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Restriction of a late neoproterozoic biotope: Ediacaran faunas, microbial structures, and trace fossils from the proterozoic-phanerozoic transition, Great Basin, United States of America

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Restriction of a late neoproterozoic biotope: Ediacaran faunas, microbial structures, and trace fossils from the proterozoic-phanerozoic transition, Great Basin, United States of America

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter Ace, 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 UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, b%inning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book. 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 UMI directly to order. UMI A Bell & Howell Informaticn Company 300 North Zeeb Road, Ann Arbor MI 48106-1346 USA 313/761-4700 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. NOTE TO USERS This reproduction is the best copy avaiiable UM I Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RESTRICTION OF A LATE NEOPROTEROZOIC BIOTOPE: EDIACARAN FAUNAS, MICROBIAL STRUCTURES, AND TRACE FOSSILS FROM THE PROTEROZOIC-PHANEROZOIC TRANSITION, GREAT BASIN, USA by James Whitey Hagadom A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Geological Sciences) December, 1998 Copyright 1998 James Whitey Hagadom Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 9931899 UMI Microform 9931899 Copyright 1999, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY p a r k LOS ANGELES. CALIFORNIA «0 0 7 This dissertation, written by Jam e s W h ite y H a g a d o rn under the direction of hxs. Dissertation Committee, and approved by ail its members, has been presented to and accepted by The Gradual School in partial fulfillment of re- quiremenis for the degree of DOCTOR OF PHILOSOPHY Diatt'of Graduate Studies DISSERTATION COMMITTEE OH J iL H f — ^ OHi*f;wr»n .. ... ...... .. . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS External Advisors; Discussicms with John Cooper, Mary Droser, Jean Durham, Chris Fedo, Stan Hnney, Jim Gehling, Duncan McIIroy, Paul Myrow, Guy Narbonne, Frieder PflUger, Chad Pridgen, Bruce Runnegar, Dolf Seilacher, Rachel Sobrero, and Ben Waggoner greatly aided the development of ideas presented herein. Funding Sources: Dissertation field research was supported by grants from the Achievement Rewards fcr College Scientists, American Museum of Natural History, Geological Society of America, Paleontological Society, Sigma Xi, Trojan League, USC Earth Science Dept, and the White Mountain Research Station. I am grateful for your support Museums & Libraries: I am grateful for the loan of specimens from the University of California Museum of Paleontology and the Los Angeles County Natural Hstory Museum, and help from curators Diane Erwin, Lindsay Groves, and Karen Wetmore. Also for the help from all the folks at my favorite library - Susan, Jean, and Mila at Hancock - you are a very special bunch. Geodept Oficina: Cindy, Desser, John, Macy, Rene, Sue, and Virginia - over the years, you’ve been the glue that held this department together. Each one of you put your heart and soul into this department and made it better than the day you started - acting like an extended family for all the graduate students. Thanks for making the extra effort during all the crises. Bedrockers: Over the years, intramural and extramural sports have brought physical temper to my life - and cemented a number of friendships. Dave, Jerry, Josh, Reese, Steve - thanks for all the workouts - or better yet - all of the "out-of-world’. u Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Earthsci Gradstus: Its true that I learned as much from you as anywhere else, but two things stick out in my mind. First: the 4 basic graduate student conversations, which at any moment, occupy 50% of graduate student activities: 1) Moan about how much work you have to do; 2) Moan about your advisor; 3) Moan about your ofTice/lab-mate; 4) Moan about how little mtmey you make. Second: the solution to all 4 - drink more beer (p e rh ^ the other 50% of their time?). Neighbors: Rumor has it that during my tenure here 1 grew up. Special credit is due the first-year “international student group”, the marine biologists, and the Venice neighbor microcommunities. You challenged me in ways geology never could - helping me grow as a person, gain perspective, and maintain enthusiasm for geology. Ben: From the first day you wore a “Steven J. Gould” nametag to the UCLA CalPaleo meeting, 1 knew you were way out there. After working with you for a couple of years, 1 now know its true. Strange thing is, 1 feel like I’m out there, too. So thanks for the company. PaleoLabbers Past and Present: From my first day in Paleobiology where 1 satin awe.... to today where I feel like an old fart - it has been a long, strange, tough, fun road. Thanks for all the inspiration, for the feedback, the camaraderie, the new high scores, and for being a role model each in your own way. Al, Dave, Gors, Jerry, Loren, Steve: Thanks for bearing with me all these years, providing insightful suggestions, pointing me in the direction of appropriate sources, writing endless letters of recommendation, and teaching me how to wiggle my square peg into a round hole. 1 couldn’t have asked for a better committee. m Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mom and Dad: Thanks fc x * a lifetime of inspiration and support I took care of number 1. And had lots of fun. Your little dirt-digging shell-collecting flower-smelling toebead grew up to do more d* the same. Let’s hope the pattern continues. Katherine and Janet You two are the best Weird, but deflnitely the best Thanks for all the support and long phone conversations over the years - was great to hear about the real worid and vent through an occasional “lecture” now and then. Grandpa Ray: You can’t hear me now, but whenever I think my life is even remotely tough, I look to you as a metric - and gain immeasurable inspiration. DB: In addition to all the traditional advisorial roles you filled so well, you gave me the one thing I needed most in graduate school - freedom. As a result, I felt like one of the luckiest students in the department - able to wander all over the intellectual map. Thanks! And thanks for yanking the rope when I got too close to the edge of the precipice. With your help, I learned alot about myself, about paleontology, and perhaps most importantly, about how science is done. As a discipline, and as a person-to-person enterprise. You taught me how lucky we are to do what we love as a job. And, to do it in a city like LA, where one can always find the edge. While eating at a little Sri Lankan restaurant under a mid-winter night’s stars. Deborah: Well, now you’ re finally married to a doctor. One who couldn’t have done it without you. Thanks for being there when 1 needed you, for making me go on “vacation”, for getting out of bed every morning, and coming home every night You are and will always be my “Brick House”. 1 love you. IV Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS ACKNOW LEDGMENTS................................................................................................... ii LIST OF FIGURES..........................................................................................................vii EXTENDED ABSTRACT............................................................................................ viii CHAPTER 1: Introduction................................................................................................1 CHAPTER 2: Ediacaran Fossils from the Southwestern United States...........................8 A bstract..................................................................................................................... 9 Introduction .............................................................................................................. 9 Stratigraphie Setting.............................................................................................10 Fossil F aunas........................................................................................................ 13 C onclusions............................................................................................................ 18 CHAPTER 3: Lower Cambrian Ediacaran Fossils from the Great Basin......................20 A bstract....................................................................................................................21 Introduction........................................................................................................... 21 Geologic Setting....................................................................................................24 Sedimentology and Depositional Context.........................................................29 Systematic Paleontology....................................................................................35 Discussion and Implications..............................................................................44 SW AR TP U N TIA A S A LO N G -R A N G IN G EDIACARAN TAXON?.............................45 SWARTPUtmAAS A N E X C L U SIV E L Y CA M BRIA N PH EN O M EN A ?.......................45 C onclusions............................................................................................................47 CHAPTER 4: Restriction of a Characteristic Late Neopioterozoic Biotope: Suspect- Microbial Structures and Trace Fc^sils at the Vendian-Cambrian-Transition 48 A bstract....................................................................................................................49 Introduction........................................................................................................... 50 Previous Research................................................................................................ 51 Modern vs. A ncient..............................................................................................57 Morphologic and Sedimentologic Investigations..............................57 Comparison of Modern and Ancient Structures............................... 68 D iscu ssio n................................................................................................ 69 Temporal Patterns............................................................................................... 70 Trace fossils & wrinkle structures..................................................................... 74 R ela tio n sh ips...........................................................................................74 I nterpretation........................................................................................79 IMPUCATIONS A N D PATTERNS...................................................................80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Discussion & Conclusions............................................................................. 82 CHAPTER 5: Pîileoecology of a Large Early Cambrian Bioturbator............................84 A bstract....................................................................................................................85 Introduction............................................................................................................85 W hy T aphrhelminthopsis?....................................................................87 Previous Research................................................................................................ 91 Geologic Context.................................................................................................. 94 PaLEO EN V IR O N M EN TA L and PA LEO EO O LO G IC CONTEXT..............................98 Taphonom y.............................................................................................................99 Trace Fossil Analyses........................................................................................103 T race Characteristics........................................................................103 How Many T racemakers Were T here?.............................................. 109 How B ig W ere T hey?............................................................................ 112 W hat W ere T hey D oing?.................................................................... 114 How Did T hey Move and What Were T hey?......................................114 How Do T races Relate to Plagiogmus?.............................................120 W ere They Systematically Foraging?..............................................121 Goniogram M ethodology..........................................................122 R esu lts...........................................................................................126 Goniogram Im plications...........................................................130 Pa tch in ess................................................................................................131 D iscussion.............................................................................................................132 C onclusions..........................................................................................................133 CHAPTER 6: Paleoecology of a Slope Margin: The Earliest Deep-sea Faunas of Southwestern Laurentia..................................................................................... 135 A bstract................................................................................................................. 136 Introduction.......................................................................................................... 136 Stratigraphie & Paleoenvironmental Framework............................................ 138 Paleogeographic Context................................................................. 144 Paleoenvironmental C ontext........................................................... 144 Previous W ork..................................................................................................... 152 Trace Fossil Usage............................................................................................ 152 Data Collection M ethods.................................................................................. 153 R esu lts....................................................................................................................153 T race F ossil A ssemblages..................................................................156 ICHNOFABRIC.............................................................................................161 B ody F o ssils...........................................................................................166 D iscussion.............................................................................................................172 C onclusions..........................................................................................................174 CHAPTER 7: References...................................................................................................175 VI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Figure 2.1: Stratigraphie and geographic context for Ediacaran localities..................... 12 Figure 22: Ediacara-type fossils from the Mojave Desert region................................15 Hgure 3.1: Stratigraphie and geographic context for Cambrian Ediacaran localities 28 Figure 3.2 Swartpuntia sp. from the Kelso Mountains, CA..................................... 32 Figure 3.3: Swartpuntia sp. from the White Mountains, CA..................................... 34 Hgure 3.4: Camera lucida interpretive drawings of Swartpuntia sp............................40 Figure 4.1: Wrinkle structure surface textures......................................................... 56 Figure 4.2: Microbial mats from Redfish Bay, TX...................................................60 Hgure 4.3: Stratigraphie and geographic context for wrinkle structures......................63 Hgure 4.4: Morphologic characteristics of wrinkle structures...................................67 Hgure 4.5: Temporal patterns in wrinkle structure distribution..................................73 Figure 4.6: Trace fossils associated with wrinkle structures..................................... 78 Hgure 5.1: Taphrhelminthopsis {tom the White-Inyo Mountains, CA....................... 90 Hgure 5.2: Detailed expression of ^ ic a l bilobate burrows..................................... 93 Hgure 5.3: Stratigraphie, geo^phic, and paleoenvironmental context.....................97 Figure 5.4: Typical preservational mode of studied traces.......................................102 Hgure 5.5: Bedding plane exposures typical of studied sections..............................106 Figure 5.6: Sectioned Taphrhelminthopsis traces................................................... 108 Hgure 5.7: Cross-cutting and directional relationships of studied traces.................... I l l Hgure 5.8: Modem bilobate trails comparable to Taphrhelminthopsis...................... 119 Hgure 5.9: Goniogram methodology and representative goniogrsuns........................125 Figure 5.10: Trace path deviation distribution .......................................................129 Hgure 6.1: Stratigraphie and paleoenvironmental context of the Emigrant Formation... 141 Hgure 6.2: Stratigraphie and paleoenvironmental context of the Swarbrick, Tybo, Hales, and Whipple Cave Formations...........................................................143 Hgure 63: Localities where two deep-sea transects were examined.........................149 Hgure 6.4: Typical sedimentary structures and bedforms of studied sections..............151 Hgure 6.5: Generalized time-environment diagram for shelf-basin transects...............155 Figure 6.6: Selected trace fossils from deep-sea transects.......................................158 Hgure 6.7: Trace fossil distribution within deep-sea transects.................................160 Hgure 6.8: Ichnofabric indices of deep-sea transects.............................................. 163 Hgure 6.9: Bedding plane bioturbation indices of deep-sea transects........................165 Hgure 6.10: SelecW skeletonized fossils from deep-sea transects........................... 169 Hgure 6.11: Body fossil distribution within deep-sea transects................................171 vu Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. EXTENDED ABSTRACT The Proterozoic-Phanerozoic transition records profound paleobiologic, paleoecologic, geochemical, preservational, and related changes in marine sedimentary environments. I^ y elements of this transition include shifts in the paleoenviroiunental and temporal distribution of soft-bodied Ediacaran fossils, suspect-microbial structures, and bedding-parallel trace fossils. In latest Vendian-eariiest Cambrian siliciclastic settings, all three of these elements were conunon in shallow envirorunents, and are regarded herein as a characteristic late Neoproterozoic siliciclastic biotope. During the Cambrian, this biotope became increasingly rare in shallow settings, and by the Ordovician was completely restricted to deeper marine settings. Although the timing of this restriction coincides with increasing diversity and abundance of metazoans, the advent of skeletonization, the escalation and development of systematic and vertically-directed burrowing activity, as well as colonization of the deep-sea realms, it is not known if or how such patterns are linked. To track the nature and timing of this shifting siliciclastic biotope, live distinct aspects of the biotope were examined, using accessible Vendian, Cambrian and Ordovician strata of the Great Basin (U.S.A.). In the uppermost Vendian - Lower Cambrian Wood Canyon Formation of western Nevada, Ediacaran faunas are documented for the first time. These faunas pinpoint the first occurrence of typical late Neoproterozoic paleoconununities in the region, and are of regional and global biostratigr^hic significance. Locally, together with a suite of trace fossils spanning this sequence, they suggest that the Precambrian-Cambrian boundary occurs within or below the lowest part of the lower member of the Wood Canyon Formation. Of global significance, however, is that the two key Ediacaran taxa identified from this region provide critical paleobiogeographic constraints for plate tectonic viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. recOTStnictions of this interval. Because these taxa. Emietta and Swartpuntia, are not cosmopolitan forms and are currently restricted to Namibia and Nevada, they suggest a paleoceanogiaphic ccmnection between southwestern Laurentia and eastern Gondwanaland in latest Vendian time. Ediacaran fossil fragments are also documented from the upper Wood Canyon Formation, and from the middle member of the Poleta Formation in the White Mountains in eastern California. Together with other occurrences from the same units, they document rare late Neoproterozoic-style preservation of soft-bodied forms in Lower Cambrian shallow subtidal facies of the regiotL Because they occur at a time when intense bedding- parallel and veitically-rmented bioturbation was developing, they record the dwindling occurrence of preserved Ediacaran faunas within typical Early Cambrian paleocommunities. Fossil fragments of Swartpuntia are important on a global scale, together with other Lower Cambrian SwartpimtiaAiiœ forms from Australia, because they suggest that this taxon is the first Ediacaran form known to span the Precambrian-Cambrian boundary. Similarly, odd sedimentary structures are characteristic of late Neoproterozoic siliciclastic marine strata and provide clues to interpreting the nature of shallow sandy seafloors during the Vendian-Cambrian transition. Wrinkle structures are a class of such sedimentary structures, which - based on comparisons with modem microbially dominated settings - can be formed by microbial mats. Wrinkle structures are common in Vendian- Lower Cambrian shallow marine strata, are less common later in the Cambrian, and typically only occur in deeper marine or restricted post-Ordovician settings. The co occurrence of trace fossils with Vendian-Cambrian wrinkle structures provides evidence for early biotic interactions and predominance of bedding-parallel burrowing strategies within such suspect microbially-dominated settings. Together with other intricate horizontally- ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. oriented trace fossils, they suggest mining of suspect microbially-bound sediment - behaviors which, like the distribution of wrinkle structures, exhibit distinct onshore- offshore patterns during the Proterozoic-Phanerozoic transition. Detailed analysis of trace fossils characteristic of these settings provides insight into the paleobiology and paleoecology of such bedding-parallel bioturbating organisms. Taphrhelminthopsis is a trace fossil typical of such settings, and appears to have been made by a large echinozoan- or mollusc-grade animal actively ingesting sediment Although studied traces do not appear to reflect systematic foraging behavior, they may reflect concentrated efforts to extract food resources from suspected microbially-bound sediment Taphrhelminthopsis is notable not only because of its large size, but because it is only one of two trace fossils associated with soft-bodied Early Cambrian Ediacaran fossils. Like other intricate bedding-parallel traces and Ediacaran fossils, they exhibit a distinct onshore- offshore pattern during this interval. Examination of outer shelf, slope, and basin-plain ''deep-sea" facies during the Proterozoic-Phanerozoic transition provides insights into the nature of such onshore- offshore patterns. Two transects through Middle Cambrian-Lower Ordovician deep-sea facies of the Great Basin illustrate the nature of the earliest deep-sea paleocommunities of southwestern Laurentia. Based primarily on analysis of trace fossils, paleocommunities in deeper settings are patchily distributed, and comprised of small shallowly infaunal and epifaunal organisms. Although shallower outer shelf and upper slope settings were more densely colonized by larger, intensely bioturbating infaimal and epifaunal organisms, the vast majoriQ' of organisms outboard of the upper slope were likely soft-bodied, with the exception of small epifaunal brachiopods, sponges, and various pelagic forms. Trace fossil assemblages exhibit higher diversity of burrowing behaviors in shallower settings. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. including intense vertically>stratifîed burrowing. In contrast, deeper assemblages are largely devoid of such burrowing, but are dominated by low diversi^ assemblages of shallowly-burrowing or bedding-parallel trace fossils. In summary, whereas previously outlined onshore-offshore patterns are largely correct, this higher-resolutirm analysis of individual communities coupled with discovery of allochthonous and parautochthonous soft-bodied Ediacaran and Burgess Shale-type body fossils suggests that there is much more to learn about the nature of the transition of the Late Neoproterozoic biotope into the deep-sea. Based on these observations, the late Neoproterozoic siliciclastic biotope was characterized by soft-bodied Ediacaran fossils, bedding-parallel trace fossils, and most importantly, microbial mats. Examples of this biotope from the Great Basin of the western U.S. include rare Vendian and Early Cambrian Ediacaran fossils, common suspect- microbial structures, and common bedding-parallel trace fossils such as Taphrhelminthopsis. During the Vendian-Early Cambrian transition, this biotope became greatly restricted. However, microbial mats, soft-bodied Ediacaran faunas, and bedding- parallel tracemaking organisms were still distinct components of shallow Early Cambrian paleocommunities. Later in the Cambrian, vertically-directed bioturbation further restricted the preservational window of this biotope to deeper marine environments. In post- Ordovician and modem settings, bedding-parallel burrowing and suspect-microbial features are only documented from deep marine or stressed settings. At present, it is unclear if or how these patterns are linked. For example, if bedding-parallel burrowing reflects a highly adapted ecological strategy tied to the presence of available mat-mediated food resources, then disruption of mats by vertically-oriented burrowing may have relegated such mats and associated mat-mining communities to other xi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. realms. In this hypothesized scenario, the bedding-parallel burrowers may not have retreated to the deep-sea because they were out-competed by vertical burrowers, but because they were ecdogically linked to a shifting food resource. Furthermore, we know little about the upper biostratigraphic range of Ediacaran faunas and how or if their disappearance reflects a shift of this biotope or other écologie factors. At present, documentation of Ediacaran descendants in the Burgess Shale, their co-occurrence with suspect-microbial structures, and the similarity between Ediacaran morphologies and modem deep-sea forms suggests that Ediacaran descendants were largely restricted by preservation, rather than through biotic interactions or extrinsic catastrophic conditions. Because suspect-microbial structures reappear in the Late Devonian and during younger mass extinction recovery intervals, but Ediacaran faunas do not, perhz^s such forms became extinct at the end of the Cambrian, in the Ordovician, or in the Silurian. Close examination of post-Cambrian, pre-Late Devonian microbially- dominated deeper marine lagersthtten (with an aim of identifying soft-bodied “sea-pens” and related forms) may help resolve such questions. The contributions herein reflect the first recognition of Vendian-Lower Cambrian subtidal siliciclastic systems as a distinct “late Neoproterozoic” biotope - but are far from a comprehensive treatment Together with other recent work, they reflect a logical first step toward understanding the nature of this biotope, and toward constraining its evolution through time and environments. As the ensuing chapters illustrate, study of this biotope requires both uniformitarian and non-uniformitarian approaches - key among these being an awareness of the important role microbial mats played in early siliciclastic paleocommunities. XU Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1: Introduction Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Proterozoic-Phanerozoic transition records some of the most important changes in Earth history, including the appearance and radiation of metazoans, the advent of skeletonization, the development, diversification and escalation of metazoan behaviors, dramatic geochemical changes in world oceans, fundamental changes in the nature of sedimentary fabric resulting from increasing depth, degree, and diversity of bioturbation, and the colonization of the deep-sea realm (see syntheses in Lipps and Signor, 1992; Schopf and Klein, 1992; Bengtson, 1994; Chen et al., 1997; Conway Morris, 1998). Based on the timing and nature of these patterns, together with insights from exceptionally preserved soft-bodied deposits (i.e., Konservat-lagerstâtten; Seilacher, 1970b; Seilacher et al., 1985), well-supported hypotheses have been developed about the nature and development of early animal communities. Biotic and sedimentologic information recorded in extremely thick and relatively well-preserved late Proterozoic and Lower Paleozoic sections of the Great Basin (western U.S.) have proved instrumental in outlining and understanding many of these larger-scale patterns. A fundamental approach to deciphering the nature of these large-scale patterns is analysis of paléontologie, sedimentologic, and related information within the context of an evolving biotope. Biotopes, in this context, include all of the écologie, sedimentologic, geochemical, preservational, and related factors associated with settings inhabited by ancient communities. Like ecotopes in modem vegetated habitats, biotopes shift through geologic time, across environments, and through sometimes narrow preservational windows as a result of intrinsic (e.g., evolutionary, selective, biotic interactive) and/or extrinsic (e.g., chemical, orbital, solar, tectonic) factors, all of which may be linked in complex biogeochemical feedback loops. Based on previous work and research presented here, it is becoming clear that in Vendian-Cambrian shallow siliciclastic settings, and perhaps in post-Ordovician deep marine settings, siliciclastic biotopes operated in a very Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. different, or non-uniformitarian, manner. Their main identifying feature is that they were dominated by microbial mats. Hagadom and Bottjer (1997) documented presence of “suspect-microbial” features in a wide variety of Vendian-Cambrian siliciclastic marine settings, and together with other studies of enigmatic sedimentary features (e.g., Schieber, 1986; PflUger and Gresse, 1996; McIIroy and Walter, 1997), it appears that these mats dominated Proterozoic siliciclastic seafloors, much in the same way microbialites dominate most Precambrian carbonate facies (e.g., Walter, 1976; Riding, 1991). These microbial communities play a strong role in preservation of nearly all Konservat-lagerstatten (e.g., Briggs, 1991; Wilby et al., 1996), particularly during the Vendian and Cambrian, where the majority of fossils were originally soft-bodied, and thus difficult to preserve (e.g., Gehling, 1986). Nowhere is this late Neoproterozoic siliciclastic biotope more evident than in late Vendian deposits, where enigmatic preservation of soft-bodied Ediacaran faunas in coarse sandstones has baffled many, and led others to rather controversial paleobiologic interpretations (see summary in Gehling, 1996, in press). Despite decades of searching by numerous workers through thousands of meters of section exposed across five states, and despite occurrences of such faunas in roughly coeval deposits from every other continent but Antarctica, Ediacaran faunas in the Great Basin have been virtually unknown (but see Horodyski, 1991). However, in this dissertation soft-bodied Ediacaran fossils are documented for the first time from the western U.S. (Chapter 2) and reflect sparse occurrence of this late Neoproterozoic biotope in latest Vendian facies of western Nevada. In studied facies, these faunas are generally poorly preserved and are associated with a variety of other soft-bodied tubular fossils and suspect-microbial structures, suggesting that preservational aspects typical of Ediacaran-bearing Vendian facies do not predominate in this region. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Restriction of characteristic late Neoproterozoic preservational conditions has caused many to believe that Ediacaran faunas became “extinct” at the end of the Vendian, or perhaps that they represented a failed evolutionary experiment (e.g., Seilacher, 1989, 1992). Considered together with other coeval occurrences in the Uratanna Formation of Australia (Jensen et al., 1998), rare Ediacaran fossil fragments documented from Lower Cambrian strata of eastern California reject such hypotheses (Chapter 3). Rather, they suggest that Ediacaran forms still existed in shallow marine Early Cambrian environments, but were only rarely preserved in settings where microbial communities flourished and in instances where such surfaces were not completely disrupted by vertical or intense bedding-parallel bioturbation. Together with documentation of Ediacaran-style faunas in basinal Middle Cambrian lagerstâtten (Conway Morris, 1993) and in Upper Cambrian turbidites (Crimes et al., 1995), it seems that this biotope became restricted to deeper settings later in the Cambrian, possibly related to increasing bioturbation and disruption of mat-dominated facies suitable for preservation of such forms (Crimes and Fedonkin, 1996; Gehling, 1996). Gehling (1996) has illustrated that microbial mats typical of this late Neoproterozoic siliciclastic biotope are critical to preservation of Ediacaran faunas, and Seilacher and Pfliiger (1994) have taken this approach one step further, proposing that many Vendian- Cambrian organisms had life-styles which were adapted to mat-dominated siliciclastic settings, such as rooting of frond-like organisms in apparently “unconsolidated” sandy sediment. Chapter 4 contains research results which demonstrate that there were, in fact, a variety of animals occupying niches within suspected mat-dominated siliciclastic settings. In particular, bedding-parallel burrowing appears to have been the predominant mode of bioturbation, and this suggests that animals in such early paleocommunities may have been highly adapted to extract horizontally-stratified food resources associated with mat-bound siliciclastic sediment. Through examination of temporal shifts in the distribution of such Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. metazoan-mat associations, it appears that bedding-parallel bioturbation and suspect- microbial structures exhibit coeval retreat from shallow marine settings into deeper marine facies, suggesting a possible écologie link between the two systems. Close examination of early tracemaking organisms which may have colonized such mat-bound substrates is thus needed, especially since it has been proposed that organisms increased their behavioral complexity during this interval, and that they may have developed highly specialized approaches to obtaining food resources (e.g., Seilacher, 1967a, 1974, 1977; Crimes, 1992a). Such hypotheses are based on appearance and evolution of meandering, spiraling, and patterned trace fossils during the Vendian- Cambrian transition, and through documentation of increasing vertically-directed bioturbation to deeper tiers later in the Cambrian-Ordovician transition (Ausich and Bottjer, 1982; Bottjer and Ausich, 1986; Droser, 1987, 1991; Droser and Bottjer, 1988, 1989; Crimes et al., 1992; Crimes and Droser, 1992; Crimes and Fedonkin, 1994; Droser et al., 1994; McIIroy and Logan, in press). In order to test such hypotheses about development of burrowing complexity, more information is needed about what kinds of organisms existed in ancient suspect mat-bound communities, and what exactly these animals were doing. Extremely large, well-preserved exposures of the trace fossil Taphrhelminthopsis provide an example of such activities (Chapter 5) and analysis of these traces provides insights on the behavior of a large soft-bodied grazing echinozoan- or mollusc-grade animal. Such traces are extremely common in shallow Lower Cambrian siliciclastic facies, and based on evidence presented herein, appear to reflect active ingestion of microbially bound sediment. Not coincidentally, Taphrhelminthopsis is also unique among trace fossils in that it is only one of two trace fossils associated with Lower Cambrian Ediacaran- bearing horizons. These horizons are also characterized by suspect-microbial structures - indicating that the typical late Neoproterozoic-style microbial mantling of sediment surfaces may have persisted into this interval. Furthermore, like other aspects of this late Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Neoproterozoic siliciclastic biotope, later in the Phanerozoic these trace fossils are only found in deep-sea flysches, and barring preservational effects, suggest a similar restriction to deep-sea settings after the Early Cambrian. To evaluate the mounting evidence for deep-sea restriction of soft-bodied fossils, suspect-microbial structures, and trace fossils characteristic of this late Neoproterozoic biotope, careful examination of early deep-sea (i.e., at or below the zone of turbidiie deposition) environments and paleocommunities has been needed. Thus far, deep-sea facies have only been the focus of descriptive studies of trace fossils and surveys documenting large-scale “onshore-offshore” patterns of deep-sea colonization. Although such studies have proved instrumental in outlining shifts of patterned trace fossils from shallow settings into deeper marine habitats, little is actually known about the nature of the individual communities, or their role in larger-scale patterns. To learn more about the nature and composition of these early deep-sea communities, the oldest deep-sea strata in the western U.S. were examined (Chapter 6). Deep-sea facies in this region include two outer shelf - basin plain sequences of Middle Cambrian-Lower Ordovician strata exposed in eastern California and Nevada. Deep-sea facies deposited along the margin of southwestern Laurentia are largely carbonate-dominated, and although allochthonous soft- bodied Burgess Shale faunas occur, no suspect-microbial or Ediacaran faunas were noted. The majority of biotic information in these settings is recorded in trace fossils. Analysis of trace fossil assemblages from two shelf-to-basin transects suggests that the diversity and intensity of bioturbation decreases from outer shelf to basin plain settings. Like coeval shallow shelf settings, bedding-parallel bioturbation is common, and appears to increase from the Middle to Upper Cambrian, particularly in distal settings. Unlike coeval shallow shelf settings, however, vertically-directed bioturbation is minimal in most slope and basin facies. Although observations presented herein suggest that we have much more to learn about the paleoecology and evolution of communities within these settings, observed biotic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shifts do not contradict other evidence suggesting restriction of this late Neoproterozoic biotope. Rather, together with evidence from other regions (e.g., Vidal et al., 1997) they suggest that previous models of deep-sea colonization may in fact be oversimplified. In summary, it appears that dominant, and potentially diagnostic, components of this late Neoproterozoic siliciclastic biotope are soft-bodied animals, microbially-bound sediment, and bedding-parallel burrowing animals. This biotope existed in Vendian facies of the Great Basin, but in the Early Cambrian was restricted in shallow settings by bioturbating animals, which later forced restriction of the biotope to deeper settings by the end of the Cambrian. The following five chapters constitute an attempt to track this characteristic late Neoproterozoic biotope through the Proterozoic-Phanerozoic transition. Close proximity, year-round accessibility of exposures, and a relatively well-constrained biostratigraphic and paleoenvironmental context make Vendian, Cambrian, and Ordovician facies of the western Great Basin a logical starting place for tracking of this biotope. Although a larger-scale understanding of this biotope is the primary focus, discrete chapters focusing on specific paléontologie and sedimentologic evidence follow, and are written as manuscripts for submission to a variety of journals. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2: Ediacaran Fossils from the Southwestern United States Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT New Ediacaran fossils documented from Late Precambrian strata of the Great Basin may help constrain regional biostratigraphic and global plate tectonic interpretations of the Precambrian-Cambrian interval. Locally, trace fossils suggest the Precambrian-Cambrian boundary occurs within or below the lowest part of the lower member of the Wood Canyon Formation. Ediacaran soft-bodied fossils and tubular forms occur directly below Cambrian trace fossils, confirming the persistence of the Ediacara biota to the base of the Cambrian, and suggesting that these forms may not be of regional biostatigraphic utility. More importantly, these fossils provide paleobiogeographic links between the southwestern U.S. and southern Africa in late Vendian time. INTRODUCTION Although the southwestern Great Basin and Mojave Desert are well-known for their extensive exposures of Precambrian and Cambrian strata (Stewart, 1970), this region generally lacks Ediacaran-style fossils. Furthermore, it has been difficult to pinpoint the Precambrian-Cambrian boundary within detailed regional stratigraphie sections. This difficulty stems from the lack of carbonate-dominated strata available for constructing continuous chemostratigraphic profiles (Corsetti, 1993), the numerous regional disconformities (Corsetti and Kaufman, 1994), and the lack of diagnostic paleobiological information from the two units thought to straddle this interval, the Wood Canyon Formation and the underlying Stirling Quartzite. Recent work (Horodyski, 1991; Corsetti, 1993; Horodyski et al, 1994; Runnegar et al., 1995; Runnegar, 1998) has greatly improved understanding of this interval and recent field research corroborates suggestions that the Precambrian-Cambrian boundary lies within the lowest part of the lower member of the Wood Canyon Formation. New paleobiological information that further constrains the regional position of the boundary is documented herein. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRATIGRAPHIC SETTING In the southwestern Great Basin, the Precambrian-Cambrian boundary occurs within a kilometers-thick, northwestward-thickening siliciclastic-dominated sequence of miogeoclinal sediments (Prave et al., 1991) that records the development of a passive margin along southwestern Laurentia (Stewart, 1982). This research focuses on exposures of the Stirling Quartzite and overlying Wood Canyon Formations in the Spring Mountains, Nopah Range, and Funeral Mountains of eastern California and western Nevada (Fig. 1). In this region, the Stirling Quartzite is divided into five informal members (A through E) and the Wood Canyon Formation is divided into lower, middle, and upper members (Stewart, 1970). The upper Stirling is a medium to coarse quartz arenite that interfmgers with, and is conformably overlain by, the lower Wood Canyon Formation (Wertz, 1982). The lower and middle members of the Wood Canyon Formation record a shallow marine- continental braidplain transition (Diehl, 1979; Fedo and Cooper, 1990; Fedo and Prave, 1991) with the lower member representing a highstand systems tract consisting of three carbonate-capped parasequences (Prave et al., 1991; Horodyski et al., 1994; Runnegar et al., 1995). The last parasequence is disconformably overlain by the fluvially-dominated terrestrial braidplain and braid-delta facies of the middle Wood Canyon Formation (Diehl, 1979; Fedo and Prave, 1991). The upper Wood Canyon represents a return to dominantly tide- and storm-influenced marine conditions (Fedo and Cooper, 1990), heralded by the appearance of abundant skeletonized body and trace fossils typical of Lower Cambrian facies (Fedo and Prave, 1991; Mount et al., 1991). 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.1: Generalized regional stratigraphy, modified from Prave et al. (1991), with composite measured section indicating principal fossiliferous horizons. Locality 1; Spring Mountains. 2: southern Nopah Range. 3: Funeral Mountains. (sh=shale; vf- f-c=very fine-fine-coarse sandstone; cg=coarse-pebbly sandstone and conglomerate; sd=sandy dolostone; d=dolostone) 1 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carrara Fm Zabriskie Qte Wood Canyon Fm Stirling Qtz Johnnie ® Fm Noonday Fm M W # k WoodCyn . m I c a 1 a sh f c sd | j| | { Skolithos Tœptichnus bilobate traces annulated tubes Planolites ^ trtlobites ^ SwartpunOa Q 0 Nimbla doudiniids Gordia Low anÿe x-strat Mud chips Cross-bedding — - Hummocky x-strat 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FOSSIL FAUNAS Conical calcareous fossils have been previously documented from near the base of the studied sections, in the dolomitic portions of the D member of the Stirling Quartzite. These fossils occur within a thinly-bedded vuggy dolomitic lag (Langille, 1974a, b) and are thought to represent abraded specimens of the well-known mineralized form Cloudina (Langille, 1974a; Grant, 1990). These fossils also resemble the mollusc-like shelly fossil Wyattia, known from more northern exposures of the upper Reed Dolomite of the Inyo Mountains (Cloud and Nelson, 1966; Taylor, 1966) and may indicate stratigraphie equivalence of these units (Stewart, 1970; Langille, 1974a,b). Member E of the Stirling Quartzite is dominated by thickly bedded coarse pebbly quartzite and cross-bedded medium quartzite, with some gray siltstone and fine- to medium- quartzite. The latter beds, thought to represent tidally influenced shoreface to shallow shelf-tidal flat conditions (Wertz, 1982; 1983), contain indistinct evidence of bioturbation, including rare Planolites trace fossils. In the northwestern Spring Mountains, the uppermost E Member also contains several large, ring-shaped structures preserved in negative epirelief on several bedding planes (Fig. 2F). These structures consist of a sunken rim of even thickness, about 10 mm thick, with slightly raised edges and a flat or raised center. Although identification of circular “medusoid” fossils is problematic owing to the abundance of circular pseudofossils during this interval (Cloud, 1960; 1968), these structures do not resemble concretions, gas escape structures, load casts, mud clasts, or any other abiogenic sedimentary structures. Although the specimens were uncollectable (thus preventing serial sectioning), field inspection of partial specimens along broken slab edges exhibited no concretionary or loading features, nor any gas escape or sand volcano features. Rather, their consistent morphology and size strongly suggests that they are fossils. They are nearly identical with the simple Ediacaran 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.2: Ediacara-type fossils from the southwestern Great Basin. All except (F) are from the lower member Wood Canyon Formation. (A) Treptichnus pedum. (B) Treptichnus pedum, (c) Gordia sp. NH^Cl coated. (D) Smooth tubular fossil in epirelief, possibly an internal mold. (E) Cast of an annulated tubular fossil in hyporelief. NH^Cl coated. (F) Three of the Nimbia-\\k& forms in situ, from the upper Stirling Quartzite. (G) Fragment of an arthropod?-like form. (H-K) Swartpuntia sp. (H) Close-up showing rims, inferred to be of separate leaves. (I) Overall view of the most complete specimen, NH4 CI coated. (K) Close-up showing end of segmented rachis and segmentation of frond. NH4 CI coated. With the exception of Nimbia-like forms, all fossils were collected by the author and will be reposited at the LACMNH. 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. * m i 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. “medusoid” form Nimbia occlusa, except that they are about three times as large in diameter as typical Nimbia (Fedonkin, 1985). Horodyski (1991) was the first to report Ediacaran fossils from this region, documenting a single complete specimen and several fragments of the ribbed, sac-like fossil Emietta plateaiiensis from the Spring Mountains. Although the original nearly- complete specimen was collected from float near the Wood Canyon-Stirling contact (Horodyski, 1991), further research confirmed that it came from the lowest parasequence within the lower member of the Wood Canyon Formation (Horodyski et al., 1994). Other Ediacaran forms occur in the same interval, including specimens of the frond-like organism Swartpuntia (Narbonne et al., 1997), which occurs as close as 1 meter above the Wood Canyon-Stirling contact (Fig. 2H-K). Unlike event-bed occurrences of Ediacaran faunas from Australia, Russia, and elsewhere, the Swartpuntia specimens occur in epirelief on tops of sandy siltstones, interpreted to have been deposited in shallow subtidal settings (Prave et al., 1991). Although these faunas are associated with suspected microbial structures and sometimes have a thin limonitic coating, they lack the characteristic “elephant-skin textures” found in microbially-mantled Australian forms (Gehling, 1996). The most complete specimen has a spindle-shaped central axis 85 mm long and 11 mm at its widest point, divided into segments 7 to 8 mm long (seven are clearly visible; Fig. 21). The axis is surrounded by a flat, oval frond with fine parallel striae about 1 mm apart, radiating from the axis at approximately 45° (Fig. 2K). Examination of the edge suggests that more than one frond was present (Fig. 2H). The total width of the fossil is estimated to be about 90 mm, and the total length to be over 100 mm. Although these specimens are incomplete and lack clear evidence of a stalk or holdfast, as is commonly seen in Vendian “fronds”, they are nearly identical to the fossil species S. germsi. In the same beds are elongate fossils, 1 0 - 2 0 0 mm long, which are circular in cross- section (Fig. 2D,E). Some are annulated (Fig. 2E), whereas others are smooth and tend to 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. taper slightly; all are between 1 and 2 mm in diameter. Specimens abruptly terminate, and there is no evidence of a fecal string or morphologically distinct anterior or posterior regions. Their truncated shape and absence of self-intersection suggest they are molds of tubular fossils, rather than trace fossils such as Palaeophyciis, or compressed algal or bacterial sheaths. These tubes may have been organic, which would suggest an affinity with sabelliditids, or may have been mineralized and therefore possibly doudiniids. However, the relatively high relief of the fossils suggests that they were once mineralized; the organic-walled sabelliditids are usually found completely flattened (Fedonkin, 1985). Very similar tubular fossils, both annulated and smooth-walled, have been documented from Sonora, Mexico, in roughly coeval strata (McMenamin et al., 1983). At a different site in the Spring Mountains, also about one meter above the base of the Wood Canyon Formation, a fragment that may have come from a large arthropod-like form was recovered (Fig. 2G). There are traces of four evenly spaced, acute triangular wedges, each arising from an indistinct central ridge and bounded by another, broader ridge. This specimen may be a trace fossil made by an arthropod-like form, in which the “wedges” would represent appendage marks and the outer ridge would have been left by the edge of the carapace. It may also be a fragmentary body fossil similar to “protoarthropods” such as Vendia (Fedonkin, 1985). Relatively simple trace fossil assemblages occur above the occurrences of Swartpuntia and Emietta, typically within fine quartzites and siltstone horizons of the lower member Wood Canyon Formation. In the Spring Mountains, the lowest occurrence of the trace fossil Treptichnus (Phycodes) pedum is approximately two meters above the dolomitic top of the lowest shoaling sequence, and extends the known occurrence of the genus in this region (Horodyski et al., 1994). Although ethologic, evolutionary, facies, and preservational biases make the use of trace fossils as biostratigraphic indicators problematic, the lowest occurrence of T. pedum is considered diagnostic of basal Cambrian 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. strata (Narbonne et al., 1987). The meandering ichnofossil Gordia, known from both late Precambrian and Phanerozoic strata (Crimes, 1992b), also occurs at the same level in the Nopah Range. These observations complement 6 "C chemostratigraphy of carbonates from the Stirling Quartzite and Wood Canyon Formation, which indicates a latest Vendian positive carbon isotopic excursion in the Stirling, and a probable Lower Cambrian negative carbon isotope excursion in the lower member of the Wood Canyon Formation (Corsetti, 1993; R. Ripperdan, pers. comm. 1998). Together, these lines of evidence suggest that the base of the Cambrian occurs, at the very highest, in the middle carbonate-topped sequence within the lower member of the Wood Canyon Formation. Although there are no absolute age constraints for these exposures, and although there may be a significant hiatus present in the upper part of the lower member Wood Canyon Formation (resulting from incision by the middle member) there is no reason to believe these faunas are much older than 544 Ma. CONCLUSIONS These fossil occurrences confirm the discovery of Ediacara-type fossil assemblages extending up to the base of the Cambrian (Grotzinger et al, 1995). Although detailed studies have not yet been made of suspect-Ediacaran fossils found in the upper member Wood Canyon Formation, the highest Ediacara-type fossils in measured sections are, at most, only a few meters below the lowest occurrence of T. pedum. Together with their abrupt appearance at the onset of finer-grained deposition at the Stirling-Wood Canyon contact, and their association with suspect arthropod fossils, these lines of evidence may account for the paucity of Ediacaran faunas from this region. These fossils may represent the last gasp of the “Ediacaran-style” preservational window in the Great Basin—a window that in the latest Vendian is locally restricted by coarse-grained clastic input from ubiquitous 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. regional braidplain-delta systems (Diehl, 1979), and in the Lower Cambrian is restricted by the onset of bioturbation (Seilacher and Pflüger, 1994; Gehling, 1996). Rifting of the supercontinent Rodinia occurred in the late Proterozoic, separating the Cordilleran margin of Laurentia from east Gondwanaland. The timing of this rifting has been debated: Cordilleran rifting may have begun 150-200 million years before the Cambrian (Dalziel, 1997), or much later, in the Vendian (Veevers et al., 1997). Testing of conflicting hypotheses has been hampered by lack of diagnostic biostratigraphic and biogeographic faunal occurrences. If rifting occurred in the Vendian, faunas from the recently rifted margins should be similar. If rifting occurred significantly earlier, endemic faunas would be expected to develop along the different rifted margins, and might co-occur with wide-ranging cosmopolitan forms. Comprehensive review of all Vendian Ediacaran faunal occurrences indicates that Emietta and Swartpuntia are not cosmopolitan forms (Waggoner, 1998). Rather, they are currently known only from Namibia and southwestern North America (Waggoner and Hagadom, 1997), and doudiniids are also common to both regions (Grant, 1990). Although their co-occurrence does not disprove an earlier date for rifting, restriction of these faunas to the two regions suggests a significant biogeographic connection between southwestern Laurentia and the Kalahari terrane in late Vendian time. 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3: Lower Cambrian Ediacaran Fossils from the Great Basin, U.S.A. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Ediacaran faunas are not well known from the southwestern U.S., and Cambrian occurrences of soft-bodied Ediacaran faunas are rare. Two fragmentary specimens of frondose Ediacaran soft-bodied fossils are described from the middle member of the Lower Cambrian Poleta Formation in the White Mountains, CA and the upper member of the Lower Cambrian Wood Canyon Formation in the southern Kelso Mountains, CA. Based on similarities with fossils previously collected from the lower member Wood Canyon and from the Nomtsas Formation of Namibia, both specimens are interpreted as Swartpuntia sp. Fossils were collected in situ from strata containing diagnostic Lower Cambrian body and trace fossils. In this region, Swartpuntia may persist through several hundred meters of section, spanning at least two trilobite zones. In light of other Cambrian occurrences of similar soft-bodied fossils, and the close proximity of other occurrences to the base of the Cambrian, there exists the possibility that Swartpuntia may not have been a typical Vendian Ediacaran form-but rather may have been a long-ranging soft-bodied form more typical of Cambrian shallow marine environments. INTRODUCTION Although their paleobiologic affinity and paleoecologic habits are still being evaluated, Ediacaran faunas are now known from most continents and have been documented in a wide variety of clastic depositional environments (see summaries in Glaessner, 1984; Narbonne, 1998). In the past, lack of modem biological or preservational analogues has led to quite varied interpretations of their paleobiologic affinity (e.g.. Seilacher. 1989. 1992; Retallack, 1994; McMenamin, 1998). However, documentation of putative molluscs (Fedonkin & Waggoner; 1996), sponges (Gehling and Rigby, 1996; Brasier et al., 1997), echinoderms (Gehling, 1987), medusoid hydrozoans (Narbonne et al., 1991), cnidarians (Gehling, 1988), and other taxa with modem affinities 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Runnegar and Fedonkin, 1992) has shifted attention away from controversy about their paleobiology and more toward increasing understanding of their mode of occurrence. At one time the Precambrian-Cambrian boundary was thought to coincide with a disappearance or possibly even an extinction of all Ediacaran faunas, with subsequent replacement in the Lower Cambrian by bioturbating and skeletonized organisms (see summaries in Gehling, 1991; Runnegar & Fedonkin, 1992; Conway Morris, 1993; Hallam & Wignall, 1997). More recently, workers have begun to reinterpret assumptions about the temporal and stratigraphie distribution of Ediacaran faunas, largely through breakthroughs in understanding their unique mode of preservation (Gehling, 1986,1996, in press; but also see Wade, 1968) and through new occurrences of diagnostic post- Vendian Ediacaran fossils (Jensen et al., 1998). Although there have been many attempts to explain the preservation of Ediacaran faunas through interpretations of unique paleobiology or by analogy (e.g., Seilacher, 1984, 1989, 1992; Retallack, 1994), Gehling (1986, 1996, in press) used a variety of sedimentologic and taphonomic criteria to document microbial mantling of Ediacaran organism carcasses. Preservation of these faunas in coarse siliciclastic sediments was largely restricted by microbial mantling of burial surfaces, coupled with subsequent restriction of pore-water migration within such microbial mat-laden sediments (Gehling, 1996, in press). Because widespread preservation of Ediacaran faunas is largely contingent on presence of microbial mat-bound surfaces, disappearance of such ubiquitous mat surfaces in the Phanerozoic [largely resulting from increase in vertical bioturbation documented by increases in tiering depth and intensity of deep burrowing (Bottjer and Ausich, 1986; Droser and Bottjer, 1989)] caused a rapid shift and near-elimination of taphonomic conditions necessary for widespread soft-bodied preservation in subtidal sandy marine environments (Seilacher and Pflüger, 1994). 