Evolution of Ceratopsian Dental Microstructure David Kay
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Florida State University Libraries Honors Theses The Division of Undergraduate Studies 2014 Evolution of Ceratopsian Dental Microstructure David Kay Follow this and additional works at the FSU Digital Library. For more information, please contact [email protected] FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCES EVOLUTION OF CERATOPSIAN DENTAL MICROSTRUCTURE By David Kay A Thesis submitted to the Department of Biological Science in partial fulfillment of the requirements for graduation with Honors in the Major Degree Awarded: Summer, 2014 1 The members of the Defense Committee approve the thesis of David Kay defended on April 21st, 2014. ______________________________ Professor Gregory M. Erickson Thesis Director ______________________________ Professor William C. Parker Outside Committee Member ______________________________ Professor Scott J. Steppan Committee Member 2 Acknowledgements To be brutally honest, I almost didn’t write this section because I figured that as an undergraduate thesis that there didn’t need to be one; then I realized that by trivializing this I in effect trivialize all the time and guidance that people have given to me. First and foremost I need to thank Greg Erickson for all that he has done for me, including but not limited to: letting me into his reading group, letting me work in the lab, giving me spectacular advice on how to pursue a career in the paleo world, and being a top-notch mentor to a guy with a few screws loose. I would also like to thank Dr. Steppan, whose comments and advise are always amazing and make whatever I’m writing at least five times better. Dr. Parker for opening the geology department up to me and helping me to understand statistics as well as encouraging me to do what I want to in life. Thanks also to Ken Womble, not only for the figure help and printing out the same poster three times, but for always making me laugh. I also want to thank my lab family, Aki Watanabe, Matt Kolmann, Bonnie Garcia, Ken Gloeckner, and Stephen Hendricks. Thanks to Aki and Matt for teaching me how to read papers and analyze other people’s work as well as teaching me some ins and outs of academic life and for the input on my work and ideas. Thank you Bonnie, for being the lab mother and releasing the kraken when the going gets tough. Ken, thank you for all the good conversation about science fiction and for buying me my first beers. And thank you to Stephen for all of your collaboration and the math discussions. All of you have taught me a lot and have become what I consider some of my best friends. I would also like to thank a certain Microsoft millionaire who has shown me as well as hundreds of other young scientists what we need to avoid becoming. I need to thank my genetic family as well. You have all been there for me with love and support from the beginning before I was cool. Thank you to my parents for instilling in me a thirst for knowledge and all the good life advice. To my siblings old, young and new through marriage, thank you for being there for me, especially my sister Kristen for helping to guide me through my biology degree. I love you guys. 3 Abstract Throughout vertebrate evolution, a number of lineages evolved dental occlusion, whereby the contact faces of the teeth self-wear to their functional morphology. It has been shown that in mammals, increases in dental complexity accompany such changes. These presumably allowed for modifications in biomechanical form, function and performance relevant to dietary ecology. Recently, it was shown that a lineage of reptiles, the Hadrosauridae, evolved some of the most architecturally sophisticated teeth known, in association with their acquisition of a grinding dental unit. Independently, another lineage of ornithischian dinosaurs, the Ceratopsia, evolved dental occlusion in the form of slicing cheek teeth. Here, I have tested the hypothesis that ceratopsian teeth increased in complexity in association with their evolution of shearing. Transverse and occlusal plane histological sections were made using cheek teeth from representative Ornithischia spanning the transformation series leading to the evolution of slicing in ceratopsians. The sections were viewed with dissecting and polarizing light microscopy. The microstructure was described and phylogenetically character-mapped in association with whole tooth and wear facet morphological attributes. My results show that ceratopsian teeth are considerably more complex than those of the outgroup ornithischians in possessing four distinct tissues: enamel, orthodentine, coronal cementum, and vasodentine. Coronal cementum evolved in association with a cohesive dental battery and a shearing masticatory system in the common ancestor of