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Helicoplacoid echinoderms: Paleoecology of Cambrian soft substrate immobile suspension feeders

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Helicoplacoid echinoderms: Paleoecology of Cambrian soft substrate immobile suspension feeders

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UM I films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UM I a complete manuscript and there are missing pages, these w ill be noted. Also, if unauthorized copyright material had to be removed, a note w ill indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6” x 9” black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UM I directly to order. ProQuest Information and Learning 300 North Zeeb Road. Ann Arbor, M l 48106-1346 USA 800-521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. HELICOPLACOID ECHINODERMS: PALEOECOLOGY OF CAMBRIAN SOFT SUBSTRATE IMMOBILE SUSPENSION FEEDERS by Stephen Quinn Dombos A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (Geological Sciences) December, 1999 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U M I Number: 1409626 ___ __® UMI U M I Microform 1409626 Copyright 2002 by ProQuest Information and Learning Company. A ll 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, M l 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY O F SOUTHERN CALIFORNIA THK GNAOUATS SCHOOL U N i v c i u i r r m m LOS ANGKLSS. CALIFORNIA ( 0 0 0 7 This thesis, written by Stephen Quinn Dornbos under the direction of h^x& JThens Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of _______Master of Science____________ Dt am T in t* November 23 , 1999 THESIS COMMITTEE . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS I would like to begin by thanking the sources of financial support for this research: the USC Department of Earth Sciences Graduate Student Research Fund, the Paleontological Society, the Geological Society of America, and the Wrigiey Institute for Environmental Studies. The generous financial support from the above sources made all phases of this research, from the field to the lab. possible. The support and friendship of past and present Paleolabbers was also extremely important to me and this research. Thank you Margaret Fraiser, Nicole Fraser, Whitey H agadom . Karina Hankins. Tran Huynh, Sara Pruss, Dave Rodland, Stephen Schellenberg. Karen Whittlesey, Adam Woods, and Kate Woods for smoothing my transition into grad school and for all the fun I’ve had in the process. Discussions with you have been invaluable to this thesis, as well as my life. Special thanks to Margaret for her caring and support in the past few months, as well as for being a great officemate and friend last year when the bulk of this research was being carried out. The Department of Earth Sciences here at USC has been extremely helpful at every step of my grad school career so far. In particular. Bob Douglas and Donn Gorsline, both members of my committee, have been instrumental in the development of both this thesis and me as a scientist. I'd also like to thank everyone else in the department who has supported me and my research so well. Endless thanks must be offered to my advisor Dave Bottjer, without whom none of this research would have been possible, nor half as enjoyable. The combination of your scientific insight, guidance, and sense of humor have made grad school both rewarding and most enjoyable. Thanks for being a great advisor and friend. I look forward to our next few years of working together, as well as one day being your colleague. Finally, and most importantly. I would like to thank my family for continually supporting me and my goals not only the past few years, but throughout my life. Your Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. love and support are always appreciated and help keep me headed in the right direction when little else seems to make any sense. I owe it all to you. Thank you. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS i v A cknow ledgem ents...................................................................................................... ii List of Figures......................................................................................................... v List o f Tables............................................................................................................. ix A bstract........................................................................................................................ x Introduction................................................................................................................... 1 The Chengjiang Fauna............................................................................................. 9 Past Research on Helicoplacoids............................................................................ 37 The Origin of the Echinoderm s.............................................................................. 40 Taphonomy of Echinoderm s................................................................................... 53 Bioturbation in Siliciclastics During the Precambrian-Cambrian Transition............... 62 Petrographic Evidence Suggestive of the Presence of Microbial Mats in Siliciclastics.. 76 Soft-Substrate Adaptations of Benthic Invertebrates.............................................. 85 Geologic Setting and Paleogeography.................................................................... 86 M ethods............................................................................................................................. 93 Results and D iscussion.............................................................................................. 101 Paleoecology and Paleoenvironm ent............................................................ 101 T aphonom y......................................................................................................... 168 The Early Cambrian Extinction Event......................................................... 180 Lower Cambrian Echinoderm Plate Beds.................................................... 185 Modem Soft-Substrate Suspension Feeders................................................ 187 Other Cambrian Fauna Suspension-Feeding Benthos.................................. 196 Biological B ulldozing..................................................................................... 210 C onclusions..................................................................................................................... 219 R eferences....................................................................................................................... 222 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES v Figure Page 1. Generalized reconstruction of a helicoplacoid in life position........................ 2 2. Regional stratigraphy of the White/Inyo Mountains with location map 4 3. Familial diversity of marine invertebrates through the Cambro-Ordovician 6 4. E o re d lic h ia ................................................................................................... 10 5. R e tifa c ie s.......................................................................................... 12 6. C in d a re lla ...................................................................................................... 14 7. A n o m a lo c a ris................................................................................................ 16 8. C h o ia ................................................................................................... 19 9. X ia n g u a n g ia .................................................................................................. 21 10. D in o m isc h u s ................................................................................................... 23 11. M o a tia n sh a n ia .............................................................................................. 25 12. H a llu c ig e n ia ...................................................................................... 27 13. M ic ro d ic ty o n .................................................................................... 29 14. E ld o n ia ............................................................................................... 31 15. R o ta d isc u s ......................................................................................... 33 16. Y u n n a n o zo o n .................................................................................................. 35 17. A rk a ru a ............................................................................................................ 41 18. T rib ra ch id iu m ................................................................................... 43 19. Cambro-Ordovician echinoderm age range chart........................................ 45 20. Fortey et al.'s (1996) model of metazoan diversification............................. 49 21. Wray et al.’ s (1996) model of metazoan diversification................................ 51 22. Taphonomic grades of echinoderm s........................................................... 55 23. Distribution of different types of obrution deposits on the continental shelf..... 60 24. A) Average ichnofabric indices of Cambro-Ordovician shelf carbonates in North America; B) Ichnofabric percentages of Cambrian inner shelf siliciclastics 64 25. Photograph of typical wrinkle structures..................................................... 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26. Diagram displaying the shift in dominant factors influencing siiiciclastic sedimentary fabrics during the Precambrian-Cambrian transition.................. 69 27. Diagram showing the effects of increased bioturbation on biomat-reiated lifestyles during the Precambrian-Cambrian transition................................. 72 28. Tiering of soft-substrate suspension-feeding communities during the Phanerozoic.................................................................................................... 74 29. Schematic displaying the relationship between dissolved oxygen and composition o f the benthic fauna................................................................ 77 30. Photograph of wavy-crinkly carbonaceous laminae...................................... 80 31. Photograph of cohesive behavior in carbonaceous beds............................... 82 32. Diagram of sediment stickers employing the "iceberg" strategy...................... 87 33. Global Early Cambrian paleogeography........................................................ 90 34. Photograph of the new helicopiacoid locality............................................... 95 35. Photograph of an outcrop of the Middle Member of the Poleta Formation after sam pling.......................................................................................................... 97 36. Photograph of horizontal Planolites trace fossils in the Middle Member of the Poleta Form ation........................................................................................... 102 37. Photograph displaying the unbioturbated (ii 1) field appearance of the Middle Member of the Poleta Formation................................................................ 104 38. Photograph of a well-preserved helicopiacoid specimen............................... 106 39. Photograph of an in situ helicopiacoid specimen.......................................... 108 40. Photograph of an in situ helicopiacoid specimen.......................................... I l l 41. Generalized reconstruction of a helicopiacoid in life position with important morphological features labelled.................................................................. 113 42. Photograph of an Olenellid trilobite.............................................................. 116 43. Photograph of a bed of archaeocyathids....................................................... 118 44. Relative stratigraphic position of the six outcrops sampled........................... 121 45. Uppermost 25 cm of the Site B x-radiograph core...................................... 123 46. Interval of Site B x-radiograph core from 25 to 50.5 cm............................. 125 47. Interval of Site B x-radiograph core from 50.5 to 75 cm ............................. 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure v" Page 48. Interval of Site B x-radiograph core from 75 to 83.5 cm............................. 129 49. Uppermost 22.4 cm o f the Site C x-radiograph core................................... 135 50. Interval of Site C x-radiograph core from 22.4 to 31.8 cm.......................... 137 51. The Site D x-radiograph core..................................................................... 142 52. The Site E x-radiograph.core.................................................................... 145 53. The Site A x-radiograph core..................................................................... 148 54. The Site F x-radiograph core.................................................................... 151 55. Photograph of chloritized trilobite fragments in thin section.......................... 155 56. Photograph of an echinoderm plate bed in thin section................................. 157 57. Photograph of a slightly altered echinoderm plate in thin section.................. 159 58. Photograph of micaceous laminae in thin section........................................ 161 59. Photograph of a sand-mica intraclast in thin section.................................... 163 60. Photograph of a planar concentration of heavy minerals in thin section 166 6 1. Photograph of wrinkle structures collected at the new helicopiacoid locality.... 169 62. Photograph of wrinkle structures in thin section......................................... 171 63. Rose diagrams of the orientations of 33 helicopiacoid specimens................. 177 64. Generic diversity of reef biota through the Early Cambrian extinction event.... 181 65. Diagram of a typical bryozoan.................................................................... 189 66. Idealized cross-section of an Antedon bifida pinnule................................... 192 67. L ic h e n o id e s .................................................................................................... 197 68. Basal cup and individual plate of Cym bionites........................................... 199 69. Basal cup of P e rio d in ite s.......................................................................... 202 70. Gogia guntheri............................................................................................. 204 71. Idealized edrioasteroid............................. 207 72. A) Number of brachiopod genera in functional groups through the Phanerozoic; B) Percentage of brachiopod genera in functional groups through the P hanerozoic....................................................................................................... 212 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure v*" Page 73. A) Number of bivalve genera in functional groups through the Phanerozoic; B) Percentage of bivalve genera in functional groups through the P hanerozoic...................................................................................................... 214 74. A) Number of families of ail other skeletonized marine benthic invertebrates in functional groups through the Phanerozoic; B) Percentage of families of all other skeletonized marine benthic invertebrates in functional groups through the Phanerozoic............................................................................................... 216 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ix LIST OF TABLES Table Page 1. Percentage of helicopiacoid specimens in each taphonomic group................ 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT x Bioturbation in non-nearshore siliciclastic settings during the Proterozoic- Phanerozoic transition increased in depth and intensity (Droser. 19S7: Mcllroy and Logan. 1999), causing a change from the matgrounds characteristic of the Proterozoic. to the mixgrounds characteristic of the Phanerozoic (Seilacher. 1999). This increase in bioturbation would have softened the consistency of the substrate and blurred the sediment- water interface (Rhoads. 1970). having a severe impact on any benthic metazoans. particularly sessile suspension feeders, that were well-adapted for survival on relatively unbioturbated Proterozoic substrates. The unusual Early Cambrian helicopiacoid echinoderms, found in abundance in the shales of the Middle Member of the Poleta Formation in Westgard Pass of the White/Inyo Mountains, eastern central California, are an example o f one such metazoan. The examination of 107 new helicopiacoid specimens collected during this study, combined with field, x-radiographic, and petrographic studies o f the rocks in which they are preserved, indicate that helicoplacoids lived as mud stickers on a substrate that only underwent low- to moderate levels of strictly horizontal bioturbation. These bioturbation conditions would have created a- relatively firm substrate and sharp sediment-water interface, on which the helicoplacoids, based on their functional morphology, small size, and vulnerable life mode (Thayer, 1983), were dependent for survival. As bioturbation increased through the Cambrian in non-nearshore siliciclastic settings, therefore, it may have led to the extinction of the helicoplacoids. Other similarly adapted sessile suspension-feeding echinoderms, such as stemless eocrinoids. may have also been driven to extinction by the bioturbation-induced softening of siliciclastic substrates during the Proterozoic-Phanerozoic transition. Cambrian suspension-feeding echinoderms that could attach to hard substrates, meanwhile, such as edrioasteroids and possibly crinoids. survived to flourish as part of the Paleozoic fauna. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I INTRODUCTION Unique Cambrian echinoderms have long intrigued paleontologists. Perhaps the strangest are the helicoplacoids, small echinoderms covered with unusual helically-arranged columns of calcite plates and triradiate ambulacra that are found only in the Lower Cambrian of North America (Figure 1) (Durham, 1993). They occur most abundantly in the green to brown shales of the Middle Member of the Lower Cambrian Poleta Formation of Westgard Pass in the White-Inyo Mountains of eastern California (Figure 2) (Durham, 1993). Their appearance may seem bizarre, but, as this research will display, helicoplacoids were simply well-adapted for survival in conditions that no longer exist in normal shallow marine settings. Helicoplacoids are a part of the Cambrian fauna, but only a minor part (Sepkoski, 1981). The Cambrian fauna, as defined by Sepkoski (1981), radiated rapidly in the Early Cambrian and, afterward, reached a diversity plateau that lasted through the remainder of the Cambrian (Figure 3). This plateau crashed dramatically in the Early Ordovician and continued to decline until near the end of the Paleozoic, whereafter it existed at extremely low levels (Sepkoski, 1981). On the class level the Cambrian fauna is overwhelming dominated by the trilobites, which account for 77% of the fauna (Sepkoski, 1981). The trilobites are followed in abundance by the polychaetes, which alone account for 6% of the fauna, and the monoplacophorans, inarticulates, and hyoliths, which together account for 10% of the fauna (Sepkoski, 1981). Just 4% of the Cambrian fauna is accounted for by the conodonts, graptolites, pogonophorans, eocrinoids, merostomatans, scyphozoans, and malacostracans (Sepkoski, 1981). All of the remaining Cambrian fauna organisms, including helicoplacoids, account for the final 3% of the fauna (Sepkoski, 1981). Although Sepkoski (1981) does not Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1- Generalized reconstruction of a helicopiacoid echinoderm in life position based on fossil evidence. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 Interambulacral Columns Ambulacra 1 cm Mouth (?) Sediment-Water Interface Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 Figure 2- Regional stratigraphy of the White-Inyo Mountains, eastern central California; Occurrence of helicoplacoids marked by large arrow; Map showing location of Westgard Pass in California, as indicated by the black dot (stratigraphy based on Stewart, 1970; Nelson, 1976; and Corsetti and Kaufman, 1994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 u 2 Monola Fm Mule Spring Ls " S 3 Saline Valley Fm s .2 'u O J o .2 S c Harkless Fm • O E c 0 2 u h a u ■ S - 2 13 Poleta Fm u M L 0 1 C Q > z[ 2 1 ‘ E C /5 C 9 J O it Campito Fm M AM ----- U U Q - Deep Springs Fm M L CA 40*N- I20*W Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 Figure 3- Familial diversity of marine invertebrates through the Cambro-Ordovician; C- Cambrian Fauna; Pz- Paleozoic Fauna; Md- Modem Fauna (modified from Sepkoski, 1981). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 s 1 £ C m e I m 9 * J9 s 3 z Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 400 Md 300 200 100 Geologic Time 8 include them in his analysis because they have such an anomolous diversity pattern, the archaeocyathids were also a key organism is Cambrian ecosystems. The surviving rem nants o f the Cambrian fauna consist of a few families of inarticulates, monoplacophorans, and pogonophorans (Sepkoski, 1981). While previous workers (Durham and Caster, 1963; Durham, 1967, 1993; Derstler, 1982; Paul and Smith, 1984) studied the functional morphology and life mode of helicoplacoids, as well as classifying them, they never closely examined the rocks in which they were found, nor did they undertake a detailed taphonomic study of these unusual echinoderms. The goal of this research was to examine the rocks in which the helicoplacoids are preserved, through x-radiography and petrography, in order to characterize their paleoecology and the paleoenvironment in which they lived. The 107 helicopiacoid specimens recovered during this study from a new site in the Lower Cambrian Poleta Formation of Westgard Pass were also examined in order to search for further evidence of the helicopiacoid life mode and to perform a detailed taphonomic study. The results indicate that helicoplacoids lived as mud-stickers on a relatively firm substrate with comparatively low water content resulting from only minimal bioturbation by horizontal Planolites, and were preserved in mass-mortality obrution deposits. Given their small size (l-5cm) and unique functional morphology, it seems unlikely that helicoplacoids could have survived on more thoroughly bioturbated sediment, which would have had a blurred sediment-water interface and high water content. Increasing levels of vertical bioturbation in offshore siliciclastic settings during the remainder of the Cambrian, therefore, may have led to their extinction. It is likely that other members of the Cambrian Fauna, well adapted to the relatively unbioturbated conditions in which they had originally evolved, were also unable to adapt to increasing levels of bioturbation, and suffered the same fate as these early suspension-feeding echinoderms. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 THE CHENGJIANG FAUNA The Cambrian fauna (Sepkoski, 1981) described above contains only organisms with easily preservable mineralized skeletons. There were, however, many soft-bodied metazoans which lived during the Early Cambrian, and any consideration of the paleoecology during this time would be sorely incomplete without including these metazoans. Located in the Yunnan Province of China, the Chengjiang fauna (Hou and Sun, 1988), the most diverse and well-preserved soft-bodied fauna of the Early Cambrian , is clearly the ideal place to look in order to complete the paleoecological picture of the Early Cambrian. Arthropods dominate the Chengjiang fauna, comprising over 40% of the species (Hou et al., 1991), with only 4 genera (Chen et al„ 1996), all trilobites, possessing commonly preservable mineralized skeletal elements (Figure 4) (Hou et al., 1991). A wide array o f soft-bodied arthropods is found in the Changjiang fauna, including the Trilobitomorphs, such as Retifacies and Acanthomeridion, which resemble trilobites (Figure 5) (Hou et al., 1989). Many species of Naraoiids, Xanderellids, Tegopeltids, and Fuxianhuids, all arthropod groups, are also important components of the Chengjiang fauna (Figure 6) (Hou, 1987; Chen et al., 1996). Arthropods also comprise over 40% of the Middle Cambrian Burgess Shale fauna (Whittington, 1985). 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 which are also found in the Burgess Shale fauna (Figure 7) (Hou et al., 1995). Three genera of anomalocarids, A nom alocaris, Amplectobulua, and Peytoia, are found in the Chengjiang fauna (Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10 Figure 4- The trilobite Eoredlichia from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11 1 cm Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 Figure 5- The triiobitomorph Retifacies from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 Figure 6- The arthropod Cindarella from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 Figure 7- The nektonic predator Anomalocaris from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. r Further reproduction prohibited without perm ission. ■ „ thp rnnvriaht owner. Further lepiuuu R eproduced with perm ission of the copy g 18 Opabinids, another group of strange nektonic arthropod-like metazoans, are also found in the Chengjiang fauna, as well as the Burgess Shale fauna (Hou, 1987). The Chengjiang fauna also contains a diverse array of sponges and cnidarians. Ten genera of sponges, some of which, such as Choia, are also present in the Burgess shale fauna, are found in the Chengjiang fauna (Figure 8) (Chen et al., 1996). Meanwhile, just one undisputed cnidarian genus, Xianguangia, an anemone-like benthic suspension feeder, is present in the Chengjiang fauna (Figure 9) (Chen and Erdtmann, 1991). The enigmatic suspension feeder Dinomischus is also known from Chengjiang (Figure 10) (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 (Figure 11) (Chen et al., 1996). The well- known Burgess Shale lobopodian Hallucigenia is present, as are five previously unknown genera of these unusual creatures: Microdicry on, Onychodictyon, Cardiodictyon, Luolishania, and Paucipodia (Figure 12; Figure 13) (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 referrable 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 (Figure 14) (Walcott, 1911; Sun and Hou, 1987). Thought to be planktonic (Chen et al., 1995), Eldonia is accompanied by Rotadiscus, another medusiform (Figure 15) (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. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 Figure 8- The sponge Choia from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 Figure 9- The cnidarian Xianguangia from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sediment-Water Interface Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 Figure 10- The suspension-feeder Dinomischus, of unknown affinity, from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sediment-Water Interface F Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25 Figure 11- The priapuiid Maotianshania from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27 Figure 12- The lobopodian Hallucigenia from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29 Figure 13- The lobopodian Microdictyon from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31 Figure 14- The medusoid Eldonia from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33 Figure 15- The medusoid Rotadiscus from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 Figure 16- The chordate Yunnanozoon from the Chengjiang fauna (modified from Chen et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36 C '5 C Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37 Phoronids, brachiopods, hyolithes, 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 linguiids, complete with preserved pedicles (Chen et al., 1996). Finally, the Chengjiang fauna contains one chordate species, Yunnanozoon (Figure 16) (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). Because the Chenjiang fauna is found in the Upper Adtabanian (Qian and Bengston, 1989), as are helicoplacoids (Durham, 1993), many similar or even identical metazoans as those in the Chengjiang fauna were probably living in association with helicoplacoids. The explanation for why helicoplacoids are not, therefore, found in the Chengjiang fauna is unknown at this time. Possible explanations could include that the depositional environment of the Middle Member of the Poleta Formation, where helicoplacoids are preserved, is different than that of the Maotianshan Shale of the Yuanshan Formation, in which the Chengjiang fauna is found (Hou et al., 1991). Pending further Field work, the Chengjiang fauna preliminarily appears to have been preserved in shallower water than the helicoplacoids of the Middle Member of the Poleta Formation (Moore, 1976a; Hou et al., 1991). Helicoplacoids may also be absent from the Chengjiang fauna simply because they were endemic to the northern coast of Laurentia, now western North America. PAST RESEARCH ON HELICOPLACOIDS Durham and Caster (1963) first described helicoplacoids, but it was not until 1967 that Durham discussed in detail helicoplacoid preservation, functional morphology, and classification. In this article, Durham (1967) also considered the diversity of Early Cambrian echinoderms. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38 Durham (1967) observed that by the Early Cambrian echinoderms had already diversified considerably, leading to the conclusion that echinoderms began evolving and diversifying sometime in the Precambrian. The helicoplacoids, eocrinoids, edrioasteroids, and two other echinoderm classes, unnamed at the time of this publication, already were present by the Early Cambrian (Durham, 1967). Helicoplacoids are relatively abundant at the Westgard Pass locality, and most of the helicoplacoid specimens with plates still in life position are found as external molds beneath graded shale beds, indicating somewhat rapid burial (Durham, 1967). Disassociated plates are the most common mode of helicoplacoid preservation, and these plates can sometimes accumulate to form bioclastic limestone lenses (Durham, 1967). The next subject Durham (1967) discussed is the water vascular system in echinoderms and, more specifically, helicoplacoids. First, Durham (1967) claimed it is safe to assume, based on their morphological similarity with modem echinoderms, that extinct echinoderm species had complete water vascular systems for respiration, food gathering, and sometimes locomotion. This assumption being made, Durham (1967) described the morphology of helicoplacoids. The tests of helicoplacoids were constructed of columns of plates arranged spirally around a central axis. Most of these columns are interpreted to be in the interambulacra of the organisms, with two thin ambulacra that spiral down from the top and eventually join to form one ambulacrum (Durham, 1967). The ambulacra of helicoplacoids are small and do not extend to the apical pole (Durham, 1967). They have two rows of sutural pores through which tube feet probably extended. These rows of tube feet probably extended from a medial radial canal, and ampullae may have been present (Durham, 1967). Durham continued his work on helicoplacoids and later published an article that further discussed the preservation and functional morphology of helicoplacoids (Durham, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39 1993). Again, Durham (1993) noted that disassociated plates are the most common mode of preservation for helicoplacoids, because the soft parts which held together the helical skeleton of helicoplacoids probably decayed shortly after death, scattering the plates. Rarely, specimens with plates in life position are found, usually along bedding planes beneath graded beds. The position of these well preserved specimens beneath graded beds indicates that the helicoplacoids were buried during some sort of rapid depositional event (Durham, 1993). Durham (1993) attempted to determine the strati graphic range of the various helicoplacoid taxa in the Poleta Formation, but was unsuccessful because the stratigraphic position of many specimens was never recorded and the area is geologically complex, making the determination of stratigraphic position extremely difficult. Durham (1993) also hypothesized about the life mode and position of helicoplacoids. In several bedding planes of well-preserved helicoplacoids, presumably preserved by rapid burial, there are specimens which appear to be preserved in a vertical position (Durham, 1993). It appears that they may have died and collapsed upon themselves before significant decay or burial. Based on these specimens, and others like them found by Dombos and Bottjer (1998), helicoplacoids were almost certainly suspension-feeding mud-stickers (Durham, 1993). Durham (1993) also postulated that the mouth of helicoplacoids was located at the pole from which the ambulacra extend, as opposed to the point at which the ambulacrum branches. Derstler (1982) and Paul and Smith (1984), however, showed convincingly that the mouth was most likely at the branching point of the ambulacra, making the ambulacra of the helicoplacoids triradiate. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 THE ORIGIN OF THE ECHINODERMS A review of the origin and early evolution of echinoderms, based on previous work, is critical to putting this research in the proper perspective. The Late Neoproterozoic soft-bodied Ediacaran Fauna contains no undisputed echinoderms, but several members of this fauna do resemble echinoderms. Gehling (1987) described the Ediacaran fossil Arkania as a possible example of an early edrioasteroid (Figure 17). Another Ediacaran fossil, Tribrachidium, described by Glaessner and Wade (1966), resembles a triradiate echinoderm, but has since been interpreted as both a coelenterate (Sokolov and Fedonkin, 1984) and a sponge-grade organism (Figure 18) (Seilacher, 1999). The appearance of skeletonized helicoplacoids and edrioasteroids, both undisputed echinoderms, in the middle of the Early Cambrian of western North America, just after the first trilobites and archaeocyathids appeared, marks the earliest accepted occurrence of echinoderms in the fossil record (Figure 19) (Durham and Caster, 1963; Durham, 1967, 1993; Sprinkle, 1976, 1992). More echinoderms occur in the late Early Cambrian of eastern North America, with the appearance of camptostromid edrioasteroids and lepidocystoid eocrinoids (Figure 19) (Durham, 1966, 1968; Paul and Smith, 1984). After getting off to an early start, the helicoplacoids disappeared from the fossil record by the late Early Cambrian (Figure 19) (Durham, 1993). These Early Cambrian echinoderms had primitive morphological features such as imbricate plates, adjacent unarticulated plates, epispires through the oral surface, and triradiate or pentameral symmetry reflected only in the ambulacra (Sprinkle and Guensburg, 1997). They all lived as low- to medium-level mudsticking or attached suspension feeders (Sprinkle and Guensburg, 1997). Diversification of the echinoderms continued in the Middle Cambrian, with six new groups appearing during this period (Figure 19) (Sprinkle and Guensburg, 1997). While the eocriniods and edrioasteroids continued to diversify in the Middle Cambrian, stylophorans, homosteleans, homoiosteleans, and ctenocystoids, all homalozoans, make Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41 Figure 17- The Ediacaran fossil Arkarna (modified from McMenamin, 1998). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43 Figure 18- The Ediacaran fossil Tribrachidium (modified from Seilacher, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45 Figure 19- Age-range chart showing generic diversity of echinoderm classes or orders during the Cambrian and Ordovician radiations (modified from Sprinkle and Guensburg, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46 a ORDOVICIAN CAMBRIAN Croixian fremjArenifl Lnv Lnd Ctsnocystoids Comute Stytopnorara M itrmte Stytophorans Homoiostateans Homosteieans Eocnnoids Glyptocystitid Hetnk osmttid Rhombiferins Fistuliponte Oiploporitea Coranoids Blastoids Inadunate Crinoids Dispand Hybocrinids Crinoids Echmatocrinids ? Crinoids Crinoids Dipiobathrid Monobathrid Came rata Crinoids Came rate Helicoplacoids Opriiodstioids Echinoids Somastaroids Asteroids Number of Genera , 3 5 10 Ophiurmds Low-diversity interval Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47 their first apearances in the Middle Cambrian (Figure 19) (Sprinkle and Guensburg, 1997). One possible crinoid and several suspect holothurians also appear in the Middle Cambrian (Figure 19) (Sprinkle and Guensburg, 1997). Middle Cambrian echinoderm faunas were more common and diverse than their Early Cambrian predecessors, and their members were somewhat more advanced, with fewer echinoderms displaying the primitive features mentioned above (Sprinkle and Guensburg, 1997). While some homalozoans lived as deposit feeders (Parsley, 1991), low- to medium-level suspension feeding was still the most common life mode for Middle Cambrian echinoderms (Sprinkle and Guensburg, 1997). The fossil record of echinoderms in the Late Cambrian is somewhat poor, with only eocrinoids, edrioasteroids, stylophorans, and homoiosteleans present (Figure 19) (Ubaghs, 1963; Brett et al., 1983; Jell et al., 1985; Smith, 1988; Smith and Jell, 1990). Homosteieans and ctenocystoids had presumably gone extinct, while crinoids and holothurians, both possibly present in the Middle Cambrian, are absent in the Late Cambrian only to return to the record again in the Early-Middle Ordovician (Figure 19) (Sprinkle and Guensburg, 1997). Many Late Cambrian echinoderms are more advanced than Middle Cambrian echinoderms, with fewer thecal plates, better pentameral symmetry, longer, more organized columnal-bearing stems, branched arms, and the loss of aboral plating (Jell et al., 1985; Smith and Jell, 1990; Dzik and Orlowski, 1993). Many attached suspension-feeding echinoderms occupied the hardgrounds of the Late Cambrian, while mobile suspension feeders and deposit feeders remained on muddy substrates (Sprinkle and Guensburg, 1995, 1997). The Ordovician was primarily a time of major diversification for echinoderms, as the Paleozoic Evolutionary Fauna radiated in the Middle-Late Ordovician (Figure 3), allowing echinoderms to reach their apex in diversity and disparity (Sprinkle and Guensburg, 1995, 1997). Most notably, crinoids, blastoids, echinoids, asteroids, and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48 ophiuroids all appeared during this radiation, with crinoids easily the most diverse and abundant of these groups (Figure 19) (Sprinkle and Guensburg, 1997). Although few major groups of echinoderms went extinct, the Late Ordovician mass extinction hit echinoderms hard, with class-level diversity not recovering to reasonable levels until the latter part o f the Silurian (Eckert, 1988; Foote, 1992, 1994). Echinoderm class-level diversity declined through the remainder of the Paleozoic, with only Five classes, all of which are extant today, surviving the end-Permian mass extinction (Sprinkle and Guensburg, 1997). While the early fossil record of echinoderms is somewhat good, recent phylogenetic and molecular studies suggest that the appearance of helicoplacoids and edrioasteroids in the middle Early Cambrian does not actually mark the evolution of the first echinoderms. Fortey et al. (1996) support the Vendian origin o f m etazoans based on a fairly straightforward argument. They emphasize that the broad cladisitic picture of metazoan evolution is rather well known. That is, the evolution of individual phyla, such as echinoderms, was preceded by the protostome/deuterostome split, which was in turn preceded by the Cnidaria/Bilateria split (Fortey et al., 1996). Having established that, the presence of any conventional metazoan phyla in the Ediacaran fauna, regardless of the presence o f V endobionta (Seilacher, 1984), th erefore indicates that the protostome/deuterostome split and Cnidaria/Bilateria split both took place in the Vendian, pushing the protostome/deuterostome split back about 50 my from the Cambrian explosion (Figure 20) (Fortey et al., 1996). A molecular study by Wray et al. (1996) suggests an even earlier metazoan origin. Their results, based on the use of sequence similarities in species of various phyla to calculate the times of divergence of the major metazoan groups, indicate that the protostome/deuterostome split took place 1200 mya, and that the echinoderms first Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49 Figure 20- Fortey et al.'s (1996) model of metazoan diversification; Note the diversification of dueterostomes, and, hence, echinoderms, during the Vendian (modified from Fortey et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 li l t :u S 1 Vendobionta (partim) 1 s ^ l DeutoDHomes Proconome* "Phanerozoic" Metazoa I Larger, fossilizable forms. Smaller. unfossilizable forms. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51 Figure 21- Wray et al.'s (1996) model of metazoan diversification; Note the diversification of echinoderms about 1000 Ma ago (modified from Wray et al., 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 Protostomia Oautaroatomia • o « a s o E V o E o ■ • c e Cambrian Vandian Ma Eon 4 0 0 - 600 - 1000 - 1200 - Divergence of echinoderms. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 53 diverged about 1000 mya (Figure 21) (Wray et al., 1996). This study, if correct, pushes the protostome/deuterstome split back about 650 my before the Cambrian explosion, and the origin of echinoderms back over 450 my from their First undisputed appearance, as helicoplacoids and edrioasteroids, in the Early Cambrian (Wray et al., 1996). While these two studies (Fortey et al, 1996; Wray et al., 1996) differ in the actual timing of the origin of metazoans, they both agree that it took place well before the Cambrian explosion, during Vendian or pre-Vendian times. If the earliest metazoans, and hence echinoderms, did indeed exist in the Vendian, which seems probable given the studies discussed above, then what were they like and why were they not, as far as we know, fossilized? While there is currently not a fully satisfactory answer to these questions, it seems possible that these early metazoans, while by definition multicellular, were small, perhaps even microscopic, and unskeletonized, precluding their preservation (Davidson et al., 1995, Fortey et al., 1996). If echinoderms were indeed present in the Vendian, therefore, it is possible that most of them were extremely small, although some may have been large enough, possibly Arkarua or Tribracliidiiun, to be preserved as Ediacaran fossils. It is difficult to speculate on the life modes of these earliest echinoderms, but, if they were benthic, it is likely that they had a biomat-related lifestyle as defined by Seilacher (1999) and discussed in detail in a later chapter. Perhaps some of them even lived as minute mat-stickers (Seilacher, 1999), which, if they indeed existed, is most likely how the tiny soft-bodied ancestors of helicoplacoids lived. TAPHONOMY OF ECHINODERMS Before discussing the results of the taphonomic portion of this research, it is important to review the taphonomy of echinoderms in general. There are many important taphonomic studies which are relevant to this research. O f particular interest, in addition to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54 studies of fossil echinoderms, which will be discussed later, are studies of the decay processes of modem echinoderms. Studies have shown that echinoderms in normal marine conditions usually completely disarticulate into individual ossicles within one to two weeks, depending on their construction and environmental factors (Meyer, 1971; Liddell, 1975; Kidwell and Baumiller, 1990; Greenstein, 1991; Donovan, 1991). More specifically, the arms and cirri of modem crinoids become disarticulated within three days of death, and six days after death, only the calyx and certain arm segments are still articulated (Meyer, 1971; Liddell, 1975; Lewis, 1986). The spines of echinoids are the first skeletal elements lost, followed by the disarticulation of the lantern and the breaking apart of the corona, following the decay of the membranes holding it together (Kidwell and Baumiller, 1990; Greenstein, 1991). The effects of physical disturbance on the decay of modem echinoids have been studied by Kidwell and Baumiller (1990). These were laboratory studies, and their results indicate that freshly killed echinoids remain articulated through hours of physical disturbance, while decayed echinoids disarticulate rapidly when physically disturbed (Kidwell and Baumiller, 1990). These results, and those of the other workers discussed above, have important implications for the study of fossil echinoderms because the length of time which an echinoderm decayed on the seafloor can be estimated based on how well- preserved the specimen is. A particularly imformative study of the taphonomy of fossil echinoderms was performed by Brett, Moffat, and Taylor (1997). They divided echinoderms into three different taphonomic grades based on the ease by which they become disarticulated. Then they used these three taphonomic groups to define various "taphofacies" characteristic of certain sedimentary environments (Brett, Moffat, and Taylor, 1997). These taphofacies are Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55 Figure 22- The taphonomic grades of the three types of echinoderms and their corresponding time of post-mortem seafloor exposure (modified from Brett, Moffat, and Taylor, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. C ategory example* Conditions with Varying Post-Mortem Exposure Hours to 1 Day 1 Day to 2 Weeks 2 Weeks to 1 Year More Than 1 Year T Y P E 1 : ophlurolds, asteriods, •carpolds,* paleoechlnolds, oocrinolds, edrioasterolde I ^ p jC : V /* .* V -- * • • • * • * j* * * ■ « i* f a .• * * .* • Q .»• * T Y P E 2 : most crinoids, ‘cystolds,* regular echinoids M . e ^ <'.