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Comparative Cranial Ecomorphology and Functional Morphology of Semiaquatic Faunivorous Crurotarsans a Dissertation Presented To Comparative Cranial Ecomorphology and Functional Morphology of Semiaquatic Faunivorous Crurotarsans 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 Waymon L. Holloway December 2018 © 2018 Waymon L. Holloway. All Rights Reserved. 2 This dissertation titled Comparative Cranial Ecomorphology and Functional Morphology of Semiaquatic Faunivorous Crurotarsans by WAYMON L. HOLLOWAY has been approved for the Department of Biological Sciences and the College of Arts and Sciences by Patrick M. O’Connor Professor of Biomedical Sciences Joseph Shields Interim Dean, College of Arts and Sciences 3 ABSTRACT HOLLOWAY, WAYMON L., Ph.D., December 2018, Biological Sciences Comparative Cranial Ecomorphology and Functional Morphology of Semiaquatic Faunivorous Crurotarsans Director of Dissertation: Patrick M. O’Connor Crurotarsi are a clade of archosauromorphs ranging in age from the Middle Triassic to Recent that includes two semiaquatic, faunivorous subclades: Crocodylia and the predominantly late-Triassic Phytosauria. Phytosaurs and crocodylians exhibit generally similar overall body morphology, and each exhibits a range of narrow to broad rostral cranium morphotypes. These morphological similarities lead to the commonly adopted hypothesis that the two clades exhibited a number of ecological and behavioral similarities. One such hypothesized ecological similarity is that phytosaurs utilized a range of food item types that was the same as that utilized by extant crocodylians. In particular, phytosaurs possessing slender rostra with a high aspect ratio were previously hypothesized to have been strictly or primarily piscivorous, much like extant crocodylians with slender, high aspect ratio rostra that have been described as piscivorous. However, a review of available literature reporting on direct observations and other dietary data in extant crocodylians revealed that no extant crocodylian taxa are either strictly piscivorous or lacking at least one population that consumes teleosts as a primary food source. Instead of being correlated with consumption of a specific food type, then, rostrum morphology in extant crocodylians appears to be correlated with the relative size of food items that can be consumed by a given individual. Cranium shape, 4 jaw musculature, and biomechanical performance were assessed in both phytosaurs and extant crocodylians to test hypotheses of morphological and functional similarities of the cranium between these two clades. Results of these analyses were interpreted in the context of prey:predator size ratios correlating with rostrum morphology in extant crocodylians in order to better constrain inferred diet ranges and variation among phytosaur taxa. In general, phytosaurs were more similar to crocodylians with very high aspect ratio rostra in most facets of cranium shape than crocodylians with lower aspect ratio rostra. This trend supports the interpretation that the diets of phytosaurs were probably most like those of extant crocodylians with high aspect ratio rostra. However, no overlap in any tested aspect of cranium shape was found between phytosaurs and crocodylians, precluding an inference of phytosaur diet as being the same as any sampled crocodylian taxon. The topology, individual origin and insertion area size proportions, and individual muscle force proportions of phytosaur and extant crocodylian jaw musculature all exhibited a great deal of consistency, regardless of variation in rostrum morphology among the sampled taxa. Clear indicators of greater jaw musculature similarity among particular taxa that would support inferences of dietary similarities between those taxa were thus not found. Biomechanical modeling revealed that bite forces produced by phytosaurs generally surpassed those of extant crocodylians, though the crania of most phytosaurs performed worse, from a structural perspective, than most extant crocodylians. These results, though somewhat conflicting, indicate that phytosaurs were typically able to produce higher bite forces than similarly sized extant crocodylians but were less capable of withstanding the resultant force experienced by the cranium 5 during performance of a maximally powerful bite or when biting a hard object. These diverse results, synthesized into a single conclusion, indicate that smaller phytosaurs with relatively narrow, gracile rostra were probably restricted to utilizing food items even smaller, relative to their own size, than do any extant crocodylians. Larger phytosaurs and those with somewhat more robust, if also narrow, rostra were probably capable of utilizing food items of a prey:predator size ratio similar to or exceeding those utilized by extant crocodylians with relatively low rostrum aspect ratios. These new inferences highlight the complexity of interactions between cranium shape and jaw muscle performance that result in differences in trophic niche occupation. Better appreciation for these interactions will allow for a greater understanding of the mechanisms that lead to phenomena such as niche partitioning. Furthermore, more accurately constrained reconstructions of phytosaur ecology will enable further investigations into the complex ecosystem structure and changes during the Late Triassic. 6 DEDICATION To my Mother, Patricia, who has always supported and encouraged my education, from its beginning to now; my wife and love, Katharine, for her tireless support, assistance, and motivation throughout this work; Molly, my loving inspiration, that this work may be a worthy tribute to her memory; and Frankie, for keeping me company as I write this. 7 ACKNOWLEDGMENTS I would like to acknowledge my advisor, Patrick O’Connor, for the massive amount of guidance and support that he provided throughout my graduate career and committee members John Cotton, Shawn Kuchta, Susan Williams, and Lawrence Witmer for their invaluable input and efforts to improve this work. Discussions and technical assistance from my lab mates, other Ohio University Ecology and Evolutionary Biology program graduate students, and C. Holliday and K. Sellers of the University of Missouri greatly helped me throughout this process. Specimen access and CT scanning was facilitated by C. Pugh and B. Keene of Holzer Clinic; R. Irmis and C. Levitt-Bussian of UMNH; B. Parker, A. Marsh, and M. Smith of PEFO; J. Payne of Summit Healthcare; M. Brown and C. Sagebiel of TMM; and Liz Daigle of ARA Wilson Parke. L. Witmer, C. Brochu, J. Maisano, S. Pierce, and E. Gold provided additional CT scan data. Financial support for this work was provided through an Osteopathic Heritage Foundations Graduate Research Fellowship, Ohio University Graduate Student Senate Original Work Grant, and College of Arts and Sciences Graduate Student Research Fund. 8 TABLE OF CONTENTS Page Abstract ...........................................................................................................................3 Dedication .......................................................................................................................6 Acknowledgments ...........................................................................................................7 List of Tables................................................................................................................. 10 List of Figures ............................................................................................................... 11 Chapter 1: Introduction .................................................................................................. 13 Phylogenetic Relationships of Phytosauria ............................................................... 13 Occurrence of Phytosauria ....................................................................................... 15 Hypothesized Ecology of Phytosauria ...................................................................... 20 Research Outline...................................................................................................... 31 Chapter 2: Geometric Morphometric Analysis of Cranium Shape Variation in Crurotarsans in an Ecomorphological Context ............................................................... 35 Introduction ............................................................................................................. 35 Rostrum Morphotypes ........................................................................................ 37 Geometric Morphometric Characterization of Shape .......................................... 40 Materials and Methods ............................................................................................. 43 Taxon Sampling ................................................................................................. 43 Cranial Shape and Landmark Placement ............................................................ 47 Analytical Approaches ....................................................................................... 49 Results ..................................................................................................................... 52 Principal Coordinates Analyses .......................................................................... 52 Statistical Analyses ............................................................................................ 62 Discussion ..............................................................................................................
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