. Sedimentologic observations from the Tulip Beds are fully consistent with the specific lithology that characterizes Burgess Shale type deposits found globally (Gaines et al. 2012a), and are directly comparable to data from the Walcott Quarry (Gabbott et al. 2008). The most important differences between the two localities are the presence of rare, thicker event-deposited claystone beds in the Walcott Quarry, most prominently the Great Marrella Layer and the Great Eldonia Layer of Walcott, and the presence of frequent interbedded carbonate horizons up to several cm in thickness throughout the Greater Phyllopod Bed (Gabbott et al. 2008). There are no significant breaks in sedimentation observed within the Greater Phyllopod Bed (which spans most of the Walcott Quarry) and it has been suggested that sedimentation was the result of pulsing dense slurrylike flows (Gabbott et al. 2008). As compared to the Walcott Quarry, the uniformly thin nature of the beds suggests that the Tulip Beds locality lay farther from the Cathedral Escarpment. This interpretation is consistent with that of Fletcher and Collins (2009), who proposed a more distal setting for the Tulip Beds. Like other Burgess Shale fossils, nonbiomineralized taxa found in the Tulip Beds are typically preserved as reflective two-dimensional carbonaceous compressions (Butterfield 1995; Gaines et al. 2008). In the Burgess Shale, authigenic aluminosilicate minerals also occur in association with the carbonaceous fossils (Orr et al. 1998); however, these mineral templates precipitated during late stage metamorphism (Butterfield et al. 2007) (Fig. 7). The large range of body plans and organic tissues preserved, including cyanobacteria and algae as well as complex animals (e.g., lobopodians, arthropods, and priapulids), suggests no particular taphonomic bias against the preservation of certain groups of organisms or tissue types. However, some taxa are primarily represented by poorly preserved specimens, indicating some qualitative biases among different taxa, possibly due to differential decay resistance of soft tissues, or to preburial decay. For example, 96.6% of Hallucigenia sp., from the Tulip Beds (n 5 119) are known only from spines; only four specimens show traces of the body (Fig. 5A, B). Diverse taxonomic groups across multiple body plans reoccur throughout the sampled section, which suggests that the conditions for preservation of soft tissues were recurrent across the sampled section (Fig. 3, Supplementary Data File 1). Similar to other Burgess Shale type deposits, taxa with biomineralized exoskeletons (e.g., trilobites) or shells (e.g., brachiopods) retain some aspect of original three-dimensionality. In some instances, part or all of a fossil may be partially infilled with partially oxidized pyrite; this mainly occurs in originally biomineralized fossils (e.g., brachiopods, trilobites, and echinoderms) (Fig. 8). However, some algae/cyanobacteria, sponges, and soft-bodied animals may also be completely replaced by partially oxidized pyrite, and/or iron stains from pyrite weathering. As in other Burgess Shale type localities, the gut tracts of some arthropods and priapulids that are otherwise two-dimensional are preserved in relief by phosphate (Fig. 9). The preserved material ranges from pristine complete specimens to disarticulated and decayed specimens across most nonbiomineralized taxa (Fig. 5). Complete taphoseries (preservational variants) of taxa, such as Siphusauctum gregarium (O Brien and Caron 2012) are observed when looking at the collection in its entirety, while taphonomic gradients are also evident in other taxa (e.g., leanchoiliids and many sponges). These taphoseries may be the result of both pre- and postburial effects. For r FIG. 7. Slab A) showing variation in diversity of taxa, quality of preservation and fossil size at the Tulip Beds locality, ROM 59505. B) Inset of Part A showing, I: Peytoia sp.; II: Anomalocaris sp.; III: Indet. Porifera. Scale 5 100 mm (A); 25 mm (B).