&a • D ° « A N. T Y P E 3 : robust camerates, microcrinoids, biastolds, irregular echinoids f * i > i l P ^ N - i / ■ * • • I • * • ' # -a * * * C/1 Os 57 therefore helpful in reconstructing the paleoenvironments in which fossil echinoderms were living. The first taphonomic group is that of Type 1 echinoderms (Figure 22) (Brett, Moffat, and Taylor, 1997). Type 1 echinoderms have plates that are held together only by soft tissues, i.e. ligaments and muscles. It is not likely that Type 1 echinoderms will remain articulated for very long after death because their plates will be rapidly dissociated by decay (Figure 22) (Brett, Moffat, and Taylor, 1997). Examples of Type I echinoderms include asteroids, ophiuroids, eocrinoids, some edrioasteroids, and some homalozoans (Figure 22) (Brett, Moffat, and Taylor, 1997), not to mention helicoplacoids. Naturally, Type 1 echinoderms are rarely preserved in the fossil record except as disarticulated skeletal ossicles, which are usually unidentifiable (Brett, Moffat, and Taylor, 1997). Only very rarely are Type 1 echinoderms preserved partially or wholly articulated (Brett, Moffat, and Taylor, 1997). Type 2 echinoderms contain portions of their skeleton which are more tightly articulated, and other portions which are loosely articulated (Figure 22) (Brett, Moffat, and Taylor, 1997). Examples include crinoids, cystoids, and many regular echinoids (Brett, Moffat, and Taylor, 1997). Because of the variation in their skeletal construction. Type 2 echinoderms are preserved in a wide range of taphonomic grades (Figure 22) (Brett, Moffat, and Taylor, 1997). The best preserved Type 2 echinoderms have articulated bodies and appendages. Crinoids, for instance, would have articulated calyxes, stalks, and arms (Figure 22) (Brett, Moffat, and Taylor, 1997). Articulated Type 1 echinoderms are also commonly found with these kinds of Type 2 specimens. Given their excellent preservation, this taphonomic grade of Type 2 echinoderms is the result of nearly instantaneous burial following death (Brett, Moffat, and Taylor, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58 The next taphonomic grade of Type 2 echinoderms consists of specimens which include both articulated and disarticulated portions of the body (Brett, Moffat, and Taylor, 1997). A crinoid of this grade, for instance, might have a fully or partially preserved calyx with some columnals still attached, and detached, disarticulated arms (Figure 22) (Brett, Moffat, and Taylor, 1997). This grade of Type 2 echinoderms indicates relatively rapid burial, probably within a few months after death, as opposed to instantaneous burial (Brett, Moffat, and Taylor, 1997). The third taphonomic grade of Type 2 echinoderms includes specimens that are mostly disarticulated, with a few small portions of the skeleton still articulated (Brett, Moffat, and Taylor, 1997). This grade of Type 2 echinoderms usually consists of partially articulated criniod or cystoid columnals (Figure 22) (Brett, Moffat, and Taylor, 1997), which are commonly more resistant to disarticulation than the rest of the skeleton (Baumiller and Ausich, 1992). The final Type 2 taphonomic grade consists of entirely disarticulated specimens (Brett, Moffat, and Taylor, 1997). Typically, the only recognizable skeletal elements in these kinds of assemblages are thick echinoid spines, plates of pelmatozoans, and pelmatozoan holdfasts (Figure 22) (Brett, Moffat, and Taylor, 1997). These assemblages represent years of decay on the seafloor (Brett, Moffat, and Taylor, 1997). The third, and final, group of echinoderms are the Type 3 echinoderms (Figure 22) (Brett, Moffat, and Taylor, 1997). These echinoderms have a skeleton which is almost entirely tightly articulated or sutured, as is typical of irregular echinoids and some crinoids and blastoids (Figure 22) (Brett, Moffat, and Taylor, 1997). This group of echinoderms is very resistant to decay, which often entails breakage along sutures, so heavily disarticulated specimens indicate prologed post-mortem seafloor exposure, probably in high energy environments (Brett, Moffat, and Taylor, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59 Although not discussed in detail here, Brett, Moffat, and Taylor (1997) used the taphonomic grades described above to define taphofacies which are characteristic of certain depositional environments. The feature of this study most relevant to this research is the division of fossil echinoderms into taphonomic groups and grades, allowing for the determination of the conditions under which helicoplacoids were preserved based on the study of their fossil remains. The methods and results of this helicoplacoid taphonomy study will, of course, be discussed later. Given that the preservation of echinoderms, and, specifically, helicoplacoids, is dependent on the conditions of their physical burial, it is now important to consider the burial, or obrution, events that are responsible for the preservation of exceptional fossils such as helicoplacoids. Obrution deposits represent extremely rapid burial events during which the bodies of organisms were quickly and permanently buried by a large influx of sediment, not allowing for their decay on the seafloor (Brett, Baird, and Speyer, 1997). There are four factors identified by Brett, Baird, and Speyer (1997) as necessary for an obrution deposit to form. First of all, numerous intact organisms, dead or alive, must be present on the seafloor during burial. Secondly, a relatively thick (1 mm to 1 cm) bed of sediment must be rapidly deposited. Thirdly, there must be no later physical or biological reworking of the obrution layer. And, Finally, there must be a favorable early diagenetic environment (Brett, Baird, and Speyer, 1997). Obrution events can have a variety of causes, such as seismites, turbidites, and ash falls or flows (Seilacher, 1982; Clifton, 1988). In marine shelves, however, the most common creators of obrution deposits are storms (Brett, Baird, and Speyer, 1997). Storms cause erosional winnowing in coarse-grained nearshore settings, resulting in the basinward transport of fine-grained suspended sediment (Aigner, 1985; Clifton, 1988). These fine-grained suspended sediments blanket offshore areas with a layer of fine Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60 Figure 23- Distribution of the four types of obrution deposits on a schematic continental shelf, based on differing energy and depth regimes; WB=wave base (modified from Brett, Baird, and Speyer, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Shoreline Normal WB Avoraaa Storm W ! Maximum Storm WB Rare TYPE 1 Common TYPE 2a & 2b Rare TYPE 3 Stagnation - Obrution Deposits (TYPE 4 ?? Stagnation Deposits ?? O N 62 sediment, resulting in the preservation in obrution deposits of any organisms unfortunate enough to be present (Aigner, 1985; Clifton, 1988). Brett, Baird, and Speyer (1997) classify obrution deposits into four groups based essentially on their proximity to shore (Figure 23). Type 1 obrution deposits are patchy and discontinuous because they were formed in high-energy nearshore areas with rapidly shifting coarse sediment (Brett, Baird, and Speyer, 1997). This shifting sediment often disturbs earlier obrution deposits, making Type 1 obrution deposits rather rare. More common than Type I obrution deposits. Type 2 obrution deposits consist of well-preserved fossil beds underneath proximal sandy storm beds (Figure 23) (Brett, Baird, and Speyer, 1997). Type 3 obrution deposits occur slighty below storm wave base where there is the largest influx of fine-grained suspended sediment and obrution deposits are unlikely to be disturbed by later events, optimal conditions for the formation and preservation of magnificent obrution deposits (Figure 23) (Brett, Baird, and Speyer, 1997). Finally, Type 4 obrution deposits are associated with distal tempestites or microturbidites that bury organisms in organic-rich muds, sometimes allowing for the preservation of soft parts as carbonized films or pyrite coatings, which is not possible in the other three types of obrution deposits (Figure 23) (Brett, Baird, and Speyer, 1997). Type 4 obrution deposits are typically termed stagnation/obrution deposits, and they comprise most conservation Lagerstatten (Seilacher et al., 1985; Brett, Baird, and Speyer, 1997). BIOTURBATION IN SILICICLASTICS DURING THE PRECAMBRIAN-CAMBRIAN TRANSITION Changes in the level and type of bioturbation in siliciclastics through the Precambrian-Cambrian transition have been well-studied for ichnostratigraphic reasons, but the ecological impact of these changes is only beginning to be examined. The first undisputed trace fossil evidence for metazoan activity is found at the end of the last Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63 Neoproterozoic glaciation, about 594 mya (Narbonne et ai., 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 of bioturbation, as observed in inner shelf carbonates of the Great Basin by Droser and Bottjer (1988) (Figure 24). This increase in intensity is reflected by an increase in the ichnofabric indices of carbonate rocks in the Fallotapsis zone of North America, or Atdabanian of Siberia (Droser and Bottjer, 1988). An ichnofabric index (ii) of 1, signifying no evident bioturbation, dominates Precambrian carbonates, while by the Atdabanian the ii's range from 1 all the way to 5, the last of which signifies intense bioturbation (Droser and Bottjer, 1988). Mcllroy and Logan (1999) also observed similar changes in bioturbation in shelf siliciclastics in Wales, Newfoundland, and Norway through the Precambrian-Cambrian boundary. They detected a gradual increase in bioturbation during the Early Cambrian, as opposed to the rather abrupt increase found by Droser and Bottjer (1988) in the Atdabanian (Mcllroy and Logan, 1999). This difference indicates that the rapid change observed by Droser and Bottjer (1988) may have only taken place in carbonates (Mcllroy and Logan, 1999). Whether gradual or rapid, however, the increase in bioturbation in siliciclastics and carbonates during the Precambrian-Cambrian transition is well-documented. It has been hypothesized by Mcllroy and Logan (1999) that this increase in bioturbation intensity was due to a positive feedback system wherein the increased transfer of organic matter, in the form of fecal material, to the sediment by grazing metazoans stimulated an increase in microbial mat formation, which, in turn, led to increased depth and intensity in bioturbation as metazoans fed on these microbial communities. The process continued as the deeper burrowers increased the oxygenation of the sediment, allowing even more metazoans to occupy the infaunal realm (Mcllroy and Logan, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 Figure 24- A) The average ichnofabric indices (ii) of inner shelf carbonates through the Cambrian and Ordovician western North America (modified from Droser and Bottjer, 1988). B) Histogram displaying the ichnofabric index percentages of Cambrian inner shelf siliciclastics of western North America (modified from Droser, 1987). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 A) 5 4 3 2 1 C lastic Inner Shelf 100 m Q M l i i B 5 n Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66 These deeper burrowers further strengthened the feedback loop by transferring even more organic matter to the sediment (Mcllroy and Logan, 1999). Whatever the cause of the increase in bioturbation during the Early Cambrian, its ecological effects are of primary interest to this research. By studying the interrelationships between wrinkle structures (Figure 25), which are suspect-microbial structures, and trace fossils during the Precambrian-Cambrian transition, Hagadom and Bottjer (1999) documented major ecological changes which took place during this time interval. They discovered that the traces associated with wrinkle structures are horizontally- oriented and represent active and passive sediment ingestion on, in, and under these suspect-microbial structures (Hagadom and Bottjer, 1999). During the Early Cambrian, however, vertical burrowers evolved onshore and moved offshore, relegating the microbial mats to the deep ocean and marginal marine stressed settings (Droser and Bottjer, 1993; Hagadom and Bottjer, 1999). This increase in vertical bioturbation in shelf deposits thereby restricted a Late Neoproterozoic marine biotope dominated by microbial mats and the metazoans which fed on them to the deep ocean, while replacing it with a biotope dominated by metazoans (Droser and Bottjer, 1993; Hagadom and Bottjer, 1999). This replacement resulted in a shift from Proterozoic siliciclastic sedimentary fabrics dominated by microbial and physical processes, to Phanerozoic siliciclastic sedimentary fabrics dominated by metazoan and physical processes (Figure 26) (Hagadom and Bottjer, 1999). All available evidence indicates that the Early Cambrian was a time of transition between these two siliciclastic sedimentary fabric realms and their corresponding biotopes (Hagadom and Bottjer, 1999). Seilacher (1999) also explored the ecological changes associated with increasing levels of vertical bioturbation during the Precambrian-Cambrian transition. Before the Cambrian, the seafloors were only bioturbated by horizontal burrowers, allowing for the formation of extensive microbial mats on the seafloor (Seilacher, 1999). These microbial Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67 Figure 25- Photograph of typical wrinkle structures from the Lower Cambrian of western North America; Scale bar is 1 cm (modified from Hagadom and Bottjer, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 Figure 26- Diagram representing the shift in the dominant factors influencing marine siliciclastic sedimentary fabrics during the Proterozoic-Phanerozoic transition and the approximate stratigraphic range of helicoplacoids (modified from Hagadom and Bottjer, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. physical Non-stressed, Non-disaster Siliciclastic Facies microbial microbial \ j ' \jmetazoan microbial metazoan Crvogenian \ Vendian Cambrian \ Ordovician ^ Proterozoic Paleozoic Helicoplacoids present Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71 mats created matgrounds wherein the sediment-water interface was sharp and the substrate was relatively firm (Seilacher, 1999). The organisms of the Ediacaran fauna lived in these matground environs and were dependent on them for survival. There were mat encrusters, which lived attached to the microbial mats, mat scratchers, which 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 the decomposing mat remnants (Figure 27) (Seilacher, 1999). W hen the levels of vertical bioturbation increased in the Cambrian, these matgrounds were replaced by mixgrounds, making the mat-dependent Ediacaran fauna life modes obsolete (Figure 27) (Seilacher, 1999). The development of mixgrounds in the Early Cambrian changed the ecological scene dramatically, requiring metazoans to adapt to the softening of the substrate and blurring of the sediment-water interface which would have accompanied this transition (Rhoads, 1970). Extensive geochemical changes also took place during this transition to mixgrounds, with the redox boundary moving deeper in the substrate, allowing for better oxygenation and, perhaps, more infaunal colonization by metazoans (Aller, 1982; Mcllroy and Logan, 1999). Several possible explanations for this Early Cambrian increase in depth and intensity of bioturbation have been presented. It is likely that increased predation played a role in this bioturbation change, as organisms sought to escape assault from above by burrowing deeper into the sediment (Bottjer and Ausich, 1986). Bottjer and Ausich (1986) concluded that much of the increase in bioturbation depth and intensity through the Phanerozoic developed in response to increased predation from above (Figure 28). As evidence, they point to the increase in infaunal bioturbation depth at the beginning of the Devonian, which coincides nicely with the increase in Phanerozoic predation documented by Signor and Brett (1984) (Bottjer and Ausich, 1986). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 Figure 27- Diagram showing how increased bioturbation caused the biomat-related lifestyles of the Precambrian to nearly disappear in the Phanerozoic as matgrounds were restricted to extreme environments (modified from Seilacher, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. m nt e n c r u s t « r s a E sca la tio n m .its lic k e rs u n d o rm n t m n e r s microbial grain coatings vertical biotur bation f ■ j p£>£ Phanerozoic $ 1 ^ 1 1 Mixgrounds I Precambrian Matgrounds scnlinQ Biomat-relaied lifestyles dominate the Precambrian. Biomat-related lifestyles rendered obsolete by increased vertical bioutrbation. --4 u > 74 Figure 28- Tiering of soft-substrate suspension-feeding communities during the Phanerozoic; Thick lines represent maximum tiering levels above or below the sediment-water interface, while thinner lines represent tiering subdivisions within these maxima; Dotted lines represent inferred tiering levels (modified from Bottjer and Ausich, 1986). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75 S I s g K t tc so z o c -100 100 9 u •90 - 5 0 - 100 -1 0 0 C am brian expansion o f infaunal tiering. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76 Another possible factor in this Early Cambrian change in bioturbation is increased oxygen concentrations in the oceans (Rhoads and Morse, 1971). Rhoads and Morse (1971) tested this hypothesis by examining the distribution of benthic invertebrates in various oxygen deficient basins: the Black Sea, the Gulf of California, and the continental borderlands of California. Their results indicate that water with less than 0.1 ml Cb/l is devoid of metazoans, water with between 0.3 and 1.0 ml Cb/l contains low-diversity populations of small infaunal soft-bodied metazoans, and water with more than 1.0 ml Cb/I contains more diverse epifaunal and infaunal communities, including metazoans with calcareous skeletons (Figure 29) (Rhoads and Morse, 1971). This distribution of modem metazoans in oxygen deficient basins mimics the Precambrian-Cambrian increase in bioturbation and the subsequent evolution of calcareous metazoans, possibly indicating a connection between these events and an increase in the dissolved oxygen concentration of seawater during the same interval (Figure 29) (Rhoads and Morse, 1971). Given the results of these studies (Bottjer and Ausich, 1986; Rhoads and Morse, 1971), both increased nektonic predation and increased seawater oxygen concentrations probably played important roles in the increase in the depth and intensity of bioturbation during the Proterozoic-Phanerozoic transition. PETROGRAPHIC EVIDENCE SUGGESTIVE OF THE PRESENCE OF MICROBIAL MATS IN SILICICLASTICS As mentioned above, the Early Cambrian was a time of transition between the matgrounds of the Proterozoic and the mixgrounds of the Phanerozoic (Seilacher, 1999). It is important, therefore, to review the possible evidence suggestive of the presence of microbial mats in siliciclastic settings before discussing the petrographic results of this Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77 Figure 29- Schematic cross-section of an idealized modem marine basin displaying the relationship between dissolved oxygen and the composition o f the benthic fauna: Right hand axis displays how appearance of trace fossils and calcified metazoans during the Proterozoic-Phanerozoic transition parallels the distribution of the modem benthic faunas based on dissolved oxygen levels (modified from Rhoads and Morse, 1971). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 Oivtr** C0C03 founo Sott-l infauna (tract*) Azoic 6*10 1 . 0 » 1 . 0 DISSOLVED O X Y G EN (ml/1) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GEOLOGIC TIME (my) 79 research. Schieber (1986. 1990, 1998, 1999) has completed pioneering work in identifying petrographic evidence for microbial mats in siliciclastics, and he recently (Schieber, 1999) compiled petrographic criteria for suggesting the presence of microbial mats in siliciclastics. This petrographic evidence, discussed in detail below, includes wavy-crinkly laminae, cohesive behavior, carbonaceous or micaceous laminae, and mat- decay mineralization (Schieber, 1999). This petrographic evidence can also be bolstered by field observations such as domal buildups, irregular wrinkled bedding surfaces, ripple patches, and irregular curved- wrinkled impressions on bedding surfaces (Schieber, 1999). Due to the petrographic nature of this study, however, this field evidence will not be discussed further in this section. If a rock possesses several, or all, of these petrographic and field criteria, then the presence of microbial mats is suggested (Schieber, 1999). None of these criteria occurring alone, however, are suggestive of microbial mats (Schieber, 1999). It should be emphasized that all of this evidence is merely suggestive of the presence of microbial mats. The only irrefutable evidence of microbial mats in rocks is preserved bacterial filaments (Schieber, 1999). A microbial mat origin for wavy-crinkly laminae is suggested by both their similarity to modem microbial laminae and the contrast between them and non-mat laminae, which are typically planar and parallel (Figure 30) (Schieber, 1999). Often associated with carbonaceous layers, they can occur in both sandstones and mudstones (Schieber, 1986). When soft-sediment deformation can be ruled out, these wavy-crinkly laminae, in conjunction with ample other evidence, are suggestive of microbial mats (Schieber, 1999). Cohesive behavior of sediment that would not be expected to exhibit such behavior is also suggestive of microbial mats (Schieber, 1999). In fine-grained sediments, this cohesive behavior is expressed by thin layers of sediment that were rolled up, tom, or Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 80 Figure 30- Hand-sample picture o f wavy-crinkly carbonaceous laminae from the Proterozoic Belt Supergroup of western North America, interpreted as subtidal microbial mat deposits by Schieber (1986); Scale bar is 0.5 mm long (modified from Schieber, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81 Wavy-crinkly laminae Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 82 Figure 31- Hand-sample picture of carbonaceous beds displaying cohesive behavior in the form of tight folds, interpreted as microbial mat deposits by Schieber (1986); Sample from the Proterozoic Belt Supergroup of western North America; Scale bar is 1 mm long (modified from Schieber, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 83 Reproduced with permission of the coperinh. „ uie copyright owner. Further rpnmri,,^- reproduction prohibited without permission. 84 folded during transport, indicating that they were held together by some binding agent, most likely microbial mats (Figure 31) (Schieber, 1999). In coarser-grained sediments, such as siltstones and sandstones, the breaking up of the sediment into intraclasts when there is no evidence for synsedimentary cementation is a strong indicator of binding by microbial mats (Pfluger and Gresse, 1996). In conjunction with other evidence, carbonaceous and micaceous laminae also suggest microbial mat presence. Carbonaceous laminae, laminae enriched in organic material, can result from the burial and incomplete decay of microbial mats in the low- oxygen conditions that would exist under the mat-water interface (Schieber, 1986, 1998). Without supporting evidence, however, carbonaceous laminae indicate little more than a reducing environment. If, on the other hand, carbonaceous laminae exhibit wavy-crinkly form and cohesive behavior, then they support the presence of microbial mats (Schieber, 1999). In addition to carbonaceous laminae, microbial mats also tend to form mica-rich laminae by trapping micas from weak currents devoid of coarser siliciclastic material (Schieber, 1999). Microbial mats trap these micas because their surfaces are often sticky, thereby causing the sediments involved to be mica-enriched (Schieber, 1999). Again, corroborative evidence is necessary to implicate microbial mats in this mica-enrichment process. Mat-decay mineralization forms when the reducing conditions created beneath the living mat results in the formation of heavy, anoxic minerals such as pyrite, siderite, and ferroan dolomite in the sediment below (Schieber, 1999). These minerals are particularly telling if they occur in well-defined, thin horizons, which may indicate the former presence of microbial mats along these bedding planes (Garlick, 1988). The decay of small pieces of microbial mats may also result in the formation of these heavy minerals in small, thin areas, often displaying cohesive behavior (Schieber, 1999). In the absence of any other Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 85 evidence for microbial mats, of course, all these minerals indicate is a reducing environment. SOFT-SUBSTRATE ADAPTATIONS OF BENTHIC INVERTEBRATES Because some helicoplacoids are found preserved in situ in shales, as will be discussed later, it is critical to examine the adaptations of benthic invertebrates to unstable soft substrates, which have a high water content and, consequently, a very low density (Thayer, 1975). Such substrate conditions are generally inhospitable to benthic invertebrates because if they are not well adapted, then they will sink into the fluid substrate (Thayer, 1975). In addition, Fine-grained material in soft substrates is often resuspended by currents or deposit feeders. This resuspended material, in large doses, clogs the respiratory and feeding structures of benthic invertebrates, especially suspension feeders (Thayer, 1975). Thayer (1975) performed an informative study on the soft-substrate adaptations of benthic invertebrates. This study focused primarily on how certain characteristics of a benthic organism's body affect the stress which that organism applies to the seafloor (Thayer, 1975). Those bodily characteristics are: bulk density, body shape, weight bearing surface area, and overall size (Thayer, 1975). The static stress created on the seafloor by the interaction of these characteristics should not exceed the strength of the sediment on which the organism is living. Otherwise, the organism will sink into the sediment and die (Thayer, 1975). Thayer's (1975) results identify four adaptations of benthic invertebrates to life on soft substrates. The first, which applies to bivalved invertebrates, is to reduce the bulk density of the organism by growing thin, non-costate shells, thereby decreasing the stress applied to the substrate by the organism (Thayer, 1975). The second adaptation is termed the "Iceberg” adaptation because it involves the submersion of a major portion of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 86 organism in the fluid sediment on which it lives (Thayer, 1975). This strategy allows the organism to stay "afloat" in the sediment because the long extension of the body, usually just skeletal material, reaches down to firmer sediment which can support the organism (Thayer, 1975). The soft parts of the organism, meanwhile, only reside in the uppermost portion of the skeleton, above the sediment/water interface (Figure 32) (Thayer, 1975; Seilacher, 1999). Mudstickers (Seilacher, 1999) employ this strategy for survival on soft substrates. The third soft substrate adaptation of benthic invertebrates is called the "Snowshoe" adaptation (Thayer, 1975). This adaptation involves the flattening of the body in order to increase the weight bearing surface area of the organism, in turn decreasing the stress applied to the sediment by the organism (Thayer, 1975). Finally, benthic invertebrates can significantly decrease the stress they apply to the sediment by simply decreasing their body size (Thayer, 1975.) GEOLOGIC SETTING AND PALEOGEOGRAPHY The Lower Cambrian Poleta Formation, which is exposed throughout western central Nevada and eastern central California, consists of marine carbonates and siliciclastics (Figure 2). Under the system of Nelson (1966, 1971), the Poleta is divided into three members. Moore (1976a, 1976b) adheres to this three-member division of the Poleta, as will this research (Figure 2). The Poleta Formation is conformably underlain by the siltstones and shales of the Montenegro Member of the Campito Formation. The Harkless Formation, with its siltstones, shales, occasional sandstones, and rare limestones, conformably overlies the Poleta (Figure 2) (Moore, 1976a). The terrigenous components of the Poleta Formation were transported from a source area to the southeast, while the carbonate-bank components were elongate parallel to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 87 Figure 32- Diagram displaying various modem and one ancient (the Devonian coral Heterozaphrentis) examples of sediment stickers employing the "iceberg" adaptation (Thayer, 1975) (modified from Seilacher, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. corals 88 MUD STICKERS Heterozaphrentis Ptychophyllum Monlhvallia Gemellaroia Richlhofenia Stnostrea Durania Vermicularia Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 89 the paleoshoreline, which had a northeast-southwest trend (Moore, 1976a). The shifts from carbonate-bank-dominated environments to siliciclastic-dominated environments represented in the Poleta Formation are interpreted as having been controlled by changes in the influx in terrigenous sediment from the continetal source (Moore, 1976a). During the Early Cambrian, what is now the western shoreline of North America was located on the northern coast of Laurentia, within 30°N latitude of the equator (Figure 33) (Sundberg and McCollum, 1997). This coastline was not on a tectonically active margin, so deposition took place on a rather broad, shallow continental shelf (Moore, 1976a). The three members of the Poleta Formation represent shifts from a carbonate-bank- dominated shelf environment in the Lower Member to a siliciclastic-dominated shelf environment in the Middle Member and then back to a carbonate-bank-dominated shelf environment in the Upper Member (Moore, 1976a). Each of these members has distinctive facies and faunas which are discussed below. The Lower Member of the Poleta Formation, as mentioned above, is dominated by carbonate-bank depositional environments (Moore, 1976a, 1976b). Ranging in thickness from 35 m to 140 m, it contains five distinctive facies: archaeocyathid-bearing bioclastic limestone, archaeocyathid-bearing biohermal limestone, oolitic limestone, bioclastic and oolitic limestone, and shale (Moore, 1976a, 1976b). The Middle Member, in which helicoplacoids are preserved, represents a shift to terrigenous sedimentation following the carbonate-bank deposition of the Lower Member (Moore, 1976a, 1976b). The thickness of the Middle Member ranges from 70 m to over 230 m (Moore, 1976a). In the White-Inyo Mountains, the Middle Member consists of four distinct units: the lower siltstone unit, the lower sandstone-siltstone unit, the middle limestone-siltstone unit, and the upper sandstone unit (Moore, 1976a, 1976b). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90 Figure 33- Global Early Cambrian paleogeography; Approximate location of Westgard Pass indicated by black circle (modified from Sundberg and McCallum, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92 The lower siltstone unit usually comprises approximately two-thirds of the Middle Member in the White-Inyo Mountains (Moore, 1976a). This unit is divided into upper and lower parts by a limestone marker bed. Below this marker bed, the shales appear to have been deposited in a subtidal environment (Moore, 1976a, 1976b). These shales contain abundant beds of trilobite fragments, archaeocyathids, and echinoderm plates, which were most likely storm-deposited. This part of the Middle M ember is where the new helicopiacoid site is located, and where almost all helicoplacoid individuals have been found preserved (Durham, 1993). Above the limestone marker bed, Moore (1976a, 1976b) interpreted the shales as being deposited in intertidal mud flats based on the presence of ripples and Runzelmarken. Hagadom and Bottjer (1997, 1999) have since shown that wrinkle structures, such as Runzelmarken, are not indicative of any particular depositional environment but, rather, indicate the possible presence of microbial mats. Although this new information does mean that the shale above the limestone marker bed was most likely not deposited in a mud-flat environment, it still does appear to have been deposited in a shallower environment than the shale below the marker bed based on the presence of more ripples. Perhaps, based on the sedimentary structures present, this upper shale was deposited above normal wave base, while the lower shale was deposited below normal wave base, and just below storm wave base. Above the lower siltstone unit is the lower sandstone-siltstone unit. This unit is relatively thin, about 20 m, and represents deposition in a wave-dominated nearshore- offshore environment (Moore, 1976a). Again, features interpreted by Moore (1976a, 1976b) as supratidal, such as Runzelmarken, are now known as possible indicators of microbial mats, independent of any particular depositional environment (Hagadom and Bottjer, 1997). The nearshore affinity of this sandstone, with its abundant ripples, still seems clear, however (Moore, 1976a. 1976b). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93 The middle limestone-siltstone unit is above the lower sandstone-siltstone unit. It is composed of gray mottled limestone interbedded with thin subunits of shale (Moore, 1976a). This unit represents a brief return of carbonate-bank formation, with some offshore shale deposition (Moore, 1976a, 1976b). The upper sandstone unit is above the middle limestone-siltstone unit and caps the Middle Member. It consists of sandstone with abundant Skolithos vertical burrows (M oore, 1976a, 1976b). Sedimentary structures indicate a nearshore depositional environment for this unit, which is in places nearly homogenized by bioturbation (Moore, 1976a, 1976b). The Upper Member of the Poleta Formation marks a return to carbonate-bank deposition (Moore, 1976a, 1976b). It consists of four major facies: buff limestone and interbedded siltstone, oolitic limestone, mottled oolitic-bioclastic limestone, and structureless crystalline limestone (Moore, 1976a). The carbonate-bank environment represented by the Upper M ember was more restricted than the carbonate-bank environment represented by the Lower Member, so no reefs developed (Moore, 1976a). This member is overlain by the nearshore siliciclastics of the Harkless Formation, representing an end to the carbonate-bank environment of the Upper Member (Moore, 1976a). METHODS Many techniques were used to reconstruct the paleoecology of helicoplacoid echinoderms: field observations, examination of the specimens collected in the field and numerous museum specimens, and x-radiographic and petrographic studies of the helicoplacoid-bearing shales in the Poleta Formation. All field observations, specimen collecting, and rock sampling took place at a new helicoplacoid-rich locality in Westgard Pass, at approximately 37°17'45" N, 118°08T5" W (Figure 34). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94 Field observations were made regarding the depositional environment in which the helicoplacoids lived and the level and type of bioturbation in the substrate on which they were living. The faunal elements associated with helicoplacoids at this location were also noted. In addition, attempts were made in the field to identify any indicators of microbial stabilization of the sediment, such as wrinkle structures (Hagadom and Bottjer, 1997). The 107 specimens collected at the new locality (specimen numbers PI-FI to Pl- F4, P4-F1, P4-F3 to P4-F23, P4-F25 to P4-F37, and P4-F43 to P4-F47) along with many from the University of California Museum of Paleontology (UCMP) and the Los Angeles County Museum of Natural History (LACMNH), were examined in order to gain insight into the life mode and position of helicoplacoids. The calcite plates of most specimens had dissolved away, leaving behind external molds. The calcite plates of specimens that still contained them were dissolved away in the lab in order to aid in their study. Latex casts were then made of several specimens in order to allow for more thorough examination. All of these specimens were collected from the talus slope present at this site, despite the excavation of six shale outcrops. As a result, unfortunately, the relative stratigraphic position of the specimens is unknown. The rocks in which they were preserved, however, were still analyzed for paleoenvironmental information. As noted above, six outcrops of the shale at this site, totalling just under 2 stratigraphic meters, were excavated in order to collect 81 rock samples for x-radiographic and petrographic studies (Figure 35). The stratigraphic position of the outcrops relative to one another was measured, and the relative stratigraphic position of the rock samples within each outcrop was recorded. Every stratigraphic interval of each outcrop is represented in the rock samples collected. These rock samples were then slabbed and x-rayed. The resulting x-radiographs were placed in stratigraphic order for each outcrop, thereby creating what amounts to an x- Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95 Figure 34- Photograph of the new helicoplacoid locality utilized in this research in Westgard Pass of the White-Inyo Mountains of eastern central California; All specimens were collected from the talus slope seen here, despite extensive outcrop excavation; Hammer handle is approximately 1 m long. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 97 Figure 35- Photograph of an outcrop of the Middle Member of the Poleta Formation at the new helicoplacoid locality after it was excavated to collect samples for x- radiography and petrography; Outcrop is approximately 80 cm in height. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 S Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 99 radiograph core o f each outcrop. These "cores" allow for convenient and detailed observation of bioturbation levels and sedimentary structures in 2 m of the substrate on which the helicoplacoids lived. The rocks containing the helicoplacoid specimens were also x-radiographed. Thin sections were made of 52 of the slabbed and x-rayed samples. Bioturbation levels in the cores were measured using the ichnofabric index method developed by Droser and Bottjer (1986). This method groups the extent of bioturbation into six categories, called ichnofabric indices (ii), based on visual observation: ii 1) no bioturbation observed, original sedimentary structures are completely preserved; ii 2) isolated trace fossils present, up to 10% of the original sedimentary structures are disturbed; ii 3) 10-40% of the original sedimentary structures are disturbed, most trace fossils are isolated but overlap occasionally; ii 4) only a small amount of original bedding is visible, 40-60% of the original bedding is disturbed; ii 5) original sedimentary structures are completely disturbed, but some individual traces are still discernible; ii 6) sediment is nearly completely homogenized (Droser and Bottjer, 1986). A total of 49 thin sections were made from the same rock samples used in the x- radiographic study. These thin sections served as the basis for a petrographic study of the helicoplacoid-rich Middle Member of the Poleta Formation. The common mineral assemblages were observed and recorded in order to better understand both the deposition and diagenesis of these rocks. These thin sections were also examined for any of the features discussed above which are suggestive of the presence of microbial mats. In order to examine the taphonomy of helicoplacoids, 107 specimens collected during this study and 39 specimens from LACMNH were carefully examined. These specimens were grouped into one of three groups based on the quality of their preservation: Group I) well-preserved with slight degrees of disassociation. Group 2) partially disassociated, and Group 3) almost fully disassociated. X-radiographs of the rocks in which these helicoplacoid specimens are preserved were then examined to search for any Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 100 sedimentological correlations between the three taphonomic groups. The percentage of total helicoplacoid specimens in each of these groups was calculated as well as the percentage of specimens that share a bedding plane with other individuals. These percentages were then used to characterize the preservation of helicoplacoids. It should be noted that there may be a bias in this research toward decreased occurrences of almost fully disassociated, or Group 3, helicoplacoid specimens. This bias stems from the fact that during the collection of specimens almost fully disassociated specimens would have been more difficult to locate and collect because of their distinct lack of any significant structure. As a result of this collection bias, a disproportionately low number of Group 3 helicoplacoids were probably collected. In order to better examine the preservation of the helicoplacoid specimens, any original calcite in these specimens was dissolved in HC1 to allow for closer examination of the resulting external molds. All of the specimens were also examined to search for evidence of preservational differences among the regions of the helicoplacoid body, testing the idea that the lower end of their body may have been more solidly constructed because it was designed for insertion into the substrate. The orientations of 33 specimens from three slabs were measured in order to assess their alignment. These orientations were plotted on rose diagrams to reveal any alignment of the specimens, more evidence for the preservation o f helicoplacoids in obrution deposits. Because these three slabs were found in a talus slope the quadrants on the rose diagrams are meaningless, only the alignment of the specimens, and not their actual orientations, is significant. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 101 RESULTS AND DISCUSSION Paleoecologv and Paleoenvironment Field observations indicate that the shales of the Middle Member o f the Poleta Formation are only minimally bioturbated. Evidence for bioturbation was seen on rare bedding plane exposures on which horizontal Planolites were visible (Figure 36). Otherwise, outcrops at this site have an ichnofabric index (ii) of I because the primary sedimentary structures are readily observable and no bioturbation is evident (Figure 37). These sedimentary structures, as observed in outcrop, consist of thick (>Icm ) graded beds with skeletal debris, either echinoderm plates, trilobite fragments, or archaeocyathids, at their bases, bioclastic limestone beds, thin (<2cm) somewhat planar beds, and fine (<lm m ) laminations. No ripples or other such structures were observed. When considered together, it seems likely that these structures were created in a calm offshore depositional environment periodically disturbed by turbidity events, most likely storm-derived. Of the specimens examined in this study, two of them were preserved in life position. As will be discussed below, helicoplacoid specimens were preserved by rapid burial in obrution deposits. As would be expected in an obrution deposit, helicoplacoids are almost always preserved lying flat on their sides (Figure 38) (Brett, Baird, and Speyer, 1997). Any specimen, then, in which part of the individual is preserved vertically-oriented must have been preserved in situ. The first specimen that was preserved in situ, P4-F34, has its lower end preserved vertically inserted into the rock, while the upper portion is lying flat on the uppermost bedding plane (Figure 39). Well-preserved columns of plates can be seen spiralling upwards in the depression left by the lower end, indicating that the depression is not just a random accumulation of helicoplacoid plates. Loose plates surround the depression, evidence that the individual was partially ruptured or tom by the force of the burial (Figure Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 102 Figure 36- Photograph taken in Westgard Pass of relatively rare horizontal Planolites trace fossils in the Middle Member of the Poleta Formation; Rock hammer for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10? Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 104 Figure 37- Photograph taken at the new helicoplacoid locality of an outcrop of the Middle Member of the Poleta Formation, displaying the relatively unbioturbated (ii L ) field appearance of the shales in which the helicoplacoids are preserved; field of view is approximately 45 cm in height. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i t K 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 106 Figure 38- Photograph of well-preserved helicoplacoid specimen P4-F37; As with the vast majority of helicoplacoid specimens, it has been preserved lying flat on its side because it was preserved in an obrution event; U.S. one cent coin for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10" Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 Figure 39- Photograph of in situ helicoplacoid specimen P4-F34; See text for description and discussion; U.S. one cent coin 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. 110 39). Based on how it is preserved, this specimen was clearly living in an upright position with its lower end inserted in the sediment, a mud-sticker. A second specimen, P4-F20, also was preserved in life position. This specimen, unlike P4-F34, is a relatively large circular depression (Figure 40). Inside this depression, the body of the helicoplacoid spirals upward from the base. Most of the plates are rather chaotically arranged, indicating that this individual had decayed slightly before burial. Well-preserved columns of plates concentrically fringe some of the outer edges of the depression. Based on these observations, this individual apparently was living upright as a mud-sticker and then, after death, collapsed upon itself shortly before rapid burial. The functional morphology of helicoplacoids also indicates that they were living as suspension-feeding mud-stickers. The triradiate ambulacra are restricted to approximately the upper two-thirds of the helicoplacoid body, while the lowermost third contains no ambulacra and is often composed of interambulacral columns that are arranged straight upward as opposed to helical (Figure 41). This arrangement of the ambulacra is ideally suited for both suspension feeding and mud sticking. The ambulacra, by nature of their helical arrangement, are present on every surface of the helicoplacoid that would have been in contact with the water column. Assuming that these ambulacra played a central role in feeding, this arrangement would be advantagous in suspension feeding. The length of these ambulacra is also maximized by their helical arrangment. The lower third meanwhile, with no ambulacra, is well-adapted to insertion in the sediment. The straight interambulacral columns in this area indicate that this portion of the body grew upward, perhaps to lengthen itself for insertion into the sediment or to keep up with slowly accumulating sediment (Figure 41). The lack of ambulacra in this body region also indicates that it was inserted in the sediment, because it is highly doubtful that their ambulacra would be located in an area permanently inserted in the substrate. What is quite interesting to note about the functional morphology of helicoplacoids Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ill Figure 40- Photograph of in situ helicoplacoid specimen P4-F20; See text for description and discussion; U.S. one cent coin 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. 113 Figure 41- Generalized reconstruction of a helicoplacoid echinoderm in life position, with important morphological features labelled; See text for discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A m b u la c r a p r e s e n t o n ly o n — u p p e r 2 /3 of b o d y , w e ll-p la c e d fo r su sp e n sio n fe e d in g . L o w e r 1 /3 of b o d y w e ll-su ite d fo r in s e r tio n in th e s e d im e n t: d ev o id of a m b u la c r a a n d p a r tia lly c o m p o sed of u p r ig h t c o lu m n s. 115 is that, although they were apparently living on a muddy seafloor, they show none o f the typical suspension-feeder adaptations for survival on soft substrates. There is no evidence that they encrusted or attached to any hard substrate, a common survival technique for soft- substrate suspension feeders. Nor did they have root-like holdfasts to stabilize themselves in soft substrates, a common technique for fossil crinoids (Sprinkle and Guensburg, 1995). Their morphology also reveals that their body mass was centered over a single point, which would have caused them to easily sink into a soft substrate (Thayer, 1975). Clearly, then, the morphology of helicoplacoids is completely inconsistent with survival on soft substrates. The sediment on which they were living, therefore, regardless of its composition, had to have been relatively firm. Other fossils preserved in the Middle Member of the Poleta Formation provide insight into the synecology of helicoplacoids. Triiobites and archaecyathids are the other benthic metazoans preserved with helicoplacoids. As discussed earlier, there was undoubtedly a rather diverse unpreserved soft-bodied fauna, probably resembling the Chengjiang fauna, with which helicoplacoids interacted. In any case, triiobites are easily the most abundant fossils found in the Middle Member of the Poleta Formation (Figure 42). Most of them were probably deposit feeders living at or near the sediment-water interface. Fragments of their skeletons are common and easily detectable in the x-radiographs and thin sections utilized in this research. While less abundant than the triiobites and helicoplacoids, archaeocyathids are also relatively common in the Middle Member of the Poleta Formation (Figure 43). Like helicoplacoids, they lived as suspension feeders. Clearly the record is incomplete, but helicoplacoids certainly shared the seafloor with triiobites, archaeocyathids, and numerous unpreserved soft-bodied metazoans. The x-radiograph cores of the six outcrops allowed for a more detailed examination of the sedimentary and biogenic structures in the substrate on which the mud-sticking Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 116 Figure 42- Photograph of an Olenellid trilobite cephalon collected from the new helicoplacoid locality in Westgard Pass; triiobites are easily the most common metazoan fossils in the Middle Member of the Poleta Formation; U.S. one cent coin 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. 118 Figure 43- Photograph of a bed of archaeocyathids from the new helicoplacoid locality in Westgard Pass; archaeocyathids are the least common metazoan fossil at the new helicoplacoid locality; U.S. one cent coin 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. 120 helicoplacoids were living. The relative stratigraphic positions and thicknesses of these six outcrops are shown in Figure 44. The same sedimentary structures visible in the field were more clearly displayed in the x-radiographs. Small horizontally-bioturbated intervals that were invisible in the field were, however, visible in the x-radiographs. The largest outcrop which was sampled, encompassing approximately 83.5 cm of stratigraphy, is Site B (Figures 45-48). Its x-radiograph core contains many interesting sedimentary structures and otherwise obscure biogenic features. A total of 30 samples were x-radiographed from this outcrop and reassembled to form the core. The uppermost 7 cm of this core contain three relatively thick beds 2 cm, 4 cm, and I cm in thickness (Figure 45). The contacts between them are well-defined and do not appear to be significantly disturbed. Small objects, possibly echinoderm plates, can be seen preserved in a small area along the contact between the first two beds. The numerous small, dark spots in these beds are probably diagenetic pyrite or chlorite crystals which are readily identifiable in thin sections. It is possible, even likely, that some unobservable bioturbation exists in this portion of the core, but it certainly does not extend vertically any greater than 4 cm, the thickness of the thickest bed in this interval, because the contacts between these 3 beds are undisturbed. Many interesting features are visible in the 7-12 cm interval of the core (Figure 45). The first 2 cm consist of two distinct beds of differing composition, as evidenced by their constrasting colors. The upper bed is lighter in color and is even lighter in its lowermost 2 mm. The lower bed, by contrast, is much darker than the overlying bed indicating that it is much more dense. Although it is very dark, some smaller, somewhat lighter, beds are visible within this larger bed. Small (<lmm) laminations are barely identifable in the lower right portion of this bed. There does appear to be some bioturbation in this bed (ii 2), but it could not extend vertically beyond 1 cm because the upper and lower contacts of this bed are so sharp. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 121 Figure 44- Relative stratigraphic positions above the Lower Member of the Poleta Formation of the six outcrops sampled in this research; See text for mm- scale description and discussion of the x-radiograph cores of these intervals. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 122 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 123 Figure 45- The uppermost 25 cm of the Site B x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 125 Figure 46- The interval of the Site B x-radiograph core from 25 to 50.5 cm; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 127 Figure 47- The interval of the Site B x-radiograph core from 50.5 to 75 cm; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 129 Figure 48- The lowermost interval of the Site B x-radiograph core, from 75 to 83.5 cm; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13i • Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131 Below these two beds, from 9-12 cm, is a series of beds which nicely display some biogenic features (Figure 45). In the upper portion of this interval are two wavy beds, one light and one dark. Below these two beds, from about 10-12 cm, is an interval of obviously disturbed (ii 3), even slightly mottled, beds and laminations. Ovoid to subcircular light-colored traces are visible in these beds and laminations. This bioturbation appears horizontal in nature because the shape of the visible traces is consistent with cross- sections through horizontal burrows, and because of the way in which the original sedimentary structures do not appear to be significantly vertically-mixed. The next 2 cm of core, from 12-14 cm, closely resemble the preceding 3 cm. There are two beds of differing colors, and the boundary between them is somewhat mottled (Figure 45). The beds in this interval have clearly been disturbed (ii 3), and several cross- sections of horizontal traces are visible. Again, this bioturbation appears to be horizontally- oriented. At the base of this somewhat bioturbated interval is a 3 mm thick interval of sub millimeter laminations, indicating a distinctive lack of bioturbation. Below these laminations, from 14.3-16.8 cm, are 2.5 cm consisting of three distinct beds, 0.9 cm, 0.8 cm, and 0.8 cm thick (Figure 45). The upper bed appears to be slightly bioturbated (ii 2) while the lower two show no evidence of bioturbation. So the pattern of bioturbation being restricted to distinct beds, or groups of beds, continues. Horizontal bioturbation returns, however, in the next 6 cm of core, from 16.8-22.8 cm (Figure 45). The upper 1.7 cm of this interval is composed of sub-millimeter laminations which are periodically slightly disturbed by bioturbation (ii 2). The beds in the remaining 4.3 cm of this interval progressively thicken downward and are somewhat bioturbated (ii 3). Cross-sections of individual traces can be observed, particularly in the first 1 cm of this interval, and, more specifically, along the base of the laminated interval above. Fine laminations are also readily observable in the upper part of this interval, but the beds in the lowermost 3 cm are somewhat blurred. Even though this interval is slightly Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132 bioturbated, the original sedimentary structures are still readily detectable and the bioturbation is once again horizontal in nature. There is a lenticular bed, from about 22.8-24.3 cm, of completely unbioturbated, somewhat wavy, sub-millimeter laminations below the interval described above (Figure 45). It extends for approximately 10 cm horizontally and is about 1.5 cm thick at its thickest point. The beds from 24.3-25 cm, below these unbioturbated laminae, appear to be slightly bioturbated (ii 3), and, again, several recognizable horizontal traces are concentrated at the base of the overlying laminations. The next 17.5 cm of the core, from 25-42.5 cm, is difficult to interpret (Figure 46). Except for the beds of bioclastic debris at about 36.5 cm and at its base, there are no clear signs of either primary sedimentary structures or bioturbation in this interval. It is either possible that this interval is heavily bioturbated by horizontal burrowers (ii 6), most likely during a period of decreased sedimentation, or that it represents a single depositional event involving homogeneous sediment. While it is unclear exactly what this interval represents, it would not be unexpected to have occasional heavily bioturbated (ii 6) intervals in a setting in transition between the matgrounds of the Proterozoic and the mixgrounds of most of the Phanerozoic. The interval from about 42.5 cm to 50.5 cm is also difficult to interpret (Figure 46). There are several bioclastic beds, but they are not planar. They appear to contain mostly echinoderm plates with some trilobite fragments. Several large pyrite crystals are also visible in this interval. The lowermost 4 cm or so resembles a turbiditic event bed, but it is not well graded or, with the exception of the lower 0.5 cm, evenly bedded. The large dark object on the lower left of the interval is most likely an archaeocyathid, although one might expect it to be lying more horizontally. This lower 4 cm possibly represents a thick event bed that has lost some of its primary sedimentary structure due to horizontal bioturbation (ii 3). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133 Three distinct beds are present in the next 5 cm or so o f the core, from about 50.5 to 55.5 cm, with their contacts visible at about 52 and 54 cm (Figure 47). Bioclastic debris and pyrite crystals are concentrated at these contacts. No visible signs of bioturbation exist in these beds, but it is possible, however, that horizontal bioturbation took place within each of these beds. The well-defined contacts of these beds indicate that this possible bioturbation would have been horizontal in nature and limited vertically to less than 2 cm of the substrate. An abundance of bioclastic debris characterizes the next 4.5 cm of the core, from 55.5 to 60 cm (Figure 47). The small white objects are echinoderm plates and the very thin black lines are trilobite fragments. The large black ovoid objects toward the bottom of the bed are archaeocyathids, one of which is white because it has dissolved away. The archaeocyathids are concentrated near the base of this bed, revealing its graded nature. This bed was clearly deposited in higher energy conditions and is probably storm-derived. No clear evidence for bioturbation is evident in this interval. The next 4 cm of the core, from 60 to 64 cm, contains relatively thin beds and some laminations (Figure 47). Consistent with previous observations, these thin beds and laminations are slightly disturbed by horizontal bioturbation (ii 2). What appears to be a particularly large oval trace is visible about half-way down this interval. It cuts through the laminations, deforming the beds above it. Laminated beds at the base of this interval are cut into from below by many small horizontal burrows (ii 3). These burrows are concentrated in one area, indicating that these horizontal burrowers, perhaps undermat miners (Seilacher, 1999), were seeking to reach an area particularly rich in organic material. Indeed, these burrows do cut into a bed that is dark with diagenetic minerals, including an obvious pyrite crystal, indicating that it was at one point organic-rich. Evidence for horizontal bioturbation abounds in the next 3 cm of core, from aproximately 64 to 67 cm (Figure 47). This interval consists of thin beds periodically Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 134 disturbed by easily recognizable horizontal burrows (ii 3), which are visible as round to oval-shaped white spots. Many of these burrows do appear to be concentrated at the upper boundary of this interval, just below the laminations in the interval above. As mentioned above, if microbial mats were indeed present in the laminations in question, this preferential distribution of burrows suggests that they were made by undermat miners (Seilacher, 1999). Although darker in x-radiograph, the next 4 cm of core, from 67 to 71 cm, is similar in many regards to the 3 cm of core which it underlies (Figure 47). Somewhat thicker-bedded than the interval above it, this interval also appears to be somewhat more heavily bioturbated (ii 3). Although the original bedding is still plainly visible, it is mottled at times by numerous horizontal burrows which are seen here as gray to white ovals. The final 12.5 cm of this core, from 71 to 83.5 cm, is extremely thin bedded, almost laminated at times, and occasionally disturbed by horizontal burrows (ii 2) (Figures 47 and 48). These burrows are often isolated, but are also concentrated on bedding planes as seen at about 73 and 80.5 cm where the beds are white and mottled in places. Individual traces are readily visible disturbing the thin beds just below and to the left of the somewhat mottled bed at 73 cm. Another zone of mottled beds is also evident at about 74 cm. Bioclastic material is not present in this interval, but dark diagenetic minerals can be seen. Site C contains the most dramatic storm-deposited beds observed in this study. The 7 samples from this site represent approximately 31.8 cm of stratigraphy (Figures 49 and 50). The uppermost 6 cm of this site consists of a massive somewhat-graded bed containing numerous trilobite fragments, visible as thin dark slivers, some echinoderm plates, and even a large archaeocyathid in the lower left at the bottom of the bed (Figure 49). The bottom of this bed is highly uneven indicating that this turbiditic event scoured the beds beneath it during deposition. Thin sections reveal that this interval is highly chloritized and that, in fact, all of the trilobite fragments are replaced by chlorite. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 135 Figure 49- The uppermost 22.4 cm of Site C x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137 Figure 50- The lowermost interval, from 22.4 to 31.8 cm, of the Site C x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 139 The next 3.2 cm or so of this core, from about 6-9.3 cm, contains 3 distinct graded beds, each representing another turbiditic event (Figure 49). All three have a relatively thick, about 0.5 cm, layer of echinoderm plates and trilobite fragments at their base. The base of the uppermost bed is rather uneven, again indicating scour, while the two lowermost beds are fairly planar. The base of the second bed is rather difficult to see because it is near the interface between the two rock samples involved, and it is badly weathered in the upper sample. No evidence for bioturbation of any kind is present in this core so far (ii 1), and, as has been noted before, if there is any undetectable bioturbation present it must be horizontal in nature and restricted to narrow stratigraphic intervals because the contacts between all these beds are so sharp. Although difficult to detect due to the lightness of this particular x-ray, there are several planar beds which are lighter than the surrounding x-ray about 2-2.5 cm below the last graded bed discussed above (Figure 49). Laminae amongst these beds are also visible on the left side of this x-ray, as are several white rectangles which are most likely helicoplacoid plates. Several of these helicoplacoid plates can also be seen above these planar beds. These plates are aligned with the planar beds, indicating that they have not been disturbed since deposition. This is the only evidence for bedding in this portion of the core because of the weathering of this sample (the lines shaped like a wishbone are the result of weathering), and because of its low-grade metamorphosis as revealed by the numerous large chlorite crystals which are the dark spots in this interval. The evidence, although somewhat obscured, indicates that the interval of the core from 9.3-13.1 cm also has undergone no significant vertical bioturbation (ii 1). The next 4.9 cm of core, from 13.1-18.0 cm, is another thick graded bed with abundant trilobite fragments and echinoderm plates (Figure 49). Many echinoderm plates are particularly visible near the base of the bed revealing its graded nature. This bed, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140 because it is graded and contains so much bioclastic material, was clearly deposited in a turbidity event, probably a storm. No bioturbation is evident in this interval. Abundant bioclastic material is present in the next 2 cm of core, from 18 to 20 cm (Figure 49). This interval appears to be a somewhat thin-bedded, but it is full of trilobite fragments, particularly at its base, and echinoderm plates. It should be noted that this interval was heavily weathered in the outcrop, perhaps in part because of its high content of calcite echinoderm plates, which dissolve easily compared to more siliciclastic facies. Even then, no evidence for bioturbation exists in this interval. The next 8.2 cm of the core, from 20 to 28.2 cm, also were in this heavily weathered zone, making this interval difficult to interpret from the x-radiograph alone (Figures 49 and 50). The uppermost 2.4 cm o f this interval, which is too dark to be informative in x-ray but is readily observable in hand sample and thin section, is the weathered, and perhaps slightly metamorphosed, remains of a bioclastic limestone bed which probably consisted almost entirely of echinoderm plates. Similarly, the remaining 5.8 cm of this interval is difficult to interpret in x-ray, but appears in hand sample to have been a bioclastic-rich zone which has since been heavily weathered. Because it is so weathered, this interval is uninformative as far as bioturbation and paleoenvironment is concerned. The final 3.6 cm of this core, from 28.2 to 31.8 cm, however, displays no clear evidence for bioturbation (Figure 50). This interval is marked by the presence of two thin, wavy bioclastic beds consisting almost entirely of chloritized trilobite fragments. About I cm below these bioclastic beds are several thin beds that contain some echinoderm plates. Thin beds can also be seen near the base of the core, and appear to be undisturbed by bioturbation. Again, if any bioturbation is present, it is vertically restricted within thin beds because the contacts between beds are so sharp. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 141 The core from Site D is the third longest in this study, with 10 samples totalling approximately 28 cm of stratigraphy (Figure 51). The upper 3.2 cm of this core displays clear evidence for horizontal bioturbation (ii 2) (Figure 51). The cluster of white and gray ovals between 1 and 2 cm are cross sections of horizontal burrows. Even where there are no discernible individual traces, the beds are slightly mottled, particularly at the base of the interval. Bioclastic material is rare in this interval, with just a few trilobite fragments around 1.5 cm. The next interval of this core, from 3.2 to about 8 cm, is thin-bedded, sometimes laminated, and somewhat bioturbated in places (Figure 51). The upper 1.5 cm or so of this interval contains thin laminations and beds with no signs of bioturbation (ii I). In the next 2.5 cm, however, the laminations and beds are less distinct, giving this portion of the core a mottled appearance (ii 2). This mottling is due to horizontal bioturbation because it is restricted to such a thin, 2.5 cm, vertical interval. The lower 0.8 cm of this interval marks the return of the non bioturbated laminations and thin beds (ii 1), culminating in the relatively thick, 2-3 mm, dark bioclastic bed at its base. The interval from approximately 8 to 13 cm is thin-bedded, with many undisturbed planar contacts visible between these beds (Figure 51). A bioclastic bed consisting entirely of trilobite fragments marks the base of this interval. Many of the thin beds near the top of this interval appear to be disturbed by horizontal bioturbation (ii 3), and some gray ovals are visible, cross-sections of horizontal burrows. Some bioturbation is also evident on the right just above the basal bioclastic bed. Again, all of this bioturbation is horizontal and limited to narrow vertical intervals, either individual beds or sets of beds. A thick bioclastic limestone bed characterizes the next 7.4 cm of this core, from 13 to 20.4 cm (Figure 51). This limestone bed is the thick, lenticular, gray bed from 16.5 to 19.2 cm. Although not visible in the x-ray because they have probably been slightly altered to siderite, this bed consists entirely of echinoderm plates. The beds directly above and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 142 Figure 51- The Site D x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 144 below this limestone mimic its lenticular shape and are relatively thick, with a few small laminations visible just above the limestone bed. The uppermost 1.5 cm or so of this interval contains roughly planar beds that are slightly disturbed by bioturbation (ii 2) and contain a few trilobite fragments. Otherwise, this interval is free of evidence for bioturbation (ii 1). The final 7.6 cm of this core contains many thin beds and some laminations (Figure 51). No bioclastic material is present in this interval, and it appears to be unbioturbated (ii 1). If there is any bioturbation, it is vertically restricted within these thin (< 5 mm) beds. Diagenetic minerals occur in distinct bedding planes near the base of this interval, possibly indicating that these beds were originally rich in organic material. Comprising approximately 18.8 cm of stratigraphy in 8 samples, the Site E core contains features ranging from thick storm-deposited bioclastic beds to thin laminations, all of which exhibit little direct evidence for bioturbation (Figure 52). The uppermost 4.2 cm of the core show little sign of either primary sedimentary structures or bioturbation (Figure 52). This may be more an artifact of the poor quality of the x-ray, which had degraded before positives could be made. Even then it is possible, as has been discussed previously, that this interval has been thoroughly bioturbated (ii 6). The relatively sharp contact between this interval and the bed below it, though, indicates that this bioturbation would have been vertically limited to this interval and, thereby, was horizontal in nature. The next interval of this core, from about 4.2 to 9.8 cm, contains abundant bioclastic material, with a concentration of echinoderm plates at its base (Figure 52). It appears that this interval consists of several beds because some relatively thin beds are somewhat visible within the bioclastic material, which consists mostly of echinoderm plates. So several storm events probably were involved in the deposition of this interval. No evidence for bioturbation exists in this interval (ii 1). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145 Figure 52- The Site E x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 147 Although not much is detectable in the next 4.1 cm of core, from 9.8 to 13.9 cm, several bedding planes are visible as horizontal white lines (Figure 52). Even though the evidence is minimal, the fact that the original bedding o f this interval appears to be preserved indicates that any bioturbation present is vertically restricted to these beds. Thin beds and laminations dominate the interval o f this core from 13.9 to 17.6 cm (Figure 52). The uppermost 3 mm o f this interval contains laminations and several echinoderm plates. A thin bed below separates these laminations from a thicker interval of laminations. The lower portion of these laminations is obscured by the poor quality of the x-ray, again because it degraded before positives could be made. Clearly, there is no bioturbation in this interval (ii 1), as the laminations are undisturbed. The final interval in this core, from 17.6 to 18.8 cm, exhibits the only clear evidence for bioturbation in this core (Figure 52). This interval is heavily mottled (ii 4), presumably because bioturbation has destroyed most evidence of its original sedimentary structures. While this bioturbation is thorough, it is horizontal in nature because, as far as can be detected from this core, it does not extend vertically beyond at least 1.2 cm. The Site A core is relatively small, consisting of 6 samples comprising approximately 17.5 cm of stratigraphy (Figure 53). The uppermost 2.9 cm of this core contains at least 4 thin to medium beds with the top bed underlain by a thin layer of trilobite fragments (Figure 53). The two beds beneath the top bed are less distinct, but are visible because they have some echinoderm plates at their bases. The bottom bed in this interval has an uneven base which is somewhat blurry, perhaps due to horizontal bioturbation (ii 2). Otherwise, there is no evidence for bioturbation in this interval (ii 1). The next 5 cm of the core, from 2.9 to 7.9, closely resembles the uppermost interval, but is somewhat thicker-bedded until near its base where it is partially laminated (Figure 53). The contacts between the thicker upper beds are rather sharp and appear to be unbioturbated (ii 1). There probably was some horizontal bioturbation within these beds, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148 Figure 53- The Site A x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. l-W Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 150 but it certainly was not vertically extensive. A relatively thick bed, about 4 mm, of trilobite fragments and echinoderm plates is at the base of this interval, indicating storm deposition of these beds. The next interval, which extends from 7.9 to 10.9 cm, consists of graded beds with layers of trilobite fragments and echinoderm plates at their bases (Figure 53). This interval, while containing more bioclastic material than the beds above it, also appears to be more bioturbated (ii 2) because the contacts between the beds are less distinct and some horizontal burrows are visible as white ovals. The teardrop-shaped white objects scattered throughout this core are a product of the x-ray development process. The final interval of this core, from 10.9 to 17.5 cm, is difficult to interpret because no sedimentary or biogenic structures are readily apparent (Figure 53). A similar interval is found in the Site B core (Figure 46). As discussed earlier, this interval could either represent a single event involving homogeneous sediment or, perhaps more likely, an interval in which horizontal bioturbation has obliterated any primary sedimentary structures (ii 6). The Site F core is the smallest in this study. It consists of 4 samples which comprise just 12.7 cm of stratigraphy (Figure 54). The upper 8 cm or so of this interval consists of thin beds, some of which contain bioclastic material, in this case echinoderm plates and trolibite fragments (Figure 54). Signs of horizontal bioturbation are present in this interval (ii 2) in the form of disturbed beds at about 1.5 and 2 cm and a few individual horizontal burrows seen as white ovals. The lowermost 4.7 cm of this core, from 8 to 12.7 cm, contains several bioclastic- rich storm-deposited beds (Figure 54). The uppermost of these beds is not planar, possibly because it scoured the underlying sediment during deposition. The base of this bed contains echinoderm plates and trilobite fragments. Below this upper bed is another graded bed with bioclastic material at its base. In this case, the base of this bed is planar Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 151 Figure 54- The Site F x-radiograph core; See text for mm-scale description and discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 153 and appears to contain more trilobite fragments than the bed above it. The remaining 3.5 cm of this core, from 9.2 to 12.7 cm, contains abundant bioclastic material, mostly echinoderm plates seen as white rectangles. The base of this bed cannot be seen, but it was presumably another relatively thick graded bed rich in bioclastic material, which was most likely storm-deposited. There is no clear evidence for bioturbation in this last interval of the core (ii I). The 1.92 m of x-radiograph core described above contain several significant, recurring, features. Perhaps the most interesting trend in these cores is the pervasive low- to-moderate levels (ii 2-ii 3) of horizontal bioturbation. This bioturbation is often represented by cross-sections of horizontal traces usually less than 5 mm wide (ii 2) and slightly mottled intervals of very limited vertical extent (ii 3). Some traces appear to be concentrated at the bases of laminated intervals, possibly indicating, if microbial mats were actually present, that they were formed by undermat miners. This bioturbation, with a few possible exceptions, is restricted to individual beds and sets of beds. These generally low levels (ii 2-ii 3) of horizontal bioturbation would not have significantly increased the water content of the substrate, causing it be relatively firm. The sediment-water interface would have also been sharp in these settings because of the complete lack of significant vertical bioturbation. The sedimentary structures visible in these x-radiograph cores also provide insight into the paleoenvironment in which helicoplacoids lived. The common sedimentary structures in these cores are thin (generally <1 cm) beds which are often graded in thin section, laminations (generally <1 mm thick), and thick (generally >2 cm) graded beds rich in bioclastic material (Figures 45-54). These sedimentary structures are interpreted as representing a calm outer shelf environment that is periodically disturbed by turbiditic events, probably storms. The laminations were deposited during the calm time intervals, but, because these laminated intervals are relatively rare and thin, most of the deposition in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 154 these cores took place during influxes of sediment during storm events. The thick, bioclastic-rich graded beds are clearly the product of storm events, and even many of the thin beds are graded in thin section. The most prominent feature of these rocks in thin section is the abundance of chlorite, giving the shale its strong green color. The pervasive presence of chlorite in these rocks indicates that they were subjected to low-grade metamorphism. Fortunately, in this location at least, this metamorphism was not powerful enough to destroy original sedimentary and biogenic structures, nor alter fossil remains beyond recognizability, although some trilobite cephalons have clearly been slightly deformed. The common mineral assemblage in these rocks consists of chlorite, quartz, heavy minerals (pyrite, etc.), and in some cases micas. Bioclasts are also common and often consist of trilobite fragments, which have been replaced by chlorite, and echinoderm plates (Figures 55 and 56). The echinoderm plates are usually altered, probably to siderite, because their stereom has been destroyed by cleavage planes (Figure 57), they commonly have a deep orange color suggestive of iron enrichment, and Alizarin Red staining of a thick bed of echinoderm plates indicates that they were extremely rich in iron. The size of the quartz grains ranges from silt to very fine sand. No carbonaceous laminae were found in this study, but micaceous laminae are rather common. These micas are usually roughly aligned to bedding and are sometimes concentrated along bedding planes. These laminated intervals tend to consist of alternating quartz silt-very fine sand and chlorite-rich layers, with aligned micas present throughout or concentrated in the chlorite layers (Figure 58). Micas do not appear as common in non laminated intervals. Heavy minerals such as pyrite are common throughout all of these thin sections. No wavy-crinkly laminae were found in this study. Cohesive behavior of very fine sand is evident in these rocks (Figure 59). In a bioclastic-rich bed deposited under turbid conditions, most likely a storm event, small (mm Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155 Figure 55- Photograph of a bed of chloritized trilobite fragments, in thin section C9, under cross-polarized light; Field of view is approximately 6 mm across. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157 Figure 56- Photograph of an echinoderm plate bed, in thin section D4, under cross polarized light; Field of view is approximately 6 mm in height. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I5S Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 159 Figure 57- Photograph of an echinoderm plate, in thin section C7b, under cross-polarized light; Notice the typical "spongy" echinoderm stereom near the center of the plate surrounded by altered stereom featuring cleavage on the outer portion of the plate; Field of view is approximately 3 mm across. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 161 Figure 58- Photograph of micaceous laminae under cross-polarized light in thin section B11; See text for discussion; Field of view is approximately 3 mm wide. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 163 Figure 59- Photograph of a sand-mica intraclast under cross-polarized light in thin section B21; See text for discussion; Field of view is approximately 3 mm across. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 164 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 165 scale) roughly rectangular intraclasts consisting of very fine sand-sized quartz grains and micas, somewhat aligned to the long direction of the clasts, are readily visible in the silty matrix (Figure 59). The clasts are easily distinguished from their matrix by their larger grain size and relative lack of chlorite. They closely resemble the lithology common to the laminated intervals described above, but are of a larger grain size. Clearly these clasts of quartz and mica grains must have been bound together by some agent in order for them to behave as a cohesive unit. No evidence of any cement is found in these particular clasts, so perhaps, as with microbial sand chips (Pfluger and Gresse, 1996), microbial mats gave them their cohesive character. Cohesive behavior is also displayed by another, much larger, intraclast. This intraclast is too large to be fully observed or photographed in thin section, but is easily observed in the slab from which the thin section was cut. It is rather long and wide (about 2 cm x 1 cm) and is resting perpendicular to bedding. It consists of very fine quartz grains and mica grains aligned with the bedding in the intraclast, but it has the altered remnant of a calcite cement, making this intraclast a calcareous very fine sandstone. Because of its perpendicular orientation and unfolded form, this clast displays very rigid behavior unlikely to be produced by microbial binding. It is more likely, therefore, that this clast was cemented by the calcite during or shortly after deposition, before it was disturbed and redeposited in its present position. The presence of this clast sheds doubt on the microbial origin of the other smaller intraclasts, which are found in close stratigraphic proximity to this large clast. Perhaps they too originally had a calcite cement which simply dissolved away during diagenesis. As noted earlier, heavy minerals such as pyrite are abundant in these rocks, indicating reducing conditions. These minerals are occasionally concentrated in thin horizons along bedding planes, but these horizons are usually not laterally extensive (Figure 60). The occurrence of these minerals themselves does not indicate the presence of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 166 Figure 60- Photograph of a planar concentration of heavy minerals under plane polarized light in thin section B23; See text for discussion; Field of view is approximately 3 mm across. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16" Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 168 microbial mats in these rocks, but, when considered with other evidence, these minerals can support the suggestion of microbial mat presence by serving as evidence for mat-decay mineralization (Schieber, 1999). The most compelling suggestion of microbial mats in these rocks actually comes from the presence of wrinkle structures. These structures, consisting o f oddly wrinkled bedding surfaces, have been shown by Hagadom and Bottjer (1997, 1999) to be suggestive of microbially-bound sediment. Two pieces of fine sandstone float from the Middle Member of the Poleta Formation contain these wrinkle structures (Figure 61). Thin sections reveal that these wrinkle structures contain a high concentration of mica (Figure 62). Looking back at the criteria set by Schieber (1999) for suggesting the presence of microbial mats in siliciclastic rocks, there is mild support for the suggestion that microbial mats were present in the sediments represented by the shales of the Middle Member of the Poleta Formation. While no wavy-crinkly laminae, carbonaceous laminae, or cohesive behavior, at least not without the possibility of synsedimentary cementation, were found in these rocks, micaceous laminae, mat-decay mineralization, and wrinkle structures are found in these rocks. If microbial mats were indeed present in this environment, they were most probably rather rare, as might be expected in an Early Cambrian environment in transition between the matgrounds of the Proterozoic and the mixgrounds o f the Phanerozoic (Seilacher, 1999). Taphonomv Given that their plates probably became disassociated under post-mortem seafloor exposure in a matter of days (Brett, Moffat, and Taylor, 1997) and that they commonly are preserved at the base of graded beds (Durham, 1967, 1993), all helicoplacoid specimens Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 169 Figure 61- Photograph of wrinkle structures, identifiable as Kinneyia, collected from the talus at the new helicoplacoid locality in Westgard Pass; They are suggestive of the presence of microbial mats in these rocks (Hagadom and Bottjer, 1997, 1999); See text for discussion; U.S. one cent coin for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 170 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 171 Figure 62- Photograph of wrinkle structures under cross-polarized light, in thin section P4- F38, collected at the new helicoplacoid locality in Westgard Pass; Notice the high concentration of mica, seen here as gold in color, along the surface of the wrinkle structure, probably because of mica capture by the sticky upper surface of a microbial mat (Schieber, 1999); Field of view is approximately 3 mm across. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 172 * V-;> k - VZ ^ V ' ..•*■■ 1 v ''" - ^ • W -.T v : Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 173 were most likely preserved in obrution deposits (Brett, Baird, and Speyer, 1997). The vast majority o f the specimens collected in this study (69%) and at LACMNH (62%) are Group 2 (partially disassociated) specimens (Table 1). This pattern is possibly due to a combination of two factors: 1) most helicoplacoid specimens may have been buried under only a thin (cO.lcm) mud layer, allowing for their partial decay following burial (Brett, Baird, and Speyer, 1997), and 2) many helicoplacoid specimens may have decayed on the seafloor before their rapid burial in obrution events. Given that few, if any, graded beds observed in x-radiographs of the helicoplacoid-bearing rocks were thinner than 0.1 cm, it appears that the predominance of Group 2 specimens has more to do with pre-burial seafloor decay of helicoplacoids than it does with their possible burial under thin mud blankets. Examination of x-radiographs of the rocks in which these helicoplacoid specimens are preserved indicates that there are some sedimentological differences between Groups 1 and 3. Group I specimens are generally associated with thin (<lcm ), sometimes graded, beds, but none of the 20 Group 1 specimens are associated with thick graded beds rich in bioclastic material. On the other hand, 6, or nearly half, of the 13 Group 3 specimens are associated with thick (>lcm ) graded beds rich in bioclastic material. Group 2 specimens are found associated with both of these end-members. It is also interesting to note that Group 1 and 3 specimens are never found together, but they are both found with Group 2 specimens. This x-radiographic evidence indicates that Group 3 specimens are generally preserved in higher energy regimes than Group 1 specimens, while Group 2 specimens are preserved in a broad range of energy regimes. An explanation for these observed differences may be that higher energy conditions aided in the almost complete disassociation of many Group 3 specimens by further disassociating them during transport (Kidwell and Baumiller, 1990). Because transport was probably minimal, even in these Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 174 Table 1- The percentage of helicoplacoid specimens at USC and LACMNH in each taphonomic group; See text for explanation of groups. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 175 Taphonomic USC LACMNH Group % % 1 19 8 2 69 62 3 12 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 176 higher energy obrution events, this process would require that the helicoplacoid in question had decayed substantially on the seafloor before the occurrence of the obrution event. Not only were most helicoplacoid specimens collected in this study in Group 2, but 73% were found preserved on the same bedding plane with at least one other individual, and 39% were found preserved on a bedding plane containing at least 10 individuals. The fact that helicoplacoids are most often preserved with other individuals, and often with numerous other individuals, attests to both their gregarious nature and their preservation in mass mortality obrution events in which numerous individuals are killed either shortly before or during rapid burial (Brett, Baird, and Speyer, 1997). As discussed earlier, the specimens collected in this study were carefully examined to test the hypothesis that the lowermost region of the helicoplacoid body was more solidly conctructed than the rest of the organism. This examination revealed that 78% of the specimens showed no preferential preservation, so no particular region of the body was more well-preserved than the rest of the individual. The lower region of the body was preferentially preserved in 15% of the specimens, while the upper region of the body was preferentially preserved in 7% of the specimens. Clearly, based on these numbers, there is no good evidence for the lower region of the helicoplacoid body being more solidly constructed than the rest of the individual. The orientations of 33 specimens on 3 separate slabs show some alignment, within each slab, when plotted on rose diagrams. The orientations of the specimens are usually concentrated in one quadrant of the rose diagrams, with a few outlying orientations (Figure 63). This alignment, while not ideal, provides even more evidence for the preservation of helicoplacoids in obrution deposits. There are several other factors which probably aided in the preservation of helicoplacoids. First of the all, the low levels of bioturbation in the substrate on which the helicoplacoids were living, and in which they were preserved, allowed for helicoplacoids to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 177 Figure 63- Rose diagrams of the orientations of helicoplacoid specimens on three separate slabs (P4-F19, P4-F43, and P l-Fl); Only the alignment of the specimens within each slab is significant, not their orientations; Percentages indicate the percentage of specimens in the largest "petals"; N- number of specimens; See text for discussion. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 178 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 179 remain relatively undisturbed by bioturbators following their burial (Brett, Baird, and Speyer, 1997). These low levels of bioturbation may have also allowed for the microbial stabilization of the sediment in which helicoplacoids were buried, as evidenced by the presence of suspect-microbial structures such as wrinkle structures throughout the Middle Member of the Poleta Formation, including the most abundant helicoplacoid fossil locality (Hagadom and Bottjer, 1999). The combination of minimal bioturbation and possible microbial stabilization of the substrate would have led to a redox boundary that was very close to the sediment-water interface. This shallow redox boundary may have aided in the preservation of helicoplacoids because once they were buried to only a shallow depth, they would have been in a reducing environment. This reducing environment would have hindered the further decay of the helicoplacoids. The usually calm depositional environment in which the helicoplacoids lived also helped facilitate their preservation. While this may sound strange because they are preserved in higher energy obrution events, most likely storms in this case, the preservation of the resulting obrution deposits is dependent on a relatively calm depositional environment (Brett, Baird, and Speyer, 1997). In an environment where obrution events occur one after the other, such as in many nearshore settings, the obrution deposits themselves are continually obliterated by the subsuequent obrution event, not allowing for their preservation (Brett, Baird, and Speyer, 1997). A delicate balance of energy regimes is required to form extensive obrution deposits like those in which the helicoplacoids are preserved. The ideal energy conditions for the formation and preservation of obrution deposits is therefore a calm, low-energy environment punctuated by occasional higher-energy events such as storms (Brett, Baird, and Speyer, 1997). Because the wonderful preservation of helicoplacoid specimens is restricted to the Middle Member of the Poleta Formation in the Westgard Pass area, it Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180 seems that the proper balance of energy regimes, in conjunction with the favorable factors discussed above, was achieved in this region, allowing for the small taphonomic window in which helicoplacoids were preserved. It is interesting to note that many of the attributes of the Late Neoproterozoic biotope defined by Hagadom and Bottjer (1999), such as low levels of bioturbation and microbial stabilization of the substrate, also probably aided in the preservation of the helicoplacoids. This research also demonstrates that the helicoplacoids were well-adapted to, and dependent on, these paleoenvironmental conditions. Because of this dependence, the helicoplacoids may have gone extinct during the increase in vertical bioturbation in shelf environments which accompanied the restriction of the Late Neoproterozoic biotope during the Cambrian (Hagadom and Bottjer, 1999). It appears, then, that the preservation of helicoplacoids was aided by the same environmental conditions on which they depended for survival. The Early Cambrian Extinction Event While it seems clear from the results of this research that helicoplacoids could not have survived on a more heavily bioturbated substrate, it still could be possible that there is some other explanation for their extinction. Perhaps some other event caused the extinction of helicoplacoids before bioturbation levels in siliciclastic shelf settings could increase enough to bring about their demise. With this possibility in mind, it might be fruitful to explore the possibility that a rather important biological event in the late Early Cambrian, a two-phase extinction event which is known to have had a profound effect on the reefal biota of the time, could possibly have played a role in the extinction of helicoplacoids. The two phases of this extinction are known as the Sinsk and Hawke Bay Events, and they took place during the mid-Botomian Stage and the Early Toyonian Stage respectively (Figure 64) (Zhuravlev, 1996). These stages are directly above the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181 Figure 64- Diagram of generic diversity of reef biota through the Early Cambrian extinction event and into the Early Ordovician; See text for discussion (modified from Zhuravlev, 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 182 1 II Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 183 Atdabanian, the latter part of which contains the helicoplacoids (Durham, 1993). So this extinction event took place after the helicoplacoids had already left the fossil record, suggesting, if their fossil record is indeed reflective of their existence, that helicoplacoids were already extinct by the time the first phase of this extinction began. The Sinsk Event is the first phase of this extinction event, and it is thought that it may have been caused by extensive eutrophy-induced anoxia (Zhuravlev, 1996). The evidence for this explanation is found in the form of black shale deposits in the mid- Botomian Sinsk Formation and its equivalents in Siberia, China, Australia, Iran, Kazakhstan, and Mongolia (Zhuravlev, 1996). These black shales, not unexpectedly, are unbioturbated, contain sub-mm laminations, abundant primary pyrite crystals, and a high organic carbon content (Zhuravlev, 1996). The global distribution of these shales suggests to Zhuravlev (1996) that they are the result of widespread anoxia due to eutrophication. Acritarch-rich calcareous laminae within these black shales are sited as evidence for this eutrophication because they indicate large-scale phytoplankton blooms Zhuravlev, 1996). The second phase of this extinction event is known as the Hawke Bay Event (Zhuravlev, 1996). Zhuravlev (1996) theorizes that the Hawke Bay Event, which may have been caused by a regression, might have been triggered by the Sinsk Event. According to Zhuravlev's (1996) model, the over-productivity which caused the anoxia of the Sinsk Event also weakened greenhouse conditions because the resultant carbon burial would have released oxygen to the atmosphere. This release of atmospheric oxygen would have then led to polar ice cap formation because of the compromised greenhouse conditions (Zhuravlev, 1996). Ice cap formation would then have caused the regression, which, in turn, restricted shelf area, causing the biological devastation of the Hawke Bay Event (Zhuravlev, 1996). Evidence for the above model is limited. The regression itself is well-documented by the global presence of Skolithos piperock and other nearshore facies in the lower Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 184 Toyonian (Zhuravlev, 1996). There is little tangible evidence, however, for the causal connection between the anoxia represented by the black shales of the Sinsk Formation and the regression of the Hawke Bay Event. While the precise causes o f the two phases of this extinction event and the relationship of these two phases to one another remain somewhat unclear, the biological impacts of the Early Cambrian extinction event are better known. This extinction event is essentially restricted to the reefal biota of the time, namely archaeocyathids, coralomorphs, radiocyathids, and calcified microbes (Figure 64), probably because they would have been least able to adapt to the decrease in shelf area associated with the Hawke Bay Event regression (Zhuravlev, 1996). Hardest-hit were the archaeocyathids and radiocyathids, which were both extinct by the end of the Toyonian, during the early Middle Cambrian (Figure 64) (Zhuravlev, 1996). The coralomorphs existed into the earliest Amgan Stage, just above the Toyonian, before going extinct (Figure 64) (Zhuravlev, 1996). Reef-associated grazing metazoans, such as molluscs and possibly some trilobites, also suffered heavily during the Sinsk Event (Zhuravlev, 1996). The effects of this event are not seen in the fossil record of echinoderms, which were primarily suspension-feeders during this time (Zhuravlev, 1996). Following the extinction of these three reef-building metazoans, calcified microbes dominated Cambrian reef ecosystems (Figure 64), possibly due to decreased grazing pressure by metazoans, some of which suffered from the extinction event (Zhuravlev, 1996). Calcified microbes flourished as reef-builders under these decreased grazing conditions, and metazoans did not return as significant reef-builders until the Ordovician (Zhuravlev, 1996). It was then that sponges and corals filled the niche o f metazoan reef- builders once occupied by archaeocyathids, radiocyathids, and coralomorphs (Figure 64). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 185 There is a small possibility that helicoplacoids were rendered extinct by the Early Cambrian extinction event, but several facts indicate otherwise. First of all, as mentioned earlier, helicoplacoids leave the fossil record in the Atdabanian, before this extinction event even took place, while the primary victims of this event linger on in the fossil record for a short time afterward (Figure 64). Secondly, there is no sign of this extinction event effecting echinoderms in any way (Zhuravlev, 1996), and, likewise, there is no sign of it impacting the Cambrian fauna in general (Figure 3) (Sepkoski, 1981). It does appear that this event was one which had a profound effect on the reefal biota of the Cambrian, but its effect seems otherwise limited. While still possible, it does not seem likely, based on the evidence available at this time, that helicoplacoids suffered extinction as a result of the Early Cambrian extinction event. Most likely, helicoplacoids had already gone extinct because of increasing levels of vertical bioturbation in siliciclastic settings by the time this extinction event took place. Lower Cambrian Echinoderm Plate Beds Considering that helicoplacoids are preserved in a taphonomic window exposed in the Westgard Pass area of California, it is reasonable to think that they lived in other depositional environments but simply were not preserved. Because most helicoplacoids are preserved as beds of disassociated plates (Durham, 1993), the presence of echinoderm plate beds in Lower Cambrian rocks may indicate the presence of helicoplacoids in these depositional environments, since they, along with a few edrioasteroids, are among the only skeletonized echinoderms found in the Early Cambrian. With these ideas in mind, a search of the literature was undertaken in order to determine whether helicoplacoids were specialists restricted to offshore shales, or whether they were generalists living in a wide range of depositional environments, as indicated by the facies distribution of Lower Cambrian echinoderm plate beds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 186 Durham (1993), while not exclusively dealing with echinoderm plate beds, does provide all known locations at which helicoplacoid specimens have been found. In most cases, however, Durham (1993) does not give information on the lithologies in which these specimens are preserved. Besides Westgard Pass, where the vast majority of helicoplacoid specimens have been recovered, helicoplacoids have been found in the Wood Canyon Formation of Death Valley, the Silver Peak area of Nevada, and at a locality in British Columbia (Durham, 1993). In these instances, except for the Wood Canyon Formation which contains shales and carbonates, Durham (1993) does not provide information on what facies these specimens are preserved in. Durham (1993) also mentions that disassociated helicoplacoid plates have been found in southwestern Nevada and northeastern Washington. But, again, no information was provided on the facies in which these plates were found (Durham, 1993). It does seem clear, based on the information provided by Durham (1993), that helicoplacoids were widely distributed along the northern coast of Laurentia during the Early Cambrian. Helicoplacoid specimens in the Poleta Formation are most commonly found in the shales of the Middle Member. Rare specimens have also been found, however, in the sandstones and bioclastic limestones of the Middle Member (Durham, 1993), indicating that helicoplacoids were not restricted to living in offshore shale environments. Although little work, then, has been done on the facies distribution of helicoplacoids and Early Cambrian echinoderm plate beds, the work that has been done indicates that helicoplacoids had a relatively broad geographical distribution and were probably generalists not restricted to living in a single depositional environment. As discussed above, they are found in abundance in Westgard Pass not because that is the only place in which they lived, but because of a taphonomic window. The fact that helicoplacoids, based on limited evidence, appear to have been generalists is not detrimental to the hypothesis that their survival was dependent on the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 187 sharp sediment-water interface and firmer substrate provided by low levels of vertical bioturbation. As long as the above conditions existed, helicoplacoids could probably survive regardless of the composition of the substrate on which they lived. In fact, they probably were better prepared to live on coarser sediments because their water vascular systems were probably particulary sensitive to fine suspended sediment (Seilacher et al., 1985). It just so happens that the correct conditions for their preservation existed in the offshore shales of the Poleta Formation, which was likely not their ideal living environment. Li and Droser (1997) conducted a study of Cambrian shell beds, but found no shell beds dominated by echinoderm plates in the Early Cambrian. This is probably because helicoplacoid plate beds are relatively rare during this time period, and also because helicoplacoid plate beds are usually very thin (<5mm) and, in outcrop, virtually unrecognizable to the untrained eye. Significant echinoderm plate beds do not appear until the Middle Cambrian (Li and Droser, 1997), when the helicoplacoids are no longer present in the fossil record. Modem Soft-Substrate Suspension Feeders In order to assert that helicoplacoids could not have survived on a thoroughly bioturbated substrate, the survival and feeding strategies of modem suspension feeders on soft substrates must be surveyed. The functional morphology of helicoplacoids can then be compared to that of these modem suspension feeders, which are presumably well-adapted to survive on the generally well-bioturbated substrates of today. This comparison will enable a comprehensive evaluation of the helicoplacoids' suitability for survival on soft, bioturbated substrates. Though minute, bryozoans are able to survive on soft substrates by encrusting whatever hard objects are available on the seafloor, often the skeletons of other, usually Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 188 dead, organisms. They feed with a lophophore, a group of ciliated tentacles arranged in the shape of a funnel (Figure 65) (Winston, 1977). Controversy exists as to exactly how bryozoans use their lophophores in feeding. Bullivant (1968) observed that in gymnolaemate bryozoans the lateral cilia on the tentacles of the lophophores beat outward, drawing water down into the lophophore and out between the tentacles. Food particles in the water were then caught in the low-energy area above the mouth and subsequently sucked into the mouth by muscular dilations of the pharynx (Bullivant, 1968). This type of feeding was termed "impingement feeding" by Bullivant (1968). Strathmann (1973) thinks that impingement feeding is unlikely to be the feeding mechanism of bryozoans because the density difference between the suspended particles and seawater is not large enough. The particles would most likely just move right through the lophophores without settling above the mouth (Strathmann, 1973). Instead, Strathmann (1973) argues that ciliary reversal is responsible for particle capture by bryozoans. In ciliary reversal, the lateral cilia, which normally beat outward, reverse their motion when a particle is detected by the cilia (Strathmann, 1973). This reversal of motion pushes the food particles in toward the mouth instead of out between the tentacles (Strathmann, 1973). Winston (1977) speculates that both impingement feeding and ciliary reversal likely play roles during bryozoan particle capture. Bryozoans can reject unwanted particles in a couple of ways. Some bryozoans reject particles by contracting the pharynx, thus sending the particles back out the mouth (Winston, 1977). Others have a tract of cilia in the mouth that reject particles by beating outward (Bullivant, 1968). In these ways, bryozoans are able to deal with unwanted particulate matter in the water column. The most important survival strategy of bryozoans for the purpose of this study is their ability to encrust hard substrates in otherwise soft, muddy areas. This adaptation to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 189 Figure 65- Diagram of a typical bryozoan showing the structure of the lophopore; Arrows show direction of feeding currents (modified from Winston, 1977). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 190 Tentacle Lophophore Mouth Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 191 soft substrates is thus far unseen in helicoplacoids, which were merely mud-stickers, indicating that the substrate on which the helicoplacoids lived was firm enough to support them, and that the sharpness of the sediment-water interface allowed for the proper functioning of their water vascular system. These substrate characteristics were provided by the lack of significant vertical bioturbation in the sediment on which helicoplacoids lived. Perhaps because of their prominence in the Paleozoic marine fossil record, the most well-studied modem suspension feeding echinoderms are the crinoids. In the past, crinoids survived on soft substrates, starting in the Late Ordovician, by either attaching to available hard substrates, using root-like holdfasts, or even wrapping their stems around other already-stabilized crinoids (Sprinkle and Guensburg, 1995). Today crinoids are dominated by the mobile comatulid crinoids. Crinoids suspension feed by direct interception, a process during which tentacles, in this case tube feet, are used to capture particles directly from the water. Direct interception in crinoids and other echinoderms involves the complex interaction of tube feet and cilia, allowing for great selectivity in particle retention (Holland et al., 1986; Lahaye and Jangoux, 1985). Polychaetes and cnidarians, in addition to echinoderms, employ this suspension feeding technique (Shimeta and Koehl, 1997). Lahaye and Jangoux (1985) performed a comprehensive study of the feeding technique of one particular comatulid crinoid, Antedon bifida. They found that Antedon fed using a repetitive system of "feeding units" consisting of three podia of different sizes, which are present along their pinnules (Figure 66) (Lahaye and Jangoux, 1985). The two largest podia, the primary and secondary, are responsible for capturing particles by direct mucous interception and then placing the individual particles on the ciliary tract running down the center of the pinnules, which carries the particles to the mouth (Figure 66) (Lahaye and Jangoux, 1985). The smallest, or tertiary, podia, meanwhile rub against the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 192 Figure 66- Idealized cross-section of an Antedon bifida pinnule; C- ciliary tract; L- protective lappet; P- primary podium; S- secondary podium; T- tertiary podium; TC- transverse cilia tract; Arrow indicates ciliary current direction (modified from Lahaye and Jangoux, 1985). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 193 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Muscle 194 ciliary current in order to embed the particles in mucous (Figure 66) (Lahaye and Jangoux, 1985). Holland et al. (1986) studied the feeding of another crinoid, Oligometra serripinna. They exposed Oligometra to nutritive and non-nutritive particles in a flume tank in order to study its particle selection techniques (Holland et al., 1986). Feeding in Oligometra is first stimulated by mechanoreception of the tube feet, which detect contact with particles and then curl over them slightly to retain them (Holland et al., 1986). Chemoreception may also play a role in the retention of particles by tube feet, and almost certainly plays a role in retention of the particle by the ciliary tract, which usually rejects non-nutritive particles (Holland et al., 1986). Most crinoids, therefore, have a complex feeding system involving rejection of undesirable particles (Lahaye and Jangoux, 1985; Holland et al., 1986). Although impossible to tell, it seems unlikely that helicoplacoids, given their stem- group status in the echinoderms, had such a complex system of feeding. More importantly, however, they did not employ the feature which today allows crinoids to live on soft substrates: mobility (Thayer, 1983). Most crinoids today are mobile, while during the Paleozoic, when bioturbation levels were lower, the vast majority of crinoids lived attached to hard substrates (Thayer, 1983). Mobility is also what allows the irregular echinoid Dendraster, which is usually an infaunal deposit feeder, to survive in dense populations as a suspension feeder just below the surf zone by standing on edge with part of its body protruding from the soft substrate (Thayer, 1983). If its suspension feeding becomes impossible to perform, perhaps because of bioturbation, Dendraster cm simply utilize its mobility to start deposit feeding, thereby doing some bioturbating of its own (Thayer, 1983). Interestingly, the part-time suspension feeder Dendraster has been known to exclude the sessile suspension-feeding sea pansy Renilla kollikeri when it creates bioturbation as a deposit feeder (Kastendiek, 1982). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 195 Other examples of modem soft substrate suspension feeders include various organisms well-adapted for survival in the deep sea. Other than the already-discussed crinoids, these organisms include sponges and pennatulids. Both of these groups of deep- sea suspension feeders are well-adapted for survival on the deep-sea floor, where there is significantly less bioturbation than on the shelves (Guinasso and Schink, 1975; Schink and Guinasso, 1977; Turekian et al., 1978; Peng et al., 1978; Cochran and Aller, 1979; Lee and Swartz, 1980). Consequently, many species of deep-sea sponges live with a long slender stalk inserted into the sediment, and the encrusting habit of shallow water sponges in greatly reduced in deep-sea sponges (Gage and Tyler, 1991), most likely because of the lower bioturbation levels in the substrate on which they live. Similarly, modem deep-sea pennatulids live with their stalks inserted in the sediment (Gage and Tyler, 1991), just as the pennatulacean Thaumaptilon, found in the Middle Cambrian Burgess Shale, lived (Conway Morris, 1993). It seems likely, then, that modem deep-sea sponges and pennatulids resemble sponges and pennatulids found preserved in the Early Cambrian Chengjiang Fauna and the Middle Cambrian Burgess Shale because they were all adapted to similar substrate conditions. The substrate characteristics of the modem deep-sea are comparable, it seems, to those of the Early Cambrian shelves (Thayer, 1983). All available evidence indicates that helicoplacoids did not employ any of the strategies utilized by modem suspension feeders on non-deep-sea soft substrates. Helicoplacoids probably did not have a complex system of particle retention and rejection comparable to that of modem crinoids, which would have enabled them to handle large amounts of suspended sediment, they did not attach to hard substrates, they were not mobile, they did not employ "iceberg" or "snowshoe" strategies in order to stay atop the substrate, and they were not inordinately large (Thayer, 1975, 1983; Lahaye and Jangoux, 1985; Holland et al., 1986). In order for helicoplacoids to survive, therefore, the substrate Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 196 on which they were living as mudstickers had to be relatively firm with a sharp sediment- water interface. Other Cambrian Fauna Suspension-Feeding Benthos Helicoplacoids are not the only Cambrian Fauna suspension feeding echinoderms that were probably dependent on the firmer substrate provided by low levels of vertical bioturbation in offshore siliciclastic settings during the Early to Middle Cambrian. Based on their sizes and unique morphologies, several stemless eocrinoids were most likely dependent on a rather sharp sediment-water interface in these settings for survival. Lichenoides is one of these stemless suspension-feeding eocrinoids (Figure 67). Lichenoides is found in the Middle Cambrian of Czechoslovakia. It has a prominent theca with three circlets of five plates, numerous sutural pores, and epispires (Ubaghs, 1968a). The two uppermost circlets have 10 to 21 brachioles attached to them, and the lowermost is highly bulged (Figure 67). The key features of Lichenoides, for the purpose of this study, are the lack of a stem and its small size (thecal height is about 1-2 cm) (Ubaghs, 1968a). Its stemless nature has led to the interpretation that it rested on the seafloor as a suspension feeder (Ubaghs, 1968a). Because of the lower bulge in the theca, Lichenoides probably had a fairly low center of gravity, allowing it to be relatively stable when resting on the seafloor. Because it was such a small immobile suspension feeder that lived resting on the seafloor, it is reasonable to think that Lichenoides, much like the helicoplacoids, would have been unable to survive on heavily bioturbated sediment. A sharp sediment- water interface and a rather firm substrate, regardless of its composition, would have been essential for its survival. Another strange stemless eocrinoid is the early Middle Cambrian Cymbionites (Figure 68). It was first described by Whitehouse (1941) from disarticulated plates found in Queensland, Australia. Although a complete specimen has never been found, much can Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 197 Figure 67- Middle Cambrian stemless eocrinoid Lichenoides with thecal plates labelled (modified from Ubaghs, 1968a). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 198 r a d i a l l a t e r a l Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 199 Figure 68- Basal cup and individual plate of the Middle Cambrian stemless eocrinoid Cvmbionites ; Cup is approximately I cm in height (modified from Ubaghs, 1968b). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 200 Individual plate Basal cup Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 201 be told from the remnants that have been recovered. Ubaghs (1968b), Sprinkle (1973), and Smith (1982) all have worked on Cymbionites since its initial description. Cymbionites consists of a small (about I cm in height) calcite cup comprised of five plates (Figure 68). These cups are often found associated with epispire-bearing polygonal plates, probably from higher on the theca, and U-shaped plates that probably supported brachioles (Smith, 1982). When reconstructed, C ym bionites closely resembles Lichenoides in that it appears to have been a small, immobile, stemless, suspension-feeding eocrioid that sat upright on the seafloor with the heavy basal cup providing stability (Smith, 1982). Just like Lichenoides, then, Cymbionites would have been dependent on the relatively sharp sediment-water interface and firmer substrate provided by low-levels of bioturbation. Similar in many regards to Cymbionites, Periodinites is another example of a small early Middle Cambrian stemless eocrinoid (Figure 69). It too was first described by Whitehouse (1941) and is found in Queensland, Australia. It also has a small (about 4 mm in height) basal cup consisting of five calcite plates, but it is bilaterally symmetrical (Figure 69) (Smith, 1982). Polygonal plates with epispires are also found associated with Peridionites cups. Because of its bilateral symmetry, Peridionites had a flattened shape and, therefore, most likely lived as a suspension feeder laying flat on the seafloor with its brachioles lifted up into the water column (Smith, 1982). If this is indeed how Peridionites lived, then it almost certainly would have required low levels of bioturbation in the substrate on which it lived. The remaining Cambrian eocrinoid genera, such as Gogia, were able to attach to small pieces of hard substrate amidst the mud, such as trilobite fragments, by way of stems with varying morphologies (Sprinkle and Guensburg, 1995). Some stems, such as those of the Middle Cambrian G. spiralis and G. guntheri, were short, wide, and covered with small unorganized plates (Figure 70) (Ubaghs, 1968a; Paul and Smith, 1984). The first Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 202 Figure 69- Basal cup of the Middle Cambrian stemless eocrinoid Periodinites in cross- section and from above; Cup is approximately 4 mm in height (modified from Ubaghs, 1968b). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. View from above Cross-section Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 204 Figure 70- The Middle Cambrian eocrinoid Gogia guntheri (modified from Paul and Smith, 1984). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 205 Brachioles Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 206 true eocrinoid stems (those with columnals), however, are found in the late Middle Cambrian, and by the middle Late Cambrian all eocrinoids had stems with columnals and lived attached to hard substrates (Sprinkle, 1976). Eocrinoids, therefore, show a general evolutionary trend through the Middle-Late Cambrian toward columnai-bearing stems that can attach to hard substrates and away from the stemless forms, such as Lichenoides, Cymbionites, and Peridionites, of the Middle Cambrian (Sprinkle, 1976). Because all of these Cambrian eocrinoids, except for those living on early hardgrounds in the Late Cambrian, are preserved in shales (Sprinkle, 1976; Sprinkle and Guensburg, 1995), this trend toward attachment in the Cambrian eocrinoids was most likely driven by the softening of the muddy substrate on which they lived by the increase in significant vertical bioturbation in such settings through the Cambrian (Hagadom and Bottjer, 1999). Those eocrinoids that evolved the ability to attach to limited hard substrates in the Middle and Late Cambrian were then well-adapted to occupy the hardgrounds that developed in the Late Cambrian and Early Ordovician (Sprinkle and Guensburg, 1995). From that point forth, competition for resources and available hard substrate were the primary driving forces in their evolution, as it was for the crinoids and edrioasteroids (Sprinkle and Guensburg, 1995). The evolution of eocrinoids during the Middle and Late Cambrian, however, appears to have been strongly influenced by increasing vertical bioturbation levels in offshore shales, which pressured them to evolve attachment abilities and preadapted them for the occupation of hardgrounds available in the Late Cambrian- Early Ordovician. Edrioasteroids, along with helicoplacoids, were the oldest known echinoderms, and they were also suspension feeders (Figure 71). Edrioasteroids were, however, more successful than helicoplacoids, surviving the Cambrian to become a part of the Paleozoic Fauna (Figure 19) (Sprinkle and Guensburg, 1995). Their success in comparison to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 207 Figure 71- Idealized edrioasteroid showing the upper, or oral, surface with pentameral ambulacra; The aboral surface would have been encrusting a hard substrate; H- hydropore; Pe- periproct (modified from Paul and Smith, 1984). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 209 helicoplacoids was most likely because they lived attached to hard substrates, while helicoplacoids lived as unattached mudstickers. As bioturbation increased in siliciclastic shelf environments during the Cambrian, therefore, edrioasteroids were already using hard substrates as a stable refuge (Thayer, 1983), allowing them to survive while the helicoplacoids apparently could not adapt and went extinct. Crinoids, members of the Paleozoic Fauna, are the other attached suspension- feeding echinoderm possibly present in the Cambrian (Figure 19) (Sprinkle and Guensburg, 1995). Their Cambrian evolution is relatively unknown, because they are only known from one genus, Echm atocrinus, from the Middle Cambrian Burgess Shale (Sprinkle, 1973; Sprinkle and Collins, 1998), and their identification as crinoids has recently been questioned (Conway Morris, 1993; Ausich and Babcock, 1998). What is known is that it was not until the Middle Ordovician, when they developed root-like holdfasts, that crinoids were able to repopulate offshore soft substrates, which, by then, were heavily bioturbated, from their nearshore hardground refugia where they lived as attached suspension feeders (Sprinkle and Guensburg, 1995). In addition to helicoplacoids and stemless eocrinoids, there are many other Cambrian suspension feeders that also appear to have been well-adapted for life on soft substrates which were relatively firm and had a sharp sediment-water interface. Dinomischus, for example, an enigmatic Cambrian suspension feeder found in only the Chengjiang fauna and the Burgess Shale (Figure 10), was relatively small (just a few cm in height) and lived with its thin stem inserted in the sediment (Conway Morris, 1977; Chen et al., 1996). These features indicate that Dinomischus, just like helicoplacoids and stemless eocrinoids, was well-adapted for survival on a substrate that was relatively firm with a sharp sediment-water interface, and, therefore, may have also been rendered extinct by increasing vertical bioturbation in siliciclastic settings during the Cambrian. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 210 Other examples of similarly-adapted Cambrian fauna suspension feeders are provided by the Chengjiang fauna cnidarians and sponges. For example, the only Chengjiang fauna cnidarian Xianguangia lived with its lower end embedded in the sediment and was also relatively small despite living on fine-grained siliciclastics (Figure 9) (Chen and Erdtmann, 1991). Also, the small sponge Choia, known from both the Chengjiang fauna and the Burgess Shale, was a suspension feeder that lived by simply lying flat on the substrate (Figure 8) (Chen et al., 1996). Because of their small size and lack of common survival strategies for typical soft-substrate suspension feeders (Thayer, 1975), these two Cambrian soft-substrate suspension feeders appear to have been well-adapted to survival on a relatively firm substrate with a sharp sediment-water interface. Accordingly, they most likely suffered the same fate as helicoplacoids and stemless eocrinoids as vertical bioturbation increased in siliciclastic settings in the Cambrian, increasing the water content in siliciclastic substrates as well as blurring the sediment-water interface. Biological Bulldozing The results and conclusions of this research fit well with Thayer's (1983) "biological bulldozing" hypothesis. This hypothesis claims that bioturbation has increased through the Phanerozoic, thereby severely restricting immobile organisms that lived on unconsolidated substrate (IMOUS) to various refugia (Thayer, 1983). To test this hypothesis, Thayer (1983) documented the changes in the number of genera of bivalves, brachiopods, and other metazoans in various functional groups through the Phanerozoic. If functional groups vulnerable to increased bioturbation decrease through the Phanerozoic, then the bulldozing hypthesis would be supported (Thayer, 1983). Thayer (1983) found that vulnerable functional groups in bivalves, brachiopods, and all other benthic invertebrates did indeed decrease through the Phanerozoic, while immune functional groups actually grew in taxonomic size. More specifically, the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 211 bioturbation-vulnerable free-lying brachiopod genera virtually disappear after the Paleozoic, while the immune functional groups, the pedunculate, cemented, and burrowing brachiopods, remained relatively constant in size, but occupied a greater generic percentage, through the Phanerozoic (Figure 72) (Thayer, 1983). The bivalves show a similar trend, with the vulnerable endobyssate and free-lying bivalves decreasing dramatically in percentage of bivalve genera through the Phanerozoic (Figure 73) (Thayer, 1983). Conversely, free-burrowing bivalves, and all other bioturbation-immune bivalves have increased in size and generic percentage over the Phanerozoic (Figure 73) (Thayer, 1983). Just as with brachiopods, this bivalve trend in functional groups composition supports the bulldozing hypothesis (Thayer, 1983). The remaining benthic invertebrates show a similar trend to brachiopods and bivalves. Immobile soft-substrate dwellers and immobile soft-substrate dwellers that initially begin growing on small pieces of hard substrate, both vulnerable to bioturbation, decrease through the Phanerozoic, with the immobile soft-substrate dwellers disappearing in the Carboniferous (Figure 74) (Thayer, 1983). The remaining functional groups, all immune to bioturbation, have increased during the Phanerozoic, particularly since the Paleozoic (Figure 74) (Thayer, 1983). Thayer (1983) also identified the physical, behavioral, and ontogenetic refugia that 1MOUS have occupied as they were restricted by increasing levels of bioturbation through the Phanerozoic. There are five such refugia: hard substrates, the littoral zone, the deep sea, mobility, and a refuge in size (Thayer, 1983). The move of IMOUS to hard substrates is well-documented, and is displayed well by the brachiopods (Thayer, 1983). The free- lying forms common to the Paleozoic are now extinct, and almost all living brachiopods attach to hard substrates (Figure 72) (Thayer, 1983). The littoral zone can also serve as a refuge for IMOUS because the harsh environmental conditions often inhibit bioturbation (Thayer, 1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 212 Figure 72- A) The number of genera of brachiopods in functional groups through the Phanerozoic; I- immune to bioturbation; V- vulnerable to bioturbation; See text for discussion; B) The percentage of brachiopod genera in functional groups through the Phanerozoic; I- immune to bioturbation; V- vulnerable to bioturbation; See text for discussion (modified from Thayer, 1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Phanerozoic Phanerozoic B) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 214 Figure 73- A) The number of genera of bivalves in functional groups through the Phanerozoic; See text for discussion; B) The percentage of bivalve genera in functional groups through the Phanerozoic; See text for discussion (modified from Thayer, 1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geologic Time 215 - a V . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 216 Figure 74- A) The number of families of all other skeletonized marine benthic invertebrates in functional groups through the Phanerozoic; I- immune to bioturbation; V- vulnerable to bioturbation; See text for discussion; B) The percentage of families of all other skeletonized marine benthic invertebrates in functional groups through the Phanerozoic; I- immune to bioturbation; V- vulnerable to bioturbation; See text for discussion (modified from Thayer, 1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Phanerozoic I I I I I I I V I I I Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 218 The deep sea is well-suited as a refuge for IMOUS because the rate of bioturbation, or sediment mixing coefficient, in the deep sea is two to three orders of magnitude slower than on the continental shelves (Guinasso and Schink, 1975; Schink and Guinasso, 1977; Turekian et al., 1978; Peng et al., 1978; Cochran and Aller, 1979; Lee and Swartz, 1980). These low levels of bioturbation in modem deep sea settings are approximately the same as those found on Early Paleozoic shelves (Thayer, 1983). Furthermore, deep sea bioturbators are relatively rare and probably only disturb the uppermost surface of the sediment (Thayer, 1983). Because of all these features, which resemble closely Paleozoic shelf environmental conditions, deep sea environments are the host of various "archaic" immobile suspension-feeders occupying soft substrates (ISOSS) such as crinoids, articulate brachiopods, bryozoans, and Hexactinellid sponges (Meyer and Macurda, 1977; Hampson, 1978; Cook, 1981). Vulnerable immobile invertebrates can make themselves immune to bioturbation by evolving mobility (Thayer, 1983). The crinoids provide a good example of this refuge: Paleozoic crinoids were stalked and immobile, while the Mesozoic and Cenozoic crinoids are mostly mobile comatulids (Thayer, 1983). This change was most likely in response to increasing levels of bioturbation (Thayer, 1983). Another possible refuge for IMOUS is one of size (Thayer, 1983). By growing very quickly, at least faster than the biological turnover rate of the sediment, IMOUS can keep themselves above the sediment-water interface and make up for the effects of bioturbation (Thayer, 1983). The best exemplor of this type of refuge may be the endobyssate bivalve Pinna, which is normally restricted to substrates that have been stabilized, either biologically or physically, but in some cases can survive on well- bioturbated sediments through rapid growth, up to 15 cm in their first six months (Thayer, 1979, 1983). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 219 The results of this research certainly are consistent with those of Thayer (1983), and also the faunal changes of the Cretaceous documented by Jablonski and Bottjer (1983). Cambrian immobile suspension-feeding echinoderms on soft substrates, such as helicoplacoids and stemless eocrinoids, are extinct by the end of the Cambrian, while attached suspension-feeding echinoderms, edrioasteroids and, possibly, crinoids, survive to be a part of the Paleozoic Evolutionary Fauna. This trend, as argued above, most likely developed in response to increasing levels of bioturbation in siliciclastic shelf settings through the Cambrian. CONCLUSIONS 1) Fossil evidence, in conjunction with sedimentological and ichnological evidence, indicates that helicoplacoids lived as mud-stickers in sediment that was only minimally horizontally bioturbated. 2) Because of their functional morphology, small size, and the fact that they were echinoderms, which have a water vascular system that is sensitive to large concentrations of fine suspended sediment, helicoplacoids probably could not survive on a substrate with extensive vertical bioturbation. 3) Increasing levels of vertical bioturbation in offshore siliciclastics during the Cambrian, therefore, may have led to their extinction, although other causes are certainly possible. 4) Other Cambrian suspension-feeding benthos, stemless eocrinoids such as L ich en o id es, C ym bionites, P eridionites, and some G ogia species, the cnidarian Xianguangia, the sponge Choia, and Dinomischus, which were probably adapted to similar conditions, may also have been unable to adapt to the increasing levels of vertical bioturbation in siliciclastics during the Cambrian. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 220 5) There is mild supporting petrographic evidence, in the form of micaceous laminae and mat-decay mineralization (Schieber, 1999), for the suggestion of microbial mat presence in the Middle Member of the Early Cambrian Poleta Formation. 6) Stronger supporting evidence for the presence of microbial mats in these rocks is found in the form of wrinkle structures (Hagadom and Bottjer, 1999), which, in conjunction with the evidence above, suggest the presence of microbial mats. 7) The common mineral assemblage in these rocks is chlorite, quartz, heavy minerals (pyrite, etc.), and sometimes micas. The abundant chlorite is an indicator of low- grade metamorphism. 8) If indeed microbial mats were present in the environment represented by these rocks, they were probably rather rare, as the scarcity of their evidence indicates. 9) Because their plates were held together only by soft tissue, allowing for their rapid decay and disassociation on the seafloor, helicoplacoids were preserved in obrution (rapid burial) deposits. 10) The majority of helicoplacoids collected in this study (69%) and those in the LACMNH (62%) are Group 2 (partially disassociated) specimens. 11) The predominance of Group 2 specimens is probably mostly due to the pre burial decay of helicoplacoids on the seafloor. 12) X-radiography indicates that Group 3 (almost completely disassociated) specimens are commonly associated with higher-energy regimes than are Group 1 (well- preserved, with slight degree of disassociation) specimens. This pattern may be due to the further disassociation of already well-decayed individuals during the minimal transport associated with their burial in an obrution event. 13) Most (73%) helicoplacoid specimens are preserved on the same bedding plane as at least one other individual, while many (39%) are preserved on a bedding plane Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 221 containing at least 10 individuals. These numbers indicate that helicoplacoids were gregarious and that they were preserved in mass mortality obrution events. 14) The vast majority (78%) of specimens show no preferential preservation of any particular body region, suggesting that the lower region of helicoplacoids was not more solidly constructed than the rest of the individual. 15) The orientations of 33 specimens on 3 separate slabs show some alignment, even more evidence for the preservation of helicoplacoids in obrution deposits. 16) The preservation of helicoplacoids was also aided by low levels of bioturbation, possible microbial stabilization of the substrate, a shallow redox boundary, and a calm depositional environment capable of preserving the obrution deposits once they formed. 17) It seems probable that the magnificent preservation of helicoplacoid specimens is restricted to the Middle Member of the Poleta Formation of Westgard Pass because it is in this region where the proper balance of energy regimes, in conjunction with the favorable factors discussed above, was achieved. 18) The preservation of helicoplacoids was aided by the same features of the Late Neoproterozoic biotope on which they depended for survival (Hagadom and Bottjer, 1999). 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Creator Dornbos, Stephen Quinn (author)

Core Title Helicoplacoid echinoderms: Paleoecology of Cambrian soft substrate immobile suspension feeders

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Advisor Bottjer, David (committee chair), Douglas, Robert (committee member), Gorsline, Donn (committee member)

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