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Early Silurian Terrestrial Biotas of Virginia, Ohio, and Pennsylvania: an Investigation Into the Early Colonization of Land (284 Pp.)

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Early Silurian Terrestrial Biotas of Virginia, Ohio, and Pennsylvania: an Investigation Into the Early Colonization of Land (284 Pp.) LATE ORDOVICIAN – EARLY SILURIAN TERRESTRIAL BIOTAS OF VIRGINIA, OHIO, AND PENNSYLVANIA: AN INVESTIGATION INTO THE EARLY COLONIZATION OF LAND A dissertation presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Alexandru Mihail Florian Tomescu November 2004 © 2004 Alexandru Mihail Florian Tomescu All Rights Reserved This dissertation entitled LATE ORDOVICIAN – EARLY SILURIAN TERRESTRIAL BIOTAS OF VIRGINIA, OHIO, AND PENNSYLVANIA: AN INVESTIGATION INTO THE EARLY COLONIZATION OF LAND BY ALEXANDRU MIHAIL FLORIAN TOMESCU has been approved for the Department of Biological Sciences and the College of Arts and Sciences by Gar W. Rothwell Distinguished Professor of Environmental and Plant Biology Leslie A. Flemming Dean, College of Arts and Sciences TOMESCU, ALEXANDRU MIHAIL FLORIAN. Ph.D. November 2004. Biological Sciences Late Ordovician – Early Silurian terrestrial biotas of Virginia, Ohio, and Pennsylvania: an investigation into the early colonization of land (284 pp.) Director of Dissertation: Gar W. Rothwell An early phase in the colonization of land is documented by investigation of three fossil compression biotas from Passage Creek (Silurian, Llandoverian, Virginia), Kiser Lake (Silurian, Llandoverian, Ohio), and Conococheague Mountain (Ordovician, Ashgillian, Pennsylvania). A framework for investigation of the colonization of land is constructed by (1) a review of hypotheses on the origin of land plants; (2) a summary of the fossil record of terrestrial biotas; (3) an assessment of the potential of different continental depositional environments to preserve plant remains; (4) a reevaluation of Ordovician-Silurian fluvial styles based on published data; and (5) a review of pertinent data on biological soil crusts, which are considered the closest modern analogues of early terrestrial communities. The three studied biotas are non-paradigmatic biotas, defined here as biotas whose components escape unequivocal taxonomic assignment because their features preclude assignment to any known group, or compare them to several known groups without allowing further distinction. The Passage Creek biota consists of rich, morphologically and anatomically diverse fossil assemblages. They demonstrate that a well-developed groundcover was already present at the beginning of the Silurian, occupying river floodplains. This groundcover consisted principally of thalloid organisms and the diversity of terrestrial communites included cyanobacteria, as well as organisms characterized by complex internal organization, the affinities of which remain unresolved. The Kiser Lake fossils consist of carbonaceous compressions, one of which compares to liverwort leaves and lichen rhizines. The same sediments yield scraps of cellular tissue and the earliest spores assignable to ascomycetes. These are preserved in peritidal carbonate sediments. The Conococheague Mountain biota includes rich compression assemblages from transitional shallow marine and deltaic facies. Such deposits usually harbor mixtures of marine and continental fossils, rendering interpretation of the ecology of original organisms difficult. These three biotas contain the oldest macrofossils of terrestrial (Passage Creek) and potentially terrestrial (Conococheague Mountain, Kiser Lake) organisms and demonstrate that diverse communities including complex forms were present much earlier than previously thought. The thalloid morphology represents a transitional stage in the evolution of complexity among terrestrial primary producers from exclusively microbial photosynthesizers to complex embryophytes with axial morphology. Approved: Gar W. Rothwell Distinguished Professor of Environmental and Plant Biology PREFACE When I started this study, it seemed a relatively straightforward undertaking: searching for the origin of land plants by looking at fossils. However, as I progressed in familiarizing myself with the present state of knowledge in the field, in understanding the thinking of those who had tackled this subject before me, in producing new data from fossils, and in organizing all of these into written text, I started realizing that my original pursuit was targeting only one aspect within the very tip of a vast iceberg representing the fundamental process of colonization of land surfaces by life. That this iceberg consisted of a multitude of aspects and facets frozen together as a collection of facts and observations scattered within a blurry matrix of unknown. That these facts and observations represented the accumulation of results from work by numerous people, over many years, in a very diverse array of fields of research: paleobiology, molecular biology, genetics, developmental and structural biology, biochemistry, phylogenetics, stratigraphy, sedimentology, and geochemistry. And that the answer to my initial question could only be found by melting the whole iceberg down into consistent, organized knowledge, to clarify the connections and add the missing parts between all of these facts and observations, as well as to uncover new facts and link them to what is already known. Consequently I broadened my perspective and started by building up an epistemologic framework for my pursuit. This meant defining the nature of problems engendered by the features of the fossils that I was studying; understanding what we mean by plants and land plants, and what we think currently about their phylogeny; summarizing the ideas, as well as the present state of knowledge on the colonization of land and early terrestrial biotas. I added an assessment of the potential of different sedimentary depositional environments for preserving remains of terrestrial biotas, and summarized knowledge of the structure and ecology of biological soil crusts (the most likely modern analogues of early terrestrial ecosystems), as stepping stones. Within this framework I could formulate hypotheses and begin looking at fossil biotas. The results are both frustrating and encouraging. The fossils do not give up their secrets willingly. The mode of preservation represents a redoutable obstacle to understanding their nature. But this can be overcome by detailed characterization of the fossils coupled with actualistic experimental work on morphologically similar modern groups. Additional fossil material present in abundance at several new unexplored localities can reveal better preservation and add to the characteristics documented so far. Features preserved in the fossils studied here nevertheless provide evidence for the diversity of early terrestrial life and allow for establishing a minimal date for the advent of complex life on land. But more importantly, these results show that fossils hold great potential for clarifying this crucial chapter in the history of life, of which we have only written the first pages. ACKNOWLEDGMENTS I express my deep appreciation to Dr. Gar W. Rothwell, my advisor, whose ideas have profoundly influenced my thinking, and whose enthusiasm and determination will forever inspire me in my own studies. His able guidance, his tireless help, and his generous friendship made this study possible. I wish to thank the other members of my Dissertation Committee, Dr. Morgan L. Vis, Dr. Gene K. Mapes, Dr. Royal H. Mapes, Dr. Gregory C. Nadon, and Dr. Harvey E. Ballard, Jr., who sharpened my thinking and improved my writing. An important part of the Passage Creek material has been developed in collaboration with Dr. Rosmarie Honegger, while I was visiting her laboratory. Her kind help and generous support are gratefully acknowledged. Many thanks are extended to Dr. Robert S. Hikida for permission and help in using TEM facilities. I am deeply indebted to Dr. Gregory C. Nadon, whose help in the field and ideas expressed during numerous discussions were crucial to understanding of depositional environments of the fossil localities. Numerous other people examined some of the fossil material, parts of this study, or otherwise offered helpful comments: Dr. Linda E. Graham, Dr. David L. Kidder, Dr. Jeffrey R. Johansen, Dr. Paul K. Strother, Dr. Mark A. Kleffner, Dr. Lisa M. Pratt, Dr. Dale A. Casamatta, Dr. James C. Cavender, Dr. Irwin A. Ungar, Dr. Michael C. Hansen, Dr. Charles F. Kahle, Dr. Alfred Uchman, Dr. Richard Fortey, and Dr. Stig Bergstrom. I am grateful to Michi Federer for performing some of the TEM preparations, and to Costel Constantin for help with electron-dispersive X-ray spectrometry. The Division of Geological Survey, Ohio Department of Natural Resources (Dr. Thomas M. Berg, State Geologist and Chief of the Division) is thanked for granting access to drill cores in the Ohio Geological Sample Repository, and permission to sample the cores. Help from Ronald G. Rea, Sample Repository Manager, during sampling is gratefully acknowledged. Dr. Michael T. Dunn and Genaro R. Hernandez-Castillo provided help during sampling, and participated in fruitful discussions. Support from the following grants to the author is gratefully acknowledged: National Science Foundation Doctoral Dissertation Improvement Grant DEB-0308806, Geological Society of America Graduate Student Research Grants, Ohio University John Houk Memorial Research Grants, Systematics Association Grant, Sigma Xi Grants-in- Aid of Research, Paleontological Society Student Grants-in-Aid, a Botanical Society of America Karling Graduate Student Research Award, and an Ohio Biological Survey Small Grant. 10 TABLE OF CONTENTS Page ABSTRACT.........................................................................................................................4
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