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Prior to widespread recognition of the importance of microbial mantling of soft tissues, there have been scattered reports of Lower, Middle, and Upper Cambrian “Ediacaran-style” fossils (Cloud and Nelson, 1966; Durham, 1971; Borovikov, 1976; Conway Morris, 1993; Crimes et al., 1995). Perhaps the most thoroughly documented evidence for Cambrian Ediacaran-style organisms came from the Burgess Shale, where Conway Morris (1993) made a strong case that Thaumoptilon was likely a hold-over or descendant of Ediacaran ancestors. Furthermore, recent reports of Ediacaran faunas extending mere meters beneath conformably overlying Cambrian sediments (Grotzinger et al., 1995) cast doubt on hypotheses invoking disappearance of Ediacaran faunas at the Precambrian-Cambrian boundary. Recently, diagnostic Ediacaran fossils were reported from Lower Cambrian strata of Australia (Jensen et al., 1998). These observations reiterate that the preservational conditions which favored preservation of Ediacaran faunas had been greatly reduced by the terminal Proterozoic, but may have shifted to deeper marine settings (Crimes and Fedonkin, 1996) or been restricted to narrow preservational intervals in Lower Cambrian shallow subtidal environments (Seilacher and Pflüger, 1994; see also Chapter 4). The Great Basin of the western United States is well-known for its thick, well- exposed sections of Late Proterozoic-Cambrian strata, which represent a wide variety of fluvial to marine facies (e.g., Fedo and Cooper, 1990; Link et al., 1993). Although these facies have been subjected to regional tectonism and intrusion (e.g., Burchfiel and Davis, 1981; Wernicke et al., 1988), many of them are remarkably well preserved, and provide a rich source for paleobiologic and sedimentologic information (e.g., Robison and Rowell, 1976; Taylor, 1981). Despite these merits, Ediacaran faunas have only recently been recognized from these extensive exposures (Horodyski, 1991; Waggoner & Hagadom, 1997). Coupled with reports of new Ediacaran faunas from Namibia (Narbonne et al., 1997), these new faunas led to the re-examination of other possible occurrences of suspect- 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ediacaran fossils from the Great Basin -many of which were previously discounted because they were Cambrian in age. because they were fragmentary specimens, or because comparable bauplans were not known among existing Ediacaran taxa. The following is a brief history and description of these fossils, their stratigraphie occurrence, and their relevance to the temporal distribution of Ediacaran faunas. GEOLOGIC SETTING Fossils were extracted from localities in the southern Kelso Mountains, located in the Mojave desert region of southeastern California; and the White Mountains, located in eastern California. These two localities represent sections deposited in the craton hinge zone (Kelso Mountains) and the more rapidly subsiding outer miogeocline (White Mountains; Bahde et al., 1997). The regional stratigraphy, detailed stratigraphie context, sampling horizons, and study locations are illustrated in Figure 3.1. Although absolute age constraints are not available for studied sections, consideration of available stratigraphie, paléontologie, and chemostratigraphic information (outlined below) allows one to soundly reject the possibility that study strata are older than Lower Cambrian. In the southern Kelso Mountains, samples were extracted in situ from the upper member Wood Canyon Formation, a few meters below a thin, but prominent, orange carbonate bed. The outcrop is located approximately 4.5 km NW from Kelso Station, in the Mojave desert region (LACMIP Locality Number 17108). Specimens from the Mojave region occur in the upper member Wood Canyon Formation, which contains Nevadiid trilobites and a number of typical Lower Paleozoic trace fossils, including Rusophycus and Skolithos (Langille, 1974a; Mount et al., 1991; S. Hollingsworth, pers. comm., 1998). Here, upper member strata sharply overly the middle member, whose top is ornamented with a widely developed, and locally dense Skolithos piperock. Treptichnus (Phycodes) pedum (Bahde et al., 1991) and Taphrhelminthopsis 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. also occur in a several meter thick interval at the very base of the middle member Wood Canyon Formation. In the Kelso Mountains, the middle member of the Wood Canyon Formation rests directly on middle and upper member Stirling Quartzite because of erosional truncation of the craton-miogeoclinal hinge zone. The lower member Wood Canyon Formation, whiph crops out in more miogeoclinal sections of the Death Valley area, contains T. pedum, doudiniids, and other Ediacaran fossils such as Swartpuntia (Horodyski. 1991: Horodyski et al., 1994; Runnegar et al., 1995; Waggoner and Hagadom, 1997). Carbon isotopic analysis of carbonates from the Wood Canyon suggest a negative carbon isotope excursion in the lower member Wood Canyon Formation (Corsetti, 1993). Comparison of chemostratigraphic profiles from the Wood Canyon (and underlying Stirling Quartzite) with other reference sections (Corsetti, 1993; Runnegar et al., 1995; Runnegar, 1998; R. Ripperdan, pers. comm. 1998) and available paleontological data confirms a Lower Cambrian age for the upper member Wood Canyon. The specimen from the White Mountains was collected in situ from the lower 25 meters of the lower siltstone unit of the middle member of the Poleta Formation (J. Durham, pers. comm. 1997). In this region, a three-member (i.e., upper, middle, lower member) subdivision of the Poleta Formation is utilized [after McKee and Moiola (1962) and Stewart (1970)]. The specimen was collected near Westgard Pass in the White Mountains (NWl/4 of SE 1/4, section 5, T 8 S, R35E; Westgard Pass 7 V , minute quadrangle; UCMP Locality Number B8026). In the White Mountains, the lower part of the lower siltstone unit of the middle member of the Poleta Formation contains brachiopods, helicoplacoids, and trilobites belonging to the Nevadella Zone (e.g., Durham and Caster, 1963; McKee & Gangloff, 1969; Nelson, 1976; Moore, 1976a). Immediately overlying and underlying strata also contain abundant archaeocyathids, Skolithos, and Taphrhelminthopsis (e.g., Alpert, 1975; Moore, 1976a; Hagadom et al., 1994). Carbon isotope stratigraphy and trace fossil 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. analyses of the White-Inyo sequence suggests the presence of a large hiatus at the base of the underlying Campito Formation, and reveals a negative isotopic excursion in the upper portion of the Deep Spring Formation (Corsetti & Kaufman, 1994; Runnegar, 1998). Together, these observations suggest that the overlying strata, including the Poleta Formation, are Lower Cambrian in age. 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.1: Generalized locality and stratigraphie information for study specimens. 1 ) LACMIP Locality Number 17108; 2) UCMP Locality Number B8026. 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M N Skoltha ^ M o C i M 0 ^ T m xK tn J, A a . a / W u n * . f^uotBtaiiacw 0 0 N k m n a annulaM tibw ^ aouDM M m \ 0 flanoWM <Saa la Lowan<^ »slral M uddiips CiDSS^addng ------- Hum iocliy « « m 1) White-Inyo Facies 2) Death Valley Facies Wyattia m Bonanza King S MC Bonanza # King 1 Monola LC Carrara Fm. Wj Zabriskie Harkless K A I,^'K Wood Poleta A % Campito " I # Stirling ^ Q u a r t z i t e Dolomite " 3 5 É » Wyman Fm. 5 ^ Johnnie y Noonday / Crystalline Rocks H 1 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SEDIMENTOLOGY AND DEPOSITIONAL CONTEXT The Kelso Mountains specimen is preserved on the top of a 2.5 cm-thick bed composed of very fine to fine grained quartz arenite (Fig. 3.2). Grains are subrounded to rounded and distinct laminations or grading are not visible. Along the slab margins where petaloid structures are truncated, two indistinct layers of opaque and gray interbedded quartzitic sediment appear to underlie the specimen by 5-7 mm. These layers dip obliquely toward the inferred central stalk of the specimen. The base of the sampled bed has been moderately bioturbated, and small Planolites burrows are visible in convex hyporelief. The upper member consists of interbeds of siltstone and fine sandstone that have been locally sculpted into hummocky cross stratification; a lone <Im thick carbonate occurs not far below the contact with the overlying Zabriskie quartzite. Interference ripple and wrinkle marks are locally present, as are Skolithos traces. These lithologies and structures are consistent with an interpretation of a tidal and storm swept shallow marine shelf, likely above storm wave base. The specimen figured herein is the largest fragment of a specimen extracted from steeply inclined strata exposed atop a small topographic saddle. Lack of bedding plane exposures and closely spaced fractures at this site made extraction of a complete specimen difficult. In addition to the large fragment illustrated here, a number of smaller fragments were also collected. Additional fii^gments exhibit similar petaloid ribbing and similar preservational features, but were not available for further detailed study. The White Mountains specimen is preserved on the top of a thin (3 cm) bed of very fine grained sandy siltstone (Fig. 3.3). The sample was collected from an outcrop characterized by thinly interbedded ( 1 - 1 0 cm thick) fine to very fine sandstones and siltstones/shales (Moore, 1976a). In the figured specimen, underlying sediments are generally unbioturbated and faint submillimetric laminations are visible. A thin (0.3-1.0 mm) layer of concentrated helicoplacoid and trilobite debris occurs 3-4 mm beneath the specimen surface. Comparison of swaley surface topography to the underlying layers 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (including the bioclastic layer) suggests that the primary surface topography of the sample likely reflects the shallow trough and crests of a ripple. Strata at the studied outcrop were likely deposited in a shallow subtidal environment adjacent to a carbonate-sandbar complex (Moore, 1976a,b). 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.2; Swartpuntia sp. from upper bedding plane surface of the Upper Member Wood Canyon Formation, Kelso Mountains, CA. Note semicircular striae along left margin of specimen, and infolding and termination of parallel striae at central stalk at right and lower right. Also note faint lineations on and perpendicular to the central stalk axis at lower right. 31 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. Figure 3.3: Swartpuntia sp. from upper bedding plane surface of the middle member Poleta Formation, White Mountains, CA. Note presence of two thin (-0.5 mm) layers of parallel striae, as visible at upper left of specimen where upper level of petaloid frond is preserved over lower level and striae appear to intersect one another. Although specimen has been chipped in center, striae on right side of specimen are from same layer as striae from lower left. 33 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. SYSTEMATIC PALEONTOLOGY Specimens from the Kelso Mountains are reposited with the Los Angeles County Natural History Museum (LACMIP). Figured specimen is catalogued under type number 12726. The specimen from the White Mountains is reposited with the University of California Museum of Paleontology (Berkeley; UCMP) and is catalogued as type number 37450. Terminology follows Seilacher (1964) for toponomy, Wade (1968) for preservation, and Pflug (1970) for morphology. Genus S w a r t p u n t i a Narbonne, Saylor, and Grotzinger, 1997 Type species.-Swartpuntia germsi Narbonne, Saylor, and Grotzinger, 1997. S w a r t p u n t i a g e r m s i (Narbonne, Saylor, and Grotzinger, 1997) Figure 4,6,9-10 Swartpuntia sp. Hagadom, 1998, Fig. 3.2. Description.-LACMTP 12726. Proximal fragment of multifoliate frond consisting of several petaloids. Specimen is 62 x 51 mm. Petaloid consists of a parallel sheet of ca. 22 tubular segments which extend laterally from a central stalk. Tubular segments terminate along the margin of central stalk. Tubular segments are up to 42 mm long, and range from 1.0 to 1.7 mm in diameter. Segments are oriented parallel to one another. Individual tubes are of similar diameter along the entirety of their axis. Tubular segments are truncated at distal ends by the slab edge. Tubular segments intersect the central stalk at an angle of ca. 45 degrees; their angle of orientation relative to the stalk increases distally. The stalk is partially preserved and is 1 1 mm at its widest point. The central stalk is depressed relative to the adjoining topography by up to 8 mm. The stalk region is 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. characterized by two raised projections and two faint lateral grooves. The petaloid infolds along the margin of the central stalk, and two discrete layers are visible in cross section along slab margin, each of which congrues at the central stalk. The central stalk area is not associated with any cracking, sediment veins, or other diagenetic, strain, or metamorphic features. Tubular segments mimic the topography of the slab surface. Some segments terminate in the stalk after mimicking both depressions and raised regions in the surface (both of which are directly adjacent to and leading into the central stalk). Tubular features are preserved in convex epirelief. UCMP 37450. Distal fragment of frondose petaloid. Petaloid spans an area 10.8 cm X 9.3 cm. Although semicircular in cross-sectional relief, petaloid segments are not clearly tubular and are thus referred to as striae. The two largest areas of contiguous surface striae are 6.3 x 2.8 cm (Area 1) and 9.0 x 5.3 cm (Area 2). Within Area 1, at least 30 striae are visible; 29 are visible in Area 2. Surface striae are up to 4.2 cm long, are 1-2 mm wide, and 0.1-0.3 mm in relief. Striae within the petaloid mimic the surface topography, despite slab surface relief of up to 8.5 mm (within the space covered by an individual continuous striae), including several troughs and ridges. Where some of the surface layer has flaked off the slab, surface striae are separated by as little as 4 mm or as much as 31 mm - but striae at the edges of these sets are oriented in the same direction. Striae are straight to arcuate in plan view, and are generally oriented parallel to one another. On the upper right portion of slab surface, striae curve outward from center of slab and shift directions in a sigmoidal pattern. In this region, it appears that two sets of striae may be present - one of which is on a thin (ca. 1 mm thick) upper layer (arrow) which intersects striae of the adjacent, underlying layer. At the bottom portion of the slab surface, striae curve toward one another and two striae appear to merge with one another. The petaloid is preserved in convex epirelief. 36 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Comparisons.-S. germsi is illustrated in Narbonne et al. (1997). Similar undescribed frond-like fossils have also been illustrated in Jensen et ai, (1998). Petaloids in all of these specimens are of the same size and relative dimensions; tubular segments are also comparable in size (Fig. 3.4). Intersection of tubular segments with central stalk in LACMIP 12726 suggests raised stalk projections may be similar to V-shaped ridges illustrated in Narbonne et al. (1997). Sigmoidal to straight arrangement of tubular segments are common to both specimens and previously published illustrations. Despite their low (<lmm) relief, tubular segments maintain this relief relative to cm-scale variations in slab surface topography, both in distal regions of the petaloid, and in concave depressions near/at the central stalk. Infolding of petaloid along the marginal axis of the stalk, coupled with presence of at least two discrete layers visible in cross section along slab margin, each of which congrues at the central stalk - suggests presence of additional underlying petaloids in LACMIP 12726. Abutment of striae occurring on two separate surfaces separated by a thin layer of sediment suggests presence of two petaloids in UCMP 37450. Figured specimens are much larger than published accounts of Nasepia (Germs, 1973). Presence of a central stalk, multiple petaloids, and reduced size of tubular segments reject the possibility that study specimens are Emietta (Pflug, 1972). Tubular segments are narrower, longer (relative to overall specimen width), and less regularly arching than many specimens of Pteridinium (Jenkins, 1992) and more regular than transverse striae of Dickinsonia (Sprigg, 1947). Figured specimens are most similar to the multifoliate fronds illustrated in Narbonne et al. (1997). LACMIP 12726 is directly comparable to the central stalk and proximal petaloid region of Swartpuntia (Fig. 3.3). UCMP 37450 is comparable to the petaloid region of a Swartpuntia (Fig. 3.3). Unlike S. germsi, petaloid margins and proximal stems are not preserved in studied specimens, and sinusoidal arcing of 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. tubules/striae cannot be utilized to infer ventral/dorsal orientation. Difficulties examining the tips of the petaloids and relative petaloid:stalk proportions restricts the possibility of making a confident species-level taxonomic determination. Pending collection of better preserved specimens, fossils are interpreted as Swartpuntia sp. 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.4: Camera lucida drawings and comparison of Swartpuntia sp. from this study (lower left, LACMIP 17108; upper right, UCMP 37450) with similar drawing (center) of Swartpuntia germsi (modified from Narbonne et al., 1997). Dashed regions indicate inferred fragment locations relative to specimen from Narbonne et al. (1997). Note that specimen at lower left preserves two petaloid layers, as does specimen at center, and note striae size and spacing similarity in all three specimens. All camera lucida drawings are at the same scale; scale bar (lower left) is 1 cm. 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. - 40 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Remarks.-lt is difficult to pinpoint abiogenic mechanisms which could plausibly create a surface: 1) characterized by series of up to 29 parallel surface scratch marks which extend across an uneven sediment surface and which penetrate the surface at the same depth; 2) in which the inter-scratch areas are rounded in cross-sectional view; 3) in which scratches arc in one direction and then in the other direction without crossing; and 4) on which there are no other tool marks. Slab surface topography appears to reflect original bedding plane features (although the transverse markings and surface striae appear to have draped this sedimentary surface, and together with the presence of the inferred stalk in LACMIP 12726, may have later modified this surface). In addition, well preserved underlying trace fossils and sedimentary structures suggests that these features are not metamorphic, diagenetic, or structural features. Thus, the best candidates to account for observed features might include: a) a comb-like object drifting across the sea bottom; b) scratches of an arthropoid with 20-30 appendages; c) folding or shearing of microbially-bound sediment; or d) an impression of a frond-like soft-bodied carcass. a) Ridges consistently conform to the topography of both specimens, which have a variety of bumps and depressions comprising up to 8 mm of vertical relief. The ridge- trough depth (measured along individual continuous segments) does not vary more than 0.3 mm in any individual tubular segment. Comb-like tool marks made by hard inorganic objects or skeletonized debris would not be expected to conform so precisely to surficial topography, but would rather exhibit highly varied impression depths as the structure’s edges passed across the substrate surface. A shifting current could account for the sigmoidal pattern within the surface striae, but would not account for their consistent 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. parallel orientation over a 150 cm’ surface. The only known parallel tool and drag marks made by soft tissue (Haines, 1997) are unlike those observed on study specimens. b) Ridges of tubules/striae exhibit straight, arcuate, and sigmoidal orientations, yet maintain a similar width throughout their length, as well as similar relief on surface topography. If study specimens were scratch marks made by a foraging arthropoid, one would expect splaying of impressions as appendages moved outward/inward from the torso, tapering of the scratches toward their distal tips, deep impressions (or prod marks) at one edge of impressions, and/or variations in the depth of penetration of the sediment corresponding to variations in surface topography. None of these features are present, and their absence is even more conspicuous when one considers that many presumed appendage traces are likely formed as cast undertracks (Seilacher, 1953a,b). Furthermore, on both specimens the two sets of striae occur on two different layers, are oriented obliquely to one another, and are separated by less than a millimeter of sediment. If striae represent arthropoid undertracks, one would expect both to occur on the same surface, unless the lower set was excavated and cast by another sediment layer, which was then scratched (without penetrating into the underlying layer) - a scenario which seems difficult to reconcile with such a thin interface between the layers. Furthermore, arthropoid scratch marks from other formations in this region, (including Monomorphichnus ; Alpert, 1974, 1976b; Langille, 1974a), are quite different from these features. In these traces, the size of the scratches in an individual set varies, as would be expected of an animal whose underlying surfaces and appendages are not of uniform length, size, shape, or orientation relative to its body or the substrate. Lastly, most arthropoid scratch marks preserved in convex relief are typically formed on the bottom of a bedding plane, through the casting of the scratch trough; studied samples exhibit convex ridges found on the top of bedding planes. 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c) Bunching or shearing of a microbially-bound sediment surface would not preserve such regular folds at such a small scale (relative to grain size), nor would they terminate in a central stalk in which the ridges are perpendicular and abut the edges of the frond-ridges. Surface textures of suspect-microbial features from siliciclastic strata of this region are typically more polygonally wrinkled, pustulose, and irregularly shaped than observed here (Hagadom & Bottjer, 1997). Elongate subparallel striae associated with deformation of suspect microbially-bound sediment are typically more widely spaced, radiate outward from the locus of deformation, taper distally, and are larger in size than structures figured here (Mcllroy and Walter, 1997). d) Swartpuntia has been described from Vendian strata of Namibia (Narbonne et al., 1997), and from the lower member Wood Canyon Formation in this region (Waggoner and Hagadom, 1997; Chapter 2). Fossils from both areas exhibit similar modes of preservation (including occurrence on bedding plane surfaces and apparent sand casting of petaloid segments) and are associated with similar suites of sedimentary structures, trace fossils, and body fossils. Fronds from both areas are from lithologically similar strata. Fronds figured in this study share the same morphologic features (including presence of a central stalk, multiple petaloid layers, and tubular petaloid segments) and are similar in size to previously figured fossils (Grotzinger et al., 1995; Narbonne et al., 1997). Fronds figured in this study are from roughly coeval strata as undescribed fronds illustrated from Australia (Jensen et al., 1998). Difficulty reconciling inorganic, tool, trace, weathering, or deformational origins for these structures, together with their morphologic similarities to Swartpuntia and comparable stratigraphic/preservational/paleobiogeographic occurrence, suggest that they are partial specimens of this taxon. Lastly, the history of these specimens is relevant to understanding the nature of their importance, the credit due their collectors, and the rationale for discussing them here. The UCMP specimen was originally collected by Wyatt Durham in 1970 and noted in an 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. abstract of a talk given at the 1971 Cordilleran section GSA meeting (Durham, 1971). Based on his experience working on helicoplacoids and related faunas in Lower Cambrian strata of the region, he suggested that it was not a typical Cambrian fauna! element and tentatively suggested it might be a Dickinsonia?. Conway Morris (1993) later figured the specimen in his comprehensive treatment of Ediacaran holdovers. Conway Morris (1993) thought that an assignment to Dickinsonia was plausible, but apparent convergence of the elongate segments on the upper portion of the specimen were not known from illustrated occurrences of Dickinsonia. An alternative interpretation might have been to assign the specimen to Monomorphichnus. but the closely spaced array of striae were not known from presumed arthropoid trace fossils. The size and regularity of the striae in the specimen also made assignment to a specific trace fossil genus difficult. The LACMIP specimen was collected in 1994 by C. Fedo and J. Cooper, and was thought to be a soft- bodied impression, but was discounted as an Ediacaran taxon because of the known Cambrian age. At that time, such fossils were not known and Swartpuntia had not yet been discovered. Material.-One nearly complete specimen from UCMP B8026, one incomplete specimen and numerous fragments from LACMIP 17108. DISCUSSION AND IMPUCATIONS Collectively, morphologic and sedimentologic characteristics of these specimens contradicts a non-biogenic origin and suggests they are partially preserved Ediacaran faunas. Considered together, morphologic features require description as Swartpuntia sp. Although these specimens may not provide new insights about the paleobiology and paleoecology of these faunas, they may help us to gain further insight about the temporal range and distribution of post-Vendian Ediacaran forms. 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Swartpuntia as a long-ranging Ediacaran taxon? Faunas from this region provide an example of a relatively long-ranging Cambrian Ediacaran taxon characteristic of shallow marine environments. Regional stratigraphie comparison of the White Mountains and Mojave sections suggests that the lower member Wood Canyon correlates with the Deep Spring Formation (which underlies the Campito and Poleta Formations, respectively; Runnegar, 1998). Based on this correlation, and based on occurrences lower in the Wood Canyon (Waggoner and Hagadom, 1997), Swartpuntia thus may have a stratigraphie range of up to 150 m in the Wood Canyon Formation, and if regional correlations are correct, a range of several hundred meters (spanning the pre-trilobite, Fallotaspis, and Nevadella Zones) within the western Great Basin. In light of the global eustatically-driven hiatus present at the base of the middle member Wood Canyon (Runnegar et al., 1995; Bahde et al., 1997; Runnegar, 1998) and at the base of the Campito (Runnegar, 1988; but also see Corsetti & Kaufman, 1994), Swartpuntia may have a considerably longer stratigraphie range. Swartpuntia as an exclusively Cambrian phenomena? Presence in at least two Lower Cambrian sections (U.S. and Australia) presently makes Swartpuntia the most common/widespread Ediacaran taxon found in the Cambrian. Furthermore, a review of late Vendian occurrences of this taxon leads us to question whether Swartpuntia may not just span the Vendian-Cambrian transition interval, but rather may represent an entirely Cambrian phenomena, including: i) abrupt appearance of this taxon immediately below the Precambrian-Cambrian boundary (ca. 40 m below conformably overlying occurrences of T. pedum in the southwestern U.S. and its occurrence 90 m above an ash 45 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. bed dated at 543 +/-1 Ma in Namibia; Grotzinger et al., 1995; Narbonne et al., 1997); ii) the fact that the Precambrian-Cambrian boundary has been delimited in these regions based on occurrence of the trace fossil T. pedum; and the fact that trace fossil distribution is highly dependent on behavioral, écologie, preservational, and facies variations within a given region (see summary in Bromley, 1997; specific comments on T. pedum in Jensen, 1997; and note that both sections are characterized by large erosional unconformitites which make it difficult to infer the location of the PC-C boundary); iii) the lack of strati graphically lower occurrences of Swartpuntia, including absence from Ediacaran fossil zones I or II and absence from basal/middle portions of fossiliferous strata corresponding to zone HI (Narbonne et al., 1994; Narbonne, 1998); iv) although these fossils do not exhibit any diagnostic morphologic evidence to suggest microbial mantling of soft-tissue {sensu Gehling, 1996, in press), all three Swartpuntia-type occurrences are from stratigraphie intervals characterized by suspect-microbial structures (Gehling, 1996; Hagadom & Bottjer, 1997); and v) re-establishment or modification of such microbially-mediated processes (together with other factors, such as low oxygen, restricted bioturbation, early diagenetic mineralization, and rapid burial) is thought to account for many, if not most post-Phanerozoic occurrences of soft-bodied preservation [i.e., Konservat-Lagerstâtten like the Burgess Shale (Seilacher, 1970b; Seilacher et al., 1985; see also overview in Allison and Briggs, 1991; Seilacher and PflQger, 1994)]. 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Although stratigraphie confidence intervals have not been applied to occurrences of Ediacaran taxa {sensu Marshall, 1990,1997), potentially confounding vagaries in preservation of trace fossils and soft-bodied fossils close to the boundary only heightens the likelihood that some overlap may exist between the occurrence of T. pedum and Swartpuntia. If T. pedum and Swartpuntia co-occur, then Swartpuntia may be either a taxon which spans the Vendian-Cambrian transition, or an entirely Cambrian phenomenon, rather than a characteristic Vendian taxon. At present it is not known whether the relatively long range or conspicuous Cambrian occurrence of Swartpuntia stems from biotic, écologie or preservational factors. Coupled with observations iv-v (above), it is postulated that the occurrence of Swartpuntia was restricted by microbially-mediated preservation, or perhaps unique conditions like those of other Cambrian soft-bodied Lagerstâtten. CONCLUSIONS Fragmentary remains of Ediacaran taxa documented from strata in the Great Basin suggest potential exists for further documentation of Ediacaran-style occurrences in an area where they have thus far been conspicuously absent. Locally, finer-grained Lower Cambrian facies may be more amenable to preservation of such faunas than coarse Vendian sandstones. Furthermore, analysis of Swartpuntia distribution suggests that this taxon may be an example of a long-ranging and possibly diagnostic Cambrian Ediacaran-style organism. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4: Restriction of a Characteristic Late Neoproterozoic Biotope: Suspect- Microbial Structures and Trace Fossils at the Vendian-Cambrian Transition 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Wrinkle structures are a class of oddly textured sedimentary structures common in Proterozoic-Cambrian marine siliciclastic strata, but which are uncommon in post- Ordovician subtidal marine facies. Despite a long history of study, there is little agreement on how these structures form, or their role in larger-scale sedimentologic and paléontologie contexts. Based on similarities with morphologic and sedimentologic characteristics of modem microbially dominated communities, it appears that ancient wrinkle structures could have been formed by microbial mats. Microbial genesis for wrinkle structures not only helps explain their ubiquity during the late Neoproterozoic, but also their distinct paleoenvironmental and temporal shifts related to increasing vertically-oriented metazoan bioturbation. The Vendian-Cambrian transition offers a unique opportunity to examine the influence of suspect-microbial communities on the siliciclastic sedimentary record, and their relationship with the earliest burrowing, grazing, and locomotive activities of metazoans. Based on outcrops in the Great Basin, USA, metazoans appear to have been moving on, in, and under suspect-microbially bound sediment, and exhibit features which suggest active and passive epifaunal and infaunal sediment ingestion. Together with more intricate horizontal burrow networks, this style of bedding-parallel bioturbation is common in shallow settings of the Vendian-Cambrian transition, and is hypothesized to reflect highly specialized approaches to exploiting mat-mediated organic-rich sediment layers. Post- Ordovician restriction of such bedding-parallel burrowing behavior to deeper settings mirrors a shift of suspect-microbial structures to restricted or deep-sea settings. This restriction suggests replacement (rather than progressive evolution) of metazoans with horizontally specialized ecological strategies by more vertically-oriented bioturbating organisms. 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Stromatolites and other microbial carbonates have been a significant focus in sedimentologic studies of Archean and Proterozoic sedimentary rock fabrics (e.g., Waiter, 1976; Riding, 1991; Schopf and Klein, 1992). However, it was not until modem counterparts were found in hypersaline and restricted environments that earth scientists began to understand the critical role of microbial communities during this interval, and appreciate their role in mediating the deposition and style of Precambrian sedimentary structures and fabrics (such as stromatolites; see summaries in Awramik, 1990; Riding, 1991; Schopf and Klein, 1992; Walter, 1994). Although such studies illustrated that Precambrian carbonate marine environments were dominated by microbial mats, including layered communities of phototrophic and chemotrophic microorganisms, much of the unbioturbated siliciclastic sedimentary record from this interval was thought to lack structures attributable to microbial processes. Recent studies of anomalous soft-bodied preservation (Gehling, 1996, in press) and sedimentologic structures characteristic of the later Neoproterozoic (Pflilger and Gresse, 1996; Pflueger and Sarkar, 1996; Hagadom and Bottjer, 1997; Mcllroy and Walter, 1997), have begun to change these views, suggesting that many siliciclastic sediments were likely not only to have hosted a variety of layered microbial communities, but that such communities may have actually been the dominant biotic influence on clastic seafloor environments prior to the onset of bioturbation (Seilacher and Pfltiger, 1994). Wrinkle-stmctures are an enigmatic group of sedimentary structures typical of Neoproterozoic-Cambrian marine environments, which are used as a case study to illustrate the large biotic and sedimentologic shifts that were occurring during this time interval in siliciclastic seafloor environments. In this paper. I; 1) reiterate why wrinkle-structures may be produced by microbial mats; 2) illustrate the types of trace fossils associated with these suspect-microbial structures in the Great Basin, U.S.A.; and 3) suggest how analysis of 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. these features provides insights on early biotic interactions, changing behavioral strategies, and taphonomic shifts characteristic of the Vendian-Cambrian transition. PREVIOUS RESEARCH Wrinkle structures are a class of sedimentary structures which include runzelmarken, Kinneyia ripples, micro-ripples, and related structures such as “elephant skin” (Hagadom and Bottjer, 1997). In general, wrinkle structures are characterized by oddly contorted, wrinkled, irregularly pustulose, quasi-polygonal, commonly oversteepened surface morphologies that can occur on bed tops and bottoms. Wrinkle structures such as “elephant skin” (which are formed millimeters to centimeters below the seafloor by loading of a microbially-bound sediment layer) are common on bed bottoms in coarse Neoproterozoic sandstones, and are discussed elsewhere (Gehling, 1986,1991, 1996, in press). Wrinkle structures such as sharp-edged and flat-crested “Kinneyia” are also likely formed below the sediment-water interface by gas buildup beneath buried microbial mats and are discussed by Pflüger (in press). Wrinkle structures discussed in this study largely reflect original surface features and have been described in the literature as runzelmarken. wrinkle-marks, micro-ripples, and variations of Kinneyia (Fig. 4.1). Wrinkle structures typically occur on the tops of very fine- to coarse-grained sedimentary surfaces and have been documented in a wide variety of marine environments, ranging from intertidal to abyssal facies (e.g., Dzulynksi and Walton, 1965; Fedo and Cooper, 1990; Hagadom and Bottjer, 1997; Hughes and Hesselbo, 1997). A long history of wrinkle structure study has not necessarily resulted in agreement about their mode of formation. Their history begins with a study by Charles D. Walcott, in his volume on the Algonkian algal flora of the Proterozoic Belt Series in Montana (Walcott, 1916). Walcott described odd subparallel ridges on upper surfaces of the Newland Limestone and posited that they reflected a type of “blue-green algae” which he called 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Kinneyia simulans. Subsequent studies described similar elongate forms and more polygonal-shaped surface features in sediments from a variety of siliciclastic environments, terming them eolian microridges, Krâuselrippeln, foam marks, runzelmarken, wrinkle marks, millimetre ripples, Kinneyian ripples, and microripples (see summaries in Teichert, 1970; Singh and Wunderlich, 1978; Allen, 1985; Kopaska-Merkel and Grannis, 1990; Hagadom and Bottjer, 1997). Most of these features can be described as preservational variations of wrinkle marks, which are elongate to honeycomb-shaped surface networks of sharp to round-crested ridges, or Kinneyia, which exhibit similar relief but have more parallel, typically flat-topped ridges. Such features were thought to have been formed by inorganic processes. In perhaps the most relevant (of many) examples, Reineck (1969) suggested that wrinkle structures were formed by wind blowing across shallow pools of water (less than 1 cm deep), and Dzulynski and Simpson (1966) suggested they may have been formed through dwindling current velocities deforming sediment in deeper marine facies. Later, Allen (1985) observed wrinkle-like features in interbedded sand and mud layers within a modem estuary, and suggested that they may have formed by aseismic soft- sediment loading. Unfortunately, none of these models of wrinkle genesis quite explain their ubiquity in Neoproterozoic-Cambrian environments. For example, although Alien (1985) illustrated wrinkle-like structures, he had difficulty reconciling observed intertidal examples with the great variety of depositional environments represented by ancient wrinkle mark occurrences; a loading hypothesis is also less likely in ancient environments where wrinkle- bearing facies are typically interbedded with sediment of the same grain size. In addition, structures produced in laboratory studies (e.g., Dzulynski and Simpson, 1966; Reineck, 1969; Kocurek and Fielder, 1982) are dissimilar to those found in the ancient record [e.g., compare Hantzschel and Reineck (1968) to Reineck (1969)]. Lastly, preservation mechanisms are completely lacking in many previous models, so that it is difticult to 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. substantiate preservation of wrinkle textures in unconsolidated sands (i.e., without microbial communities) in the wake of sediment pulses associated with typical burial events (e.g.. Grant and Gust, 1987; Dade et al., 1990; Noffke et al., 1997a; and references therein). 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.1: Surface textures characteristic of wrinkle structures found in Vendian- Lower Cambrian strata of the Great Basin. See also Hagadom and Bottjer (1997) and references therein for photographs o f morphologic endmembers of wrinkle structures, including KUmeyia. Examples from the Great Basin include: A) quasi- polygonal, moderately flat-topped wrinkles on multiple layers of an upper bedding plane surface from the Harkless Formation, near C^edar Flats, White-Inyo Mountains, CA; B) Quasi-rounded wrinkled upper bedding surface from the Wyman Formation, Silver Peak Range, NV; Q Very small rounded low relief wrinkle structures from an upper bedding plane surface of the Harkless Formation, near Cedar Flats, White-Inyo Mountains, CA (note cast of P lam lites burrow at left); D) Elongate rounded wrinkle structures with large crest height visible on upper bedding plane surface where overlying wrinkled counterpart layer has been removed (upper right), from the Poleta Formation near Cedar Rats, Wiite-Inyo Mountains, CA; Q Rounded slightly elongate wrinkle structures from a lower bedding surface of the Wood Canyon Formation near Johnnie in the Spring Moimtains, NV; and F) Tightly-spaced polygcmal wrinkles associated widi trilobite skeletal debris and one trace fossil; specimen is from a micaceous upper surface of a very fine grained quartzite from the Poleta Formation near Horse Thief Canyon, Last Chance Range, CA. Scale bar in all photos is 1 cm. 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Not surprisingly, in most previous accounts of wrinkle structures, biotic factors such as microbial sediment binding were discounted, or ignmed in lieu of purely physical processes (e.g., Fenton and Fenton, 1936; Dzulynski, 1966; Dzulynski and Simpson, 1966; Hunter, 1969; Reineck, 1969; Kummel and Teichert, 1970; Ricci-Lucchi, 1970; Teichert, 1970; Goldring, 1971; Klein, 1977; Vos, 1977; Mantz, 1978; Singh and Wunderlich, 1978; Reineck and Singh, 1980; Kocurek and Fielder, 1982; Seilacher, 1982; Kopaska-Merkel and Grannis, 1990; Robb, 1992; but also see MacKenzie, 1972). For example, in laboratory flume experiments seeking to re-create wrinkles, sediments were Qpically sterilized of organic (i.e., microbial, meiofaunal) material, thus preventing the re creation of a true biotically enriched marine sedimentary environment (e.g., Reineck, 1969; Dzulynski and Simpson, 1966). Ironically, by removing the microflora and microfauna, previous woikers essentially acknowledged the strong role that microbial binding plays in sediment consistency and cohesion. Modem studies, for example, have shown that even the most sparse microbial presence changes the adhesion of sediment grains by two orders of magnitude (Neumann et al., 1970; Dade et al., 1990). Thus, Charles Walcott’s original supposition that “algal” or microbial factors were involved may have been correct. To examine the possibili^ that biotic factors played a role in the formation of these structures, I made preliminary field studies of modem microbial mats from Redflsh Bay, Texas, analyzed literature reports of microbial communities and structures in marine siliciclastic settings, and compared these results with ancient suspect- microbial structures in Vendian and Cambrian strata from eastem California and western Nevada. 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MODERN VS. ANCIENT Morphologic and Sedimentologic Investigations Lack of rapid seaflow cementation, ^ ic a l of carbonate-dominated marine environments during the Proterozoic. has hampered effoits to pinpoint diagnostic microbial clues (such as a filament mold or carbon isotope signature) in ancient suspect-microbially- dmninated siliciclastic facies. Thus, for this study, a comparative morphologic and sedimentologic s^roach was used in order to implicate or reject microbial mats as a culprit in the formation wrinkle marks. In Redflsh Bay, Texas, modem supratidal flats were examined immediately after they were flooded by a storm-induced high tide (Hg. 4.2A). In areas of pooled water, microbial mats flourished immediately after the flooding event, prior to consumption and disrupticMi of the mats by cerithid gastropods, crabs, and other animals. In these environments, (and in other areas where bioturbating organisms are excluded by elevated salinities and/or subaerial exposure), patchy microbial communities flourished, forming cohesive carpets of microbially-bound fine-grained sediment (Sorensen and Conover, 1962). These sediments, which are typically laminated (up to I cm depth, depending on duration of subaqueous exposure and bioturbation; Hg. 4.2B), are characterized by pustulose, wrinkled, irregularly shaped surfaces - which are commonly contorted, tom, or folded (Hg. 4.2C.D). Sediments in this setting primarily consist of wind and river-derived fine to very fine grained quartz-rich sand, silt, and clay. Although such features were not examined in detail in Redflsh Bay, similar laminated microbial mats are often underlain by concentrations of heavy minerals, such as pyrite, magnetite, or illmenite, and are notable because their surface filaments tend to baffle and trap finer-grained micaceous sediment (e.g., Dunham, 1962; Shinn, 1983; Gerdes et al., 1985; Schieber, 1989; Noffke et al., 1997a,b). Laminations can extend to a depth of 1 cm, but in pools examined at Redflsh Bay, they typically extended only to 0.5 cm depth. In these pods, wrinkled surfaces 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. exhibited ridgertiough relief of 1- 4 mm (mean - 2.5 mm; n=20), and intercrest (i.e.. ridge to ridge) distances of 3-8 mm (mean- 4 mm; n=20). Wrinkle ridges exhibited angular to rounded bifurcation patterns, were characterized by both pointed and rounded wrinkle crests, and with individual crests extending for tens of centimeters. These surface morphologies varied depending on relative growth of mat filaments at the surface. Wrinkle troughs were typically covered with a thin layer of very fine to fine grained sand, with minor amounts of silt, clay, and organic debris. Although the relationship between mat cohesiveness, thickness and surface structure morphology remains to be quantified [sensu Dade et al, (1990) or Noffke et al., 1997a)], modem mat-dominated settings can exhibit a variety of physical and morphologic characteristics which are comparable to those observed in the geologic record. For example, in modem siliciclastic settings, mats can exhibit a variety of surface morphologies, including thirmer, less-well-developed mats which behave like the "skin" on top of a pudding. Spongy mat surfaces also have a pudding-like consistency, whereas thicker layered mat-bound sediments (i.e. where laminations can be observed at depths of at least 5 mm) have a consistency like a tough bread (Neumarm et al., 1970; Gunatilaka, 1975; Gerdes et al., 1985,1991,1993; Cameron et al., 1985; Dade et al., 1990; Noffke et al., 1996,1997a,b; or the excellent synthesis in Krumbein et al., 1994). The latter mat textures are quite corrunon in settings such as Redfish Bay, and were used in the comparative morphologic and sedimentologic approach. 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.2: Microbial mats with wrinkled surfaces from Redfish Bay, TX. A) Flooded tidal flats, where microbial mats flourish (dark areas in foreground pools) in patchy concentrations. Foreground field of view is approximately 10 m. B) Cross-section of thin microbial mat from margin of above pools. C) Plan view of wrinkled mat surface from shallow (ca. 3 cm deep) portion of pool - mat is growing in very fine quartzitic sand in which filaments are directly exposed at wrinlde ridge crests, and in which inter-crest troughs of wrinkle ridges are filled with a thin veneer of sediment. D) Plan view of patchy microbial mat from deeper portion of pool. Note wrinkled surface textures, which in this example, consist of wrinkle ridges that are quasi-elongate to dendritic and are characterized by relatively sharp to rounded wrinkle crests. Mats are slightly thicker in this portion of the pool, and intercrest regions appear to have less of a sediment veneer, possibly resulting from relatively increased mat growth. Opaque circular objects are bubbles on the surface of the ponded water. Scale bars on left of DNAG card are in cm. 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the southwestern U.S.. wrinkles were studied in a variety of Vendian and Lower Cambrian units (Fig. 4.3), including the Campito, Deep Springs, Harkless, Johnnie, Poleta, Stirling Quartzite, Wood Canyon, and Wyman Formations. Within these formations, wrinkle structures occur in units which exhibit broadly similar sedimentologic characteristics (described below), typically occurring on the tops of centimeter to decimeter- thick very fine to coarse-grained sandstones and quartzites (Fig. 4.3C). Because they commonly have less than a centimeter of vertical relief, wrinkles are difficult to identify in vertical section and are best exposed on dip-slopes, or on large bedding-plane exposures. Within studied stratigraphie sections, wrinkle structures can be found on tens to hundreds of beds within a given 100 m thick measured section. On individual bedding planes, wrinkle structures are commonly quite laterally extensive, occurring in patchy concentrations. Wrinkle structures can also occur on ripple-marked bedding planes, where they occur in the troughs and crests of uni-, bi-, and cross-directional ripple features (figure 1B,E of Hagadom and Bottjer, 1997). Furthermore, wrinkle structures are overlain and underlain by very fine to coarse quartzitic sand laminations. Within lamination-bounded vertical sequences, wrinkled layers can be as closely spaced as 3 mm. At present, it is difficult to reconcile (through pétrographie and slab analysis) if there is a systematic pattem of lamination deformation or truncation associated with ancient wrinkle structures, or if these patterns vary according to depositional environment. Many wrinkled bedding plane exposures are coated with a very thin layer of mica (Fig. 4. IF; figure l E f of Hagadom and Bottjer, 1997) which occurs in the troughs and crests of wrinkle surfaces. Mica-rich surface layers are typically 0.02 to 0.1 mm thick and include 0.1 to 0.2 mm long mica flakes that are mixed with very fine sand and silt grains. Oddly contorted surface textures, suggestive of sediment shearing or tearing are also commonly associated with bedding plane exposures of wrinkle structures (e.g., Schieber, 1986, in press; Pflueger and Sarkar, 1996; Pflueger, 1997, in press). 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.3: Context for wrinkle structures from ancient Great Basin strata. A) Generalized stratigraphie and geographic setting for observed occurrences of ancient wrinkle structures. Measured sections were made at wrinkle-laden localities in the Silver Peak Range and Spring Mountains in western Nevada (1), and in the Inyo Mountains, Last Chance Range, and White Mountains in eastern California (2). Wrinkle-bearing horizons are extremely common in the 13 study sections (arrows) in the Death Valley (Bi) and White-Inyo (B2) re^ons. Other than variation in the style and degree of bioturbation (i.e., Vendian sections are less bioturbated, and the Johnnie section in the Spring Mountains appears to be unbioturbated), the generalized section at Cedar Rats (C) in the White-Inyo Mountains is typical of studied wrinkle occurrences. Generalized regional lithostratigraphy and biostratigraphy based on Stewart (1970), Nelson (1976), Hunt (1987), Corsetti and Kaufman (1994), and S. Hollingsworth, (pers. comm., 1998). 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ocean (BI) Death Valley Facies SC Carrara Fm ZabfutacQtt Wood Canyon Fm Stirling Quartzite Johnnie Fm (82) White-Inyo Facies Monola Fm Mule Spring Ls Saline Valley Fm Harkless Fm Poleta Fm Campito Fm Deep Springs Fm Reed Dolomite M AM iL M U M Wyman Fm C) Cedar Flats, CA m E / 1 / 5^ I9ml 63 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Quartz grains range from fine- to coarse-grained sand, and quartz grains immediately underlying the surface have fewer grain-grain contacts than the underlying laminations, or the overlying (burial) layers. Wrinkle surface relief ranges from ca. 0.03 mm to 3 mm (from wrinkle crest to trough) and wrinkles have intercrest distances of 1-5 mm. Crest shape, crest length, and ridge steepness vary according to the variety of wrinkling - which includes mosaic, polygonal, bifurcating, and elongate subparallel forms (Fig. 4.4). Although wrinkle structures generally cap thin laminations, these wrinkle- bearing layers can overlie cross-bedded sediments, flat-lying sediments, as well as obliquely inclined and draped layers of laminated to thinly bedded sediments. In all of the foregoing associations, no bedding features, such as inclined foresets, grading, asymmetric ridge faces, or layer truncations were observed within individual wrinkle ridges, suggesting that wrinkle crests are not microscopic waveform features. Relatively contiguous sets of laminations underlying wrinkles extend as much as 12 cm beneath wrinkled surfaces. Laminations and underlying layers ranged in thickness from ca. 0.1 mm to 2 mm and included very fine to coarse sands. Where wrinkles have been observed in cross-section in unweathered slabs, both rounded- and flat-crested wrinkles occur where underlying and overlying sediments are lithologically similar. These observations suggest that rounded and flat wrinkle crests observed on bedding plane exposures are not preservational end-members of weathered pointed- or flat-crested wrinkles. Microscopic analysis of wrinkle horizons within unweathered polished slabs suggests sharp interfaces exist between overlying and underlying sand grains. In very fine sands, interfaces are at most 0.02 mm thick and have been measured at less than 0.01 mm (i.e., the thickness of one grain of silt). In coarser sands, wrinkle interfaces are nearly indistinguishable from overlying and underlying grains. Together, these observations suggest that wrinkle features do not likely represent loading and/or dewatering of thin mud 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. layers (as in Allen, 1985). Wrinkles are commonly associated with trilobite exuviae (Fig. 4. IF) and tubular microfossils, and in rare cases are found on adjoining bedding planes with unmineralized annulated body fossils, and mineralized sclerites of unknown origin (Hagadom and Bottjer. 1997; Waggoner and Hagadom, 1997). In general, ancient occurrences of wrinkle structures formed in subtidal marine environments, ranging from tidally influenced shallow facies to strata deposited significantly below storm wave base. These interpretations are based on sedimentary structures and vertical facies relationships within specific measured sections, and integration of these smaller-scale observations into previously established regional depositional contexts (Stewart, 1970; and volumes edited by Moore and Fritsche, 1976; Robison and Rowell, 1976; Cooper et al., 1982). 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.4: Schematic representation of morphologic characteristics used to describe wrinkle structures. All sketches (except the Crest & Trough Width section) are drawn from samples collected from study sites. All scale bars are 0,5 cm 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Intercrest Distance (cross-section) Crest Height (cross-section) I I Crest Shape (cross-section) AAA- t Surface Patterns (plan view) Crest & Trough Width ► T ‘ (cross-section) C i Q: V 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Comparison o f Modem and Ancient Structures Modem and ancient study surfaces both exhibit striking wrinkled surface morphologies, and commonly exhibit patchiness as well as sheared, tom, or other mat- related surface characteristics (Figs. 4.1,4.2). Wrinkle subsurface sedimentology is similar in both modem and ancient facies, including presence of subsurface laminations, fme-to medium sand sizes (with minor amounts of silt and clay), and accumulations of opaque minerals. Although no current database suitable for rigorous statistical comparison of morphologies of wrinkle structures and modem mat textures is currently available {sensu Sobrero et al., 1998), the height, length, intercrest distance, and angle of repose of modem and ancient wrinkles are quite similar. In addition, despite pustular and wrinkled surfaces, laminations below wrinkled surfaces are nearly flat-lying and parallel. At present, it is not known if such lamination reflects diumal growth of filaments, tidal sediment flux oscillations, concentrations of organic matter, or a combination of biological and depositional factors (Gerdes et al., 1991). Laminations beneath ancient wrinkled surfaces also extend much further beneath the substrate than in modem occurrences (e.g., up to 1 cm in Redfish Bay and up to 10 cm in ancient samples) - a phenomenon which may result from variations in duration of aqueous immersion and/or bioturbation (i.e., persistence of relatively continuous mat-growth in the same area; Kendall and Skipworth, 1968). However, modem laminations are of similar size and lateral continuity as those observed in ancient study sections (Horodyski et al., 1977; Gerdes et al., 1985). In summary, comparison of sedimentologic and morphologic characteristics of modem and ancient wrinkled surfaces provides sufficient evidence to collectively implicate microbial mats in the genesis of wrinkle structures. Burial of mat-covered surfaces by a sediment pulse, possibly associated with a spring tide in shallower facies or a storm event/turbid flow in deeper settings, is hypothesized to have sealed mat-covered surfaces, either restricting filament growth so that the mat perished, or forcing the mat community to 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. migrate upward (Gerdes and Krumbein, 1987). In patches where current velocities associated with the burial event were lower, the cohesive mat surface was not ripped firom the substrate (resulting in sand chips, c.f. Pflueger and Sarkar, 1996), and the wrinkled textures were preserved. Perhaps these mats may also mediate parting along mat-rich layers which have trapped limited amounts of clays - as authigenic micas derived from the clays provide a post-compaction cleavage interface in lithologically homogeneous beds. In coarse sandstones of the Rawnsley Quartzite in Australia, a similar phenomenon is observed in lithologically homogeneous beds, but the mat-covered interface is manifested as limonitic coatings, which, as a geologic byproduct, promote soft-bodied Ediacaran preser\'ation (Gehling, in press). Discussion Because only biotic factors from one modem marine environment have been examined, fairly generalized assumptions must be made about the microbial genesis of wrinkle structures. However, review of the literature suggests that observed wrinkled textures are not anomalous phenomena unique to the study area; rather, wrinkled surfaces are common features of modem mat-dominated sandy settings (Sorensen and Conover, 1962; Gunatilaka, 1975; Cameron et al., 1985; Reynolds, 1988; Grant, 1991; Bauldet al., 1992; Gerdes et al., 1993; Noffke et al., 1997a). Based on cumulative similarities, the assumption is made that all ancient wrinkles in this study (for which there are comparable modem analogs) are suspect-microbial structures, and that associated laminated sediments are likely to be microbially dominated, if not microbially bound. In making this assumption, one would be remiss not to stress that: a) non-microbial mechanisms may account for some of the observed structures; b) that a combination of syndepositional and postdepositional microbial and physical processes may lead to formation of specific forms of wrinkle textures [much like mat phenomena described by Gehling (1996, in press)]; and 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c) that all modem sediments, including unconsolidated sediments which appear abiogenic to the naked eye, are characterized by microscopic microbial communities (including phototrophic bacteria, chemotrophic bacteria, diatoms, and other organisms), in all depositional environments, from supratidal flats to seeps/vents to the deep abyssal plains (see overviews in Edwards, 1990; Pierson, 1992). TEMPORAL PATTERNS Wrinkle structures and related suspect-microbial structures are extremely common in Proterozoic and Lower Paleozoic marine environments (e.g., Walcott, 1916; McKee and Resser, 1945; Hantzschel, 1965; Martinsson, 1965; Diehl, 1974; Wunderlich, 1970; Moore, 1976a,b; Klein, 1977; Singh and Wunderlich, 1978; Cope, 1982; Hiscott, 1982; Wertz, 1982; Ranger et al., 1984; Fedo and Cooper, 1990; Kopaska-Merkel and Grannis, 1990; Fedonkin, 1992; Hughes and Hesselbo, 1997; Jensen, 1997; MacNaughton et al., 1997; Gehling, 1996, in press). Later in the Phanerozoic and in modem settings they typically occur in deeper marine or restricted facies where intense bioturbation is suppressed (e.g., McKee, 1954; Riicklin, 1954; Haff, 1959; Dzulynski and Walton, 1965; Hantzschel and Reineck, 1968; Hunter, 1969; Playford and Cockbain, 1969; Kummel and Teichert, 1970; Ricci-Lucchi, 1970; Goldring, 1971; Gunatilaka, 1975; Schwarz, 1975; Bloos, 1976; Playford et al., 1976; Seilacher, 1982; Bridge and Droser, 1985; Reynolds, 1988; Bernier et al., 1991; Grant, 1991). Wrinkle structures thus exhibit a distinct “onshore-offshore” pattem. For shallow marine settings, the analogy of a swinging pendulum helps illustrate temporal changes in the dominant influences on siliciclastic sedimentary fabric (Fig. 4.5). Barring secular geochemical variations, the distribution of wrinkle structure occurrences (outlined above) suggests that microbial and physical processes were the dominant factors affecting siliciclastic sediment fabric in the Proterozoic. During the Vendian-Cambrian 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. transition, the sedimentologic “pendulum” shifted towards increasing metazoan influence, and by the post-Ordovician, the dominant factors affecting marine siliciclastic sedimentary fabrics were physical and metazoan processes. Thus, the Vendian-Cambrian transition records a unique snapshot of the transition between typical Proterozoic (physical-microbial) and Phanerozoic (physical-metazoan) sedimentary fabrics. In recording this change, the Vendian-Cambrian transition affords one of the few opportunities to observe both suspect-microbial structures and diverse trace fossils in the same setting. Later in the Phanerozoic, bioturbated sediments and microbial mat structures are typically mutually exclusive in siliciclastic shallow marine settings, largely due to increased vertical mixing of sediment by bioturbation recorded after the Ordovician (e.g., Bottjer and Ausich, 1986; Droser, 1987; Droser and Bottjer, 1988,1989; Crimes and Droser, 1992; Crimes et al., 1992; Seilacher and PflUger, 1994; Mcllroy and Logan, in press). By analyzing relationships commonly observed between traces and suspect- microbial textures, one can outline some preliminary insights about early metazoan-mat relationships, how these relationships changed through time, and how such shifts correspond to larger-scale biotic changes that occurred during this critical transition interval. 