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Evolutionary paleoecology and taphonomy of the earliest animals: Evidence from the Neoproterozoic and Cambrian of southwest China

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Evolutionary paleoecology and taphonomy of the earliest animals: Evidence from the Neoproterozoic and Cambrian of southwest China

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Content EVOLUTIONARY PALEOECOLOGY AND TAPHONOMY OF THE EARLIEST ANIMALS: EVIDENCE FROM THE NEOPROTEROZOIC AND CAMBRIAN OF SOUTHWEST CHINA Copyright 2003 by Stephen Quinn Dornbos A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (GEOLOGICAL SCIENCES) May 2003 Stephen Quinn Dornbos Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U M I N um ber: 3 1 0 3 8 8 2 Copyright 2003 by Dornbos, Stephen Quinn All rights reserved. ® UMI UMI Microform 3103882 Copyright 2003 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90089-1695 This dissertation, written by Stephen Quinn Dornbos under the direction o f h l s dissertation committee, and approved by all its members, has been presented to and accepted by the Director o f Graduate and Professional Programs, in partial fulfillment o f the requirements fo r the degree o f DOCTOR OF PHILOSOPHY Director Date May 1 6 . 2 0 0 3 Dissertation Committee Chair 3 / 0? - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ii Acknowledgements First and foremost, I would like to thank Dave Bottjer for everything he has done for my development as a scientist over the course of the last six years. His advice and guidance made this dissertation possible. Special thanks also to all the other members of my dissertation committee: Frank Corsetti, Richard Deonier, Bob Douglas, Donn Gorsline, Al Fischer, and Russell Zimmer. Their insight and feedback improved this dissertation immeasurably. Thanks also to Jun-Yuan Chen, Eric Davidson, and Chia-Wei Li for their assistance that was critical to the completion of this dissertation. Of course, without Mark Wilson getting me hooked on paleo to begin with, none of this would be possible. Thanks Mark! I’d also like to thank all the members of the Paleolab during my time here: Nicole Bonuso, Matthew Clapham, Margaret Fraiser, Nicole Fraser, Gerald Grellet-Tinner, Whitey Hagadorn, Karina Hankins, Tran Huynh, Catherine Jamet, Pedro Marenco, Sara Pruss, Dave Rodland, Stephen Schellenberg, Karen Whittlesey, Adam Woods, and Kate Woods. You have all contributed in your own way to my life and to my research. The USC Department of Earth Sciences, NASA, the National Geographic Society, the USC Wrigley Institute for Environmental Sciences, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. iii the Paleontological Society, and Sigma Xi all deserve special thanks for their financial support of this research. Finally, I’d like to thank my family and Margaret for their undying support. I would be nowhere without you guys! Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Page Acknowledgements........................................................................................... i i List of Tables...................................................................................................viii List of Figures..................................................................................................ix Abstract.................................................................................................... xii Chapter 1: Introduction..................................................................................... 1 Introduction.............................................................................................1 The Neoproterozoic-Cambrian Animal Fossil Record..........................2 Phosphatic Fossils of the Neoproterozoic Doushantuo Formation 7 The Early Cambrian Chengjiang Fauna..............................................10 The Middle Cambrian Burgess Shale Fauna...................................... 15 The Cambrian Substrate Revolution................................................... 19 Global Geologic and Paleogeographic Setting....................................23 Chapter 2: Sedimentology and Microfacies Analysis of the Weng’an Phosphate Member of the Neoproterozoic Doushantuo Formation, Southwest China.............................................................................................27 Introduction...........................................................................................27 Phosphatization....................................................................................28 Preliminary Taphonomic Model............................................................29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V Page Geologic Setting..................................................................................31 Methods...............................................................................................34 Results............................... 36 Discussion.......................................................................... 59 Conclusions............................................................................. 66 Chapter 3: Taphonomy of Phosphatized Metazoan Embryos from the Neoproterozoic of Southwest China...............................................................68 Introduction............................................................................. :..........68 Previous Work...................................................................................... 70 Methods...............................................................................................71 Results................................................................................................. 74 Discussion...........................................................................................83 Conclusions..........................................................................................85 Chapter 4: Evidence for Seafloor Microbial Mats and Associated Metazoan Lifestyles in the Lower Cambrian Meishucun Formation of Southwest China............................................................................................ 87 Introduction...........................................................................................87 Previous Research...............................................................................89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vi Page Geologic Setting...................................................................................90 Methods............................................................................................... 95 Metazoan Behavior and Paleoecology............................................... 96 Discussion..........................................................................................114 Conclusions.........................................................................^............. 116 Chapter 5: Paleoecology of Benthic Metazoans in the Early Cambrian Chengjiang Fauna and the Middle Cambrian Burgess Shale Fauna: Evidence for the Cambrian Substrate Revolution........................................118 Introduction........................................................................ 118 Geologic Setting.................................................................................125 Methods..............................................................................................129 Shiyantou Formation and Maotianshan Shale Member Core Analysis..............................................................................................131 Adaptations of Benthic Suspension Feeders to Typical Phanerozoic-style Soft Substrates....................................................141 Adaptations of Benthic Suspension Feeders to Typical Proterozoic-style Soft Substrates......................................................145 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vii Page Paleoecology of Chengjiang Fauna Benthic Suspension Feeders...............................................................................................146 Paleoecology of Burgess Shale Fauna Benthic Suspension Feeders....................................................................................... 157 Paleoecology of Mobile Benthic Metazoans in the Chengjikng and Burgess Shale Faunas............................................................... 166 Discussion..........................................................................................173 Conclusions.................................... 174 Chapter 6: Conclusions ......................................................................178 References Cited..........................................................................................185 Appendix A.............................. 199 Appendix B....................................................................................................217 Appendix C....................................................................................................223 Appendix D....................................................................................................236 Appendix E....................................................................................................239 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. viii List of Tables Table Page Table 3.1. Percentage of embryos in each taphonomic grade.....................77 Table 5.1. Criteria for interpreting life modes..............................................147 Table 5.2. Chengjiang fauna benthic suspension feeders ............... 149 Table 5.3. Adaptive morphologies of Chengjiang fauna benthos............... 153 Table 5.4. Burgess Shale fauna benthic suspension feeders....................159 Table 5.5. Adaptive morphologies of Burgess Shale fauna benthos..........163 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ix List of Figures Figure Page Figure 1.1. Neoproterozoic-Cambrian animal fossil record............................ 3 Figure 1.2. Doushantuo metazoan embryo.....................................................9 Figure 1.3. Anomalocaris................................................................................12 Figure 1.4. Microdictyon.................................................................. 14 Figure 1.5. Marella splendens........................................................................16 Figure 1.6. Neoproterozoic global paleogeography ..........................26 Figure 2.1. Doushantuo Formation stratigraphy............................................32 Figure 2.2. Weng’an Phosphorite Member stratigraphy............. 35 Figure 2.3. Photograph of Weng’an Phosphorite Member........................... 37 Figure 2.4. Photograph of Weng’an black facies..........................................38 Figure 2.5. Photograph of Weng’an black facies bedding............................ 39 Figure 2.6. Photograph of Weng’an black facies phosphoclast....................40 Figure 2.7. Photograph of Weng’an black facies lower contact.....................42 Figure 2.8. Photograph of Weng’an black facies pyrite................................. 43 Figure 2.9. Photograph of Weng’an black-gray facies transition...................44 Figure 2.10. Photograph of Weng’an black-gray facies transition.................45 Figure 2.11. Photograph of Weng’an gray facies...........................................46 Figure 2.12. Photomicrographs of black facies............................................ 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. X Figure Page Figure 2.13. Photomicrograph of pyrite in black facies.................. 49 Figure 2.14. Photomicrographs of amorphous phosphate..........................51 Figure 2.15. Photomicrographs of lenticular bed in black facies................ 52 Figure 2.16. Photomicrograph of 2-3 m interval of black facies..................53 Figure 2.17. Photomicrograph of gray facies.................................. :............54 Figure 2.18. Photomicrograph of phosphatic Crust...................................... 56 Figure 2.19. Photomicrographs of fine dolomitic laminae............................57 Figure 2.20. Photomicrograph of fine-grained dolomite...............................58 Figure 2.21. Scatter plot of estimated dolomite percentages...................... 60 Figure 2.22. Southwest China Neoproterozoic-Cambrian facies belts........65 Figure 3.1. Photomicrographs of metazoan embryos..................................69 Figure 3.2. Examples of taphonomic grades of embryos........................ ...72 Figure 3.3. Stacked bar histogram of taphonomic grades........................... 78 Figure 3.4. Plot of taphonomic grade abundances...................................... 81 Figure 4.1. Stratigraphic column of Meishucun Formation.......................... 92 Figure 4.2. Photographs of Meishucun Formation.......................................93 Figure 4.3. Photographs of Radulichnus......................................................97 Figure 4.4. Sketches of Radulichnus............................................................99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. xi Figure Page Figure 4.5. Photographs of discoidal impressions....................................103 Figure 4.6. Photographs of modern Radulichnus.....................................106 Figure 4.7. Photographs of red seams..................................................... 109 Figure 4.8. Photographs of horizontal Thalassinoides..............................112 Figure 5.1. Diagram of dominant sedimentary fabrics...............................120 Figure 5.2. Lower Cambrian stratigraphy of Yunnan Province.................. 126 Figure 5.3. Photographs of Shiyantou Formation cores............................ 133 Figure 5.4. Graph of ii percentages .................................................. 135 Figure 5.5. Photographs of Moatianshan Shale Member cores.................139 Figure 5.6. Chengjiang fauna benthic suspension feeders........................ 150 Figure 5.7. Box diagram of Chengjiang suspension feeders..................... 155 Figure 5.8. Burgess Shale fauna benthic suspension feeders...................160 Figure 5.9. Chengjiang and Burgess Shale fauna mobile benthos............ 168 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Abstract i The Neoproterozoic and Cambrian soft-bodied fossil deposits, or lagerstatten, of southwest China provide invaluable windows into life during the initial radiation of animals. This dissertation examines various aspects of two of these lagerstatten: the Neoproterozoic Doushantuo Formation and the Early Cambrian Chengjiang fauna. Of primary interest is the taphonomy of the earliest known animal fossils, the phosphatized animal embryos of the Neoproterozoic Doushantuo Formation, and the evolutionary paleoecology of animals during the Cambrian “explosion”, as preserved in the Early Cambrian Chengjiang fauna and adjacent strata. A sedimentological and petrographic study of the embryo-bearing interval of the Doushantuo Formation demonstrates that there are two distinct phosphogenic environments in which the Doushantuo fossils were phosphatized. These results may explain the distribution of probable fossils described from the Doushantuo Formation. A detailed specimen-based taphonomic study of the Doushantuo embryos indicates that there is a taphonomic bias toward early cleavage stages and away from later cleavage stages and adults. One possible explanation for this pattern is that earlier cleavage stages were more physically robust and thereby better able to withstand the abundant reworking inherent in phosphogenic settings. A study of the trace fossils and sedimentology of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Early Cambrian Meishucun Formation of southwest China, just stratigraphically beneath the Chengjiang fauna, reveals that many animals during the early stages of the Cambrian radiation still had lifestyles adapted to the presence of abundant seafloor microbial mats in normal marine settings even as bioturbation levels began increasing. Analysis of the paleoecology of benthic metazoans in the Chengjiang fauna and the i sediments in which they are preserved indicates that a majority of these benthic metazoans were adapted to survive on substrates characterized by low levels of bioturbation more typical of the Neoproterozoic. Furthermore, a comparison between the paleoecology of benthic metazoans in the Chengjiang fauna and the younger Middle Cambrian Burgess Shale fauna of British Columbia, Canada reveals that the Burgess Shale fauna contains a larger percentage of benthic metazoans adapted to survive on more intensely bioturbated substrates more characteristic of the Phanerozoic. Increasing bioturbation levels thereby likely had a profound effect upon the early evolution of animals. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 CHAPTER 1: Introduction Introduction Deposits in which the soft tissues of animals are preserved, or fossil Lagerstatten, have proven invaluable in the study of early animals. This dissertation focuses on various aspects of several of these crucial deposits, and adjacent strata, primarily in the Neoproterozoic and Lower Cambrian of Yunnan and Guizhou Provinces in Southwest China, but also in the Middle Cambrian of British Columbia, Canada. In particular, this research aims to broaden our understanding of the evolutionary paleoecology of early benthic animals and the taphonomy of possibly the oldest evidence for animals currently known in the fossil record. The taphonomy of what are probably the earliest animal fossils is studied through a specimen-based taphonomic study of the phosphatized animal embryos from the Neoproterozoic Doushantuo Formation of Southwest China, as well as a sedimentological and petrographic study of the Doushantuo Formation itself. The evolutionary paleoecology of early animals, meanwhile, is studied through examination of their adaptive morphologies and the substrates on which they were living as preserved in the rocks and fossils of the Lower Cambrian Chengjiang fauna Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 and Meishucun Phosphorite of Southwest China, and the Middle Cambrian Burgess Shale fauna of British Columbia. The Neoproterozoic-Cambrian Animal Fossil Record The earliest possible evidence for animals consists of cm-scale, Ediacaran-type, radially symmetrical, discoidal impressions called Twitya discs (Hofmann et al., 1990). Found in siliciclastic turbidites of the Neoproterozoic Twitya Formation, which is stratigraphically between two glacial diamictites in the Mackenzie Mountains of northwestern Canada, these discs are probably slightly older than, or concurrent with, the last glacial interval (Marinoan glaciation) of the Neoproterozoic (Fig. 1.1) (Hofmann et al., 1990). This stratigraphic position places them as the oldest possible fossil evidence for animal life (around 610 Ma) (Hofmann et al., 1990), although their interpretation as such is still somewhat controversial because there are multiple possible interpretations of simple discoidal fossils (e.g. Grazhdankin, 2001; Jensen et al., 2001). Probably the earliest unambiguous evidence for animal life is, therefore, the phosphatized animal embryos of the Neoproterozoic Doushantuo Formation of Southwest China (Fig. 1.1). Discussed in more Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 5 2 0 - SSFs Siliceous Sponge Spicules Diverse Kriincaran Fossils 5 7 0 - Marinoan Glaciation Twitya Disks Fig. 1.1. Chart displaying the Neoproterozoic-Cambrian animal fossil record. Ages are approximate (modified from Knoll, 2000). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 detail in the next section, as well as in Chapter 2, these embryos, along with other possible developmental stages and adult forms, are found in rocks just stratigraphically above a diamictite, the Nantuo Formation, thought to have been deposited during the last glacial interval of the Neoproterozoic. The rocks bearing these phosphatized animal embryos have recently been dated at 586+26 Ma (Lu-Hf dating) and 599.3±4.2 Ma (Pb-Pb dating) (Barfod et al., 2002). These dates, if upheld, suggest that the phosphatized animal embryos of the Doushantuo Formation may be the oldest definitive evidence for animals known in the fossil record (Fig. 1.1) (Barfod et al., 2002). The next clear evidence for animals in the fossil record consists of the Ediacaran fauna and associated trace fossils (Fig. 1.1). All Ediacaran fossils are preserved as impressions in siliciclastic rocks, and the precise taxonomic affinity of most of them is still unknown. There is a general consensus, however, that most of them were animals of some form or another (e.g. Bottjer, 2002). The oldest known diverse assemblage of Ediacaran fossils is found at Mistaken Point, Newfoundland, which are between 595 and 565 Ma in age (Narbonne and Gehling, 2003). The Ediacaran fauna is preserved in siliciclastics around the world and displays complex community structure and tiering (e.g. Clapham and Narbonne, 2002). Several microbial-mat dependent life modes evolved in the Ediacaran fauna including: 1) mat stickers and mat encrusters, that in many cases were probably suspension Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 feeders; 2) mat scratchers, that scratched microbial mats for food; and 3) undermat miners, that burrowed horizontally underneath microbial mats eating buried organic material (Seilacher, 1999). The Ediacaran fauna is also associated, particularly closer to the Precambrian-Cambrian boundary, with simple horizontal trace fossils such as Planolites (Fig. 1.1), probably produced by bilaterians (e.g. Bottjer, 2002). In the terminal Proterozoic, a group of calcareous skeletonized animals begin appearing in the fossil record, signifying the nascent beginning of biomineralization (Fig. 1.1) (e.g. Knoll, 2000). The conical macrofossil Cloudina and cup-shaped macrofossil Namacalathus are typical of the stratigraphic interval just below the Precambrian-Cambrian boundary, the latter in thrombolite-stromatolite reef settings (e.g. Grotzinger et al., 2000; Corsetti and Hagadorn, 2000; Watters and Grotzinger, 2001). The conical small shelly fossils Anabarites and Cambrotubulus are also known from terminal Proterozoic strata, but are comparatively rare (Knoll et al., 1995; Brasieret al., 1997). Also found in the terminal Proterozoic of southwestern Mongolia are the earliest definitive siliceous sponge spicules (Fig. 1.1) (Brasier et al., 1997). Consisting of monaxial, triaxial, and polyactine morphologies, they are readily identifiable as Hexactinellid spicules (Brasier et al., 1997). Probable siliceous sponge spicules have been reported from older rocks in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 China, coeval with, or even older than, the Ediacaran fauna (Fig. 1.1) (Tang et al., 1978; Ding et al., 1988; Steiner et al., 1993; Li et al., 1997), but this is the oldest instance of undeniable sponge spicules assignable to an extant taxonomic group (Knoll, 2000). The Precambrian-Cambrian boundary is marked by the first occurrence of the trace fossil Treptichnus pedum, the earliest vertically penetrative trace fossil (Fig. 1.1). Trace fossils continued to diversify rapidly during the Early and Middle Cambrian, while also becoming increasingly vertically-oriented. The cause of this increase in the depth and intensity of bioturbation was, of course, the Cambrian radiation of animal life, or “Cambrian Explosion”. During the pre-trilobite Tommotian Stage of the Lower Cambrian, this radiation is evident not only in the trace fossils, but also in the proliferation and diversification of small shelly fossils (Fig. 1.1). These fossils, consisting of often microscopic sclerites and spicules thought to have comprised parts of the scleritome of mostly unknown animals, represent the next stage in animal biomineralization. In a few cases, such as the lobopod Microdictyon and the slug-like Halkieria, these animals have been found preserved with their sclerites in place, allowing for the identification of loose sclerites that belong to them. A host of fully skeletonized animals, including trilobites, brachiopods, archaeocyathids, and echinoderms, appeared in the fossil record and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 diversified rapidly during the Atdabanian Stage of the Lower Cambrian. Because it is thought to be latest Atdabanian in age, the soft-bodied Chengjiang fauna of southwest China provides an exquisitely detailed portrait of animal life during this early pulse of the Cambrian radiation (Fig. 1.1). Discussed in detail later in this chapter, as well as in Chapter 5, the Chengjiang fauna contains a diverse array of animals, including arthropods, sponges, and worms, many of which would normally not be preserved because they lack skeletons. The Cambrian radiation continued into the Middle Cambrian, wherein another fossil Lagerstatte provides the most complete snapshot of animal life: the renowned Burgess Shale fauna of British Columbia (Fig. 1.1). Discussed in detail later in this chapter, as well as in Chapter 5, the Burgess Shale fauna is thought to be approximately 10 Ma younger than the Chengjiang fauna. Like the Chengjiang fauna, the Burgess Shale fauna also contains a diverse assortment of soft-bodied animals not typically preserved in the fossil record. Phosphatic Fossils of the Neoproterozoic Doushantuo Formation As noted above, the Neoproterozoic Doushantuo Formation of South China has yielded what may be the earliest known unambiguous fossil Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 evidence for metazoans in the form of phosphatized animal eggs and embryos (Fig. 1.2) (Xiao et al., 1998; Li et al., 1998; Xiao and Knoll, 2000). While the importance of these animal embryos is undeniable, an assortment of other possible phosphatic fossils has also been described from the Doushantuo phosphorites. These potential fossils include possible gastrulae, larvae, and microscopic adults of sponges and cnidarians (Li et al., 1998; Chen et al., 2000; Xiao et al., 2000; Chen et al., 2002). No definitive evidence for bilaterians has thus far been described from the Doushantuo Formation. While these recent fossil discoveries have attracted much attention, the Doushantuo Formation has actually yielded significant fossils for the last two decades. These fossils include cyanobacteria (e.g. Zhang 1981; Awramik et al., 1985), acritarchs (e.g. Yin and Li, 1978), and red algae (e.g. Zhang, 1989) preserved in the cherts and phosphorites of the Doushantuo. Xiao et al.(1998) and Li et al. (1998) first reported probable metazoan eggs and embryos from the Doushantuo. Multicellular algae were also reported (Xiao et al., 1998), as well as possible microscopic adult sponges (Li et al., 1998). The embryos were found in various stages, one, two, four, eight, sixteen or more cells, yet were all around 500-700 microns in diameter, consistent with an embryo undergoing its earliest stages of cell division but not increasing in volume (Xiao et al., 1998). Chen et al. (2000) reported Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 Fig, 1,2. Photomicrograph of a 4-cei! stage metazoan embryo from the Neoproterozoic Doushantuo Formation of southwest China in plane-polarized light. Embryo is approximately 750 microns in diameter. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 possible bilaterian gastrula-stage embryos, as well as possible cnidarian and sponge larvae from the Doushantuo. Xiao et al. (2000) have reported possible microscopic stem-group cnidarians in the form of small (100-300 microns in diameter), branching, tabulate forms, while Chen et al. (2002) have also preliminarily identified phosphatized microscopic adult hydrozoid cnidarians as well as various possible developmental stages of cnidarians. The Early Cambrian Chengjiang Fauna The Chengjiang fauna is the oldest well-studied fossil Lagerstatte of the Phanerozoic. As such, it provides a critical picture of animal life during the early stages of the relatively rapid Cambrian radiation. Arthropods dominate the Chengjiang fauna, comprising over 40% of the species (Hou et al., 1991), with only 4 genera of trilobites (Chen et al., 1996) possessing commonly preservable mineralized skeletal elements (Hou et al., 1991). A wide array of soft-bodied arthropods are found in the Chengjiang fauna, including the Trilobitomorphs, such as Retifacies and Acanthomeridion, that resemble trilobites (Hou et al., 1989). Many species of Naraoiids, Xanderellids, Tegopeltids, and Fuxianhuids, all arthropod groups, are also important components of the Chengjiang fauna (Hou, 1987; Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11 Rather unusual arthropod-like creatures of uncertain taxonomic status are also present in the Chengjiang fauna. Most dramatic among these enigmatic metazoans are the Anomalocarids, nektonic predators that are also found in the Burgess Shale fauna (Hou et al., 1995). Three genera of anomalocarids, Anomalocaris (Fig. 1.3), Amplectobulua, and Peytoia, are found in the Chengjiang fauna (Chen et al., 1996). Opabinids, another group of strange nektonic arthropod-like metazoans, are also found in the Chengjiang fauna and the Burgess Shale fauna (Hou, 1987). The Chengjiang fauna also contains a diverse array of sponges and at least one cnidarian. Ten genera of benthic suspension-feeding sponges, some of which, such as Choia, are also present in the Burgess shale fauna, are found in the Chengjiang fauna (Chen et al., 1996). Meanwhile, just one undisputed cnidarian genus, Xianguangia, an anemone-like benthic suspension feeder, is present in the Chengjiang fauna (Chen and Erdtmann, 1991). The enigmatic suspension feeder Dinomischus is also known from Chengjiang (Chen et al., 1996). Worms and lobopodians are also common in the Chengjiang fauna. Three genera, Maotianshania, Circocosmia, and Palaeoscolex, of burrowing priapulid worms are present, in addition to a wide array of lobopodians (Chen et al., 1996). The well-known Burgess Shale lobopodian Hallucigenia is present, as are five previously unknown genera of these Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 10 cm Fig. 1.3. The arthropod-like predator Anomalocaris from the Early Cambrian Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13 unusual creatures: Microdictyon (Fig. 1.4), Onychodictyon, Cardiodictyon, Luolishania, and Paucipodia (Conway Morris, 1977; Hou and Chen, 1989; Hou et al., 1991; Chen et al., 1995). While some workers (Hou et al., 1991) suggested that these lobopodians are referable to the Onychophora, their classification remains contentious (Chen et al., 1996). Medusiform animals of uncertain affinities are found at Chengjiang, most prominent of which is Eldonia, also known from the Burgess Shale (Walcott, 1911; Sun and Hou, 1987). Thought to be planktonic (Chen et al., 1995), Eldonia is accompanied by Rotadiscus, another medusiform (Chen et al., 1996). The lobopod Microdictyon is often found preserved with Eldonia, interpreted by Chen et al. (1996) to indicate some sort of symbiotic relationship between these two very different animals. Phoronids, brachiopods, hyoliths, and chordates are also known from Chengjiang. While phoronids and hyolithes are represented by only one genus each, there are four genera of brachiopods (Chen et al., 1996). Three of these brachiopod genera are lingulids, complete with preserved pedicles (Chen et al., 1996). Finally, the Chengjiang fauna contains one chordate species, Yunnanozoon (Hou et al., 1991). Yunnanozoon, similar in appearance to Pikaia, the Burgess Shale chordate, was nektonic, and its notochord is usually well-preserved (Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 Posterior Anterior Fig. 1.4. The lobopodian Microdictyon from the Early Cambrian Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15 The Middle Cambrian Burgess Shale Fauna The Middle Cambrian Burgess Shale fauna of British Columbia is one of the most famous Lagerstatten in the world. Approximately 10 million years younger than the Chengjiang fauna (e.g. Bowring et al., 1993; Landing et al., 1998), the Burgess Shale fauna provides the most well-studied glimpse into marine life during the Middle Cambrian, arguably the heart of the Cambrian radiation. Much like the Chengjiang fauna, arthropods dominate the Burgess Shale fauna, likewise comprising over 40% of the fauna (Whittington, 1985; Conway Morris, 1986). These arthropods include crustaceans, chelicerates, trilobites, possible ostracods, possible cirripeds, and archnomorphs, as well as a variety of taxonomically enigmatic animals (e.g. Briggs and Fortey, 1989; Briggs et al., 1994). Among the taxonomically enigmatic forms is Marella splendens, the most abundant and well-preserved Burgess Shale animal (Fig. 1.5) (e.g. Briggs and Whittington, 1985). The second most common Burgess Shale animal is the crustacean Canadaspis perfecta (Briggs, 1978, 1992; Hagadorn, 2002). Sponges are the second most diverse and abundant group of animals in the Burgess Shale fauna (e.g. Rigby, 1986; Hagadorn, 2002). Primarily Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 1cm Fig. 1.5. The taxonomically enigmatic Marella splendens from the Middle Cambrian Burgess Shale fauna (modified from Briggs et al., 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17 composed of demospoges, but also containing hexactinellid and calcareous forms, sponges are the dominant Burgess Shale sessile epifaunal suspension feeders (Rigby, 1986). The bush-like Vauxia gracilenta is the most common demosponge, while the calcareous sponges include the round Eiffelia globosa, and the hexactinellids include the sac-like Diagoniella hindei (Rigby, 1986). Polychaetes and priapulids are also fairly common in the Burgess Shale fauna. The polychaetes are preserved in exquisite detail with setae, tentacles, appendages, and internal organs typically visible (e.g. Conway Morris, 1979). The most common Burgess Shale polychaete is the likely infaunal species Burgessochaeta setigera, characterized by long tentacles and at least 24 pairs of setae (Conway Morris, 1979). Probably the best known Burgess Shale priapulid is Ottoia. With its large proboscis and oral hooks, Ottoia is typically interpreted as an infaunal predator (Conway Morris, 1977a). A few cnidarians and ctenophores are also preserved in the Burgess Shale fauna. Among the cnidarians are the elongate anthozoan Mackenzia and the frondose pennatulacean Thaumaptilon (Briggs and Conway Morris, 1986; Conway Morris, 1993). The ctenophores include the bowl-shaped Fasciculus and the globular Ctenorhabdotus (Conway Morris and Collins, 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18 Echinoderms such as the eocrinoid Gogia and the edrioasteroid Walcottidiscus are also found in the Burgess Shale fauna (Bassler, 1936; Sprinkle, 1973). Other possible echinoderms include Echmatocrinus, which has been interpreted as a crinoid (Sprinkle and Collins, 1998) but also as an octocoral (Ausich and Babcock, 1998), and Eldonia, which has been interpreted as a pelagic holothurian (e.g. Durham, 1974). One possible cephalochordate, Pikaia gracilens, exhibits features resembling a notochord and myotomic muscle tissue (Conway Morris and Whittington, 1979). If Pikaia was indeed a cephalochordate, then it was the earliest representative of the group. The Burgess Shale fauna also contains more typically preserved animals with mineralized skeletons such as inarticulated and articulated brachiopods, molluscs, hyoliths, and trilobites. The brachiopods include the inarticulated Lingulella and the articulated Nisusia (Walcott, 1924). The hyolith Haplophrentis is often extremely well-preserved with its helens, operculum, and conch all in life position (Babcock and Robison, 1988). A large number of problematic taxa are also found in the Burgess Shale fauna. These problematic forms include perhaps some of the most dramatic animals in the fauna, including the spiny, slug-like Wiwaxia (Conway Morris, 1985, 1992), the large swimming predator Anomalocaris (e.g. Whittington and Briggs, 1985), and the lobopodians. Hallucigenia is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 perhaps the best-known lobopodian, with long spines on its back that were initially interpreted as ventral walking legs (Conway Morris, 1977b; Ramskold and Hou, 1991). The Cambrian Substrate Revolution Studies of modern soft substrate environments have shown that increased bioturbation levels have various negative impacts on sessile benthic suspension feeders (e.g. Rhoads and Young, 1970; Rhoads, 1974; Rhoads and Boyer, 1982; McCall and Tevesz, 1982). In a pioneering study, for example, Rhoads and Young (1970) examined the effects of bioturbation by deposit feeders on benthic suspension feeders in siliciclastic soft substrate environments of Buzzards Bay, Massachusetts. They sampled benthic macrofauna, took bottom photographs, analyzed sedimentary structures, texture, organic content and water content of the sediments, and measured bottom currents and suspended sediment levels at 11 stations along two transects in Buzzards Bay (Rhoads and Young, 1970). These transects included bottom sediments composed of silt to medium sand at water depths of 3 to 10 m (Rhoads and Young, 1970). Scuba divers collected most of the samples and made most of the observations (Rhoads and Young, 1970). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 Their results indicate that intense bioturbation levels, mostly by deposit-feeding bivalves in this case, result in bottom sediments characterized by an uncompacted, low density surface layer, up to 2-3 cm thick, rich in fecal material and reworked mud clasts that typically has a water content of greater than 60% (Rhoads and Young, 1970), known today as the mixed layer (e.g. Ekdale et al., 1984). Because of their high water content and irregular upper surface, bioturbated sediments are also less physically stable than comparable unbioturbated sediments and, thereby, more easily resuspended by currents and storms (Rhoads and Young, 1970). In this study, this physical instability leads to high turnover rates in bottom sediments by current activity, high turbidity near the sediment-water interface, and grading of resuspended sediments to the depth of intense bioturbation (Rhoads and Young, 1970). Rhoads and Young (1970) conclude that these intensely bioturbated, physically unstable sediments are stressful for benthic suspension feeders because they do not provide a stable substrate to which the benthos can maintain a firm connection, they clog filter-feeding structures, they resuspend and bury newly settled larvae, and they discourage the settlement of suspension-feeding larvae. They found, in fact, that in Buzzards Bay benthic suspension feeders could not live where the sediment was being actively bioturbated and resuspended (Rhoads and Young, 1970). Benthic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 suspension feeders in Buzzards Bay were thereby limited to living in environments where either deposit feeding bioturbators were excluded by a lack of buried trophic resources, or where the bottom sediment was bioturbated but not strongly resuspended (Rhoads and Young, 1970). Results of subsequent studies of modern marine environments, as well as laboratory experiments, generally support the conclusions of Rhoads and Young (1970). Increasing levels of porosity and water content in the upper few centimeters of bottom sediments have been attributed to bioturbation by several studies (e.g. Rhoads, 1974; Rhoads and Boyer, 1982; McCall and Tevesz, 1982; Boudreau, 1998; Mulsow et al., 1998). Many studies also demonstrate the crucial role of bioturbation levels in influencing epifaunal and infaunal community structure (e.g. Brenchley, 1981; Posey, 1986; Warwick et al., 1990a,1990b; Brey, 1991; Austen and Widdicombe, 1998; Widdicombe and Austen, 1998; Dahlgren et al., 1999). Bioturbation has also been shown to be important in sedimentary geochemical cycling and diagenesis, particularly by increasing sediment pore water oxygenation levels in the upper few centimeters of seafloor sediments (e.g. Aller and Aller, 1992, 1998; Burdige and Zheng, 1998; Widdicombe and Austen, 1998; Bianchi et al., 2000; Modig and Olafsson, 2001). Furthermore, in several studies larval settlement and egg development on soft substrates has been shown to be negatively influenced by bioturbation (e.g. Woodin et al., 1995; Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 Albertsson and Leonardsson, 2000, 2001). Clearly, a wealth of modern studies strongly implicates bioturbation as a crucial process in modern marine ecology and community structure, sediment texture and stability, sedimentary geochemistry, and diagenesis. Considering the profound impact of bioturbation on a variety of important processes in modern settings, it would be expected that the advent and proliferation of intense vertical bioturbation during the Cambrian would have had myriad ecological, geochemical, and diagenetic consequences. Of direct interest to this research are the ecological effects of this Cambrian increase in depth and intensity of bioturbation on benthic animals. In strong contrast to modern marine settings, the Precambrian was characterized by seafloor environments dominated by microbial mats, which led to the formation of ubiquitous microbial textures and stromatolites in carbonate settings (e.g. Riding and Awramik, 2000) and a variety of microbially-mediated sedimentary structures in siliciclastic settings (e.g. Hagadorn and Bottjer, 1997; Schieber, 1999). It is hypothesized that the first diverse assemblages of animals, the Neoproterozoic Ediacaran fauna, had lifestyles adapted to survival on such microbial mat bound substrates (Seilacher and Pfluger, 1994; Seilacher, 1999). With the development of intense vertical bioturbation during the Cambrian, however, there was a fundamental shift in dominant substrate types, from typical Proterozoic-style Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 soft substrates dominated by microbial mats, to typical Phanerozoic-style soft substrates with a well-developed mixed layer, a higher water content, and an easily resuspended, diffuse sediment-water interface (e.g. Droser, 1987; Droser and Bottjer, 1988; Droser et al., 1999; Droser et al., 2002; Hagadorn and Bottjer, 1999; Seilacher, 1999; Seilacher and Pfluger, 1994). In addition, abundant, well-developed seafloor microbial mats were relegated to stressed settings with inhibited metazoan activity (e.g. Hagadorn and Bottjer, 1997, 1999). This change in dominant substrate types was termed the “agronomic revolution” by Seilacher and Pfluger (1994). The effect of this change in dominant substrate types on the ecology and evolution of non-burrowing benthic metazoans has been termed the “Cambrian substrate revolution” by Bottjer et al. (2000). Global Geologic and Paleogeographic Setting All of the strata and fossils examined in this research, with the exception of the Burgess Shale fauna, were deposited and preserved on the Yangtze Platform of South China between the last known Neoproterozoic glaciation and the early stages of the Cambrian radiation of animal life, or “Cambrian Explosion”, a time period of approximately 600 to 525 Ma ago. The tectonic history of South China is still in a state of flux, but it is thought Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24 that the Yangtze Platform was uplifted during the Jinningian and Chengjiangian orogenies, approximately 850 and 700 Ma ago respectively (Ren et al., 1988; Meyerhoff et al., 1991). Following a period of erosion marked by an angular unconformity, deposition of marine sediments began on the Yangtze Platform as the ocean transgressed across it (Ren et al., 1988; Meyerhoff et al., 1991). The Neoproterozoic (Sinian) Liantuo and Nantuo Formations were the first marine sedimentary rocks deposited on the platform (Ren et al., 1988; Meyerhoff et al., 1991). The Nantuo Formation, interpreted as a glacial diamictite, is unconformably overlain by the Neoproterozoic Doushantuo Formation (e.g. Meyerhoff et al., 1991), which contains the phosphatized animal embryos examined as part of this research. The Neoproterozoic- Cambrian Dengying Formation carbonates overlie the Doushantuo Formation, and the Dengying, in turn, is overlain by the Lower Cambrian Meishucun, Shiyantuo, and Yuanshan Formations (e.g. Meyerhoff et al., 1991), all of which contain trace and body fossils examined in this research. The Doushantuo, Meishucun, Shiyantuo, and Yuanshan Formations will all be discussed in detail when appropriate in the following chapters. There is no consensus on the global paleogeography of the Neoproterozoic-Cambrian. For the purposes of this research, therefore, one of the latest reconstructions, by Seslavinsky and Maidanskaya (2001), was Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25 chosen. This reconstruction places South China on the northeast corner of the Proto-Pacific Ocean at approximately 30°N during the late Neoproterozoic (about 560 Ma) (Fig. 1.6) (Seslavinsky and Maidanskaya, 2001). In this reconstruction, South China contained an island arc on its western edge with a comparatively large epicontinental seaway (Yangtze Platform) on its eastern edge (Seslavinsky and Maidanskaya, 2001). Through the Early and Middle Cambrian South China moved approximately 15° southward, but underwent no major changes except the expansion of the epicontinental seaway (Seslavinsky and Maidanskaya, 2001). It should be noted that this paleogeographic scheme is one of many possible reconstructions and, as such, should be considered a rough estimate. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26 North \Chinn. PROTO-PAC FIC OCEAN 'Australia. Antnrtica (Siberia Laurentia Africa, Fig.1.6. Global paleogeographic reconstruction during the late Neoproterozoic. South China is colored in black (modified from Seslavinsky and Maidanskaya, 2001). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27 CHAPTER 2: Sedimentology and Microfacies Analysis of the Weng’an Phosphate Member of the Neoproterozoic Doushantuo Formation, Southwest China Introduction This chapter examines the paleoenvironmental conditions surrounding the phosphatization of what may be the earliest unambiguous fossil evidence for animals: the animal embryos preserved in the Neoproterozoic Doushantuo Formation of southwest China, the oldest fossil Lagerstatte being studied in this research. Despite the spectacular nature of this Lagerstatte, complete with well-preserved individual cells, attempts to place the Doushantuo fossils in a detailed paleoenvironmental context are only just beginning. As a result, the paleoenvironmental conditions that led to the preservation of these magnificent fossils have yet to be fully addressed. The goal of this chapter, therefore, is to use a combination of field and laboratory techniques to gain further insight into the paleoenvironmental conditions that led to the preservation of the Doushantuo embryos. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Phosphatization 28 An important consideration for this chapter is the present knowledge regarding phosphogenesis and phosphatization. Although the details of phosphatization are still poorly understood, there are some general paleoenvironmental, biological, and chemical parameters that are thought to play important rolls in phosphogenesis and phosphatization and, subsequently, are crucial to the subject of this chapter. Modern phosphorite deposits are typically, although not always, found in environments characterized by low sedimentation rates, strong seafloor currents, and a large influx of organic material due to high primary productivity in the water column above (e.g. Follmi, 1996). The low sedimentation rates and large amount of organic input in these environments combine to create sediment relatively enriched in organic material, usually considered a prerequisite for phosphogenesis (Follmi, 1996). The organic material in the sediment then begins decaying and releasing phosphate into the pore waters. Although many details about the process have yet to be worked out, the dissolved phosphate content of the pore waters reaches supersaturation within a matter of days, allowing phosphogenesis and the phosphatization of any organisms to begin (Follmi, 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29 Although sediment pore water is typically saturated with respect to phosphate, under normal seawater pH conditions (-8) calcium carbonate is more stable than phosphate and thereby inhibits phosphate precipitation. One way in which phosphate can precipitate is if the pH of the pore waters is reduced to around 7, which destabilizes calcium carbonate and allows phosphate to begin precipitating (Lucas and Prevot, 1991; Follmi, 1996; Trappe, 1998). It is thought that bacterial decay processes may serve to lower the pH of the pore waters during this process (Froelich et al., 1988). Once phosphogenesis and phosphatization have taken place, however, the strong currents remobilize the sediment, transporting and redepositing it elsewhere. A new round of phosphogenesis then begins in the newly deposited sediment and the cycle continues (Follmi, 1996). This process of remobilization and redeposition of the sediment, called "Baturin cycles" (Follmi, 1996), results in different generations of phosphate being present in the same sediment. Preliminary Taphonomic Model Based on this work on phosphorites in general (e.g. Follmi, 1996) and the Doushantuo Formation (e.g. Xiao and Knoll, 1999), the following preliminary model for the preservation of the Doushantuo Formation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 phosphatized fossils was developed for this chapter: 1) The relatively shallow seafloors were covered with microbial mats and inhabited by abundant algae and microscopic animals. 2) Occasional storms or current activity mobilized the seafloor sediment and organisms living on it. 3) The sediment and organisms were transported and redeposited elsewhere on the seafloor. 4) Organic material in this new deposit began to degrade. 5) A relatively impervious microbial mat formed quickly on the surface of the new deposit. 6) Degradation of organic matter in the new deposit led to pore water anoxia and caused a dramatic increase in the PO4 content of the pore waters. 7) Sealing by the seafloor microbial mats hindered communication between pore waters and the overlying seawater. 8) Within a few hours to days, PO4 concentrations in the pore waters reached supersaturation and phosphatization of soft tissues occurred. 9) Remobilization, transport, and redeposition of the sediments, including phosphatized eggs and embryos, may have taken place many times before final deposition. Tests of the taphonomic model detailed above include: 1) the presence or absence of evidence suggestive of the presence of seafloor microbial mats, 2) the presence or absence of paleoenvironmental evidence for high-energy conditions, and 3) the presence or absence of intermixed phosphate generations. Evidence suggestive of the presence of microbial mats might include phosphatized microbial mats or stromatolitic structures. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31 The presence of cross-bedding or graded storm beds, as well as evidence of sediment reworking, would indicate high-energy conditions. Intermixing of phosphate generations would be signified by the preservation of multiple generations of phosphate in the same deposit. Evidence for all of these features would strongly support the taphonomic model developed at the onset of this research. Geologic Setting The Neoproterozoic Doushantuo Formation is exposed throughout South China, but some of the most fossiliferous locations known thus far are in the Weng'an area of Guizhou Province in southwestern China. The Doushantuo typically overlies the Nantuo Tillite (possibly Marinoan, although no direct evidence currently exists) and underlies Dengying Formation dolomites which contain rare Ediacaran-type fossils and Cambrian small shelly fossils near their top (Wang et al., 1984). Commonly interpreted as being deposited in a shallow marine environment above storm wave base (e.g. Zhang et al., 1998), the Doushantuo at Weng'an contains two phosphorite intervals, both representing shallowing upward sequences, separated by a sub-aerial exposure surface (Xiao and Knoll, 1999) (Fig. 2.1). All of the specimens examined are from the Weng’an Phosphorite Member of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32 Fig. 2.1. General stratigraphy of Neoproterozoic Doushantuo Formation and a locality map showing Weng’an, Guizhou Province, China. (A) The stratigraphy of the Doushantuo Formation is shown here in five distinct lithologic units: (1) the Lower Dolostone Member (7.6 m thick), consisting of massive gray dolomite; (2) the Lower Phosphorite Member (17.5 m thick), composed of thin-bedded phosphorite with bedding planes that often have a surface elephant-skin texture; (3) the Middle Dolostone Member (3.5 m thick), consisting of silicified dolomite topped with an erosive karstification surface indicating subaerial weathering; (4) the Weng’an Phosphorite Member (7.4 m thick), a phosphorite unit deposited in high energy shallow water conditions, with numerous erosive surfaces, phosphatic crusts, phosphatic intraclasts, and phosphatized metazoan embryos and algae; and (5) the Upper Phosphorite Member (7.6 m thick), composed of interbedded dolomite with phosphorite. All of the embryos examined in this study were collected from the embryo-bearing interval of the Weng’an Phosphorite Member. (B) Map of China with Weng’an, Guizhou Province marked by the dot. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 3 .6 - U pper Phosphorite 4 0 - 3 0 — M iddle Dolostone Lower Phosphorite 10— Lower Dolostone KEY em bryo-bearing interval dolostone phosphorite CHINA 1000 km Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34 the Doushantuo Formation, which occurs just above this sub-aerial exposure surface (Fig. 2.1 and Fig. 2.2). Methods Field observations of the Weng’an Phosphorite Member of the Doushantuo Formation were made at Baishakan and Nanbao quarries near Weng’an, Guizhuo Province, China. Of particular interest during fieldwork were any observable paleoenvironmental indicators including sedimentary structures and composition. In order to fully characterize the phosphogenic paleoenvironments present in the Weng’an Phosphorite Member, petrographic observations of clast and matrix composition, clast or matrix support, phosphoclast grain size, phosphoclast rounding, phosphoclast sorting, bedding and sedimentary structures, any possible microbial fabrics, and estimates of percentage dolomite were made in 66 thin sections from random intervals throughout the lowermost 9.5 m of the Weng’an Phosphorite. Phosphoclast rounding was estimated using the scale of Powers (1953), and phosphoclast sorting was estimated using the scale of Compton (1985). The percentage dolomite in each thin section was estimated using the figures of Compton (1985) in a randomly selected field of view at a magnification of x5. The thin sections came from samples collected Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 9 - E 3 - 2 - Fig. 2.2. Generalized composite stratigraphic section of the Weng’an Phosphorite Member of the Neoproterozoic Doushantuo Formation as exposed in Baishakan and Nanbao Quarries near Weng’an, Guizhou Province, China. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36 at Baishakan, Nanbao, and Wusi quarries near Weng’an, and are distributed through the composite section in the following manner: 8 thin sections from the lowermost 0.15 m, 9 thin sections from 0.15 to 2 m, 9 thin sections from 2 to 3 m, 8 thin sections from 4 to 5 m, 4 thin sections from 5 to 5.5 m, 7 thin sections from 5.3 to 6 m, 4 thin sections from 6.5 to 7 m, 13 thin sections from 7.5 to 8 m, 4 thin sections from 8.5 to 9.5 m. Results Field Observations The Weng’an Phosphorite Member is clearly divisible into two distinct lithofacies: a lower black facies and an upper gray facies, both named after their general coloration (Fig. 2.2 and Fig. 2.3). The black facies unconformably overlies a karstified subaerial exposure surface, and encompasses approximately the lowermost 4 m of the Weng’an Phosphorite Member. The contact between these two facies is transitional, with interbedding occurring between approximately 2.5 to 4 m. The coal-like black facies is characterized by irregular lenticular beds millimeters to centimeters thick (Fig. 2.4 and Fig. 2.5). These beds contain visible phosphoclasts of variable size up to pebbles (Fig. 2.6) and are grouped into meters-wide nested channels (Fig. 2.4). The lowermost 2 m of this facies are Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37 Fig, 2.3. Field photograph of the lowermost 7 m of the Weng’an Phosphorite Member of the Neoproterozoic Doushantuo Formation, southwest China. Note the distinct Sower black facies overlain by the much lighter gray facies. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 Fig. 2.4. Field photograph of the lowermost 2.5 m of the black facies of the Weng’an Phosphorite Member of the Neoproterozoic Doushantuo Formation at Baishakan Quarry near Weng’an, Guizhuo Province, China. Stratigraphic up is to the upper right. Hammer for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39 Fig. 2.5. Field photograph displaying typical fine-scale bedding at approximately 1 m in the Weng’an Phosphorite Member at Baishakan Quarry. Centimeter scale visible on ruler. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 Fig, 2.6. Field photograph of phosphociast (arrow) within bed at 75 cm in the Weng’an Phosphorite Member at Baishakan Quarry. Centimeter scale visible on ruler. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41 generally very friable, with only a few beds hard enough to sample effectively. The hardness of the black facies increases dramatically above 2 m, presumably because of increasing dolomite levels during the transition to the gray facies. At its lower contact, the black facies is draped directly on the irregular karst surface (Fig. 2.7) and, in the lowermost 25 cm, contains abundant secondary pyrite both disseminated through the black host rock and in crystalline bed-parallel concentrations (Fig. 2.8). The transition between the black and gray facies, between approximately 2.5 to 4 m, is characterized by interfingering gray and black beds that exhibit lenticular cross bedding (Fig. 2.9). The black beds are thicker and more common near the base of this transitional zone, but the gray beds become thicker and more common upsection (Fig. 2.10), probably as dolomite levels increase. The beds in the lowermost 75 cm of this zone are from 1-2 mm to 1.5 cm in thickness. As noted above, these transitional rocks are generally much harder than underlying rocks, probably because of increased dolomite levels. The gray facies is typified by a light gray to white color and contains beds centimeters to decimeters thick (Fig. 2.11). These beds exhibit lenticular cross-bedding and are typically capped by black phosphatic crusts several millimeters in thickness (Fig. 2.11). These black phosphatic crusts Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42 Fig, 2.7. Field photograph of the black facies of the Weng’an Phosphorite Member at Baishakan Quarry showing its lower contact with the karst surface beneath it (approximately traced by white line). Hammer for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43 Fig. 2.8. Field photograph of a bed of secondary pyrite (arrow) within the lowermost 25 cm of the black facies of the Weng’an Phosphorite Member at Baishakan Quarry. Centimeter scale visible on ruler. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44 w m m Fig. 2.9. Field photograph of beds in the transition between the black and gray facies at 3 m of the Weng’an Phosphorite Member at Nanbao Quarry. Centimeter scale visible on ruler. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 Fig. 2.10. Field photograph showing wider view (from approximately 2 to 4 m) of the transition between the black and gray facies of the Weng’an Phosphorite Member at Nanbao Quarry. Hammer for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46 Fig. 2.11. Field photograph of beds at 7 m within the gray facies of the Weng’an Phosphorite Member at Baishakan Quarry. Note the black phosphatic crust capping the beds, and the gray and white grains (many are embryos) visible within each bed. Centimeter scale visible on ruler. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47 resemble the black beds in the transition zone of the underlying black facies. Abundant white and gray, well rounded phosphoclasts are readily visible in the gray facies (Fig. 2.11). Many of these phosphoclasts are metazoan eggs and embryos. Petrography The lower black facies of the Weng’an phosphorite is typically composed of very fine sand to granular phosphoclasts, that are black in cross-polarized light and dark orange to black in plane polarized light, within a silty phosphatic and quartzitic matrix (Appendix A). The phosphoclasts are generally subrounded to rounded, and poorly sorted to very poorly sorted. When phosphoclasts are abundant in the black facies, it is typically grain supported. Several microfacies are evident within the black facies, and there are interesting trends within the 3 m or so of the facies. Within the lowermost 0.15 m of the black facies there are two dominant microfacies. The first is characterized by irregular, phosphoclast-rich, sub-millimeter laminae topped by thin phosphatic crusts that are dark orange in plane polarized light (Fig. 2.12). This microfacies also contains abundant secondary pyrite that often replaces the phosphatic matrix (Fig. 2.13) and sometimes forms thick (>3 mm) beds of vertically oriented bladed pyrite crystals. The second Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48 Fig. 2.12. Plane-poiarized photomicrographs from the lowermost 15 cm of the black facies of the Weng’an Phosphorite Member showing irregular, phosphoclast- and pyrite-rich (black material), sub-millimeter laminae topped by thin phosphatic crusts that are dark orange in plane polarized light. Field of view in both photomicrographs is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49 Fig, 2,13. Plane-polarized photomicrograph from lowermost 15 cm of the black facies of the Weng’an Phosphorite Member showing phosphoclasts encased in secondary pyrite that has replaced the matrix. Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 microfacies within this lowermost interval is dominated by amorphous phosphate with abundant, often horizontally distributed, crystalline and amorphous pyrite (Fig. 2.14). This microfacies dominates entire thin sections in a few cases. The dominant microfacies from 0.15 to 2 m consists of lenticular beds of very fine sand to granular sized phosphoclasts within a matrix of silt to clay sized phosphoclasts and quartz grains (Fig. 2.15). These lenticular beds are typically 1 to 2 mm in thickness. A few rare pebble-sized phosphoclasts greater than 1 cm in diameter also occur, and are clearly the remnants of phosphatic crusts as they are composed of smaller phosphoclasts in a phosphatic matrix. Little to no evidence for pyrite was observed in this interval. Dolomite makes its first appearance in the Weng’an Phosphorite in the interval of the black facies from 2 to 3 m. The dominant microfacies in this interval consists of thin (< 1 mm) irregular laminae that consist of phosphoclasts in a dolomitic matrix topped with phosphatic crusts (Fig. 2.16). Lenticular beds rich in phosphoclasts and with dolomitic matrix are also present in this interval (Fig. 2.16). The gray facies of the Weng’an Phosphorite Member, from 4 to 9.5 m, is dominated by coarse sand sized (0.5 to 1 mm), well rounded, and well sorted phosphoclasts in a dolomitic matrix (Appendix A) (Fig. 2.17). In stark Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig, 2,14. Plane-polarized photomicrographs showing the amorphous phosphate dominated microfacies in the lowermost 15 cm of the black facies of the Weng’an Phosphorite Member. (A) Example with little pyrite. Field of view is approximately 3 mm wide. (B) Example with abundant pyrite. Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 Fig. 2.15. Plane-poiarized photomicrograph from between 0.15 and 1 m in the black facies of the Weng’an Phosphorite Member showing part of a lenticular bed of coarse phosphoclasts on top of a phosphatic and quartzitic silty matrix. Field of view is approximately 3 mm in height. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53 Fig. 2.16. Plane-polarized photomicrograph from between 2 and 3 m in the black facies of the Weng’an Phosphorite Member showing both thin (< 1 mm) irregular laminae that consist of phosphoclasts in a dolomitic matrix topped with phosphatic crusts near the top, and lenticular beds rich in coarser phosphoclasts with a dolomitic matrix near the bottom. Field of view is approximately 3 mm in height. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54 Fig, 2.17. Cross-polarized photomicrograph from between 7.5 and 7.8 m in the gray faces of the Weng’an Phosphorite Member showing well-rounded, well-sorted phosphoclasts in a dolomitic matrix. Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55 contrast to the black facies, all of the phosphate in this facies is clear or slightly orange in plane-polarized light, and black, but not opaque, in cross polarized light. The gray facies is also typically matrix supported, and bedding is on the centimeter to decimeter scale. While bedding and sedimentary structures are often not visible in thin sections because of their thickness, the contacts between beds are sometimes observed. These cases show that most beds are capped by thin (several millimeters thick) grain-supported phosphatic crusts that consist of phosphoclasts in a phosphatic matrix (Fig. 2.18). In a few cases at least, the beds are capped by fine-grained dolomite with irregular, wavy, sub-millimeter laminae that show cohesive and flexible behavior through folding and tearing, possibly indicating cohesion by microbial mats (Fig. 2.19). This fine-grained dolomite is locally abundant, and creates its own microfacies with a clotted, possibly microbial, texture (Fig. 2.20). Completely dolomitized remnants of phosphoclasts are also evident in this dolomitic microfacies, perhaps because they were washed into a non-phosphogenic carbonate setting. Destructive dolomitization is also observed in the phosphatic microfacies, with many phosphoclasts and phosphatic crusts partially or almost entirely replaced by coarse-grained dolomite. This destructive dolomitization is particularly common in the uppermost 2 m (7.5 to 9.5 m) of the study interval. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56 Fig. 2.18, Cross-polarized photomicrograph from between 5 and 5.5 m in the gray facies of the Weng’an Phosphorite Member showing part of a thin grain- supported phosphatic crust consisting of phosphoclasts in a phosphatic matrix. Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57 Fig, 2,19. Cross-poiarized photomicrographs from between 4 and 5 m in the gray facies of the Weng’an Member showing fine dolomitic laminae that occasionally cap the beds in the gray facies. (A) Thin, irregular dolomitic laminae capping a bed of phosphoclasts. Field of view is approximately 3 mm wide. (B) Thin, irregular laminae capping a bed of phosphoclasts that show evidence of flexible and cohesive behavior: they are folded and appear to be torn into pieces. Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58 Fig. 2.20. Cross-polarized photomicrograph from between 7.5 and 7.8 m in the gray facies of the Weng’an Phosphorite Member showing the clotted, fine-grained dolomite that is occasionally a dominant microfacies in this interval. Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59 The estimates of percentage dolomite in the Weng’an Phosphorite Member indicate a dramatic increase in dolomite levels, from none to 10- 70%, from 2 to 3 m (samples 18-26) within the Weng’an Member (Fig. 2.21). From 4 to 9.5 m, dolomite levels remain elevated (with a range of 60-100%) except when the randomly selected field of view encompassed a phosphatic crust, as happened twice (Fig. 2.21). These estimates are consistent with the petrographic observations discussed above, wherein increasing dolomite levels characterize the Weng’an Phosphorite Member as one moves stratigraphically upward. Discussion The field and petrographic data support the general taphonomic model proposed in this chapter while also revealing interesting avenues for further research. The wavy laminae in the fine-grained dolomite that occasionally caps beds in the gray facies provide suggestive evidence for the presence of microbial mats because of their cohesive and flexible behavior (Fig. 2.9). The dominance of coarse sand sized phosphoclasts in the gray facies and the presence of granule and pebble-sized phosphoclasts in the black facies both indicate high energy conditions, episodic at least, in the Weng’an Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60 Fig. 2.21. Scatter plot of estimated dolomite percentages from the thin sections examined in this chapter. See Appendix A for raw data. Samples are ordered on the x-axis as they are in Appendix A. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 0 1 61 3 )IU IO |O Q % Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sample 62 Member. Phosphatic crusts that contain phosphociasts provide direct evidence for multiple phosphate generations and, thereby, “Baturin cycling”. All of this data provides support for the general taphonomic model proposed above, but much evidence also provides more detailed information on the nature of phosphogenesis and phosphatization in the Weng’an Member. Both field observations and petrography indicate that the black and gray facies of the Weng’an Phosphorite Member represent different phosphogenic systems. The black facies is much richer in organic material and pyrite than the gray facies, and lacks dolomite except in its upper few meters. It is extremely friable because of this lack of dolomite. It is dominated by subrounded to rounded phosphociasts that are typically poorly sorted. The gray facies is characterized by the presence of abundant dolomite, well rounded and well sorted phosphociasts, little organic material, and thicker lenticular beds with cross bedding typically capped by thin phosphatic crusts. Due to the preliminary nature of this study and the unusual nature of the rocks involved, there are several possible paleoenvironmental hypotheses to explain the differences between the black and gray facies. One of these possibilities is that the black facies represents a deeper environment than the gray facies. This hypothesis is primarily based on the relatively high organic and pyrite content of the black facies, which is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63 consistent with deposition in a deeper, low-oxygen setting. In such a setting, organic material could collect without undergoing significant decomposition. In this scenario, sea level rose abruptly following the relative sea level fall represented by the subaerial exposure surface, creating a relatively deep basin in which the black facies was deposited. As the basin filled in and sea level remained relatively static, the seafloor reached a shallow enough depth to become more oxygenated. Carbonate production could then begin and the gray facies was deposited. A second hypothesis is that the black facies was deposited during the initial rise in sea level, when the basin was just beginning to fill, and sea level rose as the gray facies was deposited. Under this hypothesis, therefore, the lower part of the Weng’an Phosphorite Member deepens upward into more open marine conditions as the black facies transitions into the gray facies. The change in environments involved in this hypothesis explains the differences between the black and gray facies and their styles of phosphatization, with phosphate being more organic-rich, dolomite-poor, and less reworked in the black facies than in the gray facies. This hypothesis is supported by the sedimentary structures present in the black facies, including lenticular bedding and intense channelization, which indicate extremely shallow deposition. It is also supported by the interpretation of the Doushantuo Formation at Weng’an as being within an extremely shallow Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 facies belt within the Yangtze Platform, with the Doushantuo deepening to the east in Hunan Province (Fig. 2.22). Dramatically thicker sections of the Doushantuo Formation, over 400 m thick in some areas, outcrop in Hunan Province and are interpreted as being deposited in a continental slope setting (Fig. 2.22). Basinal facies are found farther east in Hunan Province, but the Doushantuo is not present in these settings (Fig. 2.22). As mentioned above, the Weng’an region is interpreted as lying within the shallow facies belt of this basin (Fig. 2.22), further supporting the hypothesis that the black facies represents a shallow depositional environment. Based on the sedimentary structures present in the black facies and its location within the shallow facies belt of the Yangtze Platform, the latter hypothesis, in which the black facies deepens upward into the gray facies, is currently preferred. This preference, however, is subject to change with further data. Either way, the differences between these two facies is still consistent with rising relative sea level above the subaerial exposure surface at the base of the Weng’an Member. The lower levels of reworking in the black facies, as exemplified by the dominance of subrounded to rounded and poorly sorted phosphociasts, may also provide an important taphonomic window, as less reworking means that fewer phosphatized fossils will be destroyed. This may explain why many of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 Neoproterozoic-Cambrian facies belts, southern China • W uhai • C h on g qing C hoitfiifiiif! area •Changsha • G ujyan g Mn deposits X iangm eng Fm low e r c a p carb on ate B X iu s h a n | 0 M inle Q S in a n Q jia n g k o u g X ia n g ta n g Iron lorm atio n w ithin Jian gko u diam ictites (up to 28%) Fe approx. bo un d, betw een Ba up pe r/low er slope Shallow Facies W engan ^ 2^ C hangyang Slope Facies ^ Taijlang ^ 5 ) Sinan Xiushan < D shimeng m Anhua D engying D oushantuo Nantuo Shuijingtuo Dengying -P C -C ? Doushantuo Nantu]T X ia n g m e n g l q — P C-C? D engying | D oushantuo I Y j- '- j Dolostone Phosphate-rich lim estone lam inated shale m t,ia c k s h a ,e □ shale/siltstone E i l turbidites □ Mn cap carbonate glaciogenic Q 100 m X iaoyanxi Liuchapc[ Jinjiadong Hongjiang jJS X iang rpeng Fe Jiankou (schem atic) iff; Fig. 2.22. Diagram depicting the Neoproterozoic and Cambrian facies belts in the Yangtze platform of southwest China. Note the position of the Weng’an region (Number 1) within the shallow facies belt of this basin. See text for further discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66 the most recently reported putative fossils from the Doushantuo Formation have been reported from this facies (e.g. Chen at al., 2002). Conclusions Field observations and petrography indicate that there are two distinct lithofacies within the Weng’an Phosphorite Member of the Doushantuo Formation: a black facies beneath a gray facies. Within each of these facies, phosphogenesis and phosphatization took place under different environmental conditions. Less reworking was involved in the deposition of the black facies, and dolomite is not present in the lower 2 m of this facies, which is rich in organic material. The gray facies is characterized by more abundant dolomite, primarily as a matrix for phosphociasts, and greater levels of reworking. The transition from the black to gray facies involves increasing dolomite levels and decreasing organic material levels, and is consistent with the stratigraphic context of the Weng’an Member being directly above a subaerial exposure surface. Under the current working hypothesis, the black facies, tentatively interpreted as a shallow marine, perhaps lagoonal, deposit, was deposited as sea level was just beginning to rise again, and the environment rapidly deepened upward into the gray facies, before shallowing upward through the rest of the member. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67 Overall, these results suggest that organic remains probably had a higher preservation potential when buried in the environment represented by the black facies because of the lower levels of reworking in this setting. The strong reworking of the gray facies, on the other hand, probably resulted in a strong taphonomic bias toward fossils that could withstand frequent reworking and redeposition, such as globular sponge eggs and embryos. Perhaps this explains why many of the new potential fossils described from the Doushantuo Formation are found in this lower black facies (i.e. Chen et al., 2002). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68 CHAPTER 3: Taphonomy of Phosphatized Metazoan Embryos from the Neoproterozoic of Southwest China Introduction As discussed above, the Neoproterozoic Doushantuo Formation of South China has yielded some of the earliest known fossil evidence for metazoans in the form of phosphatized animal eggs and embryos (Fig. 3.1), as well as possible gastrulae, larvae, and microscopic adults (Xiao et al., 1998; Li et al., 1998; Chen et al., 2000; Xiao and Knoll, 2000; Xiao et al., 2000; Knoll and Xiao, 2001; Chen et al., 2002; Xiao, 2002). Although the precise age of the Doushantuo Formation is still uknown, recent Pb-Pb radiometric age dating of this spectacular Lagerstatten provides an age for the embryo-bearing interval of the Doushantuo Formation is 599.3 ± 4.2 Ma old (Barfod et al., 2002). If supported by future work, this date, as mentioned above, means that the Doushantuo embryos provide the earliest unambiguous evidence for animals found thus far in the fossil record. Pending that future verifying work, however, this Pb-Pb date should be considered preliminary at best. Exclusive of this date, the age of the Doushantuo can only be confidently constrained to somewhere between 560 and 600 Ma based primarily on its acritarch fauna and stratigraphic position Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 Fig. 3.1. Cross-polarized photomicrographs of phosphatized metazaon embryos from the Neoproterozoic Doushantuo Formation of southwest China. (A) A 2-cell stage embryo approximateiy 660 microns in diameter. (B) A 4-celi stage embryo approximateiy 750 microns in diameter. (C) An 8- cell stage embryo approximately 700 microns in diameter. (D) A 16-cell stage embryo approximately 600 microns in diameter. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 (Knoll, 2000). Even with the age of the Doushantuo so widely constrained, the phosphatized fossils of the Doushantuo Formation still provide further evidence that metazoan origins lie deeper in time than the "Cambrian explosion", helping to narrow the controversial divide between molecular and fossil estimates of metazoan divergence (e.g. Smith, 2000). While the Doushantuo fossil discoveries of the past few years have been astounding, with more probably still to come, attempts to place these fossils in a detailed taphonomic context are only beginning. As a result, the taphonomic biases inherent in the preservation of these magnificent phosphatized fossils, as well as the tempo of the phosphatization process in the Doushantuo fossils have yet to be explored. The goal of this chapter is to better understand these processes by performing a detailed specimen-based taphonomic analysis of the phosphatized metazoan embryos first reported from the Doushantuo by Li et al. (1998) and Xiao et al. (1998). Previous Work Xiao and Knoll (1999) performed a taphonomic survey of the Doushantuo phosphorites, focusing on algal thalli and metazoan eggs and embryos. By studying specimens obtained through acid digestion of whole rock samples as well as thin sections, they determined that the interiors of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 1 the eggs and embryo blastomeres were composed of micro-crystalline apatite, possibly because the apatite nucleated around biomolecules during phosphatization (Xiao and Knoll, 1999). Xiao and Knoll (1999) also interpreted blastomere shrinkage and deformation in most of these metazoan embryos as evidence of organic decay, perhaps due to hydrolysis. These observations, and their interpretation as evidence of organic decay, serve as the foundation upon which this study was built. Methods A total of 65 thin sections from throughout the Weng’an Phosphorite Member were completely examined in this study, 37 of which contained possible metazoan embryos. The egg stage of these embryos was not included in this taphonomic study. These 37 thin sections contained 207 metazoan embryos of various cleavage stages, which were placed into categories, or taphonomic grades, based on their levels of pre- phosphatization organic decay. Although this method has been applied to Phanerozoic fossil groups, such as echinoderms (e.g. Brett et al., 1997), in order to better understand their taphonomy, this is the first time that it has been applied to Neoproterozoic fossils. The taphonomic grades of these embryos are as follows (Fig. 3.2): (1) well-preserved, little or no evidence of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 Fig. 3.2. Examples of the four taphonomic grades utilized in this study as seen in 8-cell and 16-cell stage embryos. Scale bar is 100 microns and applies to all photos. Cleavage planes of the grade 1 8-cell embryo are subtle (arrow) because this embryo is so well-preserved. The grade 2 8-cell embryo shows evidence of slight decay in the coarsely crystalline apatite that has precipitated along cleavage planes and the misalignment of one of the blastomeres (arrow). The grade 3 8-cell embryo shows evidence for moderate decay in the severe shrinkage of the entire embryo and subsequent precipitation of layered, coarsely crystalline apatite in the space created by this shrinkage (arrow). The grade 4 8-cell embryo shows evidence of intense decay because its blastomeres are shrunken, deformed, misaligned, and coated with coarsely crystalline apatite, and the original circular shape of the embryo has been deformed. The grade 1 16-cell embryo is well-preserved because the blastomeres are well-aligned and undeformed. The grade 2 16-cell embryo exhibits slight evidence for decay in the precipitation of coarsely crystalline apatite along cleavage planes (arrow). The grade 3 16-cell embryo shows evidence for moderate decay in the deformation and misalignment of its blastomeres, as well as the deformation of its original circular shape. The grade 4 16-cell embryo exhibits evidence of intense decay in its drastic deformation from its original circular shape, as well as the severe shrinkage and deformation of its blastomeres. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73 f a sSnig a ftA ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 74 organic decay; (2) slightly decayed, approximately less than 25% of the embryo shows evidence of organic decay; (3) moderately decayed, approximately 25-50% of the embryo shows evidence of organic decay; and (4) intensely decayed, approximately more than 50% of the embryo shows evidence of organic decay (Fig. 3.2). Blastomere shrinkage and deformation, as well as separation of blastomeres along cleavage planes, characterize this organic decay (Fig. 3.2). Both the percentage and abundance of specimens in each taphonomic grade within each embryo cleavage stage were plotted in order to reveal any taphonomic trends within and amongst embryo cleavage stages. Results Consistent with the findings of Xiao and Knoll (1999), several mineralogical and morphological trends indicate increasing levels of organic decay in these embryos. The blastomere interiors of well-preserved (grade 1) embryos are typically composed of microcrystalline apatite, and the blastomeres themselves show little evidence of shrinkage or deformation (Fig. 3.2). The cleavage planes in well-preserved embryos are typically subtle, with little space between blastomeres for the precipitation of more Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75 coarsely crystalline apatite (Fig. 3.2). In addition, the overall shape of well- preserved embryos is generally circular in cross section (Fig. 3.2). On the other hand, slightly decayed (grade 2) to intensely decayed (grade 4) embryos, while still having microcrystalline apatite blastomere interiors, show increasing levels of blastomere shrinkage and deformation (Fig. 3.2). The blastomeres lose their original shape and alignment with one another as decay levels increase (Fig. 3.2). This shrinkage and deformation is accompanied by the precipitation of more coarsely-crystalline apatite in the spaces left by the shrunken blastomeres, including along expanded and deformed cleavage planes, which become more pronounced (Fig. 3.2). Apatitic crusts also often rim the shrunken and deformed blastomeres and the interior of the space occupied by the original embryo (Fig. 3.2). Finally, the embryo typically loses its original circular shape as decay levels increase (Fig. 3.2), probably because the embryos become softer with decay and are more easily deformed by burial pressure. These morphological and mineralogical criteria were used in this study to estimate decay levels in these embryos. It should be noted, however, that this decay process can follow many pathways and lead to decayed embryos with differing characteristics (Fig. 3.2), even though the overall decay levels are comparable. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76 Interestingly, only 2-cell, 4-cell, 8-cell, and 16-cell cleavage stage embryos, not including eggs, were found in the thin sections examined. Although later cleavage stages and even possible adult stages have been reported from the Doushantuo (Li et al., 1998; Xiao and Knoll, 1998; Xiao et al., 2000; Chen et al., 2002), they are probably comparatively rare. This pattern alone suggests a strong taphonomic bias against these later cleavage stages, as well as the eventual adult stage of these embryos. The taphonomic grade percentages of the examined embryos reveal marked taphonomic trends within and amongst the embryo cleavage stages. Well- preserved, or grade 1, embryos dominate 2- cell (89%) and 4- cell (86%) cleavage stages, while slightly less than half of 8-cell embryos (49%) and a minority of 16-cell embryos (15%) exhibit nearly pristine preservation (Table 3.1 and Fig. 3.3). These percentages indicate a strong taphonomic bias toward the nearly pristine preservation of earlier (2-cell and 4-cell) cleavage stages and away from the nearly pristine preservation of later (8-cell and 16 cell) cleavage stages. It should be noted that the presence, and even dominance, of well-preserved embryos in this study indicates that phosphatization of these embryos was generally extremely rapid, a matter of hours or days (e.g. Follmi, 1996). This bias toward earlier cleavage stages is also evident when organically decayed embryos are examined. Only 1 1 % of 2-cell stage Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77 Embryo Taphonomic Grade n % ot Total Stage 1 2 3 4 Embryos 2-cell 89% 11 % - - 61 29% 4-cell 86% 11% 3% - 99 48% 8-cell 49% 27% 18% 6% 33 16% 16-cell 15% 21% 21% 43% 14 7% Table 3.1. Percentage of embryos in each taphonomic grade within each embryo cleavage stage, and numbers of embryos in each cleavage stage. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 Fig. 3.3. Stacked bar histogram of taphonomic grade percentages for each embryo cleavage stage. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79 ''tf CO CM 0) C D Q) C D T ) * o ■o * o C O C O C D C D 5 5 5 5 ! □ □ □ ■ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cleavage Stage 80 embryos are slightly decayed (grade 2), while none are moderately (grade 3) or intensely (grade 4) decayed (Fig. 3.3). Similarly, 11% of 4-cell stage embryos are slightly decayed (grade 2), but 3% of them are moderately decayed (grade 3) and none of them are intensely decayed (Fig. 3.3). Within 8-cell stage embryos, however, 27% are slightly decayed (grade 2), 18% are moderately decayed (grade 3), and 6% are intensely decayed (grade 4) (Fig. 3.3). Unlike any of the earlier cleavage stages, 16-cell stage embryos are dominated percentage-wise (43%) by intensely decayed embryos (grade 4), while also containing 21% of both slightly (grade 2) and moderately (grade 3) decayed embryos (Fig. 3.3). These percentages show increasing levels of observed organic decay with each advance in embryo cleavage stage, from 2-cell to 16-cell stages. The natural abundances of embryo cleavage stages in these thin sections also shows an interesting pattern consistent with the taphonomic trend discussed above. A plot of these abundances in each cleavage stage versus taphonomic grade reveals some interesting trends (Fig. 3.4). First of all, the plots for all cleavage stages below 16-cell have a negative slope, indicating that they are most commonly well preserved (Fig. 3.4). The 8-cell embryo plot has a much less negative slope than the 2 or 4-cell embryos, however, indicating somewhat elevated decay levels (Fig. 3.4). The plot for Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81 Fig. 3.4. Plot of abundance of each taphonomic grade for each embryo cleavage stage. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 82 soAjqui3 jo jaqiunN Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Taphonomic Grade 83 16-cell stage embryos, meanwhile, is nearly flat and slightly positive, indicating that they more commonly show evidence for organic decay (Fig. 3.4). These data also reveal a taphonomic bias toward earlier cleavage stages, with organic decay levels generally increasing with each advancement in cleavage stage. Another interesting observation is that 4- cell stage embryos are easily the most abundant cleavage stage found in this study (Fig. 3.4). This numerical dominance may indicate that the embryos spent more time in this cleavage stage during development than in other cleavage stages. Discussion The fact that there is observable evidence for organic decay in these embryos, resulting in decipherable taphonomic patterns, lends strong support to the conclusion that they are indeed biological structures. If they were merely diagenetic artifacts, then these taphonomic trends would not be present. Perhaps, therefore, the methods employed in this study will also have utility in testing the biogenicity of other probable Precambrian fossils. A possible explanation for the taphonomic bias toward earlier (2 and 4-cell) cleavage stages found in this study may be that 16-cell embryos, and to a lesser extent 8-cell embryos, were less physically robust than earlier Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 84 cleavage stages. This lack of physical robustness would make them more susceptible to damage during burial or reworking, resulting in more rapid post-burial organic decay prior to being phosphatized. This may also explain the apparent taphonomic bias against later cleavage stages and possible adult forms revealed by this study. One factor that probably contributed to the spectacular preservation of these embryos is that, because of their large size, these embryos were likely extremely yolk-rich. These yolk-rich blastomeres would have been enriched in phospholipids, which may have served as a source for much of the dissolved phosphate necessary for the rapid, fine-scale phosphatization of the blastomere interiors. Such phosphatization, in which the animal itself is the source of the necessary dissolved phosphate, has been shown to take place experimentally (Briggs and Kear, 1993). If indeed internal phosphatization was an important component of the preservation of these embryos, the most common phosphatic metazoan fossil in the Doushantuo, it suggests that many organisms living on the Doushantuo seafloor were not preserved in the fossil record. Other less robust embryos and organisms without internal enrichment in phosphate probably were less likely to be preserved than these embryos, especially considering the extensive reworking typically involved in phosphogenesis and phosphatization. That being said, probable phosphatic fossils other than Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 85 embryos are still being found in the Doushantuo phosphorite (i.e. Chen et al., 2002). These fossils are just much rarer than embryos, and probably owe their phosphatization more to dissolved phosphate super-saturation of external pore waters than to any internal enrichment of phosphate. In addition, it is likely that non-actualistic environmental conditions, such as abundant subtidal seafloor microbial mats, may play a role in the rapid phosphatization of many spectacular Neoproterozoic and Cambrian phosphatic fossils. Nevertheless, the phosphatic fossils of the Doushantuo Formation provide a vital, if incomplete, glimpse of life on Neoproterozoic seafloors and, hence, the origin of metazoans. Conclusions A strong taphonomic bias toward earlier (2 and 4-cell) cleavage stages in the Doushantuo embryos is revealed by these results. Later cleavage stages (8 and 16-cell) typically show more evidence for organic decay than, and are also less common than, earlier cleavage stages. There also appears to be a strong taphonomic bias against the eventual adult forms of these embryos, as well as cleavage stages beyond the 16-cell stage. It is hypothesized that one explanation for this taphonomic trend is that later cleavage stages and adults were more physically delicate than these more Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. robust earlier cleavage stages, especially when cleavage planes are considered as planes of weakness in the embryo or larva. These results begin to delineate the taphonomic constraints in the phosphatization process as it occurred in the Doushantuo Formation, and may explain the lack of certain types of fossils, such as adult forms, in this deposit. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 87 CHAPTER 4: Evidence for Seafloor Microbial Mats and Associated Metazoan Lifestyles in the Lower Cambrian Meishucun Formation of Southwest China Introduction As opposed to the last two chapters, which focused primarily on taphonomy, this chapter and the next will focus on the evolutionary paleoecology of benthic animals during the early stages of the Cambrian radiation. This chapter in particular focuses on strata adjacent to the Early Cambrian Chengjiang fauna, and is of critical interest because these rocks contain evidence for the first intense bioturbation in the region. As discussed earlier, this increase in bioturbation levels as metazoans began to invade infaunal ecospace is emerging as one of the most significant features of the Proterozoic-Phanerozoic transition. Increases in the intensity and vertically of bioturbation in siliciclastics and carbonates through this transition have been well-documented (e.g. Droser 1987; Droser and Bottjer 1988; Mcllroy and Logan 1999). This increase in bioturbation, particularly vertical bioturbation, caused a transition in subtidal siliciclastic environments from typical Proterozoic-style soft substrates to typical Phanerozoic-style soft substrates. Typical Proterozoic-style soft substrates are characterized by low Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 88 levels of strictly horizontal bioturbation, a low water content, a relatively sharp sediment-water interface, and seafloor microbial mats. Typical Phanerozoic- style soft substrates, on the other hand, are characterized by intense horizontal and vertical bioturbation, a high water content, a diffuse sediment- water interface, and the lack of well-developed seafloor microbial mats (e.g. Droser 1987; Droser and Bottjer 1988; Droser et al. 1999; Droser et al. 2001; Hagadorn and Bottjer 1999; Seilacher 1999; Seilacher and Pfluger 1994). A crucial characteristic of these typical Phanerozoic-style soft substrates is the presence of a well-developed mixed layer, the soupy upper few centimeters of seafloor sediment that are homogenized by bioturbation (e.g. Ekdale et al. 1984). This substrate transition was not geologically instantaneous, however, and the dominance of typical Phanerozoic-style soft substrates did not occur with the initial increase in the depth and intensity of bioturbation, as well as ichnofossil diversity, at the Proterozoic-Phanerozoic boundary (e.g. Droser et al. 2002). While the precise duration of this substrate transition has yet to be determined, bioturbation levels reach typical Phanerozoic levels in subtidal carbonates during the Ordovician (Droser and Bottjer 1988, 1989, 1993). This probably means that much of the Cambrian, certainly the Early Cambrian at least, was a unique time interval wherein vestiges of typical Proterozoic-style soft substrates and the beginnings of typical Phanerozoic- Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 89 style soft substrates co-existed in normal marine subtidal settings. Indeed, this chapter presents data from phosphorites of the Lower Cambrian Meishucun Formation of southwest China that provides evidence for an Early Cambrian setting in which typical Proterozoic-style soft substrates, complete with seafloor microbial mats, existed in conjunction with metazoans adapted to survival on and underneath such substrates. In doing so, this chapter documents the paleoecology and organism-sediment interactions of Early Cambrian benthic metazoans in a phosphogenic environment, adding to the data already collected from siliciclastic and carbonate environments. Previous Research Cambrian substrate revolution Seilacher and Pfluger (1994) termed the Proterozoic-Phanerozoic boundary substrate transition the “agronomic revolution”, and defined it as a transition from Proterozoic “matgrounds”, with well-developed seafloor microbial mats, to Phanerozoic “mixgrounds”, with intense levels of horizontal and vertical bioturbation and lacking well-developed seafloor microbial mats. They also postulated that Neoproterozoic benthic metazoan lifestyles would center on seafloor microbial mats (Seilacher 1999; Seilacher and Pfluger 1994). These lifestyles included: 1) mat encrusters, which lived attached to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90 the seafloor microbial mats; 2) mat scratchers, which grazed on the mats; 3) mat stickers, which lived with their lower ends inserted in the mats; and 4) undermat miners, which burrowed horizontally underneath the mats and fed on decomposing mat remnants (Seilacher 1999; Seilacher and Pfluger 1994). The ecological and evolutionary effects of this substrate transition on nonburrowing benthic metazoans, particularly if they were adapted to typical Proterozoic-style soft substrates, were termed the “Cambrian substrate revolution” by Bottjer et al. (2000). Evidence for the Cambrian substrate revolution is seen in the evolutionary response of Cambrian nonburrowing benthic echinoderms and the ecological response of early grazing molluscs to these substrate changes (Bottjer et al. 2000; Dornbos and Bottjer 2000). Geologic Setting Exposures of the Lower Cambrian Meishucun Formation in a quarry on Xiaowaitoushan Hill near Sanjia Village, Jinning, Yunnan Province, China were examined during this study. This locality was a proposed type section for the Precambrian-Cambrian boundary because of its abundant and diverse small shelly fossils. The Meishucun Formation is a lowermost Cambrian sequence of dolomitic phosphorite, dolomite, and bentonite a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91 maximum of 19 m thick (Chen and Zhou 1997; Zhang et al. 1997; Zhu 1997), consisting of the Lower Phosphate, White Clay, Upper Phosphate, and Dahai Members. The two phosphorite members are separated by the thin (0.5 to 2 m) White Clay Member bentonites (Chen and Zhou 1997) (Fig. 4.1). Field observations indicate that the Lower Phosphate Member is composed of relatively thin, planar beds of dolomitic granular phosphorite, while the Upper Phosphate Member contains thicker, hummocky-cross-stratified beds of dolomitic granular phosphorite (Figs. 4.1 and 4.2). This difference in sedimentary structures indicates a shallowing upward from below storm wave base to between storm and normal wave base from the Lower to Upper Phosphate Members. This shallowing continues into the uppermost cross bedded Dahai Member dolomites that are capped by a karstic dissolution surface and overlain by black shales (Chen and Zhou 1997). The Meishucun Formation, therefore, represents a shallowing upward parasequence of dolomitic phosphorite and dolomite disrupted by a central bentonite layer. Both phosphate members contain a diverse array of small shelly fossils as well as abundant phosphatic intraclasts and evidence for mm-scale phosphatic hardgrounds (Chen and Zhou 1997; Zhang et al. 1997; Zhu 1997), indicating deposition under conditions thought to be generally characteristic of phosphogenic environments. Such environments are typically characterized by high energy conditions, low terrigenous Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92 5 - CL 4 - S o 3 - 2 - 1- KEY Red beds (56) j, Radulichnus & s \ \ i 1 / circular impressions Diverse SSF Thalassinoides Fig. 4.1. Composite stratigraphic column of Lower and Upper Phosphate Members of Meishucun Formation as exposed in Xiaowaitoushan Hill quarry near Sanjia Village, Jinning, Yunnan Province, China. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93 Fig. 4.2. Field photographs of Lower and Upper Phosphate Members of the Meishucun Formation at Xiaowaitoushan Hill quarry near Jinning, Yunnan Province, China. A. Upper Phosphate Member showing thick, hummocky cross-stratified beds. Hammer for scale. Obelisk marks site of once-proposed Precambrian-Cambrian boundary. B. Lower Phosphate Member showing thin planar bedding. Hammer for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95 sedimentation rates, high organic burial rates, probably due to high export productivity, seafloor microbial mats, and the formation, erosion, and reworking of phosphatic hardgrounds (e.g. Froelich et al. 1988; Lucas and Prevot 1991; Follmi 1996; Trappe 1998; Soudry 2000). Methods A combination of field observations and thin section analysis produced the data at the core of this chapter. Field observations focused on trace fossil evidence of metazoan behavior, possible evidence for seafloor microbial mats, and diagnostic sedimentary structures. A total of 27 thin sections from the Lower and Upper Phosphate Members were examined in order to search for any evidence suggestive of the presence of seafloor microbial mats. The data resulting from these methods allow for the interpretation of metazoan paleoecology, as indicated by trace fossils, and the characterization of the substrates on which they lived. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96 Metazoan Behavior and Paleoecology Grazing traces A key piece of trace fossil evidence for metazoan paleoecology exists in the Upper Phosphate Member. On one in situ bedding plane 6 m above the base of the Meishucun Formation and two bedding planes on slabs in float, totaling 2.4 m2 in area, is the radular-grazing trace fossil Radulichnus (Figs. 4.1, 4.3 and 4.4). Radulichnus was first described by Voigt (1977) from the surfaces of Jurassic, Cretaceous, and Tertiary oyster valves, as well as from Recent bivalve and gastropod shells. These fossil and modern Radulichnus are interpreted as the radular grazing traces of gastropods and polyplacophorans created by their feeding on algal films on the surface of the mollusc shells (Voigt 1977). These Radulichnus consist of small arcuate sets of scratches less than 1 mm in length that often occur in dense ichnofabrics and occasionally in distinct meandering arrangements (Voigt 1977). Radulichnus has also been described from the terminal Proterozoic Ediacara Member of the Rawnsley Quartzite and Cambrian sandstones in Saudi Arabia (Gehling 1996; Seilacher 1997). Previously interpreted as arthropod traces, these arcuate sets of hypichnial ridges arranged in fan-shaped arrays are reinterpreted as grazing traces left by an organism which Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97 Fig. 4.3. Field photographs of Radulichnus in the Upper Phosphate Member. A, Three arcuate sets of scratches (left of arrows) preserved in concave epirelief. Coin for scale is 2.7 cm in diameter. B. A single arcuate set of scratches (left of arrow) preserved in concave epirelief. Coin for scale is 2.7 cm in diameter. C. A single arcuate set of scratches (left of arrow) preserved in concave epirelief. Coin for scale is 2.7 cm in diameter. D. A dense ichnofabric of scratches along with discoidal impressions, all preserved in convex hyporelief. Coin for scale is 2.7 cm in diameter. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 99 A \\\ ii B C Fig. 4.4. Sketches of Radulichnus photographed in Figure 4.3. A, correspond to A, B, and C in Figure 4.3. See Figure 4.3 for scale. B, and C Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 100 was feeding upon microbial mats on the unconsolidated seafloor (Gehling 1996; Seilacher 1997). Radulichnus is distinguishable from arthropod traces, such as Rusophycus and Monomorphichnus, because its paired scratches are symmetrical, radially arranged, and its arcuate sets of scratches are not found in pairs (Gehling 1996). The radial arrangement of the scratches indicates that they were created by one organism that rotated its head as it scratched the microbial mat for nutrition (Gehling 1996). The fact that the traces were left intact as the grazer moved over them, as it would have been required to do, indicates the presence of microbial mats, which would have significantly stabilized the seafloor (Gehling 1996). Radulichnus is often found preserved with Ediacaran fossils, such as Dickinsonia and Kimberella (Gehling 1996; Seilacher 1997). In one instance, the possible soft-bodied trace-maker has been preserved with Radulichnus. The ovoid body fossil is found in the middle of an array of Radulichnus traces with its anterior end near the focal point of the last 3 arcuate sets (Gehling 1996; Seilacher 1997). The fact that the Radulichnus traces are still preserved underneath this body fossil provides further evidence for microbial mats (Seilacher 1997). This ovoid body fossil is the proper size and is in the proper position to be interpreted as the possible maker of the Radulichnus traces on which it is preserved (Gehling 1996; Seilacher 1997). Although it is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 101 unclear exactly what kind of organism made these Radulichnus, it was probably an early mollusc because of its radular grazing behavior (Gehling 1996). Previously interpreted as arthropod traces by Zhu (1997) in his landmark paper on the Precambrian-Cambrian trace fossils of Yunnan Province, the Radulichnus from the Lower Cambrian of China occur in distinctive red bedding planes, visible as red seams in vertical section, within the granular phosphates of the Upper Phosphate Member (Fig. 4.1). The bedding planes on which they are preserved have an ichnofabric in which few discrete traces are visible, but those that are readily visible are arcuate sets of radially arranged epichnial grooves or hypichnial ridges (Fig. 4.3). Because of the obscuring ichnofabric, which is probably the result of extensive radular grazing and other feeding behavior on these red bedding planes, only isolated arcuate sets are visible. Evidence for the arrangement of these sets, either as radial arrays or meandering, is therefore not present. The visible arcuate sets are 2.0 to 3.8 cm wide and contain scratches 1.5 to 2.0 cm in length. The scratches themselves are approximately 1 to 2 mm in width and depth. These traces are clearly not arthropod traces because of their symmetry, radially arranged scratches, and unpaired occurrence. And because they are symmetrical arcuate sets of radially arranged grooves, these Lower Cambrian traces are identifiable as Radulichnus. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 102 Enigmatic discoidal impressions The Chinese Radulichnus are always found preserved with discoidal epichnial impressions (Fig. 4.5) with concentric circular lineations and even, in some cases, pustulose texture (Figs. 4.1 and 4.5). Previously interpreted as the resting trace Bergaueria (Zhu 1997), these impressions are typically around the same diameter, 10 cm, and are strictly two-dimensional features. Their size similarity, sharp outlines, unusual textures, and two-dimensionality indicate that these discoidal impressions could possibly be the fossil remains of soft-bodied organisms, although other interpretations are certainly possible. One even has a somewhat teardrop shape because of what appears to be a pointed extension of the possible body (Fig. 4.5A). While their identity is unknown, the preservation of these possible soft-bodied organisms was most likely dependent on the presence of microbial mats (Gehling 1999). In fact, the preservation of Radulichnus itself in these granular phosphates is probably dependent on the presence of microbial mats, which provided a preservative interface, now seen as the red bedding planes, between granular phosphate beds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 103 Fig. 4.5. Field photographs of discoidal impressions in Upper Phosphate Member. A. Two discoidal impressions preserved in convex hyporelief. Note concentric annulations and pustulose texture. Coin for scale is 2.7 cm in diameter. B. One discoidal impression preserved in concave epirelief with concentric annulations. Coin for scale is 2.7 cm in diameter. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 105 Comparison with modern grazing traces Modern Radulichnus from the Sunset Cliffs area of Point Loma, near San Diego in Southern California, closely resemble these Chinese traces. These modern scratches, most likely made by gastropods or polyplacophorans grazing on algal mats on the surface of Upper Cretaceous shales exposed in the intertidal zone, consist of radially arranged scratches less than 1 mm long (Fig. 4.6). When individual sets of scratches are visible, they are meandering in form, but most of the radular scratches occur in dense ichnofabrics resembling the occurrence of Radulichnus in China, making discrete sets of scratches difficult to locate (Fig. 4.6). In spite of their obvious size difference, with the modern forms being much smaller, the Lower Cambrian and modern Radulichnus have highly analogous morphologies consisting of arcuate sets of parallel to sub-parallel scratches. Evidence for seafloor microbial mats Thin section analysis of the red bedding planes in which the Chinese Radulichnus are preserved reveals ample suggestive evidence for the presence of microbial mats. These red bedding planes, which contain Radulichnus but also can be seen as red seams within the phosphate in outcrop (Fig. 4.1), contain dense, bed-parallel, laterally continuous concentrations of heavy minerals, such as hematite and pyrite, and mica Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 106 Fig. 4.6. Photographs of modern Radulichnus from Sunset Cliffs, California. A. Scratches showing a meandering pattern amidst a somewhat obscuring ichnofabric of scratches. Field of view is 2 cm in width. B. A dense ichnofabric of scratches. Field of view is 2 cm in width. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 grains (Fig. 4.7). The heavy minerals are likely the result of mat-decay mineralization, in which the burial and decay of microbial mats below the redox boundary causes the precipitation of heavy minerals along the bedding plane containing the organic material of the microbial mat itself (Schieber 1999). The concentration of mica grains in these red bedding planes, which are generally absent in the dolomitic phosphorite matrix, also suggests the presence of seafloor microbial mats because their sticky surface would have trapped mica grains from the water column that would not normally have been deposited in these environments (Schieber 1999). So there is strong petrographic evidence, in addition to the circumstantial evidence discussed above, which suggests the presence of microbial mats in the red bedding planes in which these Radulichnus are preserved. These microbial mats would have served as the trophic resource that the Radulichnus-maker was consuming. In addition to this petrographic evidence, field observations also provide evidence suggestive of the presence of seafloor microbial mats. In particular, probable microbially-mediated sedimentary structures such as wrinkle structures (Hagadorn and Bottjer 1997) and “elephant skin” (Gehling 1999) are present on the uppermost bedding plane surface of the Lower Phosphate Member. This bedding plane also happens to be red and enriched in heavy minerals and mica. Both petrographic and field evidence, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 109 Fig. 4.7. Photographs and photomicrographs of red bedding planes, or seams, in outcrop and thin section. A. Photograph of red bedding plane containing Radulichnus (uppermost arrow) and red seams in vertical outcrop (lower arrows). Coin for scale is 2.7 cm in diameter. B. Photomicrograph of red bedding plane showing abundant iron-rich minerals, seen here in black. Field of view is 1.5 mm in width. C. Photomicrograph of red seam (arrow) between event beds in the phosphorite. Note its wrinkled form. Field of view is 3 mm in width. D. Photomicrograph of red bedding plane showing enrichment in iron-rich minerals and mica grains (arrows). Field of view is 6 mm in width. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 111 therefore, strongly suggest the presence of seafloor microbial mats in the environments represented by the Lower and Upper Phosphate Members of the Meishucun Formation. Thalassinoides Horizontal Thalassinoides in the Lower Phosphate Member also provide a key piece of evidence for metazoan paleoecology. These Thalassinoides, previously interpreted as cross-cutting Palaeophycus (Zhu 1997), are abundant in a 109 cm interval of the Lower Phosphate Member (Fig. 4.1) and are the first evidence for intense bioturbation in Yunnan Province (Zhu 1997). They are visible on nearly all exposed bedding planes in this interval, typically produce a bedding-plane ichnofabric index (ii) of 3 (Miller and Smail 1997), and contain both T- and Y-branches (Fig. 4.8). The burrows range from 8-15 mm in diameter and form branching frameworks of unlined horizontal burrows, the “maze” geometry of Droser and Bottjer (1988) (Fig. 4.8). While post-Paleozoic Thalassinoides are typically considered to be the traces of decapod crustaceans, the identities of early Paleozoic Thalassinoides producers remain enigmatic (Myrow 1995). Considering that these Thalassinoides occur between abundant red bedding planes containing strong suggestive evidence for the presence of seafloor microbial mats (Fig. 4.1), their formation may be related to seafloor Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 112 Fig. 4.8. Field photographs of horizontal Thalassinoides in Lower Phosphate Member. A. Pocket knife for scale is 10 cm in length. B. Pencil for scale is 8 mm in width. C. Pocket knife for scale is 10 cm in length. D. Pencil for scale is 8 mm in width. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114 microbial mats. Although the animals that made these Thalassinoides lived within these burrows, they may have also fed on buried organic material, including buried microbial mats. If this was the case, then the makers of these traces were engaging in undermat mining behavior (Seilacher 1999). In addition, the unlined nature of these Thalassinoides indicates that the coarse-grained matrix in which they were formed was relatively firm, perhaps due in part to the stabilization of microbial mats. Discussion The trace fossils of the Meishucun Formation provide evidence for both mat scratching and undermat mining by metazoans, behaviors that are ecologically centered on seafloor microbial mats. Considered with the suggestive evidence for seafloor microbial mats found in the red bedding planes of the Meishucun, which typically occur on top of event deposits, these traces indicate that the metazoans living in this particular depositional environment were adapted for survival in settings dominated by seafloor microbial mats. In this way, this environment served as a subtidal refuge for metazoans with Neoproterozoic-style adaptations to seafloor microbial mats. Careful consideration of the depositional environment in which these metazoans lived is necessary in order to better understand the environmental Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115 conditions that shaped their paleoecology, allowing for such a Neoproterozoic-style refuge. As described earlier, the Meishucun Formation is a shallowing upward, condensed sequence of dolomitic phosphorite that contains abundant phosphatic intraclasts and evidence for mm-scale phosphatic hardgrounds (Chen and Zhou 1997; Zhang et al. 1997; Zhu 1997). The Lower Phosphate Member is planar-bedded and was probably deposited below storm wave base. The Upper Phosphate Member, however, consists of thick, hummocky cross stratified beds and was probably deposited between storm and normal wave base. No evidence for intertidal or supratidal environments is found in these members. Seafloor microbial mats are generally thought to play a crucial role in phosphogenesis (e.g. Briggs and Kear 1993; Follmi 1996; Wilby et al. 1996; Soudry 2000), so their presence in this environment, as our results suggest is the case, would be predicted. It seems quite likely, based on these characteristics, that the lifestyles of these metazoans were made possible by the phosphogenic environment in which they lived. The presence of seafloor microbial mats allowed for mat scratching behavior to take place, and the relatively rapid formation of phosphatic hardgrounds may have aided in the preservation of these grazing traces. It is likely that the formation of the scratches preceded hardground formation, but the seafloor must have been relatively firm for the scratches to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 116 be created, probably because of stabilization by microbial mats. In addition, these microbial mats and phosphatic hardgrounds probably aided in the formation of the horizontal networks of Thalassinoides by providing a relatively firm substrate as well as buried trophic resources. It is important to note that while these thin phosphatic hardgrounds probably contributed to the formation and preservation of these traces, seafloor microbial mats are thought to be crucial for the formation of such phosphatic crusts (e.g. Soudry 2000), and, as such, were probably the driving force behind the paleoecology of these metazoans. In this way, Early Cambrian phosphogenic environments appear to have allowed for the dominance of typical Neoproterozoic benthic lifestyles (Seilacher 1999), while siliciclastic and carbonate environments were probably more advanced in the transition to typical Phanerozoic benthic lifestyles. Conclusions The trace fossils of the Lower Cambrian Meishucun Formation of southwest China provide evidence for metazoan behavior and paleoecology that centered around seafloor microbial mats, the presence of which is suggested by both field and petrographic evidence. The paleoecology of these metazoans is comparable to typical Neoproterozoic lifestyles Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 117 associated with seafloor microbial mats (Seilacher 1999). The presence of these lifestyles in a subtidal Early Cambrian environment indicates that the increase in bioturbation levels characteristic of the Proterozoic-Phanerozoic transition was spatially and temporally variable, resulting in a mosaic of soft substrate types during much of the Cambrian. As a result, the effect of this Proterozoic-Phanerozoic increase in bioturbation levels on the paleoecology of non-burrowing benthic metazoans, termed the Cambrian substrate revolution (Bottjer et al. 2000), was also highly variable, with benthic metazoans adapted to typical Neoproterozoic-style soft substrates coexisting with those adapted to typical Phanerozoic-style soft substrates. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 118 CHAPTER 5: Paleoecology of Benthic Metazoans in the Early Cambrian Chengjiang Fauna and the Middle Cambrian Burgess Shale Fauna: Evidence for the Cambrian Substrate Revolution Introduction This chapter focuses on the evolutionary paleoecology of benthic metazoans during the Early to Middle Cambrian by examining two of the most famous Cambrian Lagerstatten in the world: the Early Cambrian Chengjiang fauna of southwest China and the Middle Cambrian Burgess Shale fauna of British Columbia, Canada. One of the signatures of the Proterozoic-Phanerozoic transition is the increase in bioturbation levels as metazoans began to inhabit the infaunal realm during the “Cambrian explosion”. Changes in the level and type of bioturbation in siliciclastics and carbonates through the Proterozoic-Phanerozoic transition have been well- documented (e.g. Droser 1987; Droser and Bottjer, 1988; Mcllroy and Logan, 1999), and these changes led to a significant transition in dominant soft substrate types in subtidal siliciclastic environments (e.g. Hagadorn and Bottjer, 1999; Seilacher, 1999; Seilacher and Pfluger, 1994). The ecological impact of this substrate transition on benthic metazoans, however, is only Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 beginning to be examined (e.g. Bottjer et al., 2000; Dornbos and Bottjer, 2000). The first undisputed trace fossil evidence for metazoan activity is found at the end of the last Neoproterozoic glaciation, about 594 mya (Narbonne et al., 1994; Brasier and Mcllroy, 1998). It was not until the Cambrian, however, that trace fossil diversity increased dramatically (Crimes, 1992). This Cambrian increase in trace fossil diversity is coupled with an increase in the intensity and verticality of bioturbation, as observed in both carbonates and siliciclastics (e.g. Droser, 1987; Droser and Bottjer, 1988; Mcllroy and Logan, 1999). This increase in bioturbation, particularly vertical bioturbation, caused a transition in subtidal siliciclastic environments from typical Proterozoic-style soft substrates to typical Phanerozoic-style soft substrates, each dominated by different sedimentary fabric forming processes (Fig. 5.1). Typical Proterozoic-style soft substrates are characterized by low levels of strictly horizontal bioturbation, a low water content, a relatively sharp sediment- water interface, and seafloor microbial mats. Typical Phanerozoic-style soft substrates, on the other hand, are characterized by intense horizontal and vertical bioturbation, a high water content, a diffuse sediment-water interface, and the lack of well-developed seafloor microbial mats (e.g. Droser, 1987; Droser and Bottjer, 1988; Droser et al., 1999; Droser et al., 2002; Hagadorn Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120 Fig. 5.1. Change in dominant processes controlling the sedimentary fabric of normal marine neritic soft substrates during the Proterozoic-Phanerozoic transition. These processes are primary physical and microbial processes, as well as secondary metazoan bioturbation. Changes in the relative dominance of these processes is indicated by the movement of the ovals within the triangle. During this transition, the dominant processes move from a combination of physical and microbial processes to a combination of physical and metazoan processes. Benthic metazoan adaptations to these transitional soft substrates is the focus of this study (modified from Hagadorn and Bottjer, 1999 and Bottjer, et al., 2000). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 121 C l Q c N c u c u c c E Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 122 and Bottjer, 1999; Seilacher, 1999; Seilacher and Pfluger, 1994). A crucial characteristic of these typical Phanerozoic-style soft substrates is the presence of a well-developed mixed layer, the soupy upper few centimeters of seafloor sediment that are homogenized by bioturbation (e.g. Ekdale et al., 1984). Seilacher and Pfluger (1994) termed this substrate transition the “agronomic revolution”, and defined it as a transition from Proterozoic “matgrounds”, with well-developed seafloor microbial mats, to Phanerozoic “mixgrounds”, with intense levels of horizontal and vertical bioturbation and lacking well-developed seafloor microbial mats. They also postulated that Neoproterozoic benthic metazoan lifestyles would center on seafloor microbial mats (Seilacher, 1999; Seilacher and Pfluger, 1994). These lifestyles included mat encrusters, who lived attached to the seafloor microbial mats; mat scratchers, who grazed on the mats; mat stickers, which lived with their lower ends inserted in the mats; and undermat miners, which burrowed horizontally underneath the mats and fed on decomposing mat remnants (Seilacher, 1999; Seilacher and Pfluger, 1994). While the paleoecology of Neoproterozoic benthic metazoans is still not well understood, the paleoecology of Cambrian benthic metazoans is not only easier to interpret, but also extremely informative because they were living during this Proterozoic-Phanerozoic substrate transition. It is these Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 123 Cambrian fossils that currently allow a thorough examination of the ecological and evolutionary effects of this substrate transition on nonburrowing benthic metazoans, effects termed the “Cambrian substrate revolution” by Bottjer et al. (2000). Previous work on the Cambrian substrate revolution has focused on the evolutionary response of Cambrian nonburrowing benthic echinoderms and the ecological response of early grazing molluscs to these substrate changes (Bottjer, et al., 2000; Dornbos and Bottjer, 2000). Early sessile suspension feeding echinoderms show varying evolutionary responses to the Cambrian substrate revolution. The unusual Early Cambrian helicoplacoid echinoderms lived as sediment stickers but showed no soft substrate adaptations typical of Phanerozoic suspension feeders, such as attachment structures or a root-like holdfast, making it unlikely that they could adapt to the substrate changes taking place in the Cambrian. The Cambrian substrate revolution may have therefore led to their extinction (Dornbos and Bottjer, 2000). The other two groups of early sessile suspension feeding echinoderms, the edrioasteroids and eocrinoids, some of which were sediment stickers in the Early-Middle Cambrian, either lived attached to hard substrates or had evolved attaching stems by the Late Cambrian, an evolutionary pattern consistent with a response to the Cambrian substrate revolution (Bottjer et al., 2000; Dornbos and Bottjer, 2000). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 124 Monoplacophorans and polyplacophorans, both groups of benthic grazing molluscs with a long history, show an environmental distribution through the Phanerozoic that is consistent with a response to the Cambrian substrate revolution (Bottjer et al., 2000). While during the terminal Proterozoic and Cambrian these grazing molluscs and their soft-bodied ancestors, as represented by radular grazing traces, were present in nearshore hard substrate and neritic soft substrate environments, by the end of the Phanerozoic they are present only in the deep sea and nearshore hard substrate environments (Bottjer et al., 2000), both modern settings in which surficial microbial mats still exist for ingestion by grazing molluscs. This environmental distribution of early grazing molluscs through the Phanerozoic is consistent with a response to the elimination of well-developed seafloor microbial mats in neritic soft substrate environments during the Cambrian substrate revolution (Bottjer et al., 2000). While this previous work reveals ecological and evolutionary patterns that are consistent with the Cambrian substrate revolution hypothesis, the goal of this research is to further test for these ecological effects on benthic metazoans by examining in detail the adaptive morphologies and paleoecology of benthic suspension feeders and mobile benthic metazoans in the soft-bodied Chengjiang (Early Cambrian) and Burgess Shale (Middle Cambrian) faunas, as well as the rocks in which the Chengjiang fauna is Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 125 preserved. Furthermore, because the Cambrian is a time of transition between typical Proterozoic-style soft substrates and typical Phanerozoic- style soft substrates one might predict that: 1) the Chengjiang and Burgess Shale faunas would contain benthic metazoans adapted to both substrate types, 2) the Burgess Shale fauna might have a greater number of benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than the Chengjiang fauna, and 3) there would be some evidence for the beginning of mixed layer development in the rocks in which the Chengjiang fauna is preserved or those underlying them. Geologic Setting The soft-bodied Chengjiang fauna is preserved in the Lower Cambrian Yuanshan Formation in the Chengjiang area of Yunnan Province, China (Chen and Zhou, 1997) (Fig. 5.2). The Yuanshan Formation is the first Lower Cambrian stratigraphic unit in the region that bears trilobites. It represents a 150 m thick, shallowing upward sequence of shallow marine siliciclastics deposited in a prodelta setting, perhaps just offshore of an estuary (Chen and Zhou, 1997; Babcock et al., 2001; Bergstrom, 2001). The Yuanshan Formation consists of three lithological members, the lowermost Black Shale Member (about 20 m thick), the Maotianshan Shale Member Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 126 £ < hH & P P § U P4 w £ o Longwangmiao Formation Changlangpu Formation Yuanshan Formation Shiyantuo Formation Meishucun Formation 1000 km 90"E — 1 — CHINA Maotianshan Shale Fauna Fig. 5.2. General Lower Cambrian stratigraphy of Yunnan Province, China showing the stratigraphic location of the Chengjiang fauna and map of China marking the location (with a star) of the Chengjiang fauna (Luo et al., 1984; Chen and Zhou, 1997; Babcock et al., 2001). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 127 (about 60 m thick, with extraordinary preservation of soft-bodied fossils), and the uppermost Siltstone Member (about 60 m thick) (Fig. 5.2). Immediately underlying the Yuanshan Formation is the pre-trilobite Shiyantou Formation, a 50 m thick shallowing upward sequence of siliciclastic deposits, with 10 m of black shale at its base and mudstone and siltstone with sandstone interbeds in its uppermost 40 m (Chen and Zhou, 1997; Babcock et al., 2001) (Fig. 5.2). Although the details of the preservation of the soft-bodied fossils in the Chengjiang fauna are still controversial, with possibilities ranging from obrution events to tidal pulses being examined (e.g. Chen and Zhou, 1997; Babcock et al., 2001), all evidence indicates that they were buried with minimal transport (Chen and Zhou, 1997). Many infaunal forms are found preserved in life position perpendicular to bedding (Chen and Zhou, 1997). This minimal transport means that the rocks in which the soft-bodied fossil fauna is preserved represent the substrate on which they were living, making a detailed characterization of that substrate desirable. The soft-bodied Burgess Shale fauna is preserved in the Middle Cambrian Stephen Formation of British Columbia, Canada. The Stephen Formation is about 150 m thick at the Burgess Shale site and consists of dark shales interpreted as being deposited in a relatively deep basinal setting Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 128 (e.g. Briggs et al., 1994). The Burgess Shale fauna is typically thought to have been preserved adjacent to a large carbonate escarpment, represented by the Cathedral Formation, over which obrution events would sweep and bury an assemblage of fossils in the deep water mud (e.g. Briggs et al., 1994). It is thought that the carbonate platform represented by the Cathedral Formation had been covered in siliciclastic muds by the time of the Burgess Shale, meaning that Burgess Shale animals most likely lived on muddy shelf seafloors (e.g. Briggs et al., 1994). This preservation process, therefore, resulted in an allochthonous assemblage of fossils that had been living in muddy shelf environments above the escarpment, but was preserved in the muds below the escarpment (e.g. Briggs et al., 1994). Because it is generally thought that the Burgess Shale fauna was not living in the environment in which it was preserved, the rocks in which the Burgess Shale fossils are preserved do not represent the substrate on which they were living, making the direct characterization of this substrate impossible. Much can still be learned, however, from studying the adaptive morphologies of Burgess Shale benthic suspension feeders, as they probably are a fairly representative allochthonous sample of Middle Cambrian benthic metazoans that lived in muddy shelf environments (e.g. Briggs et al., 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 129 Methods In order to characterize the substrate on which the Chengjiang fauna benthic suspension feeders were living, a total of 5.163 m of core from the Lower Cambrian Yuanshan and Shiyantou Formations was examined on a millimeter scale. These cores were taken in the following two separate areas: the Shanjia Village, Jinning and the Mt. Maotian area, Chengjiang, central Yunnan (South China). The 62 samples examined in this study were selected from random stratigraphic points through the Shiyantou Formation and Maotianshan Shale Member of the Yuanshan Formation. These samples were slabbed and observations were made on the polished surfaces. The samples were also X-radiographed, but the density contrast within the samples was not great enough to produce informative results. A total of 1.107 m of the Yuanshan Formation, all from within the soft-bodied fossil-containing Maotianshan Shale Member, was examined in this manner, while 4.056 m of the underlying Shiyantou Formation were examined. The levels and types of bioturbation were recorded on a millimeter scale through these core samples, utilizing the ichnofabric index method of Droser and Bottjer (1986). This semiquantitative method, designed for determination of bioturbation levels when looking at the vertical face of an outcrop or core, groups the extent of bioturbation into six categories, called Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 130 ichnofabric indices (ii), based on visual observation: ii1 - no bioturbation observed, original sedimentary structures are completely preserved; ii2 - isolated trace fossils present, up to 10% of the original sedimentary structures are disturbed; ii3 - 10-40% of the original sedimentary structures are disturbed, most trace fossils are isolated but overlap occasionally; ii4 - only a small amount of original bedding is visible, 40-60% of the original bedding is disturbed; ii5 - original sedimentary structures are completely disturbed, but some individual traces are still discernible; ii6 - sediment is nearly completely homogenized (Droser and Bottjer, 1986). Using this methodology, the average ii was calculated for both the Maotianshan Shale Member of the Yuanshan Formation and Shiyantou Formation, as well as the percentages of each ii within each unit. The dominant direction of bioturbation, horizontal or vertical, was recorded, as well as the presence of any identifiable discrete trace fossils. This bioturbation data allows for a detailed characterization of the substrate on which the Chengjiang fauna benthic suspension feeders were living. The other critical portion of this research involved a detailed examination of the paleoecology and adaptive morphology of benthic metazoans, particularly benthic suspension feeders, in the Chengjiang and Burgess Shale faunas. Of primary interest is how these metazoans interacted with their substrate and whether or not they show adaptations to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131 soft substrates with a well-developed mixed layer that are typical of Phanerozoic benthic suspension feeders. These data were compiled for each fauna, allowing for an accurate examination of the evolutionary paleoecology of benthic suspension feeders during the Cambrian explosion, as well as consideration of the broader implications of this research for the evolution of early metazoans in general. Shiyantou Formation and Maotianshan Shale Member Core Analysis Although a total of 5.463 m of core was examined from the Shiyantou and Yuanshan Members, the cross-bedded, very fine to fine sandstone interbeds in these core samples were virtually unbioturbated (avg. ii = 1.02), because they represented rapidly deposited event beds. In order to allow for a direct comparison of the siltstone and mudstone facies of the Shiyantou Formation and the Maotianshan Shale Member of the Yuanshan Formation, these sandstone interbeds were not considered when comparing the two units, or when calculating the average ii’s or ii percentages for each member. These sandstones account for 2.193 m of the stratigraphy of the Shiyantou Member samples, leaving 1.863 m of siltstones and mudstones in this member for comparison with the 1.107 m of siltstones and mudstones of the overlying Maotianshan Shale Member. No sandstone interbeds were found Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132 in the Maotianshan Shale Member core samples, although they are known to occur within this unit. The Shiyantou Formation core samples are characterized by generally moderate levels (avg. ii = 2.87) of horizontal bioturbation. In addition to mottled biofabrics (Fig. 5.3A), this bioturbation typically consists of cross sections of horizontal burrows preserved as small (< 3 -5 mm in width) ovals (Fig. 5.3B), oval cross sections of even smaller (< 1 mm in width) horizontal burrows (Fig. 5.3C), and discrete traces identifiable as the shallow infaunal horizontal feeding trace Teichichnus (Fig. 5.3D), which is typically less than 1 cm wide in these samples. There is no evidence for any larger discrete burrows, and very limited evidence for any vertical bioturbation (three small vertical burrows less than 1.7 cm in depth and 3 mm in width) (Appendix B). The ii percentages for the Shiyantou Formation reveal not only the relative dominance of moderate levels of bioturbation (ii3), but also the broad spectrum of bioturbation levels found in these rocks. These ii percentages, and the amount of stratigraphy they represent, are as follows: ii1 -16% (29.7 cm); ii2 -13% (25 cm); ii3 - 49% (90.4 cm); ii4 - 12% (23.5 cm); and ii5 -10% (17.7 cm) (Fig. 5.3). Note that while 49% of the core sample stratigraphy is moderately bioturbated (ii 3), significant amounts of the stratigraphy are also unbioturbated (16% ii1) and completely bioturbated (10% ii5) (Fig. 5.4). These percentages indicate that the substrate that these rocks represent was Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133 Fig. 5.3. Photographs of typical Shiyantou Formation bioturbation in core samples. Scale on the left side of photographs is in millimeters. (A) Mottled beds with some individual horizontal burrows seen as gray ovals (arrow); (B) Oval cross section of a larger horizontal burrow (arrow) amidst smaller burrows and mottled beds; (C) Oval cross sections of sub-millimeter horizontal burrows (arrow) and mottled beds capped by a cross-bedded very fine sandstone with no evidence of bioturbation; (D) Discrete Teichichnus trace fossils (small arrow) below possible evidence of mixed layer development in a 2-3 mm thick zone of sediment homogenized by bioturbation (large arrow). See Appendix C for more photographs of Shiyantou Formation core samples. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 135 Fig. 5.4. Graph of ii percentages of the Shiyantou Formation and the Maotianshan Shale Member of the Yuanshan Formation mudstones and siltstones. See text for definitions of ichnofabric indices (ii) and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137 subjected to highly variable levels of horizontal bioturbation, with moderate levels occurring most frequently. Even with this amount of spatial and temporal variability in horizontal bioturbation levels, however, this substrate generally lacked any sign of a mixed layer except for the 10% of its stratigraphy in which it was thoroughly bioturbated (ii5). Also of interest is that 29% of the strata in these samples showed very little (ii2) or no (ii1) evidence at all for any bioturbation (Fig. 5.4). This bioturbation data indicates that the substrates represented by the Shiyantou Formation siltstones and mudstones, while typically undergoing moderate levels of horizontal bioturbation (ii3), generally did not have a well- developed mixed layer. Consistent with the prediction of the Cambrian substrate revolution hypothesis, however, the beginnings of mixed layer development are preserved within these rocks as vertically restricted intervals of thorough bioturbation (ii5) that account for just 10% of the stratigraphy in these samples (Fig. 5.3D). This substrate was apparently undergoing the early stages of a transition from a typical Proterozoic-style soft substrate to a typical Phanerozoic-style substrate. Accordingly, one might expect to find benthic suspension feeders adapted to both substrate types co-existing in the Chengjiang fauna. In marked contrast to the underlying Shiyantou Formation, the Maotianshan Shale Member, in which the soft-bodied fossils are preserved, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 138 is almost completely unbioturbated. The average ii for these 1.107 m of stratigraphy is only 1.01 (Appendix D). Bioturbation in the core samples examined is limited to three occurrences of individual horizontal burrows visible as gray oval cross sections, leading to ii percentages of 99% ii1 and 1% ii2 (Fig. 5.3). No evidence of moderate to thorough bioturbation levels (ii3 to ii5) was found in these rocks. These extremely low levels of bioturbation suggest that whatever environmental factors contributed to the exceptional preservation of the soft-bodied fossil fauna may have also suppressed bioturbation levels in the Maotianshan Shale Member siltstones and mudstones. With these extremely low levels of bioturbation, the substrate represented by these rocks would have been relatively firm and seafloor microbial mats may have been present. Indeed, careful examination of the sedimentology of the Maotianshan Shale Member reveals possible evidence for such seafloor microbial mats. The Maotianshan Shale Member samples generally consist of thin-bedded to laminated mudstones and siltstones that are gray to black in color (Fig. 5.5A). These thin beds are typically much less than 1 cm thick, but are up to 2 cm thick in places, while the laminae are sub millimeter in thickness. Many of these thin beds appear to have been rapidly deposited as they are typically graded and sometimes exhibit small-scale Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 139 Fig. 5.5. Photographs of typical Maotianshan Shale Member core samples. Scale on the left side of photographs is in millimeters. (A) Thin beds and laminae with no bioturbation. Note the black mudstone intraclasts common in several of the beds (arrows); (B) Graded bed containing black mudstone intraclasts (arrows) amidst thin beds and laminae. See Appendix E for more photographs of Maotianshan Shale Member core samples. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 141 cross-bedding. Many of these thin event beds also contain extremely small (typically less than 1 mm in length) black to dark gray mudstone intraclasts, that are elongate but irregular in form, thereby exhibiting flexible behavior (Fig. 5.5B). This combination of cohesiveness and flexibility strongly suggests that these intraclasts were bound by seafloor microbial mats (e.g. Pfluger and Gresse, 1996; Schieber, 1999). Even if one argues against this suggestive evidence for seafloor microbial mats, the muddy source substrate of these intraclasts still had to be relatively cohesive and firm in order to be ripped-up and transported. Therefore, the sedimentology and bioturbation levels of the Maotianshan Shale Member samples examined in this study indicate that the soft substrate on which the Chengjiang fauna was living, and in which it was preserved, was relatively firm and most likely contained seafloor microbial mats. Adaptations of Benthic Suspension Feeders to Typical Phanerozoic- style Soft Substrates Phanerozoic immobile benthic suspension feeders typically show several adaptations that allow for their survival on unconsolidated substrates, and it is important to examine these adaptations carefully in order to assess Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 142 the paleoecology of Chengjiang and Burgess Shale fauna benthic suspension feeders. Perhaps the most successful of these adaptations is the ability to attach to hard substrates, such as the skeletons of other animals. Benthic bivalves and brachiopods both show strong Phanerozoic trends toward hard substrate attachment and away from free-living forms (Thayer, 1983). Benthic suspension-feeding echinoderms show a similar trend toward attachment and away from free-living forms through the Cambrian (Bottjer et al., 2000; Dornbos and Bottjer, 2000). Hard substrate attachment provides a stable refuge from soft, soupy substrates, making it a powerful metazoan adaptation to typical Phanerozoic-style soft substrates with well-developed mixed layers. Despite their decrease in generic abundance through the Phanerozoic (Thayer, 1983), free-living immobile forms also show adaptations to typical Phanerozoic soft substrates. All of these adaptations involve either decreasing the amount of stress placed on the substrate by the organism or stabilizing the organism in the soft substrate (Thayer, 1975). The first adaptation, seen in bivalves and brachiopods, is to decrease overall body mass by growing thin, non-costate shells (Thayer, 1975). This decrease in body mass places less stress on the soft substrate underneath the organism, thereby increasing its ability to survive on such substrates. Another soft- substrate adaptation of benthic organisms is to broadly distribute its body Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 143 mass on the substrate, an adaptation called the “Snowshoe” strategy by Thayer (1975). Commonly achieved by having a wide, thin body, this adaptation allows organisms to virtually float on top of a soft substrate by diffusing their stress on the seafloor (Thayer, 1975). While the above adaptations focus on decreasing the amount of stress placed on the seafloor by the animal in question, the remaining adaptations rely on stabilizing the animal in the soft substrate. One way to accomplish this goal is to have a root-like holdfast that extends into the soft substrate, thereby stabilizing the animal (Sprinkle and Guensburg, 1994). Ordovician crinoids utilized this adaptation as they inhabited soft-substrate settings during the Ordovician radiation (Sprinkle and Guensburg, 1994). Another way in which an animal can stabilize itself on a typical Phanerozoic soft substrate is to live as a deep sediment sticker, with a large skeletal element that extends deep into the seafloor and stabilizes the animal (Thayer, 1975; Seilacher, 1999). This adaptation was termed the “Iceberg” strategy by Thayer (1975), because most of the animal extends into the substrate, with only a small part of the body, that which contains the soft parts, perched above the soft seafloor. When combined with one of the above strategies, another effective technique for survival on typical Phanerozoic-style soft substrates is to dramatically increase body size, making bioturbation levels in the soft Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 144 substrate irrelevant to survival (Thayer, 1983). By growing to a large size very quickly, at least faster than the biological turnover rate of the sediment, immobile benthic suspension feeders can keep themselves above the sediment-water interface, helping to neutralize the effects of bioturbation (Thayer, 1983). An exemplar of this strategy is the modern attaching endobyssate bivalve Pinna, which is normally restricted to stabilized or hard substrates, but in some cases can survive on well-bioturbated soft substrates through unusually rapid growth, up to 15 cm in their first six months (Thayer, 1979; 1983). Although notable exceptions do exist, this strategy does suggest that free-living immobile benthic suspension feeders adapted to typical Phanerozoic-style soft substrates are generally, but certainly not always, larger than those well-adapted for survival on typical Proterozoic- style soft substrates. It is important to remember, however, that large benthic metazoans can clearly be well-adapted to typical Proterozoic-style soft substrates if they lack stabilizing or stress reducing adaptations, which would be particularly important for a large benthic metazoan to have in well- bioturbated settings. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145 Adaptations of Benthic Suspension Feeders to Typical Proterozoic- style Soft Substrates In contrast to the benthos discussed above, immobile benthic suspension feeders well adapted to typical Proterozoic-style soft substrates lack adaptations that decrease the amount of stress on the seafloor or stabilize the animal in the soft substrate. One strategy of benthic suspension feeders well adapted to typical Proterozoic-style substrates is to live as shallow sediment stickers (Seilacher, 1999; Dornbos and Bottjer, 2000). Unlike the deep sediment stickers discussed above, these animals have no large, stabilizing skeletal elements that extend deep into the seafloor. Instead, only a small portion of their body extends into the seafloor (Dornbos and Bottjer, 2000). Benthic suspension feeders with this lifestyle were well- adapted for survival on typical Proterozoic-style soft substrates because they lack the stabilizing adaptations seen in those benthos that were well-adapted to typical Phanerozoic-style soft substrates with well-developed mixed layers. Immobile benthic suspension feeders that lived freely on the seafloor as sediment resters, but lack adaptations that would reduce their stress on the seafloor, were well-adapted to living on typical Proterozoic-style soft substrates. Benthic invertebrates with this lifestyle probably could not have survived on soft substrates with well-developed mixed layers, especially Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 146 considering that even sediment resters with adaptations to well-bioturbated soft substrates, such as a broad body mass distribution, decrease in generic abundance through the Phanerozoic, as bioturbation levels steadily increase (Thayer, 1983). Another lifestyle indicative of being well adapted to typical Proterozoic- style soft substrates is soft substrate attachment. Any immobile benthic invertebrate that simply lived attached to an unconsolidated soft substrate, such as the Neoproterozoic probable cnidarian Corumbella (Leslie et al., 2001), was clearly well adapted to a typical Proterozoic-style soft substrate with a low water content and seafloor microbial mats. Any such benthic suspension feeder could probably not survive on a soft substrate with a well- developed mixed layer. Paleoecology of Chengjiang Fauna Benthic Suspension Feeders Utilizing the criteria discussed above (Table 5.1), the adaptive morphologies of Chengjiang fauna benthic suspension feeders were carefully examined in order to determine whether they were adapted to survive on either typical Proterozoic-style soft substrates, which would have been relatively firm and bound by seafloor microbial mats, or typical Phanerozoic- style soft substrates with a well-developed mixed layer. This examination Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 147 Proterozoic-style Phanerozoic-style 1) Lower end pointed for shallow insertion into sediment 2) Lower end blunt or flat for sediment resting 3) Evidence for attachment to seafloor sediment 1) Evidence of attachment to hard substrates 2) Presence of extensive root-like holdfast 3) Broad body mass distribution (wide, thin, and flat) 4) Presence of long skeletal extension for insertion into substrate Table 5.1. Criteria used to interpret the life modes of benthic suspension feeders examined in this study. In the first column, number 1 corresponds to shallow sediment stickers (sss), number 2 to sediment resters (sr), and number 3 to sediment attachers (sa). In the second column, number 1 corresponds to hard substrate attachers (hs), number 2 to root-like holdfasts (rlh), number 3 to snowshoe strategists (sno), and number 4 to iceberg strategists (ice). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148 indicates that the Chengjiang fauna contains sediment resters, shallow sediment stickers, and hard substrate attachers (Table 5.2). One example of a sediment rester is the sack-shaped demosponge Crumillospongia, which exhibits no adaptations to typical Phanerozoic-style soft substrates (Fig. 5.6A). While one specimen has been noted to be over 10 cm in height, Crumillospongia is typically only around 2 cm in height (Rigby, 1986), making this genus even more susceptible to the effects of high bioturbation levels and the resultant mixed layer. Crumillospongia, then, seems to be a good example of a Chengjiang fauna benthic suspension feeder well-adapted for survival on a typical Proterozoic-style soft substrate. Another such Chengjiang benthic suspension feeder is Takakkawia, a conical demosponge with a narrow pointed lower end well-adapted for insertion into the seafloor (Fig. 5.6B). Takakkawia, therefore, most likely lived as a shallow sediment sticker, but lacks any adaptations to either stabilize it in the soft substrate or decrease its stress on the seafloor, as exhibited by benthic suspension feeders adapted to typical Phanerozoic-style soft substrates with a well-developed mixed layer. Its adaptive morphology, as well as its small size (typically less than 3 cm in height (Rigby, 1986), both indicate that Takakkawia was well-adapted to typical Proterozoic-style soft substrates. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 149 Genus Description | Life Mode Allantospongia Archotuba Cambrorhytium Chancelloria Choia Choiaella Crum illospongia Dinomischus Heliomedusa lotuba Leptom itella Leptomitus P araleptom itella Q uadrolam iniella Saetospongia Takakkawia Tricitispongia Xianguangia globular sponge conical animal attached to hard substrates conical animal attached to hard substrates conical animal with pointed lower end globular sponge globular sponge sac-like sponge animal with blunt spine on lower end flat brachiopod long, thin animal with rounded lower end conical sponge with pointed lower end conical sponge with pointed lower end conical sponge with pointed lower end conical sponge with pointed lower end globular sponge conical animal with pointed lower end globular sponge cnidarian with blunt lower end sediment rester hard substrate attacher hard substrate attacher shallow sediment sticker sediment rester sediment rester sediment rester shallow sediment sticker sediment rester shallow sediment sticker shallow sediment sticker shallow sediment sticker shallow sediment sticker shallow sediment sticker sediment rester shallow sediment sticker sediment rester I sediment rester Table 5.2. Chengjiang fauna benthic suspension feeding genera with brief morphological descriptions and interpreted life modes. Only interpretable genera are included in this table. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 150 Fig. 5.6. Examples of Chengjiang fauna benthic suspension feeders. (A) Sediment resting demosponge Crumillospongia, typically around 2 cm in height (modified from Rigby, 1986); (B) Shallow sediment sticking demosponge Takakkawia, typically around 3-4 cm in height (modified from Rigby, 1986); (C) Hard substrate attacher Cambrorhytium, a possible cnidarian typically 2-3 cm in height (modified from Chen and Zhou, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152 As predicted earlier, benthic suspension feeders well-adapted for survival on typical Phanerozoic-style soft substrates also exist in the Chengjiang fauna. One example is the funnel-shaped possible cnidarian Cambrorhytium that lived attached to brachiopod shells or other hard substrates (Chen and Zhou, 1997) (Fig. 5.6C). Because it lived attached to hard substrates, Cambrorhytium was clearly well-adapted for survival on typical Phanerozoic-style soft substrates with a well-developed mixed layer. When the adaptive morphologies of all 19 immobile benthic suspension feeding genera in the Chengjiang fauna are analyzed in a similar manner, it becomes obvious that an overwhelming majority (88% or 15 genera) are well-adapted to typical Proterozoic-style soft substrates (Table 5.3). The remainder (12% or 2 genera) are well-adapted to typical Phanerozoic-style soft substrates (Table 5.3). The adaptive morphologies of two genera are uninterpretable because of limitations in the fossil evidence. The dominance of benthic suspension feeding genera adapted to typical Proterozoic-style soft substrates in the Chengjiang fauna suggests that the Proterozoic-Phanerozoic substrate transition was only in its early stages during this time. These results are consistent with the core sample analysis of the Maotianshan Shale Member, which indicates that the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. P r o i e r o z o i c - s t v l e P h a n e r o z o i c - s t y l e U n k n o w n Alhuuospongia (sr) Archoiuba (hs) Haliclwndriies Ciwncelloria (sss) Cambrorhytium (hs) Hazelia Ciioia (sr) Choiaella (sr) Crumillospongia (sr) Crumillospongia (sr) Dinomischus (sss) Heliomedusa (sr) lotuba (sss) Lepiomitella (sss) Lepiomiius (sss) Paralepiomiiella (sss) Quadrolaminiella (sss) Saetospongia (sr) Takakkawia (sss) Tricitispongia (sr) Xianguungia (sr) Table 5.3. Adaptive morphologies of Chengjiang fauna benthic suspension- feeding genera. Columns indicate the type of soft substrate that the benthic suspension-feeding genera are adapted to, either Proterozoic-style soft substrates (left column), typical Phanerozoic-style soft substrates (middle column), or unknown (right column), “sr” = sediment rester. “sss” = shallow sediment sticker, “hs” = hard substrate attacher. See text for discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 154 Chengjiang fauna was living on a relatively firm soft substrate with seafloor microbial mats. It is interesting to note that 11 of the 15 Chengjiang benthic suspension feeding genera well-adapted to typical Proterozoic-style soft substrates are sediment resting and shallow sediment sticking sponges (Fig. 5.7). The free-living nature of these sponges differs greatly from typical modern shallow water sponges, which are dominated by hard substrate encrusters. None of the Chengjiang fauna sponges are hard substrate encrusters, resulting in a general Phanerozoic trend toward hard substrate attachment that is consistent with the Cambrian substrate revolution. In addition, these differences between modern shallow marine sponges and Early Cambrian ones shows that the results of this paleoecological study are not merely the product of a taxonomic effect brought on by the dominance of sponges in the Chengjiang fauna. The prediction of the Cambrian substrate revolution hypothesis that benthic suspension feeders adapted to Proterozoic-style soft substrates and those adapted to Phanerozoic-style soft substrates would both be found in the Chengjiang fauna is upheld by this paleoecological analysis. Although benthic suspension feeders adapted to typical Phanerozoic-style soft substrates comprise only 12% of Chengjiang benthic suspension feeders, they are still present and indicate that the Proterozoic-Phanerozoic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155 Fig. 5.7. Box diagram reconstructing Chengjiang fauna benthic suspension feeding genera, most of which are discussed in the text, in life position on top of a virtually unbioturbated substrate with just a few horizontal burrows seen as oval cross sections a few millimeters in diameter. An example of the sediment-resting demosponge Crumillospongia is in the upper left corner. An example of the shallow sediment sticking demosponge Takakkawia is in the upper right corner. An example of the sediment-resting demosponge Choia is next to this Takakkawia. An example of the hard substrate attaching possible cnidarian Cambrorhytium is in the lower right corner. Examples of the enigmatic shallow sediment sticker Dinomischus are in the lower left corner next to the lobopodian Hallucigenia. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157 soft substrate transition was underway during this time. The core sample analysis of the underlying Shiyantou Formation strongly supports this interpretation because it indicates that the mixed layer was beginning to develop in these environments before the preservation of the Chengjiang fauna. Paleoecology of Burgess Shale Fauna Benthic Suspension Feeders While there are distinct differences between the preservational environments of the Chengjiang and Burgess Shale faunas, it is still informative to examine the paleoecology of Burgess Shale benthic suspension feeders considering that the Burgess Shale fauna provides one of the best windows into animal life in the Cambrian. In any case, the environmental differences between the Chengjiang and Burgess Shale fauna may actually provide for an interesting comparison between the two faunas because they share many taxa, leading to speculation by some that the Burgess Shale fauna may represent a deep water refuge for Early Cambrian holdovers (Conway Morris, 1998). The same criteria used to analyze the adaptive morphologies of Chengjiang fauna benthic suspension feeders (Table 5.1) were used to analyze the adaptive morphologies of Burgess Shale benthic suspension Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158 feeders. The goal, as above, was to determine whether benthic suspension feeding genera in the Burgess Shale fauna were adapted to either typical Proterozoic-style soft substrates, or typical Phanerozoic-style soft substrates. Much like the Chengjiang fauna, the Burgess Shale fauna contains shallow sediment stickers, sediment resters, and hard substrate attachers (Table 5.4). The small round to elliptical demosponge Choia is an example of a sediment rester in the Burgess Shale fauna (Fig. 5.8A). Choia, typically only a few millimeters in diameter (Rigby, 1986), also shows no stabilizing or stress-reducing adaptations despite being a free-living sponge. This lack of common adaptations to typical Phanerozoic soft substrates together with its small size indicates that Choia could not have survived on a heavily bioturbated soft substrate with a well-developed mixed layer. Choia was therefore well-adapted to typical Proterozoic-style soft substrates. An example of a shallow sediment sticker in the Burgess Shale fauna is the small conical to sack-shaped hexactinellid sponge Diagoniella (Fig. 5.8B). Reaching a maximum height of just 15 mm and showing no adaptations to stabilize itself in the soft substrate on which it lived (Rigby, 1986; Briggs et al., 1994), Diagoniella was well-adapted for survival on a typical Proterozoic-style soft substrate with a sharp sediment-water interface Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 159 Genus Description Life Mode Cambrorhytium Capsospongia Chancelloria Choia Crumillospongia D iagoniella Dinomischus Diraphora Echmatocrinus Eiffelia Gogia Leptomitella Leptomitus Mackenzia M icrom itra Nisusia Takakkawia Thaumaptilon Vauxia Walcottidiscus Wapkia conical animal attached to hard substrates sponge with pointed lower end conical animal with pointed lower end globular sponge sac-like sponge sponge with pointed lower end animal with blunt spine on lower end brachiopod attached to hard substrates animal attached to hard substrates sponge attached to hard substrates echinoderm attached to hard substrates conical sponge with pointed lower end conical sponge with pointed lower end brachiopod attached to hard substrates brachiopod attached to hard substrates brachiopod attached to hard substrates conical animal with pointed lower end cnidarian with rounded lower end branching sponge with pointed lower end echinoderm with flat lower surface sponge with pointed lower end hard substrate attacher shallow sediment sticker shallow sediment sticker sediment rester sediment rester shallow sediment sticker shallow sediment sticker hard substrate attacher hard substrate attacher hard substrate attacher hard substrate attacher shallow sediment sticker shallow sediment sticker hard substrate attacher hard substrate attacher hard substrate attacher shallow sediment sticker shallow sediment sticker shallow sediment sticker sediment rester shallow sediment sticker Table 5.4. Burgess Shale fauna benthic suspension feeding genera with brief morphological descriptions and interpreted life modes. Only interpretable genera are included in this table. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160 Fig. 5.8. Examples of Burgess Shale fauna benthic suspension feeders. (A) Sediment resting demosponge Choia, typically 8-12 mm in central-disc width (modified from Rigby, 1986); (B) Shallow sediment sticking hexactinellid sponge Diagoniella, typically up to 15 mm in height (modified from Briggs et al., 1994); (C) Hard substrate attaching orthid brachiopod Nisusia, typically around 22 mm in width (modified from Briggs et al., 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 162 and seafloor microbial mats. It almost certainly could not have survived on a soft substrate with a well-developed mixed layer. The orthid articulate brachiopod Nisusia is an example of a Burgess Shale benthic suspension feeder that was well-adapted for survival on typical Phanerozoic-style soft substrates (Fig. 5.8C). Nisusia lived as a hard substrate attacher, using its pedicle to attach to sponges, particularly the demosponge Pirania (Briggs et al., 1986). Because it lived attached to sponges, Nisusia neutralized any effect that high levels of bioturbation may have had on its ecology, making it well-adapted to typical Phanerozoic-style soft substrates with a well-developed mixed layer. This paleoecological analysis was performed on all Burgess Shale benthic suspension feeding genera in which an adaptive morphological interpretation was possible. Of these 32 genera, the adaptive morphologies of 11 genera are uninterpretable mostly because many of them are known from only a very few fragmented specimens (Rigby, 1986). Of the 21 genera for which an interpretation is possible, however, 62% (13 genera) are well- adapted to typical Proterozoic-style soft substrates, while 38% (8 genera) are well-adapted to typical Phanerozoic-style soft substrates (Table 5.5). Although a direct reconstruction is not possible, these results at least suggest that the soft substrate on which the Burgess Shale fauna lived was a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 163 P roierozoic-style Phanerozoic-style U n k n o w n Capsospongia (sss) C am brorhyiium (hs) C a n is in tm e lla C ium ceU oria (sss) D ira p h o ra (hs) F alo spo ngia C hniu (sr) E chm arocrinus (hs) F ield o sp o n g ia C n tiu illo sp o n g ia (sr) E iffelia (hs) H a lic h o n d riie s D ia g o n ie lla (sss) Gogi.a (hs) H a m p w n ia D inom ischus (sss) Mackenziei (hs) H a z e lia L e p io m iie lla (sss) M ic ro m ilra (hs) M ole cu lo sp in a Le piom iius (sss) Nisusia (hs) P ira n ia T akakkaw ia (sss) P roto spon gia T haw na ptilo n (sss) S em inelia Vauxia (sss) Siephenospongia W alconidiscus (sr) W apkia (sss) Table 5.5. Adaptive morphologies of Burgess Shale fauna benthic suspension-feeding genera. Columns indicate the type of soft substrate that the benthic suspension-feeding genera are adapted to, either Proterozoic- style soft substrates (left column), typical Phanerozoic-style soft substrates (middle column), or unknown (right column), “sr” = sediment rester. “sss” = shallow sediment sticker, “hs” = hard substrate attacher. See text for discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 164 transitional mosaic of relatively firm substrates and substrates with some mixed layer development. Some interesting observations can be made when comparing the benthic suspension feeders of the Chengjiang and Burgess Shale faunas. Although the Burgess Shale fauna is generally thought to have been preserved in a much deeper marine setting than the Chengjiang fauna, it contains an allochthonous assemblage of metazoans from shallower muddy shelf environments that has a much higher number of benthic suspension feeders adapted to typical Phanerozoic-style soft substrates, all hard substrate attachers. This difference reveals an interesting trend in that an Early Cambrian shallow marine setting probably had considerably less mixed layer development than the Middle Cambrian muddy shelf environments in which the Burgess Shale fauna benthic metazoans lived. This observation may provide more evidence for the onshore-offshore nature of the Cambrian increase in bioturbation levels (e.g. Hagadorn and Bottjer, 1999), and is consistent with increased mixed layer development in subtidal siliciclastic soft substrate environments through the Cambrian. Similar to the Chengjiang fauna, shallow sediment sticking and sediment resting sponges dominate the Burgess Shale benthic suspension feeders well-adapted to typical Proterozoic-style soft substrates. In addition, 7 of the 13 genera in this category are present in both faunas. The most Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 165 crucial differences between the two faunas are actually found in the benthic suspension feeders well-adapted to typical Phanerozoic-style soft substrates. A relatively diverse group of hard substrate attaching echinoderms (Gogia and possibly Echmatocrinus), brachiopods (Diraphora, Micromitra, and Nisusia), possible cnidarians (Cambrorhytium and Mackenzia), and one sponge genus (Eiffelia) comprise the Burgess Shale benthic suspension feeders that are well-adapted to typical Phanerozoic-style soft substrates. What this diverse group has in common is that they all lived attached to the hard skeletons of other animals, including sponges, brachiopods, priapulid worms, and hyolithids. In the Chengjiang fauna, meanwhile, only two genera, the possible cnidarian Cambrorhytium and the enigmatic Archotuba, fit into this category, both of which lived attached to brachiopod shells. This data suggests that, in general, many Early and Middle Cambrian benthic suspension feeders began seeking refuge from increasing bioturbation levels in siliciclastic soft substrate settings by attaching to the skeletons of other animals, dead or alive. It also appears, through comparison of the two faunas, that this mode of life became more common amongst benthic suspension feeders through the Early and Middle Cambrian. While the specific examples of benthic suspension feeders well- adapted to typical Proterozoic soft substrates discussed above are all relatively small in size, it is important to note that some Chengjiang and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 166 Burgess Shale fauna benthic suspension feeders adapted to such substrates are actually quite large. For instance, the large conical demosponges Leptomitus and Leptomitella, which can reach over 30 cm in length (Rigby, 1986), are found in both faunas and are interpreted as shallow sediment stickers adapted to typical Proterozoic soft substrates. This interpretation is based on their conical morphology, with Leptomitella having a sharply pointed lower end, and their lack of any stabilizing or stress reducing adaptations despite their large size, which would seemingly cause them to strongly require such adaptations. Indeed, in the post-Cambrian Paleozoic such high level tierers in neritic environments are dominated by hard substrate attaching and root-stabilized crinoids (Sprinkle and Guensburg, 1995; Ausich and Bottjer, 2001), as opposed to the free-living or soft- substrate attaching forms of the Neoproterozoic and Cambrian. Paleoecology of Mobile Benthic Metazoans in the Chengjiang and Burgess Shale Faunas While the substrate interactions of mobile benthic metazoans are more difficult to interpret than those of immobile benthic suspension feeders, some interpretations are still possible. Because of this difficulty in interpretation, however, a quantitative assessment of the paleoecology of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 167 Chengjiang and Burgess Shale fauna mobile benthic metazoans is not possible at this time. Instead, there are criteria that will help to identify key individual examples of mobile benthic metazoans adapted to both substrate types in these faunas. For instance, mobile benthic metazoans well adapted to typical Proterozoic-style soft substrates would include animals that are dependent on seafloor microbial mats as a trophic resource, such as “mat scratchers” (Seilacher, 1999), which scratched microbial mats for food without destroying them. Also included in this category would be benthic metazoans that are dependent on a relatively firm, stable substrate with a sharp sediment-water interface for survival. Using these criteria, mobile benthic metazoans that appear to have been adapted to typical Proterozoic-style soft substrates are present in the Chengjiang and Burgess Shale faunas. The unusual lobopodians, resembling onychophorans and found in both faunas, walked on the seafloor with numerous short (<1 cm) jointless legs, perhaps living as scavengers or grazing on sponges or algae (e.g. Briggs et al., 1994; Chen and Zhou, 1997). The strange lobopodian species Hallucigenia, found in both faunas, has two rows of dorsal spines and is only 0.5 to 3 cm long (Fig. 5.9A), while Microdictyon, found in only the Chengjiang fauna, has flat oval plates on its sides and is of similar size (e.g. Briggs et al., 1994; Chen and Zhou, 1997). It seems unlikely that these small animals could have survived on a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 168 Fig. 5.9. Examples of Chengjiang and Burgess Shale fauna mobile benthic metazoans. (A) The Chengjiang and Burgess Shale lobopodian Hallucigenia, typically 0.5 to 3 cm in length (modified from Briggs et al., 1994); (B) The enigmatic Burgess Shale fauna mobile benthic metazoan Wiwaxia, typically 3.5 to 55 mm in length (modified from Briggs et al., 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 170 typical Phanerozoic-style soft substrate with a diffuse sediment-water interface and high water content resulting from a well-developed mixed layer, especially considering the muddy seafloors on which they lived. They instead appear to have been well adapted for living on a relatively firm Proterozoic-style soft substrate with a sharp sediment-water interface and low water content. While the lobopodians were probably dependent on the firmness and stability of typical Proterozoic-style soft substrates, there are examples of Burgess Shale mobile benthic metazoans that were probably dependent on the seafloor microbial mats found on this type of soft substrate as a trophic resource. One example, assuming that they lived on a muddy substrate, which is the common interpretation (e.g. Conway Morris, 1985; Briggs et al., 1994), is the small, 3.5 to 55 mm long, enigmatic metazoan Wiiwaxia (Fig. 5.9B). With its dorsal side covered with flat sclerites and spines and its ventral side unarmored, Wiwaxia is commonly interpreted to have crawled along the sediment using muscular contractions while feeding on the surface of the substrate with its two rows of anterior teeth (Conway Morris, 1985), a grazing mode of life typically restricted to hard substrates today. Especially considering the muddy substrate on which Wiwaxia lived, this mode of life would have been dependent on a relatively firm typical Proterozoic-style soft Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 171 substrate with seafloor microbial mats, which may have served as an important trophic resource for Wiwaxia. Although taxonomically controversial, another possible example of a Burgess Shale mobile benthic metazoan well adapted to typical Proterozoic- style soft substrates may be the possible mollusc Scenella. Originally interpreted as a helcionelloid mollusc with a single cap-shaped dorsal shell (Rasetti, 1954), but later reinterpreted as a pelagic chondrophorine cnidarian (Yochelson and Gil Cid, 1984; Babcock and Robison, 1988), Scenella is one of the most abundant fossils found in the Burgess Shale. An important reason for this reinterpretation of Burgess Shale Scenella as pelagic cnidarians is their preservation in a deep, muddy environment (Yochelson and Gil Cid, 1984). Their presence in such an environment was considered inconsistent with the seafloor grazing life mode of helcionelloid molluscs, a life mode typically restricted to nearshore hard substrates today (Yochelson and Gil Cid, 1984). While the taxonomy of Burgess Shale Scenella needs further work, it is important to note that our increasing knowledge of the nature of Cambrian subtidal siliciclastic substrates explains their apparently inconsistent environmental distribution because relatively firm substrates with seafloor microbial mats, typical Proterozoic-style soft substrates, would not be unexpected in such environments during the Middle Cambrian. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 172 Accordingly, preservation of seafloor grazing molluscs would not be unexpected in such deep, muddy environments during this time. Mobile benthic metazoans well adapted for survival on typical Phanerozoic-style soft substrates are also probably found in the Chengjiang and Burgess Shale faunas. Although it is difficult to determine the precise life mode of many of the Chengjiang and Burgess Shale fauna trilobites, it is likely that some of them lived as benthic deposit feeders, using their legs to dig into the upper few centimeters of the substrate and ingest organic detritus. This trilobite feeding behavior is preserved in trace fossils such as Cruziana and Rusophycus, common in siliciclastics throughout the Cambrian. Not only were deposit-feeding trilobites well-adapted to survival on typical Phanerozoic-style soft substrates, but they also contributed greatly to the Cambrian substrate revolution itself by bioturbating the upper few centimeters of the sediment. It seems clear, therefore, from this paleoecological analysis that mobile benthic metazoans adapted to both typical Proterozoic-style soft substrates and typical Phanerozoic-style soft substrates coexisted in the Chengjiang and Burgess Shale faunas, as would be expected during this time of dramatic siliciclastic soft substrate transition. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 173 Discussion The results of this research provide more evidence for the Cambrian substrate revolution hypothesis by upholding the three predictions of the hypothesis outlined earlier: 1) the Chengjiang and Burgess Shale faunas do contain benthic metazoans adapted to both substrate types; 2) the Burgess Shale fauna does have a greater number of benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than the Chengjiang fauna; and 3) there is some evidence for the beginning of mixed layer development in the Shiyantou Formation mudstones and siltstones, which underlie the rocks in which the soft-bodied fossil fauna is preserved. The results of this research also have broader implications for the early evolution and ecology of metazoans that warrant some discussion. First of all, this research, as well as previous work on the Cambrian substrate revolution, suggests that the seemingly strange appearance of many early metazoans, such as helicoplacoid echinoderms and Wiwaxia, may be partially attributed to the fact that they were well-adapted to non-actualistic environmental settings. Because modern subtidal siliciclastic shelf environments are not dominated by well-developed seafloor microbial mats but, instead, are characterized by intense bioturbation, similar metazoans with comparable lifestyles do not exist in such environments today. So rather Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 174 than these unusual forms being simply early evolutionary “experiments”, as is commonly stated, they were actually just well-adapted to environments that no longer exist in subtidal siliciclastic settings today. Another implication of this research is that the metazoan invasion of the infaunal realm appears to have been an important agent of natural selection on benthic metazoans during the “Cambrian explosion”. This infaunalization and the increased bioturbation it produced began to select for immobile benthic metazoans with attachment abilities and against those with free-living lifestyles, setting the stage for this continued trend through the Phanerozoic as well as the development of the Paleozoic Fauna, in which a diverse array of hard substrate attachers, particularly crinoids, played an important role (Sprinkle and Guensburg, 1995). Because benthic metazoans were forced to adapt to this soft substrate transition, the Cambrian substrate revolution helped to fuel the adaptive radiation of metazoans during the “Cambrian explosion”. Conclusions The core analysis portion of this research reveals that the Lower Cambrian Shiyantou Formation mudstones and siltstones are characterized by moderate levels (avg. ii = 2.87) of horizontal bioturbation preserved as Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 175 oval cross sections less than 5 mm in width, and also commonly contain the horizontal feeding trace fossil Teichichnus. Evidence for the beginnings of mixed layer development is also found in these rocks, as predicted by the Cambrian substrate revolution hypothesis. The Shiyantou Formation mudstones and siltstones are dominated by ii3, but a wide range of bioturbation levels are also present in this unit. This Shiyantou Formation bioturbation data indicates that the substrate represented by these rocks was a transitional mixture of typical Proterozoic-style soft substrates and typical Phanerozoic-style soft substrates. In contrast, the mudstones and siltstones of the Maotianshan Shale Member, which contains the soft-bodied fauna, contain almost no evidence for bioturbation of any kind (avg. ii = 1.01). This lack of bioturbation may indicate that whatever environmental conditions led to the exceptional preservation of the Chengjiang fauna also suppressed bioturbation levels. In addition, small (sub-millimeter), elongate, and irregular black mudstone intraclasts in thin (up to 2 cm thick) graded beds in the Maotianshan Shale Member provide suggestive evidence for the presence of seafloor microbial mats because of their combination of cohesiveness and flexibility (Schieber, 1999). This data from the mudstones and siltstones of the Yuanshan Member indicates that the fine-grained substrate on which the Chengjiang Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 176 fauna lived was relatively firm and likely contained seafloor microbial mats. In other words, it was a typical Proterozoic-style soft substrate. Adaptive morphological and paleoecological analysis indicates that Chengjiang fauna benthic suspension feeders are dominated by genera well adapted to survival on typical Proterozoic-style soft substrates (88%), while only two genera (12%) are well adapted to typical Phanerozoic-style soft substrates. These numbers indicate that the Cambrian substrate revolution was only in its early stages when the Early Cambrian Chengjiang fauna was preserved. Burgess Shale fauna benthic suspension feeders, meanwhile, contain a majority of genera well adapted to typical Proterozoic-style soft substrates (62%), but also contain a significant number (38%) of genera well adapted to typical Phanerozoic-style soft substrates. Although no direct data exists, these numbers suggest that the soft substrate on which the Burgess Shale fauna lived was a transitional mosaic of typical Proterozoic-style soft substrates and typical Phanerozoic-style soft substrates. Although preserved in different environments, a comparison of Chengjiang and Burgess Shale benthic suspension feeders suggests that mixed layer development was more advanced in the substrate on which the Burgess Shale fauna lived. Increased mixed layer development in Middle Cambrian shelf muds, on which the Burgess Shale fauna lived, when Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 177 compared to Early Cambrian shallow marine muds provides more evidence for increasing mixed layer development in subtidal siliciclastic soft substrate environments through the Cambrian. In addition, as predicted by the Cambrian substrate revolution hypothesis, the Burgess Shale fauna contains a greater number of benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than the Chengjiang fauna. Furthermore, mobile benthic metazoans adapted to both typical Proterozoic-style soft substrates and Phanerozoic-style soft substrates co-existed in the Chengjiang and Burgess Shale faunas. Broader implications of this research include the probability that many early benthic metazoans may appear so unusual simply because they were well adapted to non-actualistic environments, not because they were just early evolutionary “experiments”. In addition, the Cambrian substrate revolution forced Cambrian benthic metazoans to adapt to mixed layer development in siliciclastic soft substrate environments, thereby helping to fuel the “Cambrian explosion”. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 178 CHAPTER 6: Conclusions Utilizing the invaluable information provided by some of the most spectacular fossil Lagerstatten known in the world, this dissertation has shed light on a number of important and interesting questions regarding both the preservation of the earliest fossil evidence for animals and the evolutionary paleoecology of some of the earliest recognizable adult animals. In Chapter 2 a combination of field observations and petrography indicates that there are two distinct lithofacies within the Weng’an Phosphorite Member of the Neoproterozoic Doushantuo Formation of southwest China: a black facies beneath a gray facies. Within each of these facies, phosphogenesis and phosphatization take place under different environmental conditions. Less reworking was involved in the deposition of the black facies, and dolomite is not present in the lower 2 m of this facies, which is rich in organic material. The gray facies is characterized by more abundant dolomite, primarily as a matrix for phosphoclasts, and greater levels of reworking. The transition from the black to gray facies involves increasing dolomite levels and decreasing organic material levels, and is consistent with the stratigraphic context of the Weng’an Member being directly above a subaerial exposure surface. The black facies, tentatively interpreted as a shallow marine, perhaps lagoonal, deposit, was deposited as sea level was just beginning to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 179 rise again, and the environment rapidly deepened upward into the gray facies, before shallowing upward through the rest of the member. Overall, these results suggest that organic remains had a higher preservation potential when buried in the environment represented by the black facies because of the lower levels of reworking in this setting. The strong reworking of the gray facies, on the other hand, probably resulted in a strong taphonomic bias toward fossils that could withstand frequent reworking and redeposition, such as globular sponge eggs and embryos. Perhaps this explains why many of the new potential fossils described from the Doushantuo Formation are found in this lower black facies (i.e. Chen et al., 2002). In Chapter 3 a strong taphonomic bias toward earlier (2 and 4-cell) cleavage stages in the animal embryos of the Neoproterozoic Doushantuo Formation of southwest China is revealed. Later cleavage stages (8 and 16- cell) typically show more evidence for organic decay than, and are also less common than, earlier cleavage stages. There also appears to be a strong taphonomic bias against the eventual adult forms of these embryos, as well as cleavage stages beyond the 16-cell stage. It is hypothesized that one explanation for this taphonomic trend is that later cleavage stages and adults were more physically delicate than these more robust earlier cleavage stages, especially when cleavage planes are considered as planes of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180 weakness in the embryo or larva. The high-yolk content of these large embryos probably also contributed to their spectacular preservation by serving as an internal source of dissolved phosphate. These results begin to delineate the taphonomic constraints in the phosphatization process as it occurred in the Doushantuo Formation, and may explain the lack of certain types of fossils, such as adult forms, in this deposit. As discussed in Chapter 4, the trace fossils of the Lower Cambrian Meishucun Formation of southwest China, which is stratigraphically adjacent to the Lower Cambrian Chengjiang fauna, provide evidence for metazoan behavior and paleoecology that centered around seafloor microbial mats, the presence of which is suggested by both field and petrographic evidence. The paleoecology of these metazoans is comparable to typical Neoproterozoic lifestyles associated with seafloor microbial mats (Seilacher 1999). The presence of these lifestyles in a subtidal Early Cambrian environment indicates that the increase in bioturbation levels characteristic of the Proterozoic-Phanerozoic transition was spatially and temporally variable, resulting in a mosaic of soft substrate types during much of the Cambrian. As a result, the effect of this Proterozoic-Phanerozoic increase in bioturbation levels on the paleoecology of non-burrowing benthic metazoans, termed the Cambrian substrate revolution (Bottjer et al. 2000), was also highly variable, with benthic metazoans adapted to typical Neoproterozoic-style soft Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181 substrates coexisting with those adapted to typical Phanerozoic-style soft substrates. Chapter 5 focused on the evolutionary paleoecology of the benthic metazoans preserved in the Early Cambrian Chengjiang fauna and the Middle Cambrian Burgess Shale fauna, arguably the two most spectacular and significant marine Lagerstatten of the Cambrian. The core analysis portion of this research reveals that the Lower Cambrian Shiyantou Formation mudstones and siltstones, which do not contain the Chengjiang fauna, are characterized by moderate levels (avg. ii = 2.87) of horizontal bioturbation preserved as oval cross sections less than 5 mm in width, and also commonly contain the horizontal feeding trace fossil Teichichnus. Evidence for the beginnings of mixed layer development is also found in these rocks, as predicted by the Cambrian substrate revolution hypothesis. The Shiyantou Formation mudstones and siltstones are dominated by ii3, but a wide range of bioturbation levels are also present in this unit. This Shiyantou Formation bioturbation data indicates that the substrate represented by these rocks was a transitional mixture of typical Proterozoic- style soft substrates and typical Phanerozoic-style soft substrates. In contrast, the mudstones and siltstones of the Maotianshan Shale Member, which contains the soft-bodied Chengjiang fauna, contain almost no evidence for bioturbation of any kind (avg. ii = 1.01). This lack of bioturbation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 182 may indicate that whatever environmental conditions led to the exceptional preservation of the Chengjiang fauna also suppressed bioturbation levels. In addition, small (sub-millimeter), elongate, and irregular black mudstone intraclasts in thin (up to 2 cm thick) graded beds in the Maotianshan Shale Member provide suggestive evidence for the presence of seafloor microbial mats because of their combination of cohesiveness and flexibility (Schieber, 1999). This data from the mudstones and siltstones of the Yuanshan Member indicates that the fine-grained substrate on which the Chengjiang fauna lived was relatively firm and likely contained seafloor microbial mats. In other words, it was a typical Proterozoic-style soft substrate. Adaptive morphological and paleoecological analysis indicates that Chengjiang fauna benthic suspension feeders are dominated by genera well adapted to survival on typical Proterozoic-style soft substrates (88%), while only two genera (12%) are well adapted to typical Phanerozoic-style soft substrates. These numbers indicate that the Cambrian substrate revolution was only in its early stages when the Early Cambrian Chengjiang fauna was preserved. Burgess Shale fauna benthic suspension feeders, meanwhile, contain a majority of genera well adapted to typical Proterozoic-style soft substrates (62%), but also contain a significant number (38%) of genera well adapted to typical Phanerozoic-style soft substrates. Although no direct data exists, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 183 these numbers suggest that the soft substrate on which the Burgess Shale fauna lived was a transitional mosaic of typical Proterozoic-style soft substrates and typical Phanerozoic-style soft substrates. Although preserved in different environments, a comparison of Chengjiang and Burgess Shale benthic suspension feeders suggests that mixed layer development was more advanced in the substrate on which the Burgess Shale fauna lived. Increased mixed layer development in Middle Cambrian shelf muds, on which the Burgess Shale fauna lived, when compared to Early Cambrian shallow marine muds provides more evidence for increasing mixed layer development in subtidal siliciclastic soft substrate environments through the Cambrian. In addition, as predicted by the Cambrian substrate revolution hypothesis, the Burgess Shale fauna contains a greater number of benthic suspension feeding genera adapted to typical Phanerozoic-style soft substrates than the Chengjiang fauna. Furthermore, mobile benthic metazoans adapted to both typical Proterozoic-style soft substrates and Phanerozoic-style soft substrates co-existed in the Chengjiang and Burgess Shale faunas. Broader implications of this research include the probability that many early benthic metazoans may appear so unusual simply because they were well adapted to non-actualistic environments, not because they were just early evolutionary “experiments”. In addition, the Cambrian substrate Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. revolution forced Cambrian benthic metazoans to adapt to mixed layer development in siliciclastic soft substrate environments, thereby helping to fuel the “Cambrian explosion”. 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Further reproduction prohibited without permission. 197 Xiao, S., 2002, Mitotic topologies and mechanics of Neoproterozoic algae and animal embryos. Paleobiology, v. 28, p. 244-250. Xiao, S. and Knoll, A.H., 1999, Fossil preservation in the Neoproterozoic Doushantuo phosphorite Lagerstatte, South China. Lethaia, v. 32, p. 219-240. Xiao S. and Knoll, A.H., 2000, Phosphatized animal embryos from the Neoproterozoic Doushantuo Formation at Weng'an, Guizhou, South China. Journal of Paleontology, v. 74, p. 767-788. Xiao, S., Zhang, Y., and Knoll, A.H., 1998, Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite. Nature, v. 391, p. 553-558. Xiao, S., Yuan, X., and Knoll, A.H., 2000, Eumetazoan fossils in terminal Proterozoic phosphorites? Proceedings of the National Academy of Science USA, v. 97, p. 13684-13689. Yin, L., and Li, Z., 1978, Precambrian microfloras of southwest China with reference to their stratigraphic significance. 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Further reproduction prohibited without permission. 198 Zhang, Z., 1981, A new Oscillatoriaceae-like filamentous microfossil from the Sinian (late Precambrian) of western Hubei Province, China. Geological Magazine, v. 118, p. 201-206. Zhu, M.Y., 1997, Trace fossils of Yunnan: Bulletin of the National Museum of Natural Science, v. 10, p. 275-312. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 199 Appendix A. Doushantuo Formation thin section data. Sample names containing “BSK” are from Baishakan Quarry near Weng’an, Guizhou Province, China. Sample names containing “WSN” are from Wusi Quarry near Weng’an, Guizhou Province, China. Sample names containing “NB” are from Nanbao Quarry near Weng’an, Guizhou Province, China. The numbers in the sample names indicate the stratigraphic range in the Weng’an Member from which the sample is derived. The precise stratigraphic location of each sample with these ranges is unknown. The capital letter at the end of the sample name is simply used to differentiate multiple samples from within the same stratigraphic interval. Sorting was estimated using the scale of Compton (1985), and rounding was estimated using the scale of Powers (1953). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D oushantuo F o rm a tio n T h in Section D a ta Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 0-.15 A Phosphate w/ abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color Subrounded to well rounded Poorly sorted Grain supported Sub-mm laminae w/ phosphatic crusts; thick (>3mm) beds of bladed pyrite; pyrite often replaces phos phatic matrix 0 BSK 0-.15B Phosphate w / abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color; rare because dominated by amorphous phosphate Subrounded to well rounded Poorly sorted Grain supported Dominated by amorphous phosphate matrix w/ secondary pyrite distributed horizontally as crystals or blobs 0 BSK 0-.15 C Phosphate w / abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color Subrounded to well rounded Poorly sorted Grain supported Sub-mm laminae w/ phosphatic crusts; pyrite often replaces phosphatic matrix 0 BSK 0-.15 D Phosphate w / abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color; rare because dominated by amorphous phosphate Subrounded to well rounded Poorly sorted Grain supported Dominated by amorphous phosphate matrix w/ secondary pyrite distributed horizontally as crystals or blobs n 200 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D o ushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite B S K0-.15E Phosphate w / abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color Subrounded to well rounded Poorly sorted Grain supported Sub-mm laminae w/ phosphatic crusts; pyrite often replaces phosphatic matrix 0 BSK0-.15 F Phosphate w / abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color; rare because dominated by amorphous phosphate Subrounded to well rounded Poorly sorted Grain supported Dominated by amorphous phosphate matrix w/ secondary pyrite distributed horizontally as crystals or blobs 0 BSK0-.15 G Phosphate w / abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color; rare because dominated by amorphous phosphate Subrounded to well rounded Poorly sorted Grain supported Dominated by amorphous phosphate matrix w / secondary pyrite distributed horizontally, as crystals or blobs 0 B S K 0-.15H Phosphate w/ abun dant secondary pyrite; phosphate black to dark orange in color Phosphate - Fine sand to granular, dark orange to black in color Subrounded to well rounded Poorly sorted Grain supported Sub-mm laminae w/ phosphatic crusts; pyrite often replaces phosphatic matrix 0 o Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D oushantuo F o rm a tio n T h in Section D ata (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK .15-1 A Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - V. fine sand to pebbles; dark orange to black in color Subrounded to rounded Poorly- V. poorly sorted Grain supported Lenticular coarse beds in silty/clay matrix; larger (-2cm ) pebbles consisting of other smaller phosphoclasts present n BSK .15-1 B Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - V . fine sand to granular; dark orange to black in color Subrounded to rounded Poorly- V . poorly sorted Grain supported Lenticular coarse beds in silty/clay matrix n BSK .15-1 C Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - V. fine sand to granular; dark orange to black in color Subrounded to rounded Poorly- V . poorly sorted Grain supported Lenticular coarse beds in silty/clay matrix 0 B S K .15-/ D Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - V, fine sand to granular; dark orange to black in color Subrounded to rounded Poorly- V . poorly sorted Grain supported Lenticular coarse beds in silty/clay matrix 0 202 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D o ushantuo F o rm atio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK .15-1 E Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - V . fine sand to granular; dark orange to black in color Subrounded to rounded Poorly- V . poorly sorted Grain supported Lenticular coarse beds in silty/clay matrix n BSK K A Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - Fine sand to granular; dark orange to black in color Subrounded Poorly- V . poorly sorted Grain supported Lenticular beds I -2 mm thick in silt/clay matrix 0 BSK K B Phosphate - silt/clay sized w / some quartz; phosphate dark orange to black in color Phosphate - Fine sand to granular; dark orange to black in color Subrounded Poorly sorted Grain supported Lenticular beds 1-2 mm thick in silt/clay matrix 0 BSK K C Phosphate - silt/clay sized w/ some quartz; phosphate dark orange to black in color Phosphate - Fine sand to granular; dark orange to black in color Subrounded Poorly sorted Grain supported Lenticular beds (-2 mm thick in silt/clay matrix 0 IV) o 0 0 Reproduced w ith permission o f th e copyright owner. 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D oush an tuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting | Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 1-2 Phosphate - silt/clay; dark orange to black in color Phosphate - Find sand to granular; dark orange to black in color Subrounded to rounded V . poorly sorted Grain supported Lenticular beds of coarser grains in silt/ clay matrix 0 BSK 2-3 A Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V . poorly sorted Matrix supported, except for coarser lenses Thin (<lm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 20 BSK 2-3 B Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V. p o o rly sorted Matrix supported, except for coarser lenses Thin (clm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 45 BSK 2-3 C Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V . poorly sorted Matrix supported, except for coarser lenses Thin (clm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 50 204 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 2-3 D Dolomite Phosphate - Very Fine sand to granular Subrounded Poorly to V . poorly sorted Matrix supported, except for coarser lenses Thin (clm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 70 BSK 2-3 E Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V . poorly sorted Matrix supported, except for coarser lenses Thin (<lm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 30 BSK 2-3 F Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V . poorly sorted Matrix supported, except for coarser lenses Thin (clm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 35 BSK 2-3 G Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V . poorly sorted Matrix supported, except for coarser lenses Thin (<lm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 10 205 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 2-3 H Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V. poorly sorted Matrix supported, except for coarser lenses Thin (clm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 20 BSK 2-3 T Dolomite Phosphate - Very fine sand to granular Subrounded Poorly to V. poorly sorted Matrix supported, except for coarser lenses Thin (<lm m ) phosphatic laminae w/ dolomite in between, as well as coarser lenticular beds 20 WSN 4-5 A Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Weil rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible, but fine grained dolomite between some beds that shows cohesive and flexible behavior 60 WSN 4-5 B Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible- 70 206 Reproduced w ith permission o f th e copyright owner. 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D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section | Matrix Composition | Clast Composition & Size L Rounding , Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite W SN 4-5 C Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible 65 W SN 4-5 D Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible 65 WSN 4-5 E Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible 60 W SN 4-5 F Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible 70 207 Reproduced w ith permission o f th e copyright owner. 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D oush an tuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite W SN 4-5 G Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible 65 W SN 4-5 H Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in denser beds of phosphoclasts Generally no bedding visible 60 W SN 5-5.5 A Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Occasional phosphatic crusts con taining phosphoclasts between beds; extensive destructive dolomitization 75 W SN 5-5.5 B Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Generally no bedding visible; extensive destructive dolomitization 90 208 Reproduced w ith permission o f th e copyright owner. 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D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite WSN 5-5.5 C Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported Generally no bedding visible; extensive destructive dolomitization 85 WSN 5-5.5 D Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported Occasional phosphatic crusts con taining phosphoclasts between beds; extensive destructive dolomitization 80 BSK 5.3-6 A Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Welt sorted Matrix supported Occasional phosphatic crusts con taining phosphoclasts between beds 0 (phosphatic crust) BSK 5.3-6 B Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported G enerally no bedding visible 65 209 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D oushantuo F o rm a tio n T h in Section D a ta (C o n t’d) Thin Section ! Matrix Composition Clast Composition & Size | Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 5.3-6 C Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Generally no bedding visible 70 BSK 5.3-6 D Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Occasional phosphatic crusts con taining phosphoclasts between beds 65 BSK 5.3-6 E Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Generally no bedding visible 70 BSK 5.3-6 F Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Generally no bedding visible 75 I\3 o Reproduced w ith permission o f th e copyright owner. 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D oushanttio F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 5.3-6 G Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distigtrish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Generally no bedding visible 70 BSK 6.5-7 A Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported Destructive dolomitization common; dolomitized remnants of phosphatic crusts present 95 BSK 6.5-7 B Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Destructive dolomitization common; dolomitized remnants of phosphatic crusts present 80 BSK 6.5-7 C Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Destructive dolomitization common; dolomitized remnants of phosphatic crusts present 80 Reproduced w ith permission o f th e copyright owner. 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D oush an tuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 6.5-7 D Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported Destructive dolomitization common; dolomitized remnants of phosphatic crusts present 85 BSK 7.5-8 A Dominated by fine grained dolomite w / clotted fabric; "shadows" of phosphoclasts present in dolomite A few phosphoclasts of typical coarse sand size and phosphatic crusts present Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Clotted dolomitic fabric dominant 100 BSK 7.5-8 B Dominated by fine grained dolomite w/ clotted fabric; "shadows" of phosphoclasts present in dolomite A few phosphoclasts of typical coarse sand size and phosphatic crusts present Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Clotted dolomitic fabric dominant 100 BSK 7.5-8 C Dominated by fine- grained dolomite w/ clotted fabric; "shadows" of phosphoclasts present in dolomite A few phosphoclasts of typical coarse sand size and phosphatic crusts present Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Clotted dolomitic fabric dominant 100 212 Reproduced w ith permission o f th e copyright owner. 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D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding^ Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 7.5-8 D Dominated by fine grained dolomite w / clotted fabric; "shadows” of phosphoclasts present in dolomite A few phosphoclasts of typical coarse sand size and phosphatic crusts present Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Clotted dolomitic fabric dominant 100 BSK 7.5-8 E Dominated by fine grained dolomite w/ clotted fabric; "shadows" of phosphoclasts present in dolomite A few phosphoclasts of typical coarse sand size and phosphatic crusts present W ell rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Clotted dolomitic fabric dominant; partially dolomitized phosphatic crusts 70 BSK 7.5-8 F Dominated by fine grained dolomite w/ clotted fabric; "shadows" of phosphoclasts present in dolomite A few phosphoclasts of typical coarse sand size and phosphatic crusts present Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Clotted dolomitic fabric dominant 99 NB 7.S-7.8 A Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported,but grain supported in phosphatic crusts Heavily dolomitized 75 213 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission D oush an tuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite NB 7.5-7.8 B- Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Heavily dolomitized, but some remnants of phosphatic crusts present 70 N B 7.5-7.8C Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Heavily dolomitized, but some remnants of phosphatic crusts present 65 NB 7.5-7.8 D Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Heavily dolomitized, but some remnants of phosphatic crusts present 65 NB 7.5-7.8 E Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Heavily dolomitized, but some remnants of phosphatic crusts present 75 214 Reproduced w ith permission o f th e copyright owner. 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D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite N B 7.5-7.8F Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Heavily dolomitized, but some remnants of phosphatic crusts present 95 NB 7.5-7.8 G Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Heavily dolomitized, but some remnants of phosphatic crusts present 90 BSK 8.5-9 Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but sometimes grain supported None observed; destructive dolomitization very common around rims of phosphoclasts 75 BSK 9-9.5 A Dolomite' Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) W ell rounded Well sorted Matrix supported, but grain supported in phosphatic crusts None observed 85 215 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. D oushantuo F o rm a tio n T h in Section D a ta (C o n t'd ) Thin Section Matrix Composition Clast Composition & Size Rounding Sorting Grain/Matrix Support Bedding/Sed Structures % Dolomite BSK 9-9.5 B Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Remnants of phosphatic crusts preserved, but heavily dolomitized 25 (phosphatic crust) BSK 9-9.5 C Dolomite Phosphate - Dominated by coarse sand; phosphate nearly colorless (hard to distiguish from dolomite in plane-polarized light) Well rounded Well sorted Matrix supported, but grain supported in phosphatic crusts Remnants of phosphatic crusts preserved, but heavily dolomitized 85 216 217 Appendix B. Shiyantuo Formation core sample data. All samples from the Shiyantuo Formation excluding those from within the Maotianshan Shale Member are included in this Appendix. Ichnofabric index (ii) was determined using the methodology of Droser and Bottjer (1986). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shiyantuo Formation Core Slab Data Sample #J Thickness _[ ii 1 Description 1A 5.3 cm 3.5 cm of ii3 1.8 cm of ii4 Greenish-gray siltstone with mottled tan interbeds containing the horizontal feeding trace Teichichnus. IB 6.3 cm 3.5 cm of ii2 2.8 cm of ii3 Dark gray siltstone with several mottled intervals. lC(a) 4.6 cm 0.6 cm of ii2 3.0 cm of ii4 1.0 cm of ii5 Greenish-gray siltstone with tan interbeds. Discrete Teichichnus burrows present in the tan interbeds. Bioturbation highly variable, with mottled intervals occuring in discrete lenses. Possible incipient development of mixed layer in 1.0 cm thick interval. lC(b) 2.6 cm 1.9 cm of ii2 0.7 cm of ii3 Gray siltstone with horizontal bioturbation seen as ovals 2-3 mm wide and "microtraces" < 1 mm in diameter. lD(a) 5.6 cm 3.8 cm of ii2 1.8 cm of ii3 Dark gray siltstone containing one interval of "microtraces" < 1 mm in diameter. lD(b) 4.3 cm ii3 Dark gray siltstone bioturbated by small light gray ovals < 2 mm wide. IF 3.9 cm 1.0 cm of ii2 2.6 cm of ii4 0.3 cm of ii5 Greenish-gray siltstone with tan interbeds. Discrete Teichichnus burrows present in the tan interbeds. Bioturbation highly variable, with mottled intervals occuring in discrete lenses. Possible incipient development of mixed layer in 0.3 cm thick interval. 1G 3.2 cm 2.8 cm of ii2 0.4 cm of ii3 Gray siltstone with occasional mottling. Brief interval (0.4 cm) of "microtraces" < 1 mm in diameter. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shiyantuo Formation Core Slab Data (Cont'd) Sample # Thickness ii | Description 1H 7.3 cm 5.0 cm of ii3 1.1 cm of ii4 1.2 cm of ii5 Laminated greenish-gray siltstone with discrete tan Teichichnus burrows as well as mottling. Possible incipient development of mixed layer in 1.2 cm thick interval. 2 10.5 cm ii2 Dark gray siltstone with some ovals < 5 mm wide and "microtraces" < 1 mm in diameter. 3A 10.3 cm ii3 Greenish-gray siltstone with thin tan interbeds. Interbeds are mottled and tan ovals < 5 mm wide are also present. 3B 5.8 cm ii3 Greenish-gray siltstone with tan interbeds mottled by "microtraces" < 1 mm in diameter. 3C 10.7 cm iil Tan, thin-bedded fine ss. 4A 19.5 cm iil Tan, thin-bedded fine ss. 4B 12.3 cm iil Tan, thin-bedded fine ss. 4C 9.5 cm iil Tan, thin-bedded fine ss. 5A 7.5 cm iil Tan, thin-bedded, cross-bedded, fine ss. 5B 8.1 cm iil Tan, thin-bedded fine ss. 5C 12.2 cm iil Tan, thin-bedded fine ss. 5D 11.5 cm iil Tan, thin-bedded fine ss. 6A 20.9 cm iil Gray, cross-bedded very fine ss. 6B 13.9 cm iil Gray, laminated very fine ss with "fuzzy" laminae possbily due to microbioturbation by meiofauna. 6C 18.4 cm iil Gray, cross-bedded very fine ss. 6D 21.6 cm iil Gray, cross-bedded very fine ss. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shiyantuo Formation Core Slab Data (Cont'd) Sample # Thickness > i 1 Description 6E 24.4 cm iil Gray, cross-bedded very fine ss. 7A 10.3 cm 5.7 cm of iil 4.6 cm of ii4 Unbioturbated interval on top of horizontally bioturbated siltstone. Bioturbation seen as ovals < 4 mm wide filled with gray sediment and mottling. Some original lamination preserved. 7B 6.6 cm iil Lower 2.8 cm laminated with massive dark siltstone above. 7C 4.6 cm 3.8 cm of iil 0.8 cm of ii2 Cross-bedded siltstone, lowermost 0.8 cm contains a few horizontal burrows. 7D 9.8 cm iil Gray, thin-bedded siltstone. 7E 11.8 cm 6.6 cm of iil 5.2 cm of ii4 Lowermost 6.6 cm is gray, laminated very fine ss. Uppermost 5.2 cm is, heavily bioturbated silty interval. This interval contains vertical burrows 1.7 to 0.4 cm in depth and 0.2 to 0.3 cm in width. These burrows are accompanied by mottled beds and gray ovals < 3 mm wide. 8A 7.4 cm 1.2 cm of ii2 6.2 cm of ii3 Dark gray siltstone with lighter gray beds of very fine ss. Siltstone rich in "microtraces" < 1 mm in diameter. Sandstone beds contain ovals < 3 mm wide. 8B 9.2 cm 0.7 cm of ii4 8.5 cm of ii5 Dark gray siltstone heavily bioturbated by gray ovals < 5 mm wide and mottling. Bedding somewhat visible in 0.7 cm. 8C 8.8 cm ii3 Mottled siltstone with horizontal bioturbation seen as ovals < 4 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shiyantuo Formation Core Slab Data (Cont'd) Sample # Thickness ii | Description 8D 8.5 cm ii3 Dark gray siltstone mottled by gray ovals < 4 mm wide. Some original bedding preserved. One possible vertical trace 7 mm in depth. 9A 5.2 cm 1.2 cm of ii2 4.0 cm of ii3 Dark gray siltstone with lighter gray beds of very fine ss. Siltstone rich in "microtraces" < 1 mm in diameter. Sandstone beds contain ovals < 3 mm wide. 9B 8.6 cm 4.1 cm of iil 0.8 cm of ii2 3.7 cm of ii5 Cross-bedded very fine ss with dark laminae and 0.8 cm interval of ii2. Part of black siltstone lens present and heavily bioturbated horizontally. Original bedding completely destroyed in this lens. 9C 8.4 cm iil Gray, cross-bedded very fine ss with darker silty laminae. 9D 4.4 cm iil Cross-bedded sandy siltstone. 9E 3.8 cm 0.8 cm of ii2 3.0 cm of ii5 Dark gray siltstone with gray ss interbed. Dark siltstone completely bioturbated by gray ovals < 5 mm wide. 10A 5.2 cm 5.1 cm of iil 0.1 cm of ii2 Black, laminated siltstone with 0.1 cm of slight bioturbation. 10B 6.1 cm ii3 Dark gray siltstone bioturbated by abundant "microtraces" < 1 mm in diameter. 11A 3.6 cm ii4 Dark gray siltstone bioturbated by abundant "microtraces" < 1 mm in diameter and larger gray ovals. 1 IB 6.3 cm 5.4 cm of ii3 0.9 cm of ii4 Dark gray siltstone. Horizontal bioturbation in ovals < 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 222 Shiyantuo Formation Core Slab Data (Cont'd) Sample # Thickness ii Description 19 16.8 cm ii3 Gray, thin-bedded siltstone. Bedding disturbed by oval horizontal burrows and mottling. Discrete traces rare, but mottled disturbance more common. Bedding is still distinguishable. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix C. Selected photographs of Shiyantou Formation core samples. All cores are 7 cm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. \e A '** '\OA z o S r i y ,\ O' flsr ,© < ■ A^et ^e v it® A u ■\\°a 0 A V \V A 0’ ,xA ? ,et«" \ s ^ 225 Sample 1A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 'e B e , P r O 0 , 060 * ith Perniis Sion 0fth e c°pyi ri9ht °H % er. ' ^ e r rePn ^ i o n Bro*tb/,, L 'a ^ ith Out Sample 1B Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 228 Sample 1C Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sample 1C Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ss, 7 H v n i er e r rePn 0a^«on D rc "v t> it, ea u,,- w « h o ut Porm;, sbn Sample 1H Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 232 Sample 7 A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sample 7 A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. > \e eP A^0 ® ' A^' o ^ e 30^’ A ^ ° & fvV ^ 0 < A^ ■i \ cA V v 0 A V # e ' A'*4 *A 0 ' a A? , e ^ ' • s ^ ' Sample 8D Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix D. Maotianshan Shale Member core sample data. Ichnofabric index (ii) was determined using the methodology of Droser and Bottjer (1986). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Moatianshan Shale Core Slab Data Sample # Thickness ii 1 Description 13A 9.5 cm 9.3 cm of iil 0.2 cm of ii2 Laminated and thin-bedded dark gray to black siltstone and mudstone. Gray ovals < 3 mm wide in brief interval. 13B 3.8 cm iil Thin-bedded dark gray mudstone. 13C 4.7 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. 13D 4.5 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. ]3E(a) 4.0 cm 3.6 cm of ii 1 0.4 cm of ii2 Thin-bedded dark gray mudstone with graded beds with black intraclasts near base (between 2-4 mm in size). Slight horizontal bioturbation evident as light gray ovals < 3 mm wide. 13E(b) 2.2 cm ii 2 Laminated and thin-bedded dark gray to black siltstone and mudstone. 15 A 7.8 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. Contains 1.1 cm thick event bed containing irregular black intraclasts. 15B 13.2 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. 35C 9.5 cm 9.3 cm of iil 0.2 cm of ii2 Laminated and thin-bedded dark gray to black siltstone and mudstone. Gray ovals < 3 mm wide in brief intervals. 16A 7.1 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. 16B 2.8 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. 16C 4.7 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 238 Moatianshan Shale Core Slab Data (Cont'd) Sample # Thickness ii 1 Description 16D 11.8 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. I6E 3.1 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. J6F 6.9 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. 17A 6.2 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. 17B 6.7 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. Contains 3.8 cm thick event bed containing irregular black intraclasts. 18 6.2 cm iil Laminated and thin-bedded dark gray to black siltstone and mudstone. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 239 Appendix E. Selected Photographs of Maotianshan Shale Member core samples. All cores are 7 cm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sample 13A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sample 13A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 242 Sample 13A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sample 15A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 244 Sample 15A Reproduced with permission of the copyright owner. Further reproduction prohibited without permission

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Creator Dornbos, Stephen Quinn (author)

Core Title Evolutionary paleoecology and taphonomy of the earliest animals: Evidence from the Neoproterozoic and Cambrian of southwest China

School Graduate School

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest,paleoecology,paleontology

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Bottjer, David (committee chair), Corsetti, Frank (committee member), Deonier, Richard (committee member), Douglas, Robert (committee member), Fischer, Alfred G. (committee member), Gorsline, Donn (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-361556

Unique identifier UC11339842

Identifier 3103882.pdf (filename),usctheses-c16-361556 (legacy record id)

Legacy Identifier 3103882.pdf

Dmrecord 361556

Document Type Dissertation

Rights Dornbos, Stephen Quinn

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA