Burgess Shale−Type Biotas Were Not Entirely Burrowed Away

Downloaded from geology.gsapubs.org on September 25, 2015 Burgess shale−type biotas were not entirely burrowed away

Robert R. Gaines1, Mary L. Droser2, Patrick J. Orr3, Daniel Garson2, Emma Hammarlund4,5, Changshi Qi6, and Donald E. Canﬁ eld4 1Geology Department, Pomona College, 185 E. Sixth Street, Claremont, California 91711, USA 2Department of Earth Sciences, University of California−Riverside, Riverside, California 92521, USA 3UCD School of Geological Sciences, University College Dublin, Dublin 4, Ireland 4Nordic Center for Earth Evolution, DK-5230 Odense M, Denmark 5Department of Palaeozoology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden 6Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming 650091, China

ABSTRACT tion, a deepened redox boundary in sediment pore waters (e.g., Thayer, Burgess Shale−type biotas occur globally in the Cambrian record 1983; Ziebis et al., 1996), and increased sulfate availability in the global and offer unparalleled insight into the Cambrian explosion, the initial ocean (Canfi eld and Farquhar, 2009), all of which would affect the preser- Phanerozoic radiation of the Metazoa. Deposits bearing exception- vation potential of labile tissues. ally preserved soft-bodied fossils are unusually common in Cambrian An increase in depth of bioturbation and intensity of sediment mix- strata; more than 40 are now known. The well-documented decline of ing accompanying the expansion of the infaunal habitat through the early soft-bodied preservation following the Middle Cambrian represents Paleozoic is well documented in carbonate platform settings (Ausich and the closure of a taphonomic window that was only intermittently Bottjer, 1982; Droser and Bottjer, 1988, 1989). In muddy settings, sedi- open in marine environments thereafter. The prevailing hypothesis ment-mixing ichnogenera of the Nerites ichnofacies became established for this secular shift in taphonomic conditions of outer shelf environ- soon after the Cambrian (Orr et al., 2003). Here we evaluate the burrowed ments is that soft-bodied biotas were literally burrowed away from away hypothesis in light of new ichnologic data and a comprehensive the fossil record by increasing infaunal activity in muddy substrate reassessment of existing data from several of the principal BST deposits, environments; this would have affected geochemical gradients and and consider the impacts of an increase in depth, extent, and complexity increased the effi ciency of organic matter recycling in sediments. New of bioturbation on the potential for BST preservation in similar settings in and recently published data, however, suggest a more complex sce- the post-Cambrian. nario. Ichnologic and microstratigraphic data from Burgess Shale− type deposits indicate that (1) bioturbation exerts a limiting effect on ICHNOLOGIC SETTING OF BST FOSSIL ASSEMBLAGES soft-bodied preservation; (2) the observed increase in the depth and Among the more than 40 BST deposits now known, eight principal extent of bioturbation following the Middle Cambrian would have BST deposits have produced extensive collections of soft-bodied fossils. restricted preservation of Burgess Shale−type biotas in a number of Among these eight, the Burgess Shale (British Columbia, Canada) and settings; but (3) increasing depth and extent of bioturbation would Chengjiang (Yunnan Province, China) are by far the two most impor- not have affected preservation in many other settings, including the tant deposits in diversity and abundance of soft-bodied fossils. Of the most richly fossiliferous portions of the Chengjiang (China) deposit eight principal deposits, microstratigraphic data are now available for six, and the Greater Phyllopod Bed of the Burgess Shale (Canada). There- including complete millimeter-scale logs through the Greater Phyllopod fore, increasing bioturbation cannot account for the apparent loss of Bed of the Burgess Shale (Gostlin, 2006; Gabbott et al., 2008) and the this pathway from the fossil record, and requires that other circum- BST interval of the Chengjiang at two principal localities. Similar data are stances, including, but not limited to, widespread benthic anoxia, also available for three other subsidiary BST deposits. facilitated widespread exceptional preservation in the Cambrian. The ichnofabric index (i.i.), which provides a semiquantitative means of assessing extent of bioturbation (Droser and Bottjer, 1991), INTRODUCTION ranges from laminated sediments lacking bioturbation (i.i. 1), through Burgess Shale−type (BST) biotas occur globally in more than 40 sediments weakly (i.i. 2) to moderately (i.i. 3–4) disrupted, to those deposits in Early and Middle Cambrian strata (e.g., Conway Morris, completely homogenized by bioturbation (i.i. 5). The data now available 1989; Steiner et al., 2005) and form the foundation of our understand- indicate that BST assemblages are preserved in two distinct microstrati- ing of the Cambrian explosion (Budd and Jensen, 2000; Conway Morris, graphic settings. In many deposits, the most diverse BST biotas occur 1989, 2000; Marshall, 2006). Deposits containing exceptionally preserved predominantly in unbioturbated (i.i. 1) beds that are closely interbed- fossils are unusually common in Cambrian strata relative to the rest of the ded (millimeter to centimeter) with weakly to moderately bioturbated Phanerozoic, a pattern that is not an artifact of outcrop area (Allison and intervals (i.i. 2–3; Figs. 1A and 1B). This pattern is characteristic of Briggs, 1993). BST preservation was thus facilitated by taphonomic con- the Wheeler and Marjum Formations (Gaines and Droser, 2005, 2010), ditions that declined precipitously after the Middle Cambrian. Spence Shale (Garson et al., 2011), and Kaili Formation (Lin et al., The primary pathway of fossilization of BST biotas was conserva- 2010), as well as the Early Cambrian Pioche (Webster et al., 2008) and tion of primary organic remains, preserved as thin carbonaceous fi lms, Latham Shale Formations (Gaines and Droser, 2002), and a new Burgess and required early diagenetic stabilization of soft tissues against the nor- Shale locality in the Middle Cambrian “thin” Stephen Formation that mal processes of decomposition in sediments (Gaines et al., 2008). The accumulated upslope of the Burgess Shale (Caron et al., 2010; Gaines, accepted explanation for the apparent closure of this taphonomic win- 2011). This pattern is also characteristic of the Raymond Quarry Mem- dow after the Middle Cambrian is that BST biotas were burrowed away ber of the Burgess Shale (Allison and Brett, 1995). In each of these eight from the fossil record by increasing depth and intensity of bioturbation deposits, BST intervals are characterized by repeated, centimeter-scale in muddy substrate environments; this reduced the likelihood of excep- laminated (i.i. 1) to bioturbated (i.i. 2–4) cycles. Although the greatest tional preservation (Allison and Briggs, 1993; Orr, 2001; Orr et al., 2003). diversity of BST fossils within these deposits occurs in successions char- In addition to direct physical disruption of carcasses by infaunal activity, acterized by high-frequency (centimeter scale) oscillation in intensity of enhanced sediment mixing led to greater rates of organic carbon oxida- bioturbation, BST fossils are typically rare (Gaines and Droser, 2010).

© 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, March March 2012; 2012 v. 40; no. 3; p. 283–286; doi:10.1130/G32555.1; 2 ﬁ gures. 283 Downloaded from geology.gsapubs.org on September 25, 2015

The second microstratigraphic setting in which BST fossils occur BST fossils are typically abundant, diversity is limited; diminutive and is typiﬁ ed by the lower parts of the Wheeler and Marjum Formations fragmented algae are dominant, but rare pelagic soft-bodied metazoans, (Gaines and Droser, 2010). Bioturbation is absent for meters of continu- including arthropod molts and medusiform fossils, also occur (Gaines ous section, and BST fossils commonly occur in each bed for hundreds and Droser, 2005, 2010). to thousands of beds in vertical succession (Figs. 1C and 1D). While In the Walcott Quarry of the Burgess Shale (Greater Phyllopod Bed) and in the Chengjiang deposit, the most abundant and diverse BST biotas occur in a similar microstratigraphic context. No bioturbation is present A B within the entire ~7 m thickness of the Greater Phyllopod Bed (Gostlin, 2006; Gabbott et al., 2008). Microstratigraphic study of two new cores through the Chengjiang deposit at the two primary fossil-bearing localities reveals that the fossil-bearing interval is also unbioturbated throughout its 27 m extent (Fig. 2). Abundant burrows are readily recognized in identi- cal claystone lithofacies of the Burgess Shale above the Greater Phyllo- pod Bed and in the Chengjiang above the primary fossil-bearing interval, which are characterized by the same range of bed thicknesses and sedimentary structures. Therefore the absence of trace fossils from the most 1 cm richly fossiliferous parts of each unit does not represent poor preservation in soupy substrates or imply that sedimentation rates that may have been C too high to permit infaunal colonization. Instead, this suggests that biotur- bators were excluded by environmental conditions, i.e., benthic anoxia, during the deposition of each interval.

ROLE OF ANOXIA The depositional settings of BST deposits have traditionally been considered restrictive, and benthic anoxia is often invoked as a prerequi- 1 cm

D A Depth (m) i.i. B Depth (m) i.i. C Depth (m) 80 25.27 26.27 15

100 Member 3 25.47 26.47 BST Interval 25

120

Yu’anshan Formation Yu’anshan 25.67 26.67 35 Member 2

140 25.87 26.87 45

cm Member 1 1 2 3 4 5 1 cm FormationYu’anshan (Member 3) Ichnofabric index (i.i.) 160 26.07 27.07 Figure 1. Examples demonstrating two distinct ichnologic settings of 55

Burgess Shale−type (BST) assemblages. A: Microstratigraphic log of Black claystone 21 cm from Cycle 4 (Liddell et al., 1997) of Spence Shale Member of BST Interval Event claystone the Langston Formation (Miner’s Hollow, Wellsville Mountains, Utah)

180 Formation Shiyantou modiﬁ ed from Garson et al. (2011). In this interval, BST fossils occur 26.27 in nonbioturbated intervals (green), although BST fossils may also LEGEND Laminated black claystone Laminated argillageous occur in weakly bioturbated horizons (ichnofabric index, i.i., 2–3). Ichnofabric Index (i.i.) with gray claystone event beds black siltstone Extensively bioturbated horizons are closely interbedded (millimeter 1 2 3 4 5 Laminated black claystone Siltstone to centimeter scale) with nonbioturbated horizons. B: Polished slab of BST interval of “thin” Stephen Formation from Stanley Glacier locality (Caron et al., 2010), Kootenay National Park, British Columbia, Figure 2. Logs of two drill cores showing extent of bioturbation. showing low-intensity bioturbation (i.i. 2) in most beds, although A: Chengjiang deposit at Maotianshan type locality; log of Maotianshan nonbioturbated intervals are also present. Royal Ontario Museum core shows absence of bioturbation from complete 27 m Burgess sample 59951. C: Polished slab of BST fossil-bearing interval of Shale–type (BST) fossil-bearing interval, and increasing extent of lower Marjum Formation (House Range, Utah) showing absence of bioturbation in same lithofacies upsection. B: Haikou, China, log bioturbation, which is often sustained for many meters of continuous showing absence of bioturbation from complete BST fossil-bearing section. White ovals are cross sections of agnostid trilobites, which interval and extensive (ichnofabric index, i.i. 4–5) bioturbation of have been heavily cemented by calcite. D: Sample from BST fossil- underlying Shiyantou Formation. C: Detailed microstratigraphic log of bearing interval of Chengjiang deposit cut from core drilled at Haikou, 1.8 m from within BST fossil-bearing interval of Haikou core, showing China, locality (23.30 m depth) showing absence of bioturbation. absence of bioturbation.