71 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.5: Schematic representation of shifts in dominant factors influencing the marine siliciclastic sedimentary record during the Vendian-Cambrian transition. Although geochemical shifts also occurred at this time, (e.g., changes in phosphatization, biomineralization, oxygenation), they are not superimposed on this diagram because their affect on temporal changes in siliciclastic sedimentary fabrics is not well known. “Non-disaster” refers to post-mass extinction intervals (Schubert and Bottjer, 1992), such as the Early Triassic or Late Devonian, where wrinkle and related structures may re-appear in non-stressed shallow subtidal settings. 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. physical Non-stressed, Non-disaster Siliciclastic Facies physical microbial metazoan microbial physical metazoan microbial metazoan . Cryogenim Vendian Cambrian Ordovician Proterozoic Paleozoic 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TRACE FOSSILS & WRINKLE STRUCTURES Many surface trails in Vendian and Lower Cambrian strata are thought to represent undertracks, produced by animals moving between sedimentary layers (e.g., Plagiogmus/Psammichnites; Mcllroy and Heys, 1997; Seilacher, 1997) or where appendages penetrated overlying layers to prod or indirectly impress features upon underlying layers (Seilacher, 1953a,b). Because undertracks are formed beneath the sediment surface, they are more likely to be preserved (vs. surficial layers) in the wake of potentially erosive sediment pulses. Most of the trace fossils observed in this study appear to post-date formation of wrinkles, and where overlying and underlying relationships are visible, overlying beds appear to drape or have been deposited over burrows, or have been displaced by the act of burrowing (as in Plagiogmus/Psammichnites). The majority of smaller surface trails seem to contour the wrinkled surface microtopography, and other trails mimic larger-scale bedding features such as ripple marks, suggesting that wrinkle structures likely formed on the surface, and that animals were subsequently burrowing on and through them while wrinkles were at the sediment surface or after burial. All examples of trace fossils and wrinkle-structures described in this paper were collected in situ, in an attempt to avoid erroneous interpretations based on museum, private collector, or other non-integrated data sets. Where possible, both overlying and underlying beds were collected, in order to better constrain sedimentologic relationships of beds. In addition, it should be noted that taxonomic interpretations of trace fossil genera are conservative in nature, and as such, may underestimate the diversity of morphologies and behavioral complexity of animals making such traces. Relationships Trace fossils found on top of wrinkle structures include Helminthoida, Palaeophycus, and Planolites (Fig. 4.6). These traces range from 1.5 - 4 mm in diameter, 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 - 100 mm in length, and exhibit 1-1.7 mm of surface relief. They are typically preserved in convex epirelief, full relief, and convex hyporelief, and contain very-fine to fine sand burrow fill. Trace fossils observed to cross-cut and disrupt wrinkle structures include Aulichnites, Gordia, Helminthoida, Helminthoidichnites, Palaeophycus, Planolites, and bilobate trails similar to "Scolicia" (Fig. 4.6). Such traces range from 0.8 to 4.3 mm in diameter, from 10 to 160 mm in length, and exhibit 0.7 -1.5 mm of surface relief. Whereas larger bilobate traces that deform adjacent wrinkle crests and margins suggest passive movement through wrinkled surfaces, sharp wrinkle-trace margins suggest excavation, ingestion and backfill behind the tracemaker. These traces are typically preserved in convex epirelief, in fine-medium-grained sands. Less commonly, trace fossils occur directly beneath (i.e., within) wrinkled horizons and include simple lined and unlined burrows, including probable Palaeophycus and Planolites, visible in polished slabs or thin section. These traces are preserved in full relief, range from 0.5 to 1.2 mm in diameter, can occur from 2 to 20 mm below wrinkled surfaces, and based on serial sectioning, extend at least 15 mm in length. It is not known if and/or how these burrows manifest themselves in the surface because thus far they have only been observed in cross-sectional view or where wrinkled layers have been chipped away from rock surfaces. Their burrow linings are visibly distinct, but petrographically indistinct, suggesting linings are likely composed of clays or other amorphous minerals. Subsurface traces occur in a variety of grain sizes ranging ftom coarse to very fine sand, and trace fill is typically the same grain size or slightly finer than surrounding grains. Unlined burrows may represent active ingestion or removal of sediment by an infaunal animal and lined burrows may represent domiciles or agricultural networks (Rôder, 1971; Seilacher, 1977; see overview in Bromley, 1997). Infaunal traces do not cross-cut grains and therefore do not reflect bioerosive features. Subrounded to ovoid suspect-fecal pellets 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. up to 0.15 mm in diameter have been noted in medium-coarse sands, and are composed of glauconitic fine to very fine silt (c.f. Pflueger and Cuomo, 1995). In many cases, trace fossils are found on the same bedding planes as patchy occurrences of wrinkle structures, but are not directly on or in the wrinkles. Sectioning of adjacent occurrences reveals the presence of laminations and other sedimentologic features similar to those underlying wrinkle structures. Well-preserved Aren/co/irej, Bergaueria, Cruziana, Diplichnites, Helminthoida, Monocraterion, Monomorphichnus, Palaeophycus, Planolites, Riisophycus, Skolithos, Taphrhelminthopsis, Treptichnus, and variations of these forms were noted. As with most surface traces in this interval, preservation includes concave epirelief, convex epirelief, and full relief. Such wrinkle-associated traces range from scratches 0.3 mm wide to trails and resting traces 10 cm wide and up to several meters in length. 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.6: Typical trace fossils found associated with suspect-microbial structures. A) Helminthoidichnites, a lined burrow which occasionally exhibits non-self intersecting looping, and which cuts through wrinkled bedding surfaces, truncating wrinkle ridges. B) Planolites, a small un lined trace fossil which truncates wrinkle crests. Note concave depression at right where burrow fill has been removed from wrinkled surface. Burrow is obscured at left, however, and together with its relatively straight and uniform cylindrical profile, suggests it could also be a poorly preserved mold of an unmineralized tubular fossil. C) Gordia, an unlined burrow that has self-intersecting loops, which directly cross-cuts and excavates wrinkled sediment surfaces, and which has backfill that is petrographically distinct relative to adjacent wrinkles - suggesting active sediment ingestion. Note indistinct wrinkle textures at upper and lower right, as well as partially obscured wrinkles at left above lower large circular loop. D) Palaeophycus, a simple thinly-lined burrow found on wrinkle surfaces. E) Slightly larger form of Palaeophycus, in which the burrow top has been partially removed. This trace has a very fine sandy burrow lining which is slightly thicker than D) and fill which is slightly coarser (fine- medium quartz sand). ^ Bilobate burrow similar to Aulichnites, in which both lobes are semi-circular in cross-section, lobes are separated by a median furrow, and in which upper bilobate portions of burrow drape adjacent sediments and are larger than underlying burrow trough, much like Taphrhelminthopsis. Burrow is partially obscured by overlying sediment layer at lefr, but is continuous with wrinkled surfaces (visible at bottom left and right), and appears to meander through wrinkled surfaces, perhaps plunging in and out of the sediment surface. All scale bars are 1 cm. Samples are from upper bedding plane surfaces of the Harkless Foimation at exposures near Cedar Flats, White-Inyo Mountains, CA. 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Interpretation Although one can sometimes make obvious inferences about the affinity of tracemakers (e.g., scratch marks are typically made by organisms with appendages), such interpretations can be highly speculative, because one animal can produce many traces through different behavioral, ontogenetic, or preservational scenarios, and many animals can make the same trace. Nevertheless, some general interpretations are possible, and observed trace-sediment relationships suggest a variety of life styles existed during deposition of the studied sections, including epifaunal crawling/swimming {Cruziana, Dipliclmites), semi-infaunal dwelling {Bergaueria), and infaunal dwelling {Monocraterion/Skolithos). However, the vast majority of traces directly associated with these suspect-microbial structures represent horizontally-oriented activities, such as epifaunal bulldozing (bilobate “ Scolicia"), epifaunal sediment excavating {e.g., Aulichnites and Taphrhelminthopsis which cut through wrinkles and are draped by overlying sediments; see Hantzschel, 1975; Hagadom et al., 1994), semi-infaunal burrowing (e.g., Helminthoida, Helminthoidichnites, Palaeophycus, and Planolites which occur on or in wrinkles and in which overlying sedimentologic relationships are unclear), infaunal sediment ingesting (e.g., Gordia and subsurface Planolites) and other infaunal activities (e.g., subsurface Palaeophycus). Based on their size, it can be inferred that the majority of traces were made by metazoans, that infaunal animals in studied habitats were generally small (mm-scale), and that epifaunal tracemakers ranged widely in size (mm- to dm-scale). The narrow diameter, long trace length relative to trace diameter, mode of preservation (i.e., dominantly full relief and epirelief), low relief relative to wrinkle relief and bed thickness, and consistent bedding-parallel orientation of rounded, bilobate, lined and unlined burrows observed on and cross-cutting wrinkle structures suggest concentration of behavioral activities on bedding planes. Such bedding-parallel burrowing, together with evidence for sediment-excavating (oftentimes referred to as grazing; 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hantzschel, 1975; Crimes, 1992a) suggests that organisms were targeting surface or subsurface concentrations of decaying organic matter or biomass characteristic of buried mats (Seilacher, 1967b; Heezen and Hollister, 1971; Gerdes et ai., 1985,1991; Seilacher and PflQger, 1994; Seilacher, 1997). Such burrowing strategies have been proposed by Seilacher and Pfliiger (1994) and documented by Seilacher (1967b, 1997) in subtidal facies of the Vendian, in deep-sea settings later in the Phanerozoic, and in modem mat-dominated tidal flats. These study occurrences add to these observations by documenting ingestion of suspected mat-bound sediment and by demonstrating that this mode of sediment mining persisted into the Early Cambrian, but only in settings with mats that were not significantly disrupted by vertical bioturbation. Implications and Patterns Clearly, wrinkle structures have a temporal and paleoenvironmental distribution that mirrors the paleoenvironmental history of bioturbation with a significant vertical component (Bottjer and Ausich. 1986; Droser and Bottjer, 1989; Hagadom and Bottjer, 1997). Like many trace fossils which show a distinct Phanerozoic onshore-offshore pattem (e.g., Crimes and Anderson, 1985; Bottjer et al., 1988; Crimes et al., 1992; Bottjer and Droser, 1994; Crimes and Fedonkin, 1994), suspect-microbial structures largely disappear from shallow marine facies after the Ordovician (through restriction to the deep-sea or stressed facies), and only reappear in such facies when bioturbation has been significantly suppressed. Why? If bedding-parallel burrowers at the Proterozoic-Phanerozoic transition were concentrating their efforts on utilizing layered mat-mediated food resources, either at or below the sediment-water interface, then predominance of bedding-parallel burrowing during this interval may reflect establishment of an ecologically complex community in which organisms optimized their feeding strategies to exploit food resources concentrated 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in bedding-parallel surface or subsurface layers (sensu Seilacher, 1967b; 1997; Rôder, 1971 ; Seilacher and Pfliiger, 1994). Surface grazing or subsurface mining may have been employed to obtain food resources such as decaying organic-rich layers of microbial mats (c.f. “Killer mats” of Gerdes et al., 1985), and other associated microbiota (see overview in Bromley, 1997). In this non-actualistic environment, the ecological barrel may have been “full” - as organisms utilized well-developed strategies to extract “seams” of organic matter concentrated in mat-rich layers (Seilacher, 1967b; Seilacher and PflUger, 1994). In shallow marine settings, such bedding-parallel burrowing dominates Vendian and Lower Cambrian strata - while vertical sediment disruption is relatively low when compared with the high diversity of trace fossils and organisms documented from this interval (with the exception of Skolithos in nearshore facies; Droser, 1987,1991; Crimes and Droser, 1992; Crimes et al., 1992; Crimes and Fedonkin, 1994; Droser et al., 1994; Mcllroy and Logan, in press). For example, meandering, patterned and spiral traces that are typically thought to be characteristic of the deep-sea first originated in shallow siliciclastic settings during this interval, and exhibit a significant onshore-offshore retreat into the deep-sea after the Cambrian (Crimes and Anderson, 1985; Crimes et al., 1992). Such bedding-parallel burrowing is gradually overshadowed in the Cambro-Ordovician transition by vertically oriented burrowing styles which thoroughly mix the sediment (e.g., Ausich and Bottjer, 1982; Bottjer and Ausich, 1986; Droser, 1987,1991; Droser and Bottjer, 1988,1989; Crimes and Droser, 1992; Droser et al., 1994; Mcllroy and Logan, in press). This increase in vertically-oriented burrowing behavior, and hence tiering, may have been initiated in response to increased predation on the surface (e.g., Bottjer and Ausich, 1986; Conway Morris, 1998), waste disposal (e.g., Kotake, 1989, 1991), avoidance of inhospitable surface conditions, or differing agricultural strategies (e.g., Valentine, 1975; Bottjer and Ausich, 1986; Seilacher and PflUger, 1994). Such increases 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in non-Skolithos vertically-directed burrowing are accompanied by a dramatic disappearance of suspect-microbial structures from shallow marine settings. Although one could argue that such vertical bioturbation merely obscured shallow bedding-parallel traces, analysis of frozen tiering profiles (Savrda and Bottjer, 1986) suggests that complex sand- dwelling vertically stratified infaunal communities did not develop until the latest Cambrian (Bjerstedt and Erickson, 1989; Pillion and Pickerill, 1990; Droser et al., 1994). Additionally, ichnofabric indices for nearshore siliciclastic facies are generally low during the Vendian-Lovver Cambrian interval (iil or ii2 for non-Skolithos piperock facies; Droser, 1987,1991 ; Mcllroy and Logan, in press). Thus, disruption of mat-fabrics by increasing levels of vertical bioturbation may have relegated horizontally-optimized burrowers to the remaining environments where well-developed mat fabrics (and associated layered organic buildups) still flourished, such as deeper marine or restricted settings. If onshore-offshore shifts of bedding-parallel burrowing and suspect-microbial structures were indeed linked, then the proliferation of vertically-directed bioturbation in shallow marine settings may represent replacement of ecologically well-adapted bedding- parallel burrowers, rather than evolution of “simpler” or more “primitive” bedding-parallel foraging approaches into more “complex” vertical approaches. DISCUSSION & CONCLUSIONS In most published reports on the generation of structures in siliciclastic sediments, the sediment is considered to be generally sterile, and the interactions between organisms within this sediment are largely approached as if it were a noncohesive passive building material, rather than as an energetic-biotic feedback system. These observations of modem microbially-bound surface structures similar to Vendian-Cambrian wrinkle textures suggest that microbial mats carpeted Proterozoic and Early Cambrian shallow subddal siliciclastic 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. seafloors. Clues to mat presence include the wrinkle structures described herein, as well as related microbial features described by others in this theme issue. Study of trace fossils in microbially-bound sediments spanning this transition interval allows one to examine biotic interactions between early metazoans and such microbi ally-bound substrates, particularly because soft-bodied animals are commonly not well-preserved within such strata. Trace fossils associated with suspect-microbial structures from Vendian to Lower Cambrian strata of the Great Basin offer a preliminary case study of such relationships. In study exposures, presumed metazoan bioturbators are documented on, in, and under suspect-microbially bound sediment, in some cases suggesting active ingestion of sediment, active and/or passive movement or disruption of such sediment, and possibly also lifestyles within (but parallel to) such microbially layered sediment. Comparison of the distribution of intricate bedding-parallel trace fossils to the distribution of wrinkle structures suggests that the two may represent an ecologically linked system which retreated to deeper or restricted settings as a result of proliferation of vertically-oriented bioturbating organisms in shallow marine niches. 83 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5: Paleoecology of a Large Early Cam brian B ioturbator 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT The Lower Cambrian Poleta Formation in the White-Inyo Mountains, CA contains well-preserved and laterally extensive exposures of the large looping and meandering trace fossil Taphrhelminthopsis circularis. Morphologic, sedimentologic, and goniogram analysis suggests that the inferred tracemaker may have been a large soft-bodied echinozoan- or mollusc-grade animal at least 14 cm^ in size. Such traces are typical of Lower Cambrian shallow marine sandstones, and likely reflect active grazing or ingestion of sediment at or close to the sediment-waier interface. Although portions of these traces appear to reflect relatively "complex" behavior, looping patterns are not strongly periodic as would be expected in a systematic foraging strategy. Rather than being evenly or widely dispersed on individual bedding planes, however, burrows appear patchy in their distribution - commonly in areas associated with suspect-microbial features. Thus, tracemakers may have been focusing efforts in specific areas, possibly targeting microbially mediated food sources. Such behaviors are typical of shallow late Neoproterozoic-Lower Cambrian settings, and like subsequent deep-sea restriction of suspect-microbial structures, these behaviors are later recorded only from deep marine or stressed settings. INTRODUCTION The Proterozoic-Phanerozoic transition records rapid chemical, biological, and sedimentological changes in Earth history, as has been documented by significant shifts in the nature of sedimentary fabrics, appearance and diversification of soft-bodied metazoans, and increases in ecospace utilization, bioturbation, and behavioral variety (see syntheses in Lipps and Signor, 1992; Schopf and Klein, 1992; Bengtson, 1994; Chen et al., 1997; Conway Morris, 1998). Although much paleobiologic information has been extracted irom the skeletonized fossil record of this interval, many important paleoecologic insights have 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. come from study of soft-bodied animals preserved in Konservat-Iagerstatten (see summaries in Whittington and Conway Morris, 1985; Allison and Briggs, 1991; Briggs, 1991; Conway Morris, 1998). For example, paleontologists have learned about the interactions between animals within early paleocommunities by analyzing their gut contents (e.g., hyoliths in intestines of early priapulid worms; Conway Morris, 1977) or noting signs of predation [e.g., healed bite marks in trilobites (Owen, 1985; Babcock, 1993); borings in early mineralized skeletons (Bengtson and Yue, 1992; Conway Morris and Bengtson, 1994)]. Although trace fossils have been instrumental in outlining broad patterns in the temporal and paleoenvironmental evolution of early animal habits (see summaries in Crimes and Droser, 1992; Bottjer and Droser, 1994; Crimes and Fedonkin, 1996), less is known about the paleobiology and paleoecology of early bioturbating animals (but see Seilacher, 1970a, 1997; Jensen, 1990; Yochelson and Fedonkin, 1993; Mcllroy and Heys, 1997). With rare exceptions, such as trilobites preserved with Rusophycus traces (Osgood, 1970), these uncertainties stem from difficulty implicating specific tracemakers with their burrows and with understanding the behavioral records left in the sediment. However, novel approaches have been developed to extract such information from the trace fossil record, and application of these approaches to the Vendian-Lower Cambrian trace fossil record provide the ability to learn more about the range of behaviors employed by early animals, and to some extent provides gross estimates of the range in size and dominant écologie strategies utilized by animals within specific paleoenvironments. For example, meandering bedding-parallel trails have been analyzed to test hypotheses suggesting increasing complexity of burrowing behavior through time (Seilacher, 1967a, 1974,1977; Crimes, 1992a), and to distinguish between different types of morphologically similar looping trace fossils (Hofmann and Patel, 1989; Hoffman, 1990). 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Despite the lack of overprinting by intense vertical bioturbation in non-Skolithos piperock facies (Droser, 1987, 1991; Bjerstedt and Erickson, 1989; Pillion and Pickerill, 1990; Droser et al., 1994), however, the Vendian-Lower Cambrian trace fossil record has typically not been analyzed for detailed paleobiologic and paleoecologic information. For example, despite the ubiquity of Skolithos piperock in this interval, it has not yet been possible to produce an understanding of the paleoecology of the Skolithos-vaaksx, or its role in early paleocommunities (but see Pemberton and Frey, 1984; Vossler and Pemberton, 1988). Recently, however, individual trace fossils common to this interval have been re-examined, and using integrated sedimentologic, morphologic, and quantitative approaches, significantly more has been learned about the paleobiology and paleoecology of these early soft-bodied burrowers, and this information has been integrated with observations from the body fossil record (Seilacher, 1967a, 1970a, 1995,1997; Yochelson and Fedonkin, 1993; Mcllroy and Heys, 1997). Herein such an analysis is attempted, by evaluating behavior recorded in the relatively common Early Cambrian trace fossil Taphrhelminthopsis, in hopes of inferring additional paleobiologic, paleoethologic, and paleoecologic information about large early metazoans rarely preserved as body fossils. Why Taphrhelminthopsis? Vendian-Lower Cambrian sequences in the White-Inyo Mountains are well-known for their diverse and well-preserved trace fossil assemblages, many of which have been the focus of descriptive and stratigraphie studies (Alpert, 1973,1974, 1975,1976a,b, 1977; Langille, 1974a). Among the many well-preserved discrete trace fossils in the Poleta Formation, Taphrhelminthopsis was chosen for this study because of its large size, exceptional preservation, and fortuitous occurrence on a number of accessible, laterally extensive bedding plane exposures (Fig. 5.1). At several of the study exposures, both 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. trace-bearing and overlying strata could be examined and sampled in situ. Traces are typically -5 cm wide and several meters long, making them significantly larger and longer than other common surface trails from coeval strata in the region. On a global scale, Taphrhelminthopsis is important because it is one of the large meandering trace fossils that dominate Lower Cambrian shallow marine siliciclastic settings, together with Cniziana and the ichnomorphs ascribed to Plagiogmus and Psammichnites, hereafter referred to as Plagiogmus (after Mcllroy and Heys, 1997). Unlike Cniziana and Plagiogmus, however (Seilacher, 1970a, 1995,1997; Mcllroy and Heys, 1997), little is known about the paleobiology and paleoecology of the Taphrhelminthopsis-maktr. Furthermore, Taphrhelminthopsis is one of only two Early Cambrian trace fossils documented to co-occur with Ediacaran fossils (Jensen et al., 1998), suggesting the tracemaker occupied environments which overlapped those suitable for Ediacaran soft-body preservation. 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.1; Taphrhelminthopsis traces from the Poleta Folds region of the White-Inyo Mountains. Bilobate looping and meandering patterns are clearly visible on upper bedding plane surfaces, including traces with single tight loops (A), broad arcuate loops (B), multiple self-crossings (C), and regions of high density and self- crossing (D). Note that none of the lobes of the backfill are splayed or deformed in a preferential direction, and that they are slightly semicircular in relief. Coin in (A) is 2.4 cm in diameter, lens cap in (B) is 5 cm in diameter, lower scale bar in scale card in (C) is in cm, and rock hammer in (D) is 30 cm long. 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PREVIOUS RESEARCH Although traces analyzed in his studies were not figured, Alpert (1974,1976) described trails from the study area which cover large bedding surfaces, commonly cross themselves, may form distinctive loops, and in which transverse markings are absent. Alpert ( 1974, 1976) classified these trace fossils as Scolicia, a group of traces thought to be formed by shell-less gastropod-like molluscs crawling or grazing horizontally on or within the substrate (see summaries in Hantzschel, 1975; Smith and Crimes, 1983). Scolicia, however, are typically preserved in convex hyporelief on bed soles and in concave epirelief on bed surfaces (Hantzschel, 1975); whereas the trace fossils of this study are preserved in full relief and concave epirelief on bed surfaces. Furthermore, after Alpert's original studies. Smith and Crimes (1983) suggested that the ichnogenus Scolicia be used only for traces produced by spatangoid echinoids. Considered in light of the known temporal distribution of echinoids and the lack of related Sco/zcza-specific morphologic features, these trace fossils are probably not Scolicia - but are perhaps better described as Taphrhelminthopsis, an ichnogenus erected by Sacco (1888) and described in detail elsewhere by Ksiazkiewicz (1970) and Hantzschel (1975). Studied traces are non-branching and characterized by a backfilled U-shaped burrow trough; the upper half of the burrow backfill has a median furrow, and the burrow backfill overlaps adjacent laminated sediment surfaces in two convex lateral ridges (Fig. 5.2). Ridge crests are typically elevated above the surrounding sediment surface, and the base of the median furrow is slightly below. Among the various ichnospecies of Taphrhelminthopsis, the studied traces are most similar to the loosely looping form Taphrhelminthopsis circularis (Crimes et al., 1977), which is characteristic of similar Early Cambrian paleoenvironments (Crimes, 1987). 