Copyright by Lauren Taylor English 2018 The Dissertation Committee for Lauren Taylor English Certifies that this is the approved version of the following Dissertation: Evolution of Functional Morphology of Osteoderms Across Crocodylomorpha Committee: Julia A. Clarke, Supervisor Christopher J. Bell Christopher Brochu Matthew W. Colbert Rowan C. Martindale Liza J. Shapiro Evolution of Functional Morphology of Osteoderms Across Crocodylomorpha by Lauren Taylor English Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin May 2018 Dedication To my parents, Pamela and Julian, and to my brothers, Lee and Logan, for being the first to inspire in me a love of science and continuing to do so when times were hard, for never allowing me to forget to think critically, and for always being there to commiserate over failures and celebrate over triumphs. Acknowledgements Many people have contributed to the research presented here, which I cannot thank enough for their assistance. I am particularly grateful to my advisor, Julia Clarke, for all the guidance and support she has gifted me from start to finish. I would also like to thank everyone who has served on my committee (Chris Bell, Chris Brochu, Matt Colbert, Brian Horton, Rowan Martindale, Tim Rowe, and Liza Shapiro) for all the feedback that has improved this manuscript. I would like to thank the following institutions and their staff for access to specimens and CT scans: the Museum of Natural History, University of Florida, the Museum of Vertebrate Zoology, University of California, Berkeley, the Saint Augustine Alligator Farm, the University of Texas at Austin – Biodiversity Collections, the Texas A&M University – Biodiversity Research and Teaching Collections, the American Museum of Natural History, the Carnegie Museum of Natural History, the Field Museum, the North Carolina Museum of Natural Sciences, the Natural History Museum in London, the Senckenberg Naturmuseum, the Science Museum of Minnesota, Stony Brook University, the University of California Museum of Paleontology, Berkeley, the National Museum of Natural History. I would also like to thank the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University and the University of Texas High- Resolution X-ray Computed Tomography Facility for scanning specimens and particularly to Matt Colbert and Jessie Maisano for advice on processing CT data. Financial support was provided in part by the Ernest and Judith Lundelius Endowment in Vertebrate Paleontology, the University of Austin off-campus research funds, and by the Jackson School of Geosciences Student Travel Grant. v I would like to thank the other students in my lab (James Proffitt, Chad Eliason, Katie Browne, Nick Crouch, Sarah Davis, Zhiheng Li, Grace Musser, Chris Torres, and Xia Wang,) for helpful discussions of methodology and presentation, the collections and specimen preparation staff (Liath Appleton, Kenneth Bader, Lisa Boucher, Matt Brown, Cissy Geigerman, Ernest Lundelius, the late Ann Molineux, Tim Rowe, Chris Sagebiel, and Deborah Wagner) for showing me the ins and outs of specimen care, Philip Guerrero for his indispensable assistance in navigating campus paperwork, and the all other staff and students at the Jackson School of Geosciences for all their assistance and companionship over the years. vi Abstract Evolution of Functional Morphology of Osteoderms Across Crocodylomorpha Lauren Taylor English, PhD The University of Texas at Austin, 2018 Supervisor: Julia A. Clarke The vertebrate integument is a complex anatomical system that has evolved a wide array of specialized structures that are related to an animal’s ecology and evolutionary history, including mineralized structures found in many clades and ecological niches called osteoderms. However, the development, functional morphology, and evolutionary history of osteoderms is still poorly understood in relation to other portions of the skeleton. Osteoderms have independently evolved multiple times and can be found in the skin of many vertebrate lineages, making up a significant portion of the evolutionary history of the tetrapod skeleton. Though several hypotheses of osteoderm function have been proposed based on their overall morphology, osteoderm function has rarely been the subject of experimental analyses and only in a few taxa. The major aims of this research are to test new and previously proposed hypotheses of osteoderms function and to better understand drivers of osteoderm morphological variation using Crocodylomorpha as the study group, both because of their long and rich history of ecological diversity and because of their exemplary fossil record compared with other osteoderm bearing taxa. Quantification of within-individual variation in extant vii crocodilians suggests that relaxed selection on osteoderm morphology only occurs in taxa thought to exhibit reduced intraspecific aggression, suggesting that osteoderms evolved in Crocodylia as a defensive structure in territorial disputes. This is corroborated by an ontogenetic analysis, which demonstrates that in at least some crocodilian species, osteoderm growth coincides with the onset of territorial behavior, contradicting previously proposed hypotheses that crocodilian osteoderms function in thermoregulation, in stabilization of the vertebral column, or as a defense against predators. In contrast with previous research that had demonstrated a reduction of body flexibility when osteoderms are present, a final comparative analysis across Crocodylomorpha found no correlation between osteoderm aspect ratio and locomotor ecology, suggesting that osteoderm dimensions have no impact on flexibility. This research disputes some previously proposed hypotheses of osteoderm function and when placed in context of other research, reveals that osteoderms may have different functions in different taxa. viii Table of Contents Chapter 1: Variation in crocodilian dorsal scute organization and geometry with a discussion of possible functional implications ...........................................................1 Abstract .......................................................................................................................1 Introduction .................................................................................................................2 Materials and Methods................................................................................................4 Variation in scute dimensions .........................................................................5 Bilateral fluctuating asymmetry of carapace ..................................................5 Scutellation pattern .........................................................................................6 Results .........................................................................................................................7 Variation in scute dimensions .........................................................................7 Lateral asymmetry of carapace .......................................................................7 Rectangular vs. pentagonal/hexagonal scute arrangement .............................8 Discussion ...................................................................................................................8 Conclusions ...............................................................................................................10 Tables ........................................................................................................................11 Figures ......................................................................................................................12 Chapter 2: Late ontogenetic development of osteoderms and their role in social behavior.....................................................................................................................15 Abstract .....................................................................................................................15 Introduction ...............................................................................................................16 Materials and Methods..............................................................................................20 Specimens .....................................................................................................20 Assessment of maturity .................................................................................20 ix Results .......................................................................................................................21 General developmental patterns in all species ..............................................21 Life history & osteoderm development in Caiman crocodilus and Crocodylus niloticus ...............................................................................22 Discussion and Conclusions .....................................................................................23 Tables ........................................................................................................................26 Figures ......................................................................................................................28 Chapter 3: Assessment of the comparative morphology of osteoderms and locomotor ecology in Crocodylomorpha ....................................................................................30