Leptoceratops + Triceratops. Vasodentine appeared in the common ancestor of Protoceratops + Triceratops with the advent of high-angled slicing. These findings represent the second demonstration of complex dental architecture outside of Mammalia, and show that some reptiles rivaled, if not exceeded, most mammals in dental complexity. It also supports the 4 hypothesis that complex histological attributes in teeth appear in association with precise dental occlusion. Introduction Teeth, serve a number of functions including display, defense, prey capture, and relevant to this study, the oral processing of food (. Variation in osseous constituents (hereafter referred to as constituents or histology) that make up teeth varies considerably between groups (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). These include: enamel, dentine, and cementum. Enamel is an extremely hard tissue composed of hydroxy-apatite crystals within a protein matrix (Peyer 1968, Schmidt and Kiel 1971). Dentine is a softer, bone-like tissue that often makes up the majority of the tooth structure (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). Cementum is primitively a root attachment tissue in vertebrates but it is found on the chewing surfaces of some animals such as horses and bison as a crest supporting and basin contributing tissue known as coronal cementum (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). Because it is composed of more ground tissue and protein matrix than enamel and dentine, cementum is relatively softer (Peyer 1968, Hillson 1986). Most gnathostomes, including nearly all reptiles, have non-occluding teeth and the tooth crowns are composed of just an enamel shell overlying a dentine core. However in some herbivorous mammals (for which precise dental occlusion is primitive) which have dentitions composed of folded layers of enamel, several forms of dentine and cementum. Such tissue complexes self-wear to file or rasp-like surfaces used for the pulverization of tough and often abrasive plants (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). 5 Notably, a few groups of Ornithischian dinosaurs, the hadrosaurids and ceratopsians evolved precise dental occlusion (Ostrom 1964, Ostrom 1966, Dodson 1996, Hailu and Dodson 2004, Horner and Weishampel 2004, Erickson et al. 2012). Hadrosaurids in particular were shown to possess up to six different dental tissues in their teeth (Erickson et al. 2012). As in herbivorous mammals with grinding dentitions, the evolution of these complexes was shown to correlate with the evolution of precise dental occlusion and selection for more complex self- wearing architecture to pulverize plants. These findings beg the question; do the other dinosaurs with precise dental occlusion, the Ceratopsia, show histological complexity beyond that typical of reptiles? Coronosaurians (Dinosauria: Marginocephalia) are a group of non-avialan dinosaurs (hereafter dinosaurs) which, like hadrosaursids, evolved dental batteries (groups of interlocked developing and functional teeth that form a single chewing surface; Figure 1) and an occluding dentition (Ostrom 1964, Ostrom 1966, Dodson 1996, Hailu and Dodson 2004). ). The teeth of coronosaurians are thought to have worked like a giant pair of shears, whereby the corresponding dental batteries, wore to high-angled slicing faces, (unlike the typically more coarse planar surfaces of hadrosaurids) for the shearing of tough plant matter, (the types of which are unknown) (Ostrom 1964, Ostrom 1966). The teeth in the lower jaw possess hard enamel only on their lingual faces. Conversely, enamel is restricted to the labial face of teeth in the lower jaw (Figure 2). These configurations allow for self-wear to blades since the enamel is highly wear resistant and the dentine core poorly wear resistant (Figure 3) (Ostrom 1964, Ostrom 1966). Notably, like all other dinosaurs, with the exception those lacking cheek teeth and possibly derived heterodontosaurs, ceratopsian had polyphyodont dentitions (Hopson 1980, Butler et al. 6 2008). Thus, the functionality of these shearing implements was maintained despite continuous tooth eruption (Ostrom 1964, Ostrom 1966). Like all reptiles, it has been traditionally assumed that all ceratopsians possessed simple tooth histology composed of just enamel over an orthodentine (dentine possessing von-Ebner’s growth lines) core. Hatcher and colleagues (1907) however reported cementum on the crowns of Triceratops (Hatcher et al. 1907, Ostrom 1964). Examination of figures from the same publication suggests a fourth tissue, evident in the circumpulpar dentine, may also be present (Hatcher et al. 1907). In this figure (Figure 4) there is a region on the occlusal face that is differentiated as being darker than the adjacent dentine. This finding, along with the discovery of advanced dental architecture