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.2; Detailed expression of typical Taphrhelminthopsis burrow crossings. In both photographs, note that part of the trough and backfill of the lower trace are excavated by the upper crossing trace, and the upper portions of the lower trace ridges are deformed by the more recent upper trace ridges and backfill. In both cases, it is clear that the underlying trace preceded the overlying form, as the upper trace’s backfill ridges are draped across the underlying trace’s ridges. These relationships suggest self-crossing occurred on the surface, rather than in the subsurface, where an intrastratal tracemaker (like the Plagiogmus-TRak&t) could cross over under, over, or through its previous trace, and where self-crossings similar to those illustrated here would cleanly excavate or tunnel into previous burrow ridges, an occurrence not noted In any of the studied trace crossings. Beetle in upper photograph is 2.6 cm long and coin in lower photograph is 2.4 cm in diameter. 92 Reproduced with permission of the copyright owner Further reproduction prohibited without permission. 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GEOLOGIC CONTEXT In the White-Inyo Mountains, the Lower Cambrian Poleta Formation is divided into three stratigraphical!y distinct lower, middle and upper members (McKee and Moiola, 1962; Stewart, 1970; but also see Nelson, 1962). The studied horizons occur in the upper sandstone unit of the middle member at a well-known locality in the “Poleta Folds” region along the western margin of Deep Springs Valley (Fig. 5.3). Studied horizons are a few meters above the base of the Bonnia-Olenellus trilobite zone and primarily consist of thin- to thick-bedded very fine- to medium-grained quartz arenite, with minor amounts of sandy siltstone and sandy limestone. The sandstone subunit of the middle member is dominated by laminated, thinly bedded, low-angle cross-laminated and mottled bedding. In general, the study exposures consist of repeating packages of massive, blocky quartzitic beds (20-40 cm thick), overlain by thinly-bedded, more easily fragmented beds (5-10 cm thick). The thinner beds cap and appear to grade upward into the thicker, more resistant massive beds (Fig. 5.2). Burrows are preserved at the interface between massive blocky beds and thinly-bedded intervals, likely reflecting fonnation during a hiatus between coarsening and thickening-upward sequences. Within the study section, T. circularis typically occur on the top of massive 20-30 cm thick sandstones, composed of white, very fine- to fine-grained, well-sorted sandy laminated layers which range in thickness from 3 cm to 4 cm (1 mm to 0.1 mm = laminae thickness), and which alternate with darker gray-to-black mottled to poorly laminated siltier layers. Laminated T. c/rcw/am-bearing layers are overlain by black-to-dark gray thinly bedded 0.5 - 2 cm-thick silty sandstones and sandy siltstones which are poorly laminated to mottled and rarely micaceous. Laminated layers are underlain by similar, but more thickly bedded and coarser grained layers of mottled dark gray-to-black well-sorted, very fine grained silty sandstones 5 mm to 6 cm thick. Where bioturbated, internal surfaces of 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. darker silty layers are irregular in cross-sectional view. Bases of the lighter laminated beds are almost always planar, suggesting that a higher energy regime removed the upper surfaces of the darker beds, and was followed by rapid deposition of laminated sediments on top. No burrow escape structures are visible in cut slabs. 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.3: Stratigraphie, geographic, and paleoenvironmental context for studied Taphrhelminthopsis traces. Traces occur at a variety of horizons at studied sections, and are most laterally extensive at one study section (lower right), where they extend for tens of meters in a given direction. Generalized paleoenvironmental context of study region in the Poleta Folds is noted with a star in upper right diagram (modified from Moore, 1976b). Generalized regional lithostratigraphy based on Nelson (1976) and Corsetti and Kaufman (1994). 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. t 16km M onola Fm Mule Spring Ls Salme Valley Fm Haiicless Rn Poieia Fm rmkiMmimitmM Campito R n Deep Spnngs Fm Reed Dolomite W yman Fm 97 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Paleoenvironmental and Paleoecologic Context In general, the middle member of the Poleta records a transition from foreshore to shoreface to offshore environments, probably within a sand bar complex which may have been locally emergent, with minor tidal influence (Fig. 5.3b; Moore, I976a,b). These larger-scale facies interpretations are based on the presence of tidal laminations, herringbone cross laminae, low-angle cross lamination, flat-topped interference ripple marks, Skolithos, and mudcracks near the study area. Although non-syneresis cracks and diagnostic subaerial exposure features (discussed in Chapter 4) were not observed in this study, smaller-scale observations of physical sedimentary structures, stratigraphie relationships, autochthonous marine trace fossils and body fossils are consistent with previous regional-scale interpretations (Moore, 1976a,b), suggesting that facies were deposited in a dominantly subtidal setting adjacent to a ddal complex. In particular, studied sequences are interpreted to reflect oscillation between strong, rapid (possibly spring tidal) fluxes of coarser-grained sediment into a region typically characterized by more quiescent muddier conditions. In this case, perhaps tidally laminated sandy layers provided a freshly deposited substrate for opportunistic organisms to exploit. Trails on studied surfaces could reflect epifaunal movement on this siuface, and perhaps semi-infaunal movement as overlying muddier sediment was deposited. Before they could extensively chum up the surface to any great depth, and prior to subsequent exploitation of this new surface by other bioturbating organisms, these organisms were buried by another sedimentation event. Alternatively, interbedded laminated and non-laminated sequences could reflect waning flow conditions. In the latter scenario, trace-making organisms may have been moving semi-infaunally through siltier sediments, and may have been targeting the more consolidated laminated layers. 98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TAPHONOMY The studied trace fossils consist of meandering, cross-cutting, backfilled ridge and trough structures preserved on upper bedding surfaces (Fig. 5.4). Trace troughs are preserved in concave epirelief and trace backfill is preserved in full relief. The center of the upper trace surfaces consist of a broad, deep v-shaped trough. Barring the absence of sediment laminations, the burrow fill is lithologically similar to adjacent sediment and burrow troughs and do not appear to be lined. Excavation of burrow fill and sectioning of traces (including furrow-parallel, furrow-perpendicular, and bedding-parallel transects) revealed no parallel laminations, meniscate fill, perpendicular striae, trough scratches, trough prodmarks, or other features typical of other large furrowed Vendian-Cambrian traces such as Plagiogmus, or Cruziana (Seilacher, 1970a, 1995,1997; Hântzschel, 1975; Mcllroy and Heys, 1997). Where counterpart soles could be extracted, traces are preserved in convex and concave hyporelief. In the study area, T. circularis is common on many bedding planes within an 8 m- thick interval in the upper sandstone subunit of the Poleta middle member. This T. circularis-nch interval was examined at 22 sites over a -10 km^ area within the Poleta Folds region (Fig. 5.3). Research presented herein primarily focuses on one of these study sites, where T. circularis occurs on 5 bedding planes and can be traced continuously for at least 50 m in several directions along the margins of a steep gorge. Individual exposures of T, circularis-nch bedding planes within this gorge range from 1.5 to 21 mi in size (Fig. 5.5). Considering its large size and extensive lateral distribution, why is Taphrhelminthopsis so well-preserved at these study localities, and elsewhere in the Lower Cambrian? In addition to obvious factors of rapid burial and minimal vertical sediment disruption by bioturbating organisms, there may be other taphonomic factors to consider. In particular, T. circularis is one of two trace fossils directly associated with Lower 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cambrian occurrences of Ediacaran faunas (Jensen et al., 1998) and Taphrhelminthopsis is only found in shallow marine settings prior to the Ordovician (Crimes et al., 1992). Although many Ediacaran forms likely lived buried within the sediment, Gehling (1996; in press) has illustrated that preservation of surface-dwelling soft-bodied Ediacarans is typically restricted to settings where sediment surfaces were covered by microbial mats. Perhaps coincidentally, at studied localities, traces are found on bedding planes along with suspect-microbial features such as wrinkle structures (Hagadom and Bottjer, 1997), together with suspected tidal laminations. Furthermore, sharpness of trace fill and lamination contacts (described below) and lack of peristaltic trackways adjacent to the burrow fill (typical features which might be expected of animals moving across unconsolidated sediment) suggests that the surface of laminated sediment layers was quite stiff. Because carbonate cements are lacking in studied samples, one questions whether the sediment stiffness in this setting may have resulted from microbial binding. Such microbial binding has been documented from adjacent, overlying, and underlying strata in the vicinity (Chapter 4), and may help explain not only the preservational sharpness of these traces but may also provide a potential rationale for concentration of grazing activity on these particular bedding planes. 100 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.4: Simplified box-model diagram illustrating the typical mode of Taphrhelminthopsis preservation. Although burrows cross-cut underlying laminations (Fig. 5.6) and are overlain by thinly bedded to mottled layers, bed thicknesses and laminations not to scale. Measurement of maximum burrow trough width indicated at (A) and maximum burrow ridge width at (B) was noted in order to distinguish between relatively uniform trough excavation processes (a possible metric for the diameter of the tracemaker body) and the more varying width of the backfill ridges, which were presumably deformed passively by both the tracemaker’s posterior and through compression by the overlying burial layer. 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 102 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TRACE FOSSIL ANALYSES Trace Characteristics Variations between trace fossils may suggest several individuals of different size, or variation in trace-making patterns due to growth, morphology, behavior, or avoidance (see overview in Bromley, 1997). Burrow width (measured as the maximum width of the excavated trough, a width which is different from the width of the overlying backfill, because the backfill overlaps the margin of the burrow trough and drapes adjacent sediment; Fig. 5.4) is not significantly different (p<0,05, X=31.4 ±0.32 mm, n=73) and burrow depth (relative to bedding surface) is also not significantly different (p<0.05, X= 12.35 ±0.49 mm, n=73), both within and between all measured traces (p<0.05, number of measured traces = 5). If one assumes the tracemaker retained a similar mass throughout its path, then the similar trace depths within and between individual traces suggest a specialized sediment ingestion strategy and/or movement across a substrate of relatively homogeneous consistency. Furthermore, lack of significant variation in the size of the excavated burrow trough also suggests that there may not have been individuals of a variety of ontogenetic stages ingesting sediment on studied bedding planes. However, the studied sample size is relatively small, and coupled with observations suggesting the laminated sediment was relatively cohesive, one cannot reject the possibility that there may have been a threshold which restricted the ability for smaller organisms to produce these traces, or that such organisms were not focusing their efforts on studied sites. In contrast, the width of the of the trace ridges (Fig. 5.4) varies significantly within and between individual traces, ranging from approximately 22 ram to 56 mm (X=35.03 ±7.52 mm, n=73). Similarly, the depth of the trace furrows is highly variable, ranging from 2 mm to 14 mm (X=7.03 ±5.24 mm, n=73). The irregularity of the ridge width, height, and shape suggests that backfill was much less consolidated than adjacent 103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. undeformed laminated sediment and that upper portions of this backfill may have been deformed passively, as indicated by the erratic knobby shapes of the burrow ridges (Fig. 5.2A). Furthermore, upper portions of backfill are not flattened (as they would be if one or more cylindrical fecal streams were compressed by overlying bedding) nor are they preferentially folded over in one direction, as might be expected with a unidirectional storm event (Fig. 5.IB). Rather, burrow ridges are roughly semicylindrical and are separated by a median furrow which typically extends to or just below the level of the adjacent sediment surface (Figs. 5.2, 5.6). This backfill has been pushed to the side of the main burrow path and rests upon adjacent unbioturbated sediment (Fig. 5.6C). These observations suggest that backfill was passively deformed as the tracemaker's body passed over it, or that a large quasi-cylindrical backfill trail was split by the organism’s posterior into two ridges which were subject to collapse and deformation, largely because backfill was less consolidated than underlying laminated sediment. 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.5: Portions of large bedding plane exposures typical of studied sections. These bedding planes are exposed in contiguous surfaces for tens of meters along strike, and exhibit centers of high trace density, or “patchy” distributions (A, C). Rock hammer scales in A, C are 30 cm long, diameter of field of view in (B) is ca. 60 cm, and left side of scale bar in (D) is in cm. 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.6: Sectioned Taphrhelminthopsis trace, including an oblique view of the trace furrow and cut slab (A), a cross-section made perpendicular to the axis of the burrow furrow, illustrating sharp truncation of laminations (B); and x-radiographic print of the counterpart to (B), indicating overlap of burrow backfill on undeformed laminated sediment (C). Note that lamination ends are not deformed at intersection with burrow trough. Field of view in (A) is 7.5 cm, 7.2 cm in (B), and 5.4 cm in (C). 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 0 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. How Many Tracemakers Were There? Because traces were only analyzed within one sequence and on several bedding planes, the likelihood that different taxa were making similar traces on the same bedding plane is low. Although one rapidly-moving organism may be responsible for studied traces, animals which make similar traces (such as echinozoans or molluscs) typically move through the sediment relatively slowly - on the order of centimeters/hour for echinozoans (Buchanan, 1966) and meters/hr for molluscs. In light of the possibility that laminations reflect tidal ly-modulated oscillations in sediment flux, organic matter, and/or microbial mat growth, such large bedding surfaces may not have been exposed long enough for one individual to rapidly move across study surfaces. Cross-cutting relationships between and within individual traces provide evidence on the number of coeval tracemakers and their morphology. Cross-cutting structures consist of a bilobate trace covering and partially excavating an earlier trace's median furrow and infilling the earlier trace’s trough (Fig. 5.2, 5.5C). There are a total of 50 cross-cutting relationships between the 35 studied Taphrhelminthopsis traces. Of the examined trace pairs having multiple intersections, one trace always cross-cuts the other and deforms the backfill ridges of the underlying trace. To reject the possibility that one tracemaker is responsible for the traces on a particular bedding plane, reversals in cross- cutting relationships were examined. At least two of the studied traces (Fig. 5.7) illustrate a reversal in cross-cutting relationships between multiple intersections of two traces, thus providing positive evidence for at least two coeval trace-makers on studied bedding planes. Although increasing density of traces often obscures such details, when one considers the high density of traces on some studied bedding planes, it seems likely that there may have been more than two tracemakers moving across individual study surfaces. 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.7: Schematic illustration of cross-cutting relationships of two looping traces outlined on acetate sheets from Fig. 5.5A, Where burrows cross-cut one another, the upper trace at each intersection is noted by parallel lines. From such intersections, burrow direction can be discerned. In (A), for example, the tracemaker must be moving from bottom to top in order to overlap its own trace (as in Fig. 5.2) the same way twice. If traces were made by an intrastratal borrower which tunneled through its own trace or could burrow under its previous trace path, one might expect to see reversals in the over-under relationships of traces with multiple loops. Such reversals were not noted in any of the studied traces. Study of trace B, however, does not allow determination of tracemaker direction. Because A and B co-occurred, however, there existed the possibility that trace B was moving from top to bottom or vice versa. Examination of cross-cutting relationships (C) allows us to discern the minimum number of tracemakers moving on this surface. If both traces are made by the same animal, cross-cutting relationships would support scenario D or E. However, variations in over-under cross-cutting relationships illustrate that there must have been two tracemakers to produce these traces (scenarios F or G). 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Y 111 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. How Big Were They? If one assumes that the studied traces do not reflect domiciles, agricultural networks, brood chambers, or other structures erected by organisms significantly smaller than their traces, one can analyze the volume of sediment processed by the tracemaker and the cross-cutting relationships of its looping trails to estimate the tracemaker's dimensions. As a first-order approximation, the upper portions of the backfill can be retrodeformed back into the burrow’s u-shaped furrow - producing a roughly half-cylindrical backfill shape formed by the excavated burrow trough. To estimate body size, two approaches can be used: one, which assumes that the organism making the trail is roughly the same width as its backfilled trough (case I); and two, that the organism was much larger (case 2) - where the burrow trough reflects the size of the organism's food gathering apparatus-and/or the backfill reflects a slightly modified fecal stream. Loop intersections may help discern the most parsimonious assumption. For example, at trace intersections, if the tracemaker was considerably wider than its bilobate trail, one would expect it to deform the underlying unconsolidated backfilled ridges as it passed through them, depositing a new bilobate trail in its wake. Such ridge deformations are common, suggesting that the tracemaker was at least as wide as its backfilled trace (Fig. 5.2). If the tracemaker was a semi-infaunal or infaunal spherical-shaped animal of roughly the same size as its burrow (case 1), and if the mean diameter of the backfill is ~3 cm, the tracemaking animal must have been at least -14 cm^ in size (roughly the size of a golf ball). Smith and Crimes (1983; fig. 7A) illustrate a bilobate trace like Taphrhelminthopsis preserved with a mold of an irregular echinoid, which can be used to calculate the size of a probable Taphrhelminthopsis-maker which is larger than its trail (case 2). Using their best-preserved example where the tracemaker has ostensibly been buried at the end of its Taphrhelminthopsis burrow, a body volume to trace width ratio of 8 cm^ to 1 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cm can be calculated (note, however, that their examples of furrowed traces are much smaller than studied specimens). Using these relationships, the tracemaker may have had a body size as large as -1372 cm’ (dimensions: 14 x 14 x 7 cm; or about the size of two standard cans of beer, sitting side-by-side). Cross-cutting relationships, or looping, within individual traces occurs once within 11 and twice within 2 of the 35 measured Taphrhelminthopsis traces and can be used to constrain the above estimates. The minimum length of these 15 loops represents the minimum documented turning radius of the trace-maker. Because the presence of trace cross-cutting at these intersections precludes the possibility that the loop formed by the organism crossing over its own body, the maximum possible body length of the smallest looping tracemaker is -24 cm. Using the logic from case 1 (above), if organisms were no wider than their trails, but were significantly longer than they are wide, then body size is estimated to be -170 cm’, (or roughly the size of a tightly rolled-up issue of Paleobiology). Assuming that tracemakers primarily grew lengthwise, traces with larger loops could be formed by longer animals. For case 2, estimates must be constrained by taking into account the radius of a relatively tightly looping circle. If one assumes the tracemaker has the tightest possible turning radius, the maximum body size for the animal producing the smallest of studied trace loops is 58 cm’ (or roughly the size of two stacked decks of standard playing cards). Lastly, the nature of the trace surfaces and trace-margins suggest that the tracemaker was massive enough to displace its own “fecal” trail, yet not massive enough to deform the adjacent laminated sediments on bed surfaces or along the margins of trace troughs. Thus, the tracemaker probably had a small ventral surface area (similar to a worm) or had a ventral appendage or posterior which cut or dragged through its sediment trail. 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. What Were They Doing? Whereas the above analyses allow inferences on abundance, locomotive flexibility, and body size, such data provide little insight into the actual mechanism of locomotion that produced Taphrhelminthopsis. This problem is compounded by the absence of lateral, oblique, and transverse striae, and the smooth surfaces of the convex ridges. Throughout serially sectioned burrows, burrow trough edges are always sharp, and laminations abruptly terminate at the margins of the u-shaped burrow trough. Surficial laminations are not deformed as might be expected if an organism were resting on the surface or deforming it through locomotive processes (as in Climactichnites; Yochelson and Fedonkin, 1993). Such sharp and uniform sediment excavation features seem highly specialized for mere locomotive purposes. Rather, they suggest the tracemaker was actively ingesting or removing sediment in the act of food processing or searching. How Did They Move and What Were They? Poorly laminated to mottled silty layers overlying these traces typically fill burrow troughs, are thinner over burrow ridges, are laterally continuous across burrow ridge crests, do not grade downward into laminated layers, and are not incorporated into burrow backfill. These observations suggest deposition of overlying layers after or concomitant with production of the Taphrhelminthopsis burrows. Since erosive features on bed surfaces and bed soles are lacking, the latter observations cast doubt on the possibility that Taphrhelminthopsis was formed as a deep subsurface trace, akin to snorkeled subsurface burrowers responsible for Plagiogmiis.. Lack of prod or scratch marks, lateral markings, and related features suggests that the tracemaker may not have had a firmly mineralized skeleton, or that such a skeleton was not in contact with the substrate. Thus, an arthropod interpretation seems difficult to 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reconcile considering the abundance of diagnostic features typically found in other coeval arthropod traces. Lack of meniscate backfill and deformation of overlying sediments suggests the animal may not have been an infaunal organism that burrowed like an echinoid. Lack of evidence for peristaltic motion and intrastratal burrowing also suggests the animal may not have been an infaunal annelid or slug-like organism, like the Plagiogmus-maker (Scilacher, 1995,1997; Mcllroy and Heys, 1997). Furthermore, despite similarities in surficial meandering and looping patterns, backfill is unlike that produced by modem enteropneusts or echiuroid worms (Bourne and Heezen, 1965). Traces of comparable size could have been made by early molluscs such as Kimberella (Fedonkin and Waggoner, 1996), which are known up to 15 cm in length and 4 cm in diameter. However, traces made by such molluscs are thought to consist of radular scratch marks on the sediment surface (Gehling, 1996; Seilâcher, 1997), rather than deep U- shaped meandering troughs. Based on these considerations, and a survey of traces produced by animals in deep marine settings (where “typical” shallow Vendian-Early Cambrian bedding-parallel trace fossils are commonly produced; Bourne and Heezen, 1965; Sei lâcher, 1967b; Heezen and Hollister, 1971), it seems that the most parsimonious culprit might be an echinozoan- or mollusc-grade animal with a soft body or at least a soft underside, likely with appendages or a mouth capable of excavating a relatively steep-sided deep burrow without preferentially prodding or scratching sediment at the base of burrow troughs. The most plausible modem echinozoan analog might be heart urchins, which produce burrows on shallow surface sediments in the deep-sea in a manner consistent with observations from this study (Fig. 5.8A3) - burrows which are similar in morphology, size, looping, and occurrence as those observed in this study, although ancient burrows lack parallel grooves along the base of furrows. Echinoid burrowing behavior and its 115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. relevance to the trace fossil record is treated extensively in Bromley and Asgaard (1975). When one considers that modem irregular urchins also burrow below the sediment surface using a snorkel, and that in roughly coeval Lower Cambrian strata there is evidence for snorkeled burrowers nearly identical in size and scale to modem deep-sea counterparts, one wonders if an unknown echinozoan-grade animal may be responsible for Taphrhelminthopsis traces. Unfortunately, lack of skeletonized evidence for such an echinozoan makes such an interpretation difficult, but in light of coeval occurrences of other echinozoans (e.g., helicoplacoids- which clearly had an earlier unskeletonized history), and other echinozoan-like animals such as Wiwaxia or Canadia, perhaps during the Early Cambrian such forms may have been soft-bodied and thus not preserved? If so, then a soft-bodied echinozoan organism such as Wiwaxia, thought to glide-crawl across the seafloor, ingesting sediment along its path, may be responsible for Taphrhelminthopsis. If the tracemaker was a mollusc-like animal, glide-crawling is the most probable means of trace production and locomotion. This type of motion is best illustrated in the hollow, muscular foot of gastropods, in which directed waves of rhythmic, altemating contractions on the sole of the gastropod foot are produced (Schafer, 1972). Waves are propagated in the direction of motion such that the contraction’s termination lifts the sole’s anterior just above and forward over the substrate. In some modem gastropods, two discrete and slightly out-of-phase contraction waves, separated by a longitudinal membrane of connective tissue, produce a ditaxical type of direct wave motion; other types include monotaxical and tetrataxical motion (e.g., Ankel, 1936; Schafer, 1972). In the rare case of extreme substrate plasticity, ditaxically glide-crawling gastropods (i.e., Nucella lapillus) can leave a divided trace with relatively smooth sides (Fig. 5.8CJ5; Ankel, 1936). This modem prosobranch gastropod trace is analogous to studied Taphrhelminthopsis. Together with roughly sequential appearance of prosobranchs and Taphrhelminthopsis in 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the Tommotian (Runnegar, 1981; Crimes, 1987) and evidence for the relatively stiff nature of studied laminated substrates, these observations collectively implicate a relatively large early mollusc as a potential trace-producer. Based on these observations, the studied T. c/rcu/am-maker is hypothesized to have been either a soft-bodied echinozoan or a mollusc. Both of these suspected tracemakers are known from modem deep marine settings, and have been documented in modem settings making similar backfilled bilobate traces on seafloor surfaces. 