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Burgess Shale−Type Biotas Were Not Entirely Burrowed Away

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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 sedi- mentary 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
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