Stratigraphic and Paleoecological Controls on Eurypterid Lagerstätten in the Mid- Paleozoic A dissertation submitted to the Graduate School University of Cincinnati In partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Geology McMicken College of Arts and Sciences May 25, 2016 by Matthew Benjamin Vrazo M.S., University of Bristol (UK), 2006 B.S. University of Bristol (UK), 2004 Dissertation Committee Dr. Carlton E. Brett, Chair Dr. Brooke E. Crowley Dr. Aaron F. Diefendorf Dr. Brenda R. Hunda Dr. Arnold I. Miller ABSTRACT The Eurypterida (Arthropoda: Chelicerata) are unique among chelicerates in having undergone a complete marine-to-freshwater transition during their history. Although the general pattern is well documented, habitats during the peak of the transition during the Silurian and Lower Devonian have remained unclear. This is due to the co-occurrence of eurypterids with euryhaline and marine faunas in environments that have been interpreted as hypersaline, and a paucity of detailed bed-level data within eurypterid-bearing occurrences, such within the numerous Lagerstätten of the Laurentia. A high-resolution field- and specimen-based analysis was carried out on eurypterid occurrences in Silurian–Lower Devonian-age exposures in the Appalachian basin. Eurypterids in the upper Silurian Tonoloway Formation of Pennsylvania occur in a transgressive succession above microbial structures (thrombolites) with a marine fauna that suggests near-euhaline conditions. The paucity of occurrences in adjacent hypersaline facies suggests that eurypterids preferentially occupied freshening conditions. A survey of all eurypterid-bearing units in the basin found that the co-occurrence of eurypterids and hypersaline indicators (evaporites) are common, but only in the northern basin. Here, eurypterid and associated fauna frequently co-occur with disruptive, isolated salt hoppers (pyramidal-shaped halite pseudomorphs), which suggests intra-sedimentary formation. Thus, eurypterid-evaporite associations appear to reflect early-stage diagenesis rather than burial conditions, and it is unlikely that eurypterids inhabited hypersaline conditions. Analysis of eurypterid-bearing strata at the centimeter-scale reveals strong sequence stratigraphic controls on preservation. Eurypterid preservation appears to be principally controlled by water depth in nearshore settings. In carbonate-dominated environments, eurypterids often occur within or above beds containing microbial structures (thrombolites, stromatolites) that are interpreted as the flooding surface within small-scale transgressive events. Shallowing-upward successions above these beds, indicated by desiccation features (evaporites, desiccation cracks), are the result of regressions and/or evaporation. A preservational model is proposed whereby eurypterids entered nearshore settings during transgressive freshening events; subsequent hypersalinity and/or dysoxia or ii anoxia in post-burial sediments following regression was favorable to excellent preservation of cuticle. In settings lacking microbial structures or hypersaline indicators, eurypterids occur in similar transgressive successions. Such stratigraphic constraint on eurypterid occurrences permits detailed assessment of morphological variation among equivocal species. A combined landmark and semi-landmark-based geometric morphometric analysis of eurypterids from contemporaneous upper Silurian populations indicates that eurypterid morphospecies cannot be distinguished using isolated carapaces, unless identifiable macro-scale characters are present. In well-defined species, however, a combined landmark/semi-landmark approach allows regional-scale variance to be quantified. iii iv ACKNOWLEDGEMENTS There are many people that I need to thank for their contributions to my doctoral journey. Firstly, thank you to my advisor, Carlton E. Brett, for allowing pursue my research interests in both familiar and unfamiliar territory. The guidance I received in the field has been invaluable and will continue with me on future endeavors. I thank Brooke E. Crowley and Aaron F. Diefendorf for introducing me to the world of stable isotopes, willingly answering my many queries once the seed was planted, and ultimately training me to do the job myself. Brenda R. Hunda is thanked for her constant enthusiasm and making a morphometrician out of me. Arnold I. Miller is thanked for his continual support, and helping me think in terms of the big picture. I must also thank the faculty in the Department of Geology at UC, many of whom provided technical, editing, or discursive assistance during the course of my Ph.D. Special thanks goes to Andrew D. Czaja, Warren D. Huff, J. Barry Maynard, and David L. Meyer who kindly provided access