284 GEOLOGY, March 2012 Downloaded from geology.gsapubs.org on September 25, 2015 site for the preservation of BST fossils (Allison, 1988; Butterfi eld, 1995; the pathway for exceptional preservation would have been unaffected in Gaines and Droser, 2005, 2010), although it is widely recognized that parts of these formations, and possibly in others. anoxia alone is insuffi cient to promote soft-bodied preservation (Allison, The most richly fossiliferous portions of the Chengjiang and Bur- 1988; Butterfi eld, 1995). A prevailing view has emerged that the deep- gess Shale would also have been only minimally affected. Because these water taphonomic window for exceptional preservation in the Cambrian portions of the units accumulated under sustained anaerobic conditions, was associated with the edge of a fl uctuating oxycline (Conway Morris, bioturbation would have been excluded from the locus of deposition 1986; Allison, 1988; Allison and Brett, 1995; Butterfi eld, 1995; Gaines regardless of increasing depth and intensity (Savrda and Bottjer, 1991). and Droser, 2003, 2005, 2010; Brett et al., 2009; Garson et al., 2011) that The stratigraphic ranges of BST fossil-bearing intervals in both deposits intersected the seafl oor and moved back and forth over time. As a result, would likely have been reduced by burrows originating from intervals BST assemblages in unbioturbated intervals commonly occur in close above. A maximum of 1.25 m of the upper part of the Greater Phyllopod association (millimeter to centimeter) with intervals that were bioturbated Bed of the Burgess Shale would have been vulnerable to disruption by when redox conditions were favorable for the development of benthic bioturbation originating from above, leaving BST preservation in at least communities (Liddell et al., 1997; Gaines and Droser, 2005, 2010; Brett the lower 5.75 m (82% of thickness) unaffected. The 27 m BST fossil- et al., 2009; Caron et al., 2010; Lin et al., 2010; Gaines, 2011; Garson et bearing interval of the Chengjiang is represented in a new drill core from al., 2011). BST fossils worldwide are characterized by their preferential the Maotianshan type locality (182–154.99 m depth). Microstratigraphic occurrence in unbioturbated sediments; they occur only rarely in beds with data indicate that the entirety of the BST fossil-bearing interval is unbio- weak postdepositional bioturbation, and are absent from beds with moder- turbated (i.i. 1, Fig. 2). The fi rst evidence of bioturbation does not appear ate to strong extents of bioturbation (Allison and Brett, 1995; Gaines and until 21 m above the top of the fossil-bearing interval (133.68 m). In a Droser, 2010; Lin et al., 2010; Garson et al., 2011). Therefore, it is clear second core drilled at Haikou, only the lower 15.74 m of the BST fossil- that bioturbation, or the benthic chemical environments that promote it, is bearing interval is present (12.96–28.70 m). This interval is also com- limiting to BST preservation. Furthermore, it has been shown that biotur- pletely unbioturbated. However, because neither core is 100% complete, bation reached the distal margin of dysoxic benthic environments in the it is possible that bioturbated horizons may occur rarely within the BST Cambrian, and therefore the presence of bioturbation is useful in delineat- interval. Rare examples of burrows in direct association with Chengji- ing dysoxic from anoxic environments in mudstones of this age (Gaines ang BST fossils have been reported (Zhang et al., 2007), indicating that and Droser, 2010). some horizons within the fossil-bearing interval are weakly bioturbated. Therefore, we assume that a post–Middle Cambrian escalation in biotur- IMPACTS OF INCREASING DEPTH, EXTENT, AND bation would have affected preservation potential in some parts of the COMPLEXITY OF BIOTURBATION Chengjiang; however, the great majority of the fossil-bearing interval Because bioturbation exerts a limiting effect upon BST preservation, would not have been affected. the post-Cambrian escalation in bioturbation potentially would have had a deleterious effect on preservation potential in similar