318 L.J. O BRIEN ET AL. PALAIOS FIG. 9. Phosphatized guts from the Tulip Beds. A) Ventral aspect of a dinocarid body with three-dimensional preservation of gut glands, ROM 62284. B) Inset of Part A, a close-up of the gut glands. C) A coiled priapulid worm with three-dimensional gut preservation, ROM 62278. Scales 5 20 mm (A); 5 mm (B); 10 mm (C). example, leanchoiliids may be found as completely disarticulated, partially articulated, or completely articulated specimens due to preburial transport or decay, and complete specimens may or may not have threedimensional gut preservation indicating possible postburial controls on preservation. In addition, while many arthropod specimens, like leanchoiliids, might represent carcasses, there is also the possibility that many disarticulated elements, especially those with no evidence of a preserved gut, represent molts (Fig. 5F, K). Either way, the presence of elements in close proximity suggests little transportation of these parts after deposition (Fig. 5F). Individual slabs can vary greatly in their taxonomic composition and abundance. For example, ROM 62292 preserves an accumulation of 130 densely packed individuals, comprising 125 individual Anomalocaris claws and 5 other taxa each represented by a single individual (Fig. 10). ROM 61421 includes 144 individuals belonging to 15 taxa and ROM 62283 contains 51 individuals representing 17 taxa. Where large numbers of individuals are found on a single bedding surface, they are generally very densely packed and there may be little or no sediment between overlapping specimens (Figs. 7, 10). Many large slabs have a comparatively low number of specimens and exhibit low diversity, with taxa limited to algal fragments, dinocarid claws, sponges, and disarticulated trilobites (Fig. 10). While there may be large variations in taxonomic composition, articulation, and size of individuals across the large slabs, there is generally a constant quality of preservation across a given slab. For example, generally all specimens on an individual slab are well defined with good internal details, or poorly defined with poor internal details. This variation in preservational quality suggests fluctuations in the quality of preservation across events (e.g., the contrast between ROM 59505, Fig. 7, and ROM 62292, Fig. 10). Weathering is unlikely to be responsible for the poor preservation observed on many slabs, in particular for those quarried in situ. Poor preservation may indicate changes in the chemical environment, or differences in residence time of fossils on the seafloor prior to burial. The timing or frequency of depositional events may have played a role in the quality of preservation as we see elsewhere in the Burgess Shale (Caron and Jackson 2006) and Chengjiang (Zhao et al. 2009). Although the overall sedimentary regime and mode of deposition did not vary during the accumulation of the Tulip Beds succession, the frequency of events might have been highly variable interspersed by variable amounts of time in the absence of pelagic sedimentation aside from skeletal debris (see below). There is a stark contrast in the mobility of the taxa and their abundance (excluding taxa whose mobility is unknown). While mobile taxa account for 52% of all taxa, they comprise only 21% of specimens (Fig. 11). In contrast, sessile organisms account for 48% of taxa and 78%

PALAIOS TAPHONOMY OF THE BURGESS SHALE TULIP BEDS 319 FIG. 10. Cluster of Anomalocaris sp. claws and disarticulated algal fragments from the Tulip Beds (Section 2; horizon 46 cm), ROM 62292. Scale 5 100 mm. of specimens. The greater number of sessile specimens may be due to the thinness of the individual event beds, which may have allowed a greater number of mobile organisms to escape burial. It may also reflect underlying ecological differences in the original abundances of taxa at the Tulip Beds locality. This difference could also be environmental. Most sessile taxa are filter feeders and these organisms thrive in environments with low sedimentation rates and turbidity levels. In that respect, the high proportion of sessile organisms is consistent with sedimentological evidence suggesting a less energetic environment compared to the Walcott FIG. 11. Number (n 5 104) and abundance (n 5 5577) of mobile and sessile taxa at the Tulip Beds. Quarry. Some taxa are preserved primarily as isolated claws and carapaces with no or little evidence of complete carcasses (e.g., Anomalocaris, Hurdia, Canadaspis, Perspicaris). These taxa are generally interpreted to have had a pelagic or nektobenthic mode of life (Caron and Jackson 2008). These isolated elements were derived from either molts or carcasses and likely experienced variable amounts of decay and transportation prior to burial. Given that isolated elements are often found with other more complete taxa across a range of sizes, their presence did not result from possible size sorting during burial, although, as noted above, living mobile specimens may have been able to escape burial, thereby skewing the data toward dead or molted remains. Some slabs show a large number of poorly preserved specimens belonging to one or a few taxa (predominantly algae, trilobites, dinocarid claws, and sponges) (Fig. 10). These types of near monotypic accumulations are similar to those known from the Trilobite Beds, where many surfaces contain only Anomalocaris claws and trilobites (mainly Ogygopsis) without soft-tissue preservation (Fig. 12B). These horizons may represent a combination of limited time averaging combined with an original lowdiversity biotic assemblage. In the case of arthropods, these accumulations might also represent a mass molt event, as at the Walcott Quarry (Haug et al. 2013). However, mass molting events followed by rapid burial do not provide a plausible explanation for large slabs covered with Anomalocaris claws, as those tend not to be paired (see paired-claw example from the Raymond Quarry, Fig. 12A) and there is no ecological evidence that this large predator lived in social groups. A background accumulation of skeletal fossils across a short time interval is more likely, but since there is generally little to no sediment between overlapping specimens in these large monotypic accumulations (Fig. 10), the amount of time averaging must have been very short. The co-occurrence of a range of articulated specimens, disassociated specimens, and molts on most slabs suggest the preservation of mixed life and death assemblages

320 L.J. O BRIEN ET AL. PALAIOS FIG. 12. Comparison of Anomolacaris sp. claws. A) Paired claws from the Raymond Quarry on Fossil Ridge, ROM 62295. B) Assemblage of unpaired claws from the Trilobite Beds, ROM 62293. Scales 5 10 mm (A); 15 mm (B). TABLE 2. Correlation of taxon/element size (length, mm) and abundance for the Tulip Beds (TB) and the Walcott Quarry. NS 5 nonsignificance, * 5 weakly significant. TB - Total TB - Quarry Site TB - Talus Site Walcott Pearson r 0.1356 0.1611 0.1294 20.05502 p value 0.1577 0.09904 0.1861 0.5325 Significance ns ns ns ns Kendall tau 0.1308 0.1451 0.1261 20.04165 p value 0.04843 0.03489 0.06238 0.487 Significance * * ns ns

PALAIOS TAPHONOMY OF THE BURGESS SHALE TULIP BEDS 321 FIG. 13. Correlations of size. A) Plot showing the relation between size of taxon/element in the Tulip Beds and abundance. B) Histogram of the size distribution of taxa/elements. For correlation and distribution statistics see Tables 2 and 3. (see Caron and Jackson 2006, for discussion), with some taxa being dead and decaying at the surface prior to burial along with a living community during the rapid deposition of the event beds (Fig. 5). Completely disarticulated elements or unidentifiable fragments may represent decaying organisms that were exposed for longer periods above the sediment-water interface, and therefore became disarticulated prior to burial. TESTS FOR TAPHONOMIC BIASES In order to determine if the community composition of the Tulip Beds locality was significantly affected by taphonomic constraints (e.g., size biases and/or sorting, resistance to decay), different measures have been compared. The two Tulip Beds sublocalities (Quarry site and Talus site) were first compared to identify potential lateral and temporal changes, and comparisons were then made with the Walcott Quarry to identify potential taphonomic differences with the Tulip Beds. Taxonomic abundance in the Tulip Beds (including all fossil assemblages from the Quarry and Talus sites) and taxon/element length (both log transformed) are not significantly correlated (Pearson s r 5 0.136; p 5 0.158) (Table 2, Fig. 13), indicating that these two factors are not statistically dependent and size does not predict higher abundance in the collection. If rank data, rather than log data are used, this dependence becomes marginally significant (Kendall s tau 5 0.131; p 5 0.048); however, very low tau values indicate that the strength of this correlation is very