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.8: Modem deep-sea echinozoan-produced bilobate trails comparable to Taphrhelminthopsis, including furrows made by irregular echinoids (A,B). and holothurians (inset in C). Deep furrows (lower portion of photograph) presumably produced by irregular echinoids on the Antarctic continental rise, Marie Byrd Land, 4840 m depth. Echinoid (B; at trace terminus, center) plowing through the surface sediment near the outer ridge of the southern Peru-Chile Trench, 3626 m depth; note actinarian which recently crossed echinoid furrow at lower right. Plowing holothurian (cf. Pseudostichopus), from the Mid-Oceanic Ridge in the Gulf of Aden, 1288 m depth. Molluscan producers of bilobate trails comparable to Taphrhelminthopsis (CJD). Bilobate furrow from the Mid-Oceanic Ridge in the Gulf of Aden, 1288 m depth, along with its inferred sea hare or sea slug producer (far right). Photos A-C from Heezen and Hollister (1971). Bilobate sirrface trails made by modem gastropods moving through exposed intertidal surfaces from the Southem Califomia coast (D; photo courtesy of Dave Bottjer). 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. How Do Traces Relate to Plagiogmus? Taphrhelminthopsis and Plagiogmus are found in similar Vendian-Lower Cambrian siliciclastic marine settings, and later in the Phanerozoic are only known from deeper marine settings (Hüntzschel, 1975; Crimes, 1987). Although both traces are large, and bedding-parallel looping is common to both, Plagiogmus differs significantly from Taphrhelminthopsis in that Plagiogmus is an intrastratal backfilled trace characterized by transverse ridges and a lack of level self-crossing. Intrastratal tracemakers responsible for Plagiogmus remain in contact with the surface through usage of a snorkel or siphon, which punctures the overlying sediment layers (Seilacher, 1995, 1997; Mcllroy and Heys, 1997). As the animal moves along, its body displaces overlying sediments, arcing them upward into a semicircular profile, which is then bisected by the snorkel - thus creating a trace which is very similar to Taphrhelminthopsis, but which has been identified under a variety of ichnogeneric names in the literature (Seilacher, 1995,1997; Mcllroy and Heys, 1997). The snorkel commonly executes a zig-zag, sinusoidal, or straight path, perhaps relating to varying locomotive or foraging strategies (Seilacher, 1995, 1997). Unfortunately, bed surfaces characterized by such bilobate trails are rarely sectioned, making the taxonomic assignment of such trails an uncertain task. However, clues may exist where looping trails intersect one another, and examination of such cross-cutting relationships is amenable to field locales where specimens are remote or uncollectable. In particular, bilobate trails found on top of Plagiogmus trails do not cross one-another at the same level (i.e., the tracemaker may “sense” the previous trail and shift its burrowing path above it, thus superimposing the upper burrow over the lower one without eradicating or excavating it; Seilacher, 1995, 1997; Mcllroy and Heys, 1997), whereas studied Taphrhelminthopsis burrows do cross at the same level, excavating and deforming previously existing burrow ridges into new burrow ridges. 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In future work, a distinction should be made between (1) bilobate traces which reflect deformation of sediments into parallel furrows (which typically occur on bed surfaces, akin to sinusoidally-furrowed bilobate traces found above Plagiogmus-, Seilacher, 1995,1997; Mcllroy and Heys, 1997; Zhu, 1997), (2) bilobate traces which reflect erosion or casting of epichnial burrows (which can be preserved on bed surfaces or bed soles, and are commonly called Scolicia; Hantzschel, 1975; Smith and Crimes, 1983), and (3) bilobate traces which reflect split burrow trails formed after excavation of sediment (and are typically preserved on bed surfaces, such as Taphrhelminthopsis described herein; Fig. 5.6). Since no previous studies have sectioned bed surface Taphrhelminthopsis or demonstrated that Taphrhelminthopsis reflect sediment deformation (vs. sediment excavation), it is suggested that evidence for active sediment excavation may be a useful defining characteristic for this ichnogenus. Were They Systematically Foraging? Conspicuous features of studied traces are their looping and meandering patterns. Such systematic looping and meandering implies that tracemaking organisms were actively mining the sediment in search of food resources. Although they have thus far only been used to describe and distinguish between looping and meandering traces, Hofinann and Patel (1989) and Hofmann (1990) developed quantitative techniques, or “goniogram” analyses which allow quantitative description of trace fossil morphology and evaluation of hypothesized “systematic” burrowing patterns. Goniograms are plots of local trace orientation against the distance along the trace path (Hofmann, 1990). Goniograms illustrate: 1) the distance of any one point on the studied trace from the origin of the trace data; 2) the local heading with respect to the origin heading; 3) relative and absolute maximum and minimum values of headings; and 4) mode 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of looping and spiral patterns (Hofmann, 1990). Goniograms also provide a straightforward presentation of rate of change between headings by plotting the angular deviation between sequential heading pairs, which is analogous to a first derivative of a line. Perhaps more importantly, trace heading data can easily be measured in the field using a compass and ruler, and thus provides investigators with a simple means for collecting large amounts of quantitative trace fossil information in inaccessible or difficult- to-sample locales. In this study, goniogram analysis is used to determine if studied traces exhibited periodic looping or meandering patterns, and to provide an initial datum for analyzing Taphrhelminthopsis' role within suspected larger-scale patterns in the evolution of burrowing behavior. G o n io g r a m M ethodolxigy In the field, individual T. circularis traces were outlined on acetate sheets at a 1:1 scale to provide an oriented record of the relative position of each trace. Only well- preserved and relatively long traces were outlined for later goniogram analysis. At the largest contiguous bedding plane exposure, T. circularis was not homogeneously distributed over the bedding plane, but instead exhibited a distinct center of high trace density. The density of traces in the locus of burrowing activity often made it difficult to distinguish between discrete, continuous trace meanders. Thus, traces used in goniogram analysis are biased toward more distal, discretely preserved trace populations. Oriented traces were reduced to relative headings and deviation between sequential headings measured at 5 cm intervals. Changes in direction to the right of the previous path vector were given positive degree values, those to the left were given negative degree values (Fig. 5.9, inset). Although cross-cutting relationships can occasionally be used to infer direction of movement (Fig. 5.7), lack of striae precludes determination of a unique 122 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. locomotive direction or defining an arbitrary, but consistent, data collection direction between traces. Thus, the “starting” point for data reduction of each trace was randomly chosen. The 5 cm interval was chosen so that the smallest observed trace loop would be represented by at least four data points. Relative heading and angular deviation between sequential heading pairs were used to construct goniograms for individual T. circularis traces. 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.9: Goniogram methodology and data. Heading was measured using a Brunton compass, and relative deviation (inset) was noted at every 5 cm sampling interval. Goniogram plots of the heading (open circles) and deviation (closed circles) are plotted below sketches of trace outlines for five of the thirty-five studied traces. 124 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 ^ IQ D 130 a s 2M MB IM 1 joL / T /W T r / __L_ ! 130 MB n o 300 330 Î = F i 0 30 100 1 3 0 an no 3 0 0 3 3 0 125 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. R e s u l t s Thirty-five distinct traces were measured for goniogram analyses, with a total length of 36.15 m. The average length was 1.03 ±0.68 m with a maximum individual trace length of 3.15 m and a minimum individual trace length of 0.25 m. Although looping was present in 37% of the traces and isolated meanders are common (defined here as -180" turns), examination of all heading-based goniograms revealed neither consistent nor strong periodicity at any scale above the 5 cm interval. Qualitative examination also failed to show any periodicity below this interval. Examples of representative goniograms are presented in Figure 5.9. Based on goniogram analyses, the reduced data from Taphrhelminthopsis specimens were not deemed sufficiently complex or long to warrant further statistical analyses, such as auto-correlation on, or cross-correlation between, individual traces. Spectral analysis was also precluded for similar reasons (i.e., insufficient length relative to Nyquest frequency). Reduced trace data were also plotted using deviation-based goniograms as an additional test for periodicity. The mean of all angular deviation datums (n=688) was -2 ® ±30° per 5 cm interval with a median of -3° and mode of 5°. Deviation data were binned at 10° intervals from 140° to 0° (right deviations) and 0 to -140° (left deviations). The resultant frequency distribution exhibited a roughly normal distribution (Fig. 5.10). Like the heading-based goniograms, these “first derivative” goniograms also lack strong or consistent periodicity. Lastly, to evaluate the possibility that tracemakers may have exhibited preferential turning directions or “handedness”, consistent angular deviation of loops was examined in the field. Because such observations were restricted to traces where burrowing direction could be determined (as indicated above), only one measured trace appeared to exhibit consistent angular deviation in one direction. Thus, unlike marks of predation on trilobites 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Babcock, 1993), one cannot yet determine if tracemaking animals exhibited a preferred direction of looping, and thus cannot evaluate if “right- and left-handedness” may have been involved in development of intricate tracemaking capabilities or presumed neurological complexity. 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5.10: Distribution of deviation measurements for all studied traces. Distribution is not heavily skewed, as would be expected with organisms who exhibit preferred turning direction (right- vs. left-handedness). 128 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160- 140- 120 - 6 100- C ë 80 - 3 6 0 . o o 40 - 20- 0 - Angular Deviation per 5 cm Interval Angular Deviation 0 _______ 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GONIOGRAM Im plica tio ns Temporal shifts in the development of such intricate foraging patterns are of interest because they may help us to better understand how metazoans increased their behavioral complexity and/or developed neurological return response mechanisms concomitant with, or perhaps in the wake of, the Cambrian explosion and shifts in bioturbation occurring across the Proterozoic-Phanerozoic transition. Previous experimental and analytical studies have examined the distribution of such tracemaking behaviors in order to test hypotheses about development of early animal behavior. For example, Seilacher (1967a, 1974,1977) originally hypothesized that the complexity of looping and meandering traces increased through time, reflecting an increased behavioral complexity. Documentation of apparently “complex” Vendian and Early Paleozoic traces restricted such shifts to the Vendian- Cambrian transition, and further suggested existence of distinct onshore-offshore patterns in trace fossil genera (see summary in Crimes, 1992a). However, recent documentation of looping and broadly meandering traces in Vendian flysches of Spain suggests such complexity and onshore-offshore hypotheses may need further refinement (Vidal et al., 1994). Together with experimental simulations of systematic foraging behavior (Raup and Seilacher, 1969; Hammer, 1998) goniogram analyses might help to quantitatively test such large-scale shifts in tracemaking behavior. In this study, goniogram analysis was used to determine if studied traces exhibited periodic looping or meandering patterns, and to collect quantitative data about a typical large Early Cambrian bioturbating organism. Looping and meanders are present in many of the studied traces, but their abundance relative to trace length precludes verirication of consistent tracemaking patterns. Although looping patterns appear to be quite rounded and loop sizes are similar - suggesting a relatively organized neurological control over looping 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. behavior, such looping can also occur as a natural result of quasi-random meandering search patterns (Hoffman and Patel, 1989; Hammer, 1998). Although systematic trace patterns were not identified, when later combined with data from other occurrences, these data may provide a first step toward analyzing Taphrhelminthopsis' role within suspected larger-scale patterns in the evolution of burrowing behavior. Together with further quantitative studies of other ancient bedding- parallel looping and meandering traces (such as Gordia and Taphrhelminthoida; Hofmann and Patel, 1989; Hofmann, 1990), and possibly with integration with predictive, actualistic, or computer-generated models, perhaps this information will help address hypotheses about evolution of foraging strategies (or bioturbating complexity) through the Vendian-Cambrian transition. Patchiness Although the field-based nature of these studies limited the ability to collect large amounts of quantitative data about the areal distribution of T. circularis relative to all studied outcrop areas, some quantitative data was collected from one outcrop area. In addition, based on examination of the same interval at 22 different sites, one can qualitatively state that larger trace-covered bedding surfaces at most of the study localities exhibit highly patchy distributions of trace density (Fig. 5.5A,B). Across sampled bedding plane exposures of nearly 950 m \ approximately 25% of outcrop surfaces are covered by a monospecific assemblage of T. circularis traces. Without taking into account “behavioral” implications of regions characterized by overlapping traces, bioturbation intensity on individual surfaces (as estimated by relative trace density) seems to modulate between scarcely disrupted bed surfaces, characterized by bedding-plane bioturbation indices (BBI; Miller and Smail, 1997) of 1-2 (reflecting areal bioturbation of less than 10%) and BBI of 131 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4-5 (mean areal bioturbation of = 62%, estimated range = 48-72%). Similarly, at various localities, the five most distinguishable bedding planes within the Taphrhelminthopsis-nch interval exhibited BBI of either 2 or 4-5. Together, these preliminary semiquantitative observations suggest two potential modes of bioturbating strategy may have existed, potentially including rigorous bioturbation of localized areas, and sparse bioturbation of intermediary regions. In better constrained and exposed sections, perhaps future work will allow one to determine if such patterns reflect proactive ecological bioturbating strategies, or if they reflect response mechanisms to changing environmental variables, duration of sediment exposure, random or preferential taphonomic effects which affect cleavage or exposure of bioiurbated bed surfaces, or other factors. DISCUSSION Although the T. circularis animal may have been actively excavating sediment in pursuit of other food resources such as infaunal prey, morphologic and sedimentologic features suggest the tracemaker was extracting sediment in a very precise and uniform manner consistent with active-sediment ingestion or sorting to extract food resources. Despite relatively uniform burrow excavation and looping in the studied traces, no systematic organization of such strategies was identified. However, tracemaking organisms appear to have concentrated their efforts in specific bedding plane regions. Does this extreme heterogeneity in areal distribution of traces reflect stochastic crawling or grazing behavior over a relatively stable sedimentary surface, or does it reflect directed behavioral activities by the tracemaking organism? Perhaps after excavating and sampling many patchily-covered bedding planes which are orders of magnitude larger than the outcrops utilized in this study (akin to surfaces at Mistaken Point, Newfoundland, Canada), one could reject the former possibility. 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Barring such circumstances, the latter alternative is considered, which suggests that centers of trace density reflect concentration of sediment-ingestion activities. On adjacent studied surfaces, wrinkle-structures occur - features which are not diagnostic of microbial communities, but are commonly formed by microbial mats (see Chapter 4; Hagadom and Bottjer, 1997). In addition, analyses of burrow:sediment relationships suggest that the tracemaker burrowed through relatively stiff and laminated sediment - typical modem tidal flats and other “stressed” settings where microbial mats may dominate. Along the periphery of such settings, and in dysaerobic realms of the deep-sea where microbial mats such as Beggiatoa flourish, semi-infaunal and epifaunal spatangoid echinoids and gastropods are well-known ingesters and bulldozers of microbially bound sediment, and often leave bilobate traces nearly identical to studied Taphrhelminthopsis (e.g., Heezen and Hollister, 1971; Grant, 1991). Furthermore, such sediment grazers have retum-response mechanisms whereby they turn, loop, or circle back towards concentrated regions of food resources, leaving highly bioturbated patches much like the apparent loci of Taphrhelminthopsis density. Seilacher and Pfliiger (1994) have hypothesized that many such Vendian-Cambrian meandering trace fossils strip-mined sediment from mat-rich layers and in Chapter 4 such activities are documented on and within suspect-microbially bound Vendian-Early Cambrian sediment. Although many other hypotheses remain to be rejected, is it possible that the T. czrcidarij-maker employed similar écologie strategies as it moved across the sediment surface? CONCLUSIONS Analysis of well-preserved, laterally extensive exposures of the looping trace fossil r. circularis provides evidence for relatively large soft-bodied epifaunal or semi-infaunal echinozoan or mollusc-like animals that fed by actively ingesting relatively stiff microbially- 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. bound sediment. Although tracemakers appear to have incorporated looping into their search strategy, no systematic or nonrandom periodicity in studied patterns were detected. Together with further quantitative analyses of patchiness, such quantitative information may aid in determining whether such patterns reflect tracemaker efforts to increase food intake per distance traveled and/or a neurological return response to patchily distributed resources. When combined with other analyses of key trace fossils characteristic of the Vendian-Cambrian transition, these analyses provide a starting point for development of a larger-scale quantitative database on early animal burrowing behavior to refine current understanding of larger-scale shifts in niche colonization, tiering, and escalation of écologie interactions that were developing during the late Neoproterozoic-Paleozoic transition. 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6: Paleoecology of a Slope Margin: The Earliest Deep-sea Faunas of Southwestern Laurentia 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Middle Cambrian to Lower Ordovician upper slope to basin plain “deep-sea” facies of the southwestern United States have yielded new faunas which provide insight on the nature of the earliest deep marine communities from southwestern Laurentia and may add to current understanding of larger-scale patterns of deep-sea colonization. Facies representing two outer shelf to basin plain transects were examined in outcrops distributed throughout eastern California and Nevada; despite large outcrop areas, deeper facies are characterized by relatively few fossiliferous intervals. Autochthonous fossils in these transects include a variety of trace fossils, and perhaps Ungulate brachiopods and hexactinellid sponges. Common ichnogenera include Arenicolites, Palaeophycus, Planolites, and Thalassinoides. In lower slope and basin plain settings, evidence of vertically-oriented or bedding-parallel bioturbation is rare - but where documented, bedding plane disruption predominates over vertically-oriented burrowing. Allochthonous faunas include trilobites, graptolites, phylocarids, indeterminate arthropods, hyoliths, gastropods and other forms. Locally, faunas reflect a relatively depauperate infaunal marine community characterized by small shallowly burrowing animals. Barring preservational biases, epifaunal marine communities were similarly small and depauperate, possibly stemming from dominance by poorly preserved or easily transported forms. Although the size, diversity, and degree of bioturbation decreases from outer shelf to basin plain environments, significant temporal changes in trace fossil assemblages are not observed between studied sections. INTRODUCTION Early metazoans underwent an unparalleled explosion in abundance and diversity during the late Neoproterozoic and early Paleozoic (e.g., Sepkoski, 1982,1992) when they 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. began to colonize a variety of marine environments (e.g., Seilacher, 1967a; Crimes, 1974; Crimes and Anderson, 1985; Bottjer et al., 1988; Crimes et al., 1992; Crimes and Fedonkin, 1994). At this time, organisms typical of shallow marine environments began occupying progressively deeper marine environments, ranging from the continental slope to abyssal plains (e.g.. Crimes, 1974; Bottjer et al., 1988; Crimes et al., 1992; Crimes and Fedonkin, 1994, 1996). During the Vendian-Cambrian transition many metazoans, and perhaps most metazoans in deep marine environments, were soft-bodied - and thus were not as likely to be preserved as organisms which possess a mineralized skeleton (e.g., Allison and Briggs, 1991; Crimes and Droser, 1992). Lack of abundant skeletal evidence, however, has not completely stymied documentation of paleobiologic, paleoecologic, and evolutionary changes which occurred in early deep-sea environments (e.g.. Crimes, 1974; Crimes and Anderson, 1985; Bottjer et al., 1988; Crimes et al., 1992; Crimes and Fedonkin, 1994, 1996). For example, based on careful documentation of trace fossils in such facies, it appears that shallow marine late Precambrian and Early Cambrian environments were characterized by “standard” shallow marine trace fossils as well as trace fossils typically characteristic of later deeper marine deposits (Crimes and Anderson, 1985; Fedonkin, 1985; Crimes, 1987; Crimes et al., 1992; and Crimes and Fedonkin, 1994). During this interval, deep marine environments were occupied by few organisms, as indicated by the scarcity of trace fossils and body fossil impressions (Crimes et al., 1992; but also note evidence of Ediacaran faunas in Vendian turbidites at Mistaken Point, Newfoundland; Anderson and Misra, 1968; and deep-sea Vendian trace fossils reported by Vidal et al., 1997). By the Early Ordovician, most deep marine epifaunal habitats are thought to have been occupied (e.g.. Crimes, 1974; Pickerill, 1980; Crimes and Fedonkin, 1994). Thus the generally accepted model is one of onshore-offshore retreat of Vendian-Ordovician deep-sea-style trace fossils. 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Deep-sea colonization models outlined in these studies are based on information collected from a variety of strata in eastern Canada, Argentina, the British Isles, and Spain (e.g.. Crimes, 1969,1987; Acenolaza and Durand, 1973; Acenolaza, 1978; Anderson, 1978; Pickerill, 1981; Pickerill and Keppie, 1981; Crimes and Anderson, 1985; Pickerill and Williams, 1989; Crimes et al., 1992). Unlike broader-scale biotic patterns for coeval shallow settings, which have been drawn from world-wide coverage of such facies, this restricts most deep-sea work to outcrops which were located along the lapetus (or proto- Atlantic) Ocean. With the exception of descriptive reports of trace fossils from Argentina (Acenolaza and Durand, 1973; Acenolaza, 1978), facies bordering other ocean basins, such as the proto-Pacific (later called the Panthalassic Ocean) have yet to be integrated and compared to the onshore-offshore model. This model is also largely based on data collected from the soles of siliciclastic turbidites, and generally lacks information from settings where siliciclastic sources are overshadowed by carbonate input. Lastly, although they provide an excellent image of the temporal and paleoenvironmental distribution, and in some cases the range of trace fossils found in individual communities, very little is known about the nature of the individual communities (but see Crimes, 1992a). Paleobiological and paleoecological study of early deep-sea faunas could thus potentially add to the growing understanding of the nature of the earliest deep marine communities. As a logical first step toward addressing some of these gaps in current understanding of early deep-sea life, autochthonous, parautochthonous, and allochthonous faunas were collected from Middle Cambrian-Lower Ordovician carbonate-dominated outer shelf, slope, and basin plain facies of the southwestern U.S. STRATIGRAPHIC & PALEOENVIRONMENTAL FRAMEWORK This study focuses on the oldest well-preserved units in southwestern North America for which there is reasonably well-constrained existing paleoenvironmental and 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. stratigraphie control. Studied units can be grouped into two categories, including: a) Middle Cambrian to Upper Cambrian exposures of the Emigrant Formation in Inyo County, California, and Esmeralda County, Nevada (Fig. 6.1); and b) Upper Cambrian to Lower Ordovician strata (including the Swarbrick Formation, Tybo Shale, and Hales Limestone) which are exposed in Nye and White Pine Counties, Nevada (Fig. 6.2). As in other larger-scale studies of deep-sea paleocommunities, only facies deposited outboard of the purported shelf margin were examined, in settings where there is clear evidence for large-scale laterally extensive erosive turbidite or mass flow systems (e.g.. Crimes, 1974). Rationale and context for choosing these units is critical to interpretation of associated faunal information and is outlined below. 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.1; Stratigraphie and paleoenvironmental context for studied Middle Cambrian- Upper Cambrian deepening upward sequences of the Emigrant Formation. The Emigrant Formation is also known as the Monola and Bonanza King Formation on various geologic maps and older publications (see summary in Kepper, 1981). Paleoenvironmental interpretations of these strata are illustrated in the far right column (from Kepper, 1981). A section measured at Horse Thief Canyon, Last Chance Range, CA is thought to reflect the outer shelf-slope break, as illustrated by the schematic reconstruction at bottom (modified from Kepper, 1981) 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. McKee & Nelson (1967) McKee (1966) Kepper (1981) Stewart (1965) Facies Distal Apron Basin Limestone and Chert Member U p p w Member Proximal Apron Merabrecda Member Lower Member Papoose Late Tongue Upper Member Limestone and Silstone Member Platform Lower Member Mule Spring Limestone Mule Spring Limestone Mule Spring Limestone Mule Spring Limestone a ml (13 km) vrfTTTfl Bbnini Sudm «rm # A p m 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.2: Stratigraphie and paleoenvironmental context for studied Upper Cambrian- Lower Ordovician shallowing upward sequences of the Swarbrick, Tybo, Hales, and Whipple Cave Formations (modified from Cook and Taylor, 1989). 