Reconstructing the Paleodiet of Ground Sloths Using Microwear Analysis
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RECONSTRUCTING THE PALEODIET OF GROUND SLOTHS USING MICROWEAR ANALYSIS A thesis submitted to the Kent State University Honors College in partial fulfillment of the requirements for General Honors by Nicholas Arthur Resar May, 2012 Thesis written by Nicholas Arthur Resar Approved by ________________________________________________________________, Advisor ______________________________________________, Chair, Department of Geology Accepted by _____________________________________________________, Dean, Honors College ii TABLE OF CONTENTS LIST OF FIGURES............................................................................................................iv LIST OF TABLES...............................................................................................................v ACKNOWLEDGMENTS..................................................................................................vi CHAPTER I. INTRODUCTION.............................................................................................1 II. MATERIALS AND METHODS.......................................................................8 III. RESULTS........................................................................................................13 IV. DISCUSSION..................................................................................................15 V. CONCLUSSION.............................................................................................18 LITERATURE CITED......................................................................................................19 FIGURES AND TABLES.................................................................................................24 APPENDIX A. RAW MICROWEAR DATA..........................................................................29 iii LIST OF FIGURES Figure 1. Mean Microwear Values for Examined Taxa............................................24 Figure 2. Example image of dental microwear.........................................................26 iv LIST OF TABLES Table 1. Summary statistics of genera.....................................................................27 Table 2. ANOVA test...............................................................................................28 Table 3. Test of Homogeneity of Variance..............................................................28 Table 4. LSD post hoc tests for significance...........................................................28 v ACKNOWLEDGMENTS I would like to thank my parents, Dale and Donna Resar, for helping me get here, and my advisor Jeremy Green for taking me under his wing. I also thank the curators at the American Museum of Natural History, New York, and the Field Museum of Natural History, Chicago, for permitting access to tooth specimens for sampling. Finally, I would like to Dr. Carrie Schweitzer, Dr. Robert Hamilton, and Dr. Brett Ellman for serving on my defense committee. Partial funding for this project was provided by a grant from the Kent State University Undergraduate Research Scholars Program. vi CHAPTER I INTRODUCTION Xenarthrans are a group of placental mammals that include extant armadillos, tree sloths, and anteaters, as well as the extinct ground sloths and glyptodonts (McKenna and Bell 1997). Among other specialized traits, such as xenarthrous articulations of the spinal column and unique articulations of the ischium (Vizcaíno and Loughry 2008), xenarthrans are differentiated from other mammals by a lack of enamel on their adult teeth (Hillson 2005). Although several clades of placental mammals have evolved partial or complete enamel loss on their teeth (Hillson 2005; Green 2009c; Ungar 2010), xenarthrans are unique in the almost universal enamel loss within the clade (Vizcaíno and Loughry 2008). The orthodentine that composes the surface of xenarthran dentition is a softer tissue than enamel (Hillson 2005), so their teeth wear much faster compared to the enamel-covered teeth of most mammals. Because dentition is mainly used for processing food, the unique, soft, simple-shaped nature of xenarthran teeth begs the question as to what extinct members of this group were feeding upon. Although numerous in the fossil record (McDonald and De Iuliis 2008), understanding the paleoecology of extinct xenarthrans, such as ground sloths, is complicated because they lack modern ecological analogues. Ground sloths inhabited a wide range of environments, from Alaska (McDonald 1995; Hoganson and McDonald 2007) to Argentina, including the Caribbean islands 1 2 (White 1993) and even Antarctica (Vizcaíno and Scillato-Yane 1995). Feeding habits ranged from grazing (Webb 1989, Shockey and Anaya 2011) and forest browsing (McDonald 