Microwear Analysis of Crab Claw Fingers: a Functional
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MICROWEAR ANALYSIS OF CRAB CLAW FINGERS: A FUNCTIONAL MORPHOLOGICAL APPROACH A thesis submitted to Kent State University in partial fulfillment of the requirements for the degree of Master of Science by Eric J. Sload August, 2014 Thesis written by Eric J. Sload B.S., Appalachian State University, Boone, NC, 2012 M.S., Kent State University, 2014 Approved by __________________________________, Rodney Feldmann, Advisor __________________________________, Daniel Holm, Chair, Department of Geology __________________________________, Janis Crowther, Dean, College of Arts and Sciences ii TABLE OF CONTENTS LIST OF FIGURES …………………………………………………………………...…iv LIST OF TABLES………………………………………………………………………...v ACKNOWLEDGEMENTS ……………………………………………………………...vi SUMMARY ...…………………………………………………………………………….1 INTRODUCTION ………………………………………………………………………..3 METHODS ....…………………………………………………………………………….7 RESULTS ……………………………………………………………………………….18 DISCUSSION …………………………………………………………………………...26 CONCLUSIONS ………………………………………………………………………..35 REFERENCES…………………………………………………………………………..37 APPENDIX………………………………………………………………………………43 iii LIST OF FIGURES Fig. 1. - SEM micrographs illustrating A) spalling, in the upper left near the denticle tip and; B) total surface destruction of a denticle……………………………….......12 Fig. 2. - Scatterplot of wear features counted versus time tumbled in sediment, classified by claw face....………………………………………………………………….……….19 Fig. 3. - SEM micrographs of the denticle surface from the right movable finger of a crusher claw of Menippe mercenaria; at 20x (B), 52x (C), and 100x (D) magnification………………………………………………………………………....24 iv LIST OF TABLES Table 1. Specimen properties for fossil claws ………..……………………………….......16 Table 2. - Output generated by generalized linear model and chi squared test in R……….....………………………………………………………………….……….19 Table 3. - Results for intraoperator error analysis completed for taphonomic study…………………………………........................................................................21 Table 4. - Results for intraoperator error analysis completed for major and minor claw study…………………………………………………………………………………..22 Table 5. - Significance values from Kruskal-Wallis test of modern claws between claw types at 20x, 52x, and 100x magnification………………………………………………….23 Table 6. - Significance values from Kruskal-Wallis test between modern and fossil crusher claws at 100x magnification…………………………………………………………..25 Table 7. - Number of microwear features through intermolt rank……………………………34 Table 8. - Raw data used for analysis of tumbled specimen in taphonomic study at low (~10x) magnification………………………………………………………………………….43 Table 9. - Raw data used for analysis of tumbled specimen in taphonomic study at high (~20x) magnification………………………………………………………………………….46 Table 10. - Raw data used in analysis of major and minor claws...…………………………..48 v ACKNOWLEDGEMENTS I would first like to thank my advisor, Dr. Rodney Feldmann for his help and support with this research, especially in regards to acquiring the materials required for making molds and casts. I would also like to thank my committee member Dr. Jeremy Green for his help and experience with microwear studies, for helping me with the statistical work, and teaching me the methodology of microwear. Dr. David Waugh and Merida Keatts helped me learn to use the scanning electron microscope at Kent State, and helped when the instrument needed fixing or maintenance. Without their aid, this research would not have been completed. I would also like to thank Ashleigh Stepp for completing the very tedious task of image randomization before each microwear analysis was carried out. Also, I thank Roger Portell at the Florida Museum of Natural History for providing the specimens used in this study. Portions of this research were presented at the Geological Society of America annual meeting in 2013, for which travel support was granted by the Graduate Student Senate at Kent State University, and GSA. vi SUMMARY Traditionally, microwear analyses have focused on scratches, pits, and other scars on the surface of the teeth of vertebrates. These methods have proven effective in reconstructing the diet of extinct and extant taxa. Such studies have been completed on a wide range of vertebrates, including conodonts, fish, non-avian dinosaurs, and mammals, exhibiting the versatility of microwear analysis. This study applies the methods used in dental microwear analyses to study the potential functional and taphonomic significance of wear patterns on the claws of the Florida stone crab, Menippe mercenaria (Say, 1818). Molds are made of the inner and outer claw surfaces, as well as the denticles using a high resolution polyvinylsiloxane compound, and casts are poured using epoxy resin. The study area was standardized by selecting the center of each claw finger molded. This replication procedure is used extensively in dental microwear studies. Scanning electron microscopy (SEM) in conjunction with the semi-automated software Microware 4.02 is used to quantify wear marks seen on cuticle. Patterns in wear features are then determined and an attempt is made to relate them to functional morphology. Number of scratches, mean scratch length, and angular dispersion were the primary factors used to make interpretations. Some differences are important to note, such as the varying hardness of decapod cuticle, as well as its softness in relation to enamel and dentine. Also, the function of claws is very different from the role teeth occupy in the life of 1 2 vertebrates. Decapod claws function to capture food, to manipulate food toward the mouthparts, and to defend against predators; claws do not masticate the food. Taphonomic effects on wear marks were investigated by tumbling a modern Menippe claw in sediment for set time intervals. Tumbling results suggest that transport in sediment does not produce new wear features and may obliterate previously deposited features. An SEM investigation was initiated using modern crab claws and fossil specimens, in which wear patterns on crusher and pincer claw types were compared statistically. These data showed no significant difference between claw types, and may have been the result of sampling site or other biomechanical and behavioral factors. This study is, to the author’s knowledge, the first of its kind attempted on an invertebrate taxon. INTRODUCTION Dental microwear studies have focused on the microscopic scratches, pits, and other wear marks found on the teeth of vertebrates. These studies are very useful for completing paleoecologic reconstructions by allowing researchers to determine feeding niche within a particular trophic