to their labs and their technical expertise. Joshua H. Miller is thanked for his input on quantitative analyses and willingness to answer any number of R programming-related questions. This dissertation is the result of discussions, input, and feedback from many outside the department. Samuel J. Ciurca, Jr. is thanked for generously dispensing his knowledge in the field, and over the course of many emails and phone calls. A significant portion of this dissertation would not have been possible were it not for the samples gathered during his tireless fieldwork over the last 40 years. I greatly benefitted from assistance by Dr. Jeffrey M. Trop, both in the field and in during manuscript preparation. Susan Butts and Jessica Utrup at the Yale Peabody Museum are thanked for their collections and specimen assistance, which was critical to the completion of this dissertation. John-Paul Zonneveld, Peter van Roy, and anonymous reviewers read earlier versions of parts of this dissertation and are acknowledged for their helpful criticism and suggestions. Finally, Simon J. Braddy is thanked for setting me down the road of arthropod paleoecology and taphonomy that led my continuing interest in this area. v Very large thanks goes to my UC graduate colleagues who listened to my endless spiels, offered their own wisdom, provided technical assistance, joined me for a necessary beverage, and ultimately kept me sane over the course of my Ph.D. This includes, but is not limited to Christopher D. Aucoin, Kelsey M. Feser, Gary J. Motz, Nicholas B. Sullivan, Cameron E. Schwalbach, Janine M. Sparks, James R. Thomka, Alexander F. Wall, Julia L. Wise, and Andrew A. Zaffos. Thanks goes to Gene Hunt at the Smithsonian NMNH for offering me a space in which to finish this dissertation, and to Laura C. Soul and Rachel C. M. Warnock for getting me through the final push. Funding and support for this dissertation was provided by the Geological Society of America, the Paleontological Society, the Schuchert and Dunbar Grants in Aid Program (Yale Peabody Museum), Sigma Xi-University of Cincinnati Chapter, the University of Cincinnati Department of Geology Caster Fund, the University of Cincinnati Graduate Student Governance Association, the University of Cincinnati University Research Council, and the University of Cincinnati Graduate School Dean’s Fellowship. This dissertation is dedicated to: My parents, who always supported my interest in natural history and pushed me to go further; Anna, for her unwavering support (and keeping me on my toes); Alex (a.k.a., my 6th committee member), whom, without her guidance, patience, and sharp-eyed critiques, this Ph.D. would have never been possible. vi TABLE OF CONTENTS Chapter I: Introduction (pp. 1–5) Chapter II: A new eurypterid Lagerstätte from the upper Silurian of Pennsylvania (pp. 6–69) Chapter III: Buried or brined? Eurypterids and evaporites in the Silurian Appalachian basin (pp. 70–118) Chapter IV: Paleoecological and stratigraphic controls on eurypterid Lagerstätten: a model for preservation in the mid-Paleozoic (pp. 119–173) Chapter V: Taphonomic bias in taxonomic assignment: a semi-landmark approach to distinguishing species of Silurian eurypterids (pp. 174–230) Chapter VI: Conclusions (pp. 231–232) Appendix: Chapter II Supplementary Materials (p. 233–236) Chapter III Supplementary Materials (p. 237–238) Chapter V Supplementary Materials (p. 239–252) vii Chapter I INTRODUCTION The transition of an entire group of organisms from one ecosystem into another is a rare occurrence in the history of life (Vermeij and Dudley 2000). Studies of specific clades that have successfully exploited new ecosystems can provide insight into the causes of these important evolutionary events. The arthropod subphylum Chelicerata contains several lineages that have independently made such a transition (Selden and Jeram 1989). These include the Order Eurypterida, a predatory and widely distributed aquatic clade that was closely related to scorpions and other arachnids (Kamenz et al. 2011). Eurypterids originated in open marine environments in the mid-Ordovician (Lamsdell et al. 2015) and were found in nearshore settings by the Late Ordovician (Young et al. 2007). In the Silurian, eurypterids first appeared in freshwater-influenced environments (Swartz and Swartz 1930; Swartz and Swartz 1931); by the beginning of the Carboniferous, they were restricted entirely to terrestrial, freshwater-dominated settings (Plotnick 1983, 1999; Tetlie 2007). Eurypterids transitioned into a new system only once (unlike other aquatic chelicerates, e.g., the Xiphosura, Lamsdell 2016), but freshwater taxa were highly successful and survived longer than any marine clade before their extinction in the end- Permian. Although