1995; Hoganson and McDonald 2007) to aquatic feeding (de Muizon et al. 2004), and they could reach large sizes (~1000–6000 kg in some taxa; Fariña et al. 1998). Modern tree sloths, however, are limited to arboreal habitats in tropical climates (Vizcaíno et al. 2008) and are relatively small compared to ground sloths (Gaudin and McDonald 2008). Previous studies have applied functional morphology and biomechanical analyses to reconstruct the diet and lifestyle of ground sloths (Vizcaíno et al. 2006; Bargo et al. 2009; Shockey and Anaya 2011). As noted by Smith and Redford (1990), anatomy is not always an accurate predictor of diet in extant xenarthrans. Therefore, it is important to pursue as many independent lines of evidence as possible when examining diet in extinct xenarthrans. A line of evidence just recently being explored more fully for xenarthran diet is dental microwear. Dental microwear is the microscopic scarring of the occlusal surface of teeth due to tooth-on-food or tooth-on-tooth interactions during food processing (Teaford 1991) and can take the form of scratches and pits of various widths and lengths. The type and number of microwear features depends on several factors. The more an animal chews its food (i.e. oral processing), the more microwear features should be deposited on the chewing (occlusal) surface of the tooth (Teaford 1991). The toughness of food particles also directly affects microwear, as tougher, more abrasive foods (e.g. grasses) are correlated with more tooth scarring (Ungar 2010). For this reason, a browser (herbivore that consumes tender leaves, fruits, etc.) should exhibit a lower density of microwear 3 features than a grazer (a herbivore that primarily eats tough, abrasive grasses), as the grazer will use more oral processing to break down tougher foods (Solounias et al. 1988; Teaford 1991; Ungar 2010). Ingested grit from either digging for food (such as roots or insects) or dust on leaves is also a major contributor to microwear formation (Williams and Kay 2001). It is also possible that the acidity of fruits in an animal’s diet will partially erase microwear (i.e. acid etching; Teaford 1988). Analysis of microwear patterns can be conducted either qualitatively (describing overall texture or complexity), or quantitatively by measuring the size and density of features (Teaford 1991). When applied to living organisms, it is possible to correlate specific diets with specific microwear patterns; these data can be used as a foundation to reconstruct paleodiet in extinct taxa. For example, Solounias et al. (1988) compared dental morphology and dental microwear in a modern giraffe (Giraffa camelopardalis, a browser) to Grant's gazelle (Gazella granti, an intermediate feeder [an herbivore that falls between browsers and grazers]) and the wildebeest (Connochaetes taurinus, a strict grazer). They found a clear relationship between the three groups, with the giraffe having the least amount of microwear, the wildebeest having the most microwear features, and the gazelle displaying an intermediate amount (Solounias et al. 1988). The authors then looked at the extinct giraffid Samotherium boissie using both the morphological evidence and microwear analysis and found that both lines of evidence pointed towards S. boissie being a grazer (i.e. S. boissie was most similar to the wildebeest in tooth morphology and number of microwear features; Solounias et al. 1988). While dental microwear is a well-established proxy for feeding patterns in 4 mammals with enamel-covered teeth, the significance of microwear on orthodentine has received comparably less attention (Oliveira 2001; Green 2009a, 2009b). However, previous studies (Oliveira 2001; Green 2009a) have shown that xenarthrans do have microwear features on their teeth that are similar to those in other mammals, which supports the application of this technique to studying xenarthran paleoecology. Previously, Green and Resar (2012; unpublished data) examined the microwear patterns of five extant Xenarthrans (Euphractus sexcinctus [Six-Banded Armadillo], Bradypus variegatus [Brown-Throated Three-Toed Sloth], Choloepus didactylus [Linnaeus’ Two- Toed Sloth], C. hoffmanni [Hoffmann’s Two-Toed Sloth], and Dasypus novemcinctus [Nine-Banded Armadillo]), which were then grouped into one of four dietary categories. Folivores consisted of B. variegatus, which consumes leaves from a narrow range of plant species (Chiarello 2008). Frugivore-folivores consisted of C. didactylus and C. hoffmanni, which eat a more varied mixture of fruits, leaves, and flowers (Chiarello 2008). Among armadillos, insectivores consisted of D. novemcinctus, and although it primarily consumes insects, some opportunistic omnivory does occur is this group (McDonough and Loughry 2008). Carnivore-omnivores consisted of E. sexcinctus due to