142 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carbonate SIope-Fan-Basin Plain Depositional Faciès & Model, Central Nevada Goodwin Lim estone Contountes S 1 9 H g Slumps & Channels lOWlK v îM S S Feeder Channels Haies Limestone (~500m) Thinning Up Distrib utary Channels Thickening Up f Lobe I Sheets fêH l»9iS Tybo Shale (-500m) ëéiftil n âiH Swarbnck Pm. (-500m) 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Paleogeographic Context During the Early Paleozoic, Laurentia straddled the equator, with its western margin facing a large newly-formed ocean basin, called the Panthalassic Ocean (see summaries in Stewart and Suczek, 1977; and Stewart, 1991). Although the timing of basin development is poorly constrained, the Panthalassic likely developed during rifting of Laurentia (and other continents) from Rodinia in the late Neoproterozoic (e.g., Dalziel, 1997; Veevers et al., 1997). A broad continental shelf developed along Laurentia's western margin, where thick sequences of shallow detrital elastics and ocean-margin carbonates were deposited (Dietz and Holden, 1966; Stewart, 1972; Stewart and Poole, 1974; Cook and Taylor, 1975, 1977, 1987; Taylor, 1976; Taylor and Cook, 1976; Stewart and Suczek, 1977; Cook and Egbert, 1981 ; Kepper, 1981). Possibly as early as the latest Early Cambrian, units in the Great Basin suggest development of restricted basins on the shelf, followed by development of a northward-trending distally-steepened passive continental slope and margin in the Middle Cambrian (e.g., Stewart and Suczek, 1977; Cook and Taylor, 1987; Stewart, 1991). Based on relatively well-constrained deposits from accreted terranes thought to be deposited outboard of the continent, abyssal environments existed westward of the continental shelf margin by at least the Ordovician (e.g., Stewart and Suczek, 1977; Stewart, 1991; Finney and Perry, 1991; Finney et al., 1995). Within this larger-scale context, a number of potential deep-sea facies thus exist for characterization of early bathyal and abyssal marine communities of southwestern Laurentia. All units are exposed in Nevada and eastern California. Paleoenvironmental Context The oldest deep marine facies in southwestern North America include aulacogenic or possibly deeper trough-like basins inferred from late Middle to Late Proterozoic 144 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. olistolith-dominated strata of the Kingston Peak Formation (Miller et ai., 1988). Unfortunately, these facies were not examined for this study partly because their paleoenvironmental and paleogeographic context is poorly constrained (Prave, 1998) and partly because the Kingston Peak lies 1500 m below the oldest known metazoan trace fossils from the Great Basin (Hagadom and Waggoner, in press), suggesting the unit likely predates the evolution of metazoans. Together with Middle Cambrian units of the House Range embayment. Middle? Cambrian strata of the Preble Formation also have been interpreted to be of "deep marine" origin. These units were likely deposited in a basin setting located on the continental shelf (Rowell et al., 1979; Rees and Rowell, 1980; Rowell and Rees, 1981; Rees, 1986; Stewart, 1991). Because their depth of deposition and the relationship of these strata to coeval slope facies (described below) is unclear at present, these strata are not included in this study (see summary of this controversy in Rowell and Rees, 1981). Two major deep marine sequences were examined for this study, including exposures of the Middle Cambrian-Upper Cambrian Emigrant Formation and the Upper Cambrian-Lower Ordovician Swarbrick, Tybo, Hales, and Whipple Cave Formations. Strata of the Emigrant Formation are also known as the Nopah, Bonanza King, Monola, and Carrara Formations in various publications and geologic maps (Kepper, 1981; Ansell, 1987). Exposures of the Emigrant Formation occur in a number of localities in the Last Chance Range, the White Mountains, and the Inyo Mountains of eastern California, as well as in the Last Chance Range, Silver Peak Range, Palmetto Mountain, Bare Mountain, General Thomas Hills, and Paymaster Ridge of western Nevada (e.g., Kepper, 1981; Brown, 1991; Fig. 6.3). At these localities, strata representing outer shelf, slope, and basin facies are exposed and can be roughly correlated across the western Great Basin based on four lithologically distinct subunits (Fig. 6.1; Kepper, 1981; Ansell, 1987). Thicknesses of the Emigrant are highly variable because of structural complexities, but in 145 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. western Nevada have been estimated to be ca. 600-700 m thick (Albers and Stewart, 1972) and in eastern California exposures are approximately 1000 m thick (Ansell, 1987). The second deep marine sequence examined for this study includes the Swarbrick, Tybo, Hales and Whipple Cave Formations. Facies of these units have been the focus of extensive biostratigraphic, sedimentologic, and paleoenvironmental studies - summarized in Cook et al. (1989). Strata were examined in two regions, including a shallowing upward sequence in the Hot Creek Range of central Nevada and a similar, but shallower sequence in the Egan Range of eastern Nevada (Fig. 6.3). Exposures consist of ca. 1500 m of cherts and shales, carbonate submarine-fan facies, and submarine slides and slumps which were deposited in a deep marine slope and ocean margin setting (Fig. 6.2; e.g.. Cook et al., 1989). Sedimentary structures, stratigraphie features, and regional facies relationships in both sequences are broadly similar, suggesting deposition in deep marine settings below storm wave base, but adjacent to a steep carbonate-dominated shelf margin (Cook and Taylor, 1975; Kepper, 1981). Although it is unclear if the two sequences are genetically linked, both sequences exhibit a variety of features typical of carbonate-dominated slope settings, including large flute and erosional casts, Bouma-style stratigraphie sequences, highly erosive megabreccias, channels, and related basin wall collapse structures (Fig. 6.4). The presence of slump blocks (tens to hundreds of meters in diameter) within otherwise planar laminated marine sediments also suggests that a significant bathymetric relief existed. Where fossiliferous, facies contain faunas typical of such settings, including spicule-rich cherts, graptolitic shales, and pelagic trilobite lags (Taylor, 1976; Kepper, 1981; Ansell, 1987; Cook et al., 1989). A suite of deep marine units of the Roberts Mountain Allochthon provide the next, and perhaps deepest examples of deep-sea facies in the region. Probable deep marine facies of this allochthon consist of “eugeoclinal” strata such as the Middle Cambrian Shwin 146 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Formation, the Upper Cambrian? exposures of the Paradise Valley Chert, Upper Cambrian to Lower Ordovician Harmony Formation, Cambrian? to Lower-Middle Ordovician exposures of the Tennessee Mountain Formadon, and Ordovician Vinini Formation (see summary in Stewart, 1991). Although lithologie features of these units all point to deposition in deep marine settings, work in such facies is hampered by stratigraphie, temporal, preservational and tectonic complications. Of these facies, the Vinini Formation, thought to have been deposited in a deep marine basin plain and continental rise setting (Finney, 1986; Madrid, 1987; Finney and Perry, 1991; Ketner, 1991), is the least deformed and has high potential for recovery of macroscopic paleobiological information. Distal and medial siliciclastic turbidite facies of the Vinini were surveyed for this study, but were not focused on because they contained a very high diversity trace fossil assemblage, indicating colonization of the deep marine realm in this region was already fairly well established by the Early-Middle Ordovician, and because at the time of study the regional stratigraphie and paleoenvironmental context was still being developed (Finney et al., 1995). At present, Finney and colleagues (summarized in Finney, 1997) have trenched continuous profiles through the region and are developing a highly detailed stratigraphie and paleoenvironmental framework for the region. 147 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.3: Localities where outcrops of the two study transects were examined. Exposures of the Emigrant Formation were examined in areas 1-9 and the Whipple Cave-Hales-Tybo-Swarbrick Formations were examined in areas 10-13. 148 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STUDY AREAS 1. Papoose Flats, Inyo Mtns. 2. Black Canyon, White Mtns. 3. Horse Thief Canyon, Last Chance Range 4. Eureka Canyon, Last Chance Range 5. Emigrant Pass, Silver Peak Range 6. Palmetto Mountain 7. General Thomas Hills 8. Paymaster Ridge 9. Bare Mountain 10. Tybo Canyon, Hot Creek Range 11 Hot Creek Canyon, Hot Creek Range 12. Sawmill Canyon, Egan Range 13. Christmas Tree Canyon, Egan Range STUDIED UNITS Emigrant Formation (1-9) Whipple Cave, Hales, Tybo, Swarbrick Formations (10-13) 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.4: Typical sedimentary structures and bedforms found within studied sequences, including intrafcrmational breccia of the middle Emigrant Formation (A), mottled lime mudstone from the lower Emigrant (B), massive intraformational conglomerate of the middle Emigrant (C), thinly bedded laminated lime mudstone of the middle Emigrant (D), thin intraformational conglomerate from distal portions of the upper Emigrant (E), and thinly bedded zebra-colored lime mudstone of the upper Emigrant (F). 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 ; 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PREVIOUS WORK Study exposures are quite extensive - but have not yet been examined in any type of paleobiologic or paleoecologic context other than biostratigraphic studies, most of which focus on allochthonous faunas (e.g.. Palmer, 1965; Kepper, 1981; Cook et al, 1989; but also see Taylor, 1976). In the context of regional mapping studies, some potentially useful faunas have been noted (e.g., McKee, 1968; Robinson et al., 1976; Kepper, 1981), but the autochthonous origin of these faunas is unclear as they have not been described in the paléontologie literature. Barring large-scale tectonic displacement of these facies into strati graphically disparate regions, these strata represent depositional environments suitable for burial and preservation of fossils (i.e., they are characterized by many rapid burial events and may represent lower oxygen environments with minimal bioturbation). TRACE FOSSIL USAGE The majority of proposed study horizons are dominated by trace fossils rather than autochthonous body fossils. Trace fossils are critical to paleobiologic studies of flysch and other deep-sea facies, because they record in situ information in settings where most fossils can easily reflect transported shelf faunas, pelagic faunas, or other transported forms (see review in Crimes and Droser, 1992). They are also useful in settings where the majority of animals are soft-bodied, and not typically preserved. However, because individual organisms may produce more than one trace, potential biases may exist in usage of trace fossil information to estimate abundances in paleoecologic studies. Similarly, one organism may produce many different types and sizes of trace fossils, either through different ontogenetic stages or though employing different behavioral strategies. Trilobites, for example, may produce several different types of trace fossils through different behavioral patterns (e.g., Seilacher, 1955,1959,1970a). With these constraints in mind, trace fossils are utilized as a qualitative metric to describe the diversity of behaviors found 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in specific environments. This information is then used to make broad inferences about community characteristics such as substrate niche and feeding preference, tiering levels, and degree of substrate disruption. DATA COLLECTION METHODS To identify fossils in studied strata, each outcrop area of units representing the two outer shelf-to-basin sequences was visited twice in the field, and surveyed for fossiliferous horizons. Exposed surfaces, bed soles, and vertical sections were examined in detail. Only in situ fossil occurrences are described below. RESULTS The two studied shelf-to-basin sequences each record a diachronous outer-shelf- basin plain transect. The first transect includes a deepening-upward sequence of the Emigrant Formation and the second includes an Upper Cambrian to Lower Ordovician shallowing-upward sequence consisting of the Swarbrick, Tybo, Hales, and Whipple Cave Formations. Based on previous research, facies in these sequences are divided into outer shelf, upper slope, lower slope, and basin plain environments and are organized based on their temporal occurrence into a simplified block diagram (Fig. 6.5). Fossil occurrences are noted within the context of the same generalized time-environment framework. 153 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.5: Generalized time-environment diagram for the two diachronous shelf-basin transects examined in this study. Paleoenvironmental interpretations are based on Cook and Taylor (1989) and Kepper (1981) and references therein. Text-filled boxes indicate facies available for study; shaded regions reflect no outcrop exposure of facies at that specific time interval. 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Stratigraphy Outer Shelf Platform Facies Proximal Apron Fan Facies Distal Apron Outer Rm Facies Basin Facies a) Whipple Cave Fm. b) Upper Habs Ls. L Lower Ordovician Hales Ls a) Lower Maes b) Tybo Shale U. Upper Cambrian a) Swarbnck Fm. ) Upper Emigrant L. Upper Cambrian U. Middle Cambrian Upper Emigrant Fm. Middle Emigrant Fm. M. Middle Cambrian Lower Emgrant Fm. 155 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Trace Fossil Assemblages In the first shelf-to-basin sequence, traces in Middle Cambrian outer shelf and platform facies are generally large and are preserved on the tops of fine-grained dolomitic siltstones, limestones with minor amounts of skeletal debris, and shales (Figs. 6.6,6.7). Traces include 3-4 cm long Arenicolites, several morphotypes of large and small Planolites, Thalassinoides, and possible Palaeophycus. Traces from Middle Cambrian proximal apron to fan facies are preserved on the tops and bottoms of fine grained gray to black calcarenites and include medium-sized (1-1.5 cm long) Arenicolites, narrower Planolites, and poorly preserved Palaeophycus. Upper Cambrian distal apron and outer fan facies have traces on the bases of fine-grained thinly bedded calcisiltites and are characterized by small but abundant Arenicolites, Palaeophycus, and Planolites. Upper Cambrian basin facies of the Emigrant are characterized by extremely small Planolites burrows (length: 0.2 -1 cm; maximum diameter: 0.3 cm) on the soles of thin shales and lime mudstones. In the second shelf-to-basin sequence. Lower Ordovician platform and outer shelf facies are characterized by abundant bioturbation, including trace fossils of a wide variety of sizes, intermixed with skeletal debris (Figs. 6.6,6.7). Although bioturbation is easily recognizable in vertical sections in these facies, discrete trace fossils are best observed on the tops of bedding planes, and include Arenicolites, Planolites, and Thalassinoides. Trace fossils from Upper Cambrian to Lower Ordovician proximal apron and fan facies are primarily preserved on the tops and bottoms of fine-medium grained limestones and include abundant Arenicolites and rare Palaeophycus. Thinly-bedded calcarenites of the Upper Cambrian distal apron facies are characterized by abundant small Arenicolites. Soles of calcareous shales in the outer fan facies are characterized by abundant Palaeophycus. Upper Cambrian basin facies are characterized by extremely rare flute and tool marks and small (maximum length: 3 mm; maximum diameter 1.5 mm) Planolites on the bottom of thinly bedded laminated calcareous mudstones. 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.6: Selected trace fossils examined in this study, including large Planolites from the lower Emigrant Formation (A), branching Thalassinoides (B) and large Aremcolites (bed surface, C) from the upper Hales Limestone, large Arenicolites (bed sole, D) from the lower Emigrant, Planolites from the lower Whipple Cave Formation (E); branching Planolites from the upper Emigrant (F); Palaeophycus and shallow Arenicolites from the middle Emigrant (G), and very small Planolites on sole of tool-marked bed from the Swarbrick Formation Scale bars in B,C,E,H are in cm, and lens cap in remaining photographs is 5 cm in diameter. 157 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.7: Simplifîed distribution of trace fossils in studied transects, using the time- environment diagram from Fig. 6.5. Hatchured areas indicate no outcrop area, rather than absence of faunas. Although their abundance, diversity, size, and preservation varied greatly between study areas, trace fossils were noted in all studied outcrops. 159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Trace Fossils Outer Shelf Platform Facies Proximal Apron Fan Facies Distal Apron Outer Rm Basm Facies a)&b) Arenicoliles Planolites Thalassinoides L Lower Ordovician Arenicoliles Palaeophycus a) Arenicoliles U. Upper Cambrian b) Palaeophycus a) Planolues b) Planollles L. Upper Cambrian Arenicoliles Palaeophycus Planollles Arenicoliles Palaeophycus Planollles U. Middle Cambrian Arenicoliles Planollles Thalassinoides M. Middle Cambrian 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ichnofabric Lack of continuous sections in much of the Emigrant, patchiness and rarity of bioturbated intervals in deeper settings from both transects, and inaccessibility of Emigrant exposures made logging of quantitative bioturbation information difficult. However, where fossiliferous intervals were accessible, semiquantitative information about sediment disruption, bioturbation strategy, and bioturbation style was noted. Maximum extent of vertical disruption of sedimentary fabric was noted in each study area, using ichnofabric indices (ii) of Droser and Bottjer (1986). Because most bioturbation did not have a strong vertical component, maximum extent of bedding plane bioturbation was also noted in each setting, using bedding plane bioturbation indices (bbi) of Miller and Small (1997). Ranges for both bioturbation metrics were not recorded because all studied settings exhibited polar distributions of ii and bbi (i.e., zero bioturbation present, or some bioturbation present). Similarly, since vertical logging was not possible in much of the Emigrant, mean values for these bioturbation metrics were not calculated. Although temporal shifts are not evident, maximum ichnofabric indices decrease with increasing depth in both studied transects, reflecting high levels of vertical sediment disruption in outer shelf settings and coarser upper slope settings, and low levels of vertical disruption in distal slope to basin plain facies (Fig. 6.8). Maximum bedding plane bioturbation indices exhibit similar bbi levels for outer shelf to upper slope settings, but better record intense shallow bioturbation occurring in deeper settings, particularly in basin plane settings, where bioturbation is not visible in vertical profiles (Fig. 6.9). In all upper slope to basin plane settings, maximum bbi increase from older to younger transects, and a large shift is visible between distal Middle-Upper Cambrian settings. 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.8: Maximum level of sediment disruption noted in vertical profile of studied sections, using the ichnofabric index (ii) of Droser and Bottjer (1986; inset at right). Distribution of maximum ii reported for each facies indicated using the same time- environment diagram from Fig. 6.5. 162 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Maximum Level o f Bedding Disruption ü u e r Shelf Pladbmi Fades Proximal Apron Fan Facies Distal Apron OtaerRm Rides % Bedding Dismption Basin Fades L. Lower Oidovidan < 10% U. upper Cambrian 1040% L. Upper Cambrian 4(W0% U. Middle Cambrian >60% M. Middle Cambrian 163 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.9: Maximum level of bioturbation noted on bedding plane exposures, using the bedding plane bioturbation index (bbi) of Miller and Smail (1997; inset at right). Distribution of maximum bbi reported for each facies indicated using the same time- environment diagram from Fig. 6.5. 164 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Maximum Bedding Plane Bioturbation Owa^hdr P!aiform Facies Proximal Apron Fan Facies Disial Apron Outer Fan Facies Basn Facies % D isruption L. Lower Ordovician U. Upper Cambnan a) bbi4 b) bbi2 L. Upper Cambnan 1 U. Middle Cambnan M. Middle Cambnan 165 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Body Fossils Rare parautochthonous macroscopic skeletonized fossils also occur in studied units, and help provide a limited picture of epifaunal components of studied facies. Although transported shell-beds, lags, and related allochthonous fossil accumulations occur in study areas, variations in preservational context suggest that some lingulate brachiopods and hexactinellid sponges may not have been subject to significant transport. In the Upper Emigrant Formation in the northern Silver Peak Range, for example, lingulate brachiopods occur which are not preserved concave down in shell pavements, do not occur oblique to bedding, do not occur in shell lags, and are not disarticulated (Fig. 6.10A). On rare bed surfaces, they occur as articulated, often compressed individuals or in patches of 2-4 individuals. Similarly, isolated 2-20 cm diameter “patches” of hexactinellid spicule concentrations occur occasionally on laminated lime mudstone surfaces, suggesting collapse or compression of a sponge. Neither of these fossils are associated with features suggesting deposition via pelleted, ellipsoidal, or elongate fecal streams. In addition to autochthonous and parautochthonous forms, allochthonous body fossils occur in concentrations on bed bases, particularly in upper slope settings, or more rarely, on bed surfaces in shales of distal slope or basin plain settings (Fig. 6.10C-D). Although not the focus of this study, documented faunas are noted in a similar time- environment block diagram (Fig. 6.11). In general, outer shelf and upper slope facies from both transects are characterized by abundant trilobites and hyoliths, Upper Cambrian distal slope and basin plain facies are characterized by graptolites, lingulate brachiopods, and hexactinellid spicules. One gastropod was documented from Upper Cambrian upper slope facies of the Hales Limestone. Although strata were not sufficiently fossiliferous to establish a finer-scale biostratigraphic framework, rare concentrations of Burgess Shale- type body fossils and trilobite-rich shales in the Emigrant Formation likely merit further study. In particular, phylocarids similar to Isoxys, unidentified non-trilobite arthropods, 166 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. algal fragments similar to Marpolia, and isolated Anomolacarid appendages may have links to pelagic components of the well-known Burgess Shale fauna. 167 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.10: Selected parautochthonous and allochthonous skeletonized fossils examined in this study, including unidentified arthropod fragments from the upper Emigrant Formation (A), lingulate brachiopod from die middle Emigrant (B), gastropod from the upper Hales Limestone (C), pyritized trilobites from the lower Emigrant (D), graptolites from the upper Emigrant (E f), phylocarid arthropod and unidentified cuticular fragments from the upper Emigrant (G, H). Width of field of view in each photograph is as follows: A - 1.3 cm, B - 2.8 cm, C - 1.3 cm, D - 2.1 cm, G - 1.4 cm, H - 1.5 cm. 168 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 169 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6.11: Simplified distribution of parautochthonous and allochthonous macroscopic skeletonized fossils in studied transects, using the time-environment diagram from Fig. 6.5. Parauthochthonous faunas are indicated by an asterisk. Hatchured areas indicate no outcrop area, rather than absence of faunas. Although their abundance, diversity, size, and preservation varied greatly between study areas, fossils were noted in all studied outcrops. 170 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Body Fossils Outer Shelf Platform Facies Proximal Apron Fan Facies Distal Apron Outer Fan Facies Basin Facies Abundant Fossils & Bioclastic Debris L. Lower Ordovician Tnlobites Gastropod Trilobites U. Upper Cambrian Sponge spicules a) Spooge Spicttle* b) Grepcolitet Rwlocands. A J ^ ? Aftttropodt. Spoage»* L Upper Cambrian U. Middle Cambrian Tnlobites Brachiopods Brachiopods* M. Middle Cambrian Trilobites Hyoliths 171 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DISCUSSION Deep-sea environments along the western margin of Laurentia were colonized by at least the Middle Cambrian. Whereas shallower outer shelf and upper slope facies exhibit relatively diverse and dense trace fossil assemblages, deeper portions of surveyed sequences are characterized by patchy occurrences of simple trace fossils and extremely rare autochthonous body fossils. Trace fossils are usually found on the tops and bottoms of fine-grained limestone beds in shallower facies, and are usually only found on the bottoms of fine-grained limestones in more distal, deeper facies. Despite significant variation in bed thickness and lithology within and between studied facies, the majority of faunas reported here occur in relatively thinly bedded (0.5 - 20 cm thick) gray to black, commonly laminated fine-medium-grained limestones, silty limestones and shales. In general, it seems that the size, complexity, and depth of bioturbation decreases from outer shelf to basinal environments. Based on studied transects, temporal changes in fossil assemblages are not observed. However, increasing level of bedding plane bioturbation is noted, particularly in deeper settings, perhaps suggesting increasing exploitation of the substrate and churning of sediments in the Late Cambrian. Because bbi could not be logged along continuous vertical transects, however, it is difficult to distinguish this pattern from other stochastic or preservational factors. For example, lack of body fossils and low diversity trace fossil assemblages in the Upper Cambrian basin plain facies of the Swarbrick Formation may result from minimal exposure and fissile nature of shales in the available study regions. Or perhaps lack of skeletal faunas may stem from preservational factors such as carbonate dissolution, scavenging or transport of soft- bodied forms, and/or écologie factors. Data collected herein add new geographic coverage to the existing database on deep marine habitats. In doing so, it provides a geographic outgroup comparison for existing 172 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. onshore-offshore deep-sea colonization models, and adds information from carbonate- dominated facies to the current siliciclastic-skewed deep-sea database. By comparison, deep-sea communities in this region may be different than those described from other regions, largely because trace fossil assemblages illustrated here are less diverse, and consist of smaller, less elaborate forms. Similar trace fossil assemblages have been observed in Middle Cambrian deposits from Canada (Young, 1972; Pratt, 1982) and suggest that similarities may exist between early bathyal benthic deep-sea communities along the western margin of Laurentia. Although significant temporal shifts cannot be readily identified using only two study transects, observed timing of infaunal colonization of the southwest Laurentian deep- sea realm fits well with the existing onshore-offshore model. Although future comparisons between similar coeval lithofacies will help resolve whether varying diversity estimates and related disparities reflect preservational bias specific to studied facies, it is possible that the nature of early deep-sea communities differed between disparate deep ocean settings. Lastly, the unanswered question which led to this study and which is key to understanding the Proterozoic-Phanerozoic transition: When, where, and how did animals first evolve? The current consensus is that the earliest metazoan communities evolved in shallow marine environments and then migrated to deeper settings (Crimes, 1974,1992a,b; Anderson and Crimes, 1985; Crimes et al, 1992; Crimes and Fedonkin, 1994,1996). However, this widespread perception seems to be contradicted by the fact that the oldest multicellular animals documented from the fossil record are preserved in situ on the upper surfaces of U-Pb zircon-dated 565 ±3 My Vendian deep marine turbidites (Anderson and Misra, 1968; Anderson, 1978; Anderson and Conway Morris, 1982; Benus, 1988; Jenkins, 1992), and the oldest undisputed trace fossils occur in situ below U-Pb zircon- dated 565 My Vendian deep marine flysches of Spain (Schafer et al., 1993; Vidal et el., 1997). This apparent contradiction suggests that we still have much to learn about the 173 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. earliest deep marine biotopes. Data collected in this study is not sufficient to reject or modify such onshore-offshore hypotheses, but together with detailed re-examination of faunas used in previous larger-scale onshore-offshore syntheses, observations presented herein provide a logical starting point for reinvestigation of these patterns. CONCLUSIONS Examination of two Middle Cambrian-Lower Ordovician deep-sea sequences deposited along the margin of southwestern Laurentia suggests the seafloor in this region was colonized as early as the upper Middle Cambrian by relatively simple, small epifaunal and shallowly infaunal communities. Extensive infaunal communities exhibiting a diverse suite of bioturbating behaviors were well established in upper slope settings throughout this interval, whereas only monotypic shallowly infaunal behaviors are recorded from deeper slope and basin plain settings, suggesting predominance of smaller and less diverse paleocommunities. Within studied transects, the size, diversity, depth, and degree of bedding-parallel bioturbation decreases from outer shelf to basinal environments. Biotic shifts within and between these transects complement and expand existing large-scale models of deep-sea colonization, but suggest that we have much more to leam about how, why, and what kinds of organisms colonized early deep-sea environments and what roles these organisms may have played in the evolution and colonization of the world's oceans. 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 7: References 175 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acenolaza, F.G., 1978, El Paleozoico de Argentina segun sus trazas fôsiles: Ameghiniana, v. 15, p. 15-64. 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Yochelson, E.L., and Fedonkin, M.A., 1993, Paleobiology of Climactichnites, an enigmatic Late Cambrian fossil: Smithsonian Contributions to Paleobiology, n. 74, 74 p. Young, F.G., 1972, Early Cambrian and older trace fossils from the Southem Cordillera of Canada: Canadian Joumal of Earth Sciences, v. 9, p. 1-17. Zhu, M., Precambrian-Cambrian trace fossils from eastem Yunnan, China: Implications for Cambrian explosion: Bulletin of the National Museum of Natural Science, n. 10, p. 275-312. 198 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. iM A n r r\/A i I lATinM ------- IV ^ I^ I TEST TARGET (Q A -3 ) / vL 6> % 1 . 0 l.l I : « 6 ü r[f IS I Ills .8 1 .2 5 1 . 4 1 . 6 150mm V < P > V / / ' / / a /A PPL IE D A IIVMGE . Inc 1653 East Main street Rocttester, NY 14609 USA Ptione: 716/4824)300 . ^ ^ ^ 5 Fax: 716/288-5989 0 1 9 9 3 . A p p l i e d I m a g e , I n c . . 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Creator Hagadorn, James Whitey (author)

Core Title Restriction of a late neoproterozoic biotope: Ediacaran faunas, microbial structures, and trace fossils from the proterozoic-phanerozoic transition, Great Basin, United States of America

Degree Doctor of Philosophy

Degree Program Geological Sciences

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

Tag OAI-PMH Harvest,paleoecology,paleontology,Paleozoology

Language English

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c17-425746

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