poor (Table 2). Similar results were obtained independently, for the Quarry site alone, and for the Talus site alone, with neither showing significant statistical correlations (Table 2). There is no significant correlation between taxon/element size and abundance for the Walcott Quarry, regardless of whether log or rank data is used (Table 2). Since the size of a taxon or element does not predict how abundant a taxon will be in the assemblages, it is clear that larger taxa do not have a higher representation than smaller taxa (or vice versa), and, thus, that there is no discernible size-related taphonomic bias. True size biases are difficult to quantify in fossil assemblages, as the original composition is unknown and not easily comparable with modern assemblages. Studies comparing living and recently dead assemblages of modern mollusc communities provide insight into potential biases (Kidwell 2001, see also Cooper et al. 2006). However, to date, such studies have focused on the potential bias within skeletal taxa and it is difficult to compare these results to exceptionally preserved soft-bodied assemblages. The Tulip Beds data shows a nonnormal distribution for taxon/element sizes; with a distinct negative skew (Fig. 13, Table 3). Additionally, the two Tulip Beds sublocalities (Quarry site and Talus site) show very similar size distributions (Table 3), with highly nonsignificant differences (KS test p 5 0.9992). The Walcott Quarry also shows a nonnormal distribution for taxon/element sizes; with a negative skew (Table 3). The taxon/element size distribution of the Walcott Quarry and the Tulip Beds are not statistically different from each other (KS test p 5 0.1479), suggesting they are drawn from the same size distribution. The range in size of the taxa from each site is indicated in Table 1, and there is a large variation in the size of taxa distributed on the large slabs. The distributions of sizes across the assemblages are different from normal in that they exhibit a negative skew; this indicates a greater diversity of large taxa than small taxa. As this distribution is similar across all the assemblages examined, it indicates that they are comparable, without distinct size-related differences in preservation between sites, regardless of their taxonomic composition. The abundance distribution of biomineralized taxa is not significantly different from that of nonbiomineralized taxa in the total Tulip Beds TABLE 3. Size distribution in the Tulip Beds (TB) and the Walcott Quarry, with tests for normality and skewness. * 5 weakly significant, *** 5 strongly significant. Size distribution TB - Total TB - Quarry Site TB - Talus Site Walcott Shapiro-Wilk p-value 0.0001871 0.000241 0.0001492 0.04386 Sig. diff. from normal *** *** *** * Skew 20.9186 21.0474 20.9612 20.3984

322 L.J. O BRIEN ET AL. PALAIOS TABLE 4. Difference in abundance between biomineralized (BM) and nonbiomineralized (NBM) taxa in the Tulip Beds (TB) and the Walcott Quarry. Distribution TB - Total TB - Quarry Site TB - Talus Site Walcott ks.test p-value 0.2053 0.5695 0.07972 0.4938 Significance ns ns ns ns Means t.test p-value 0.05810 0.07015 0.02856 0.6525 Significance ns ns * ns % taxa BM 40.91% 38.27% 40.95% 32.57% % specimens BM 16.66% 15.32% 16.68% 28.16% mean abund. BM 29.31 13.68 16.88 338.09 mean abund. NBM 101.45 52.37 58.37 416.6 (combined Quarry and Talus sites), in the Tulip Beds Quarry, and in the Walcott Quarry, but is marginally significant for the Tulip Beds Talus site (Table 4). The same results are found when the mean abundances of the biomineralized taxa and the nonbiomineralized taxa are compared (Table 4). Although significantly different in the Tulip Beds Talus site, the biomineralized taxa have markedly lower abundances than nonbiomineralized taxa. The percentage of biomineralized taxa within each site is given in Table 4. When the differences between the abundance of biomineralized taxa and nonbiomineralized taxa are considered, the Tulip Beds Talus site is marginally significant for both distributions and means of these abundances (Table 4). Although this suggests that abundance and the presence of biomineralized skeletal elements are not independent at this site, it is noteworthy that the abundance of soft-bodied taxa is statistically higher than that of biomineralized taxa. A strong taphonomic bias against soft-bodied taxa would predict the opposite trend. The other sites have no statistical dependence between the presence