Variation in Dental Morphology and Bite Force Along the Tooth Row in Anthropoids by Lynn Lucas A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Approved May 2012 by the Graduate Supervisory Committee: Mark Spencer, Chair Gary Schwartz William Kimbel ARIZONA STATE UNIVERSITY August 2012 ABSTRACT Modern primate diet is well-studied because of its considerable influence on multiple aspects of morphology, including the shape of the facial skeleton and teeth. It is well-established that differences in craniofacial form influence feeding abilities by altering the nature of bite force production. Tooth morphology, likewise, has been shown to vary with diet across primates, particularly in the details of occlusal form. It has also been suggested that tooth form (e.g., tooth root size and shape and crown size) reflects, in part, the demands of resisting the stresses generated during feeding. However, while they are central to our efforts to infer diet in past species, the relationships between bite force production, craniofacial morphology and tooth form are not well-established. The current study is separated into two parts. In Part I, the hypothesis that crown size and root surface area are adapted to resist masticatory stress is evaluated by testing whether these features show correlated variation along the tooth row in a taxonomically diverse sample of primates. To further explore the adaptive nature of this correlation, pair-wise comparisons between primates with mechanically resistant diets and closely-related species consuming less resistant foods are performed. If crown size and root surface area covary along the tooth row, past research suggests they may be related to bite force. To test this hypothesis, Part II examines the variation of these dental characteristics in comparison to theoretically-derived bite force patterns along the tooth row. Results suggest that patterns of maximum bite force magnitude along the tooth row are variable both within and between species, underscoring the i importance of individual craniofacial variation on masticatory force production. Furthermore, it is suggested that some adaptations traditionally associated with high bite force production (i.e., facial orthognathy) may increase anterior biting force at the expense of posterior biting force. Taken together, results from the current study reveal that both tooth root and crown size vary in conjunction with the mechanical properties of diet and with bite force patterns along the tooth row in anthropoids. ii DEDICATION For my dad, Dr. Michael Lucas, who has taught me to take nothing for granted and to attack every challenge, especially the unexpected ones, with gusto. iii ACKNOWLEDGMENTS This dissertation could not have been completed without the help and support of numerous people. First and foremost, I would like to extend my thanks to my committee members Gary Schwartz and Bill Kimbel and especially to my committee chair, Mark Spencer. Thank you for teaching me and challenging me to think. The process of collecting the primate µCT scans central to my dissertation was long and arduous—much more so than I ever would have imagined before I began this project. My thanks to the many people at Harvard University who made data collection possible. Thank you Judy Chupasko and Mark Omura of the Museum of Comparative Zoology, who generously let me wheel a cart full of monkey skulls across the street to the Center for Nanoscale Systems (CNS) every day (sometimes multiple times a day) for 5 weeks during the summer of 2010. Thanks also to Michele Morgan from the Peabody Museum of Archaeology and Ethnology for granting access to their chimpanzee collection. Finally, thank you to Fettah Kosar, who runs the µCT scanner at CNS, for not only teaching me how to use the scanner, but also for granting 24-hour a day 7-day a week access to it, which, it turns out, was not overkill at all, but exactly what was needed to collect all of the data that form the basis of this dissertation. Funding for this project was provided by Sigma Xi Grants-In-Aid of Research, a SHESC Special Research Grant, a GPSA Dissertation Grant, and an ASU Dean’s Writing Fellowship; thanks to Tae O’Connor, Heidi Scheier, Jodi Guyot, and Mena Bell for helping me secure and spend every cent. iv My sincerest thanks to all of my fellow graduate students, past and present, who have collectively provided hours of stimulating scientific discussion and commiseration when appropriate; thank you Caley Orr, Jeremiah Scott, Kristi Lewton, Thierra Nalley, Stephanie Meredith, Heather Smith, Laura Stroik, Amy Shapiro, Sam Russak, Kierstin Catlett, Terry Ritzman, and Halszka Glowacka. To my motley crew of undergraduate research assistants, thank you for spending hours erasing pixels, organizing files, and tolerating the lab computer! Without your help, I would still be sorting through it all: thank you, Angie Gutierrez, Brittany Tullinen, Erik Thunberg, Morgan Texeira, Courtney Bruce, Scott Hamilton, and Lawrence Fatica. Thanks go to Rebecca Fisher, my unofficial mentor, who is easily one of the best teachers I’ve ever had in my entire academic career. Thank you for your expertise and support both in and out of the classroom. Thanks also go to my latest office-mate, Hallie Edmonds, for letting me draw scores of triangles on my sciencing board and talk about them at length without fleeing or even fidgeting too much. To my friend, Joyful Scientist Lynn Copes, thank you for sciencing with me for the last 5 years. You are an inspiration. Thanks to my mother and sister, Patty and Jessica Lucas, and to my dad and stepmom, Michael Lucas and Kay Hampshire, for their constant love and support. Finally, to my husband Emory, thank you for everything you do. There are no words to convey the depth of my love for you and gratitude for your presence in my life. v TABLE OF CONTENTS Page LIST OF TABLES ....................................................................................................... x LIST OF FIGURES ................................................................................................... xii CHAPTER 1 INTRODUCTION .................................................................................. 1 2 BACKGROUND .................................................................................... 4 Tooth Function and Morphology ....................................................... 6 Tooth anatomy and development ............................................. 6 The periodontal ligament ........................................................ 10 PDL biomechanics ....................................................... 10 Sensory feedback .......................................................... 13 Tooth root comparative and functional morphology ............. 15 Tooth root and crown independence ...................................... 18 Crown size .............................................................................. 18 Crown size and metabolic rate ..................................... 19 Crown size and sexual dimorphism ............................. 20 Crown size and diet ...................................................... 21 Food material properties ......................................................... 22 Patterns of molar crown size .................................................. 26 Note about tooth enamel ......................................................... 28 Craniofacial biomechanics ............................................................... 29 Simple lever model ................................................................. 31 vi CHAPTER Page Muscle resultant inclination and raising the TMJ .................. 35 Constrained lever model ......................................................... 39 Effect of configurational changes on bite force .................... 44 The buffer zone ....................................................................... 47 Simple vs. Constrained lever model predictions ................... 49 Experimental Research ........................................................... 50 Hypotheses and Predictions .............................................................. 53 Part I: Dental relationships ..................................................... 54 Part II: Dental features and bite force patterns ...................... 55 3 MATERIALS AND METHODS ......................................................... 56 Sample ............................................................................................... 56 Platyrrhines ............................................................................. 62 Callitrichinae ................................................................. 62 Cebinae ......................................................................... 63 Pitheciinae ..................................................................... 64 Atelinae ......................................................................... 66 Catarrhines .............................................................................. 67 Colobinae ...................................................................... 67 Cercopithecinae ............................................................ 68 Macaques ............................................................ 69 African papionins ............................................... 70 Guenons .............................................................. 71 vii CHAPTER Page Homininae ....................................................................