environments later CONCLUSIONS in the Phanerozoic. Using data from nine BST deposits, we project the A post–Middle Cambrian escalation in depth, intensity, and com- impacts of the post–Middle Cambrian increase in depth, complexity, and plexity of bioturbation would have had signifi cant consequences for pres- intensity of bioturbation on preservation potential in these deposits, and ervation potential of soft tissues in later Phanerozoic environments simi- consider the implications for post-Cambrian strata deposited under similar lar to those that promoted BST preservation in the Cambrian. Escalation conditions. In order to conservatively evaluate the potential for survival of of bioturbation may have drastically reduced the likelihood of this type this taphonomic pathway, we assume a worst-case scenario in which bur- of preservation in many environments, severely limiting its geographic rows originating from weakly to moderately bioturbated horizons would extent and local stratigraphic ranges. However, we fi nd that, even con- have increased from 1–3 cm in depth to the Phanerozoic maximum of sidering a worst-case scenario for the impacts of escalating bioturbation, 2.5 m (Pemberton et al., 1976), corrected to 1.25 m using a conservative the potential for BST preservation in environments similar to those of estimate (50%) for burial compaction of mudstones. We also assume that the Chengjiang and Greater Phyllopod Bed of the Burgess Shale would intensity would have increased from i.i. 2–3 to i.i. 4–5. Because BST fos- have been only minimally affected, because they were deposited under sils do not occur in intensely bioturbated horizons of the deposits (Allison conditions of sustained benthic anoxia that precluded disturbance from and Brett, 1995; Dornbos et al., 2005; Hu, 2005; Gaines and Droser, 2010; bioturbation, except via burrows penetrating downward from above. This Lin et al., 2010; Garson et al., 2011), we assume that the potential for BST pathway would also have remained unaffected in parts of at least two other preservation of soft tissues would have been eliminated in any horizons principal BST deposits. Although escalating levels of bioturbation would affected by bioturbation. certainly have reduced the prevalence of BST preservation in outer shelf An increase in depth, intensity, and complexity of bioturbation would environments later in the Phanerozoic, it cannot explain the decline of this have had the greatest impact on preservation potential in the portions of taphonomic pathway. While anoxia alone cannot account for BST preser- the deposits in which BST fossil-bearing intervals are closely interbedded vation, widespread anoxic benthic conditions (e.g., Peters, 2007; Dahl et with bioturbated intervals. These settings include crucial fossil-bearing al., 2010; Gill et al., 2011) played an important role in promoting excep- intervals of many of the principal deposits (Figs. 1A and 1B). BST preser- tional preservation in the Cambrian. vation would have been lost from the portions of the Spence, Wheeler, and Marjum deposits that contain the most diverse BST assemblages, and also ACKNOWLEDGMENTS We thank T. Abbott, L. Allen-Williams, M. Balint, P. Burke, F. Duan, S. would have been entirely lost from the Kaili Formation, Latham Shale, Finnegan, G. Gunther, X., Hou, S. Hlohwwyskyj, P. Hong, P. Jamison, K. Le, R. Pioche Formation, and “thin” Stephen Formation, as well as the Raymond Mackenzie, B. Markle, J. Skabelund, J. Tian, X. Zhang, and F. Zhou for assistance Quarry Member of the Burgess Shale. in the fi eld. J. Shiffbauer and two anonymous reviewers provided helpful comments It is important, however, that the BST taphonomic pathway would on the manuscript. 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286 GEOLOGY, March 2012 Downloaded from geology.gsapubs.org on September 25, 2015

Geology

Burgess shale−type biotas were not entirely burrowed away

Robert R. Gaines, Mary L. Droser, Patrick J. Orr, Daniel Garson, Emma Hammarlund, Changshi Qi and Donald E. Canfield

Geology 2012;40;283-286 doi: 10.1130/G32555.1

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