of biomineralized skeletons and abundance. This indicates that biomineralization does not clearly affect the observed abundance in the assemblages and that the assemblages are not biased toward the preservation of biomineralized taxa relative to soft-bodied taxa. DEGREE OF TRANSPORT Analysis of large clusters of Siphusauctum gregarium indicates that there is no dominant orientation (table 1 and fig. 15 in O Brien and Caron 2012). Specimens tend to show a similar degree of preservation, perhaps indicating that they were buried simultaneously; certainly, there is no evidence of tearing or breakage indicating limited preburial decay and transport. Evidence of decay in some other specimens (fig. 6 in O Brien and Caron 2012) and the presence of disarticulated arthropod carcasses and molts suggest local displacement of dead individuals and molts during sedimentary events. Little is understood regarding the distances marine organisms, in particular arthropods, can be transported before disarticulation, although experimental work (Allison 1986) has suggested that in the absence of decay, nonbiomineralized arthropods may be transported significant distances without disarticulation, while more rapid fragmentation and disarticulation of taxa with more robust exoskeletons was observed. However, putative molts and/or decayed specimens are sometimes found as disarticulated fossils that remain in close association. This suggests that these specimens were not transported great distances, as complete disarticulation would be expected in this case (Fig. 5F). How far a fossil assemblage has been transported has significant implications for reconstructing the paleoenvironment and the paleocommunity. Evidence from both the sedimentology and taphonomy at the Tulip Beds locality suggests most organisms, in particular those occupying a benthic habitat, were entombed close to their living environment. This is supported by multiple lines of evidence that can be summarized as follows: 1. Presence of disarticulated individuals with elements remaining in close association. 2. Burial in thin event beds that represent limited sediment influx and low depositional energy. 3. No evidence of size sorting, lack of preferential alignment of specimens, and little or no sediment between individual specimens. CONCLUSIONS Fossils are found throughout the Tulip Beds succession, preserved in very thin event-deposited beds. The mode of preservation is typical of most Burgess Shale type deposits, with a predominance of two-dimensional carbonaceous compressions, some aspect of original three-dimensionality retained by biomineralized taxa, some phosphatization of gut tracts, and limited pyritization in the form of replacement and infilling (Figs. 5, 7 10) (Conway Morris 1986; Butterfield et al. 2007; Caron and Jackson 2008; Gaines et al. 2008). There are no quantifiable taphonomic biases within the Tulip Beds assemblages and the quality and mode of preservation does not change significantly across the studied section. This assemblage, like that of the Walcott Quarry, does not show any evidence of preservational bias associated with size of specimens or the possession of biomineralized skeletal elements. The similarities between the two localities imply that the assemblages were not significantly affected by differences in taphonomic regime. Therefore, differences in taxonomic composition and abundance between the localities likely reflect primary differences in community structure that existed between the localities. SUPPLEMENTAL MATERIAL Data is available from the PALAIOS Data Archive: http://www.sepm. org/pages.aspx?pageid5332. ACKNOWLEDGMENTS We thank Parks Canada for collection and research permits (to Desmond Collins before 2000 and J.-B. Caron for the 2008/2010 field seasons), Michael Streng and Martin Smith for field assistance, Peter Fenton for assistance in the collections, and Caleb Brown for illustration assistance. Thanks are extended to the Palaeontological Association for a Sylvester Bradley Award (2011) given to L.J. O Brien to fund work carried out at Pomona College. This study was part of L.J. O Brien s Ph.D. studies and was supported by fellowships from the University of Toronto (Dept. of Ecology and Evolution) and J.-B. 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324 L.J. O BRIEN ET AL. PALAIOS Supplementary FIG. 1. Representative X-radiographs of quarry section three (ROM 62298). Length 5 21 cm (A); 15 cm (B); 16 cm (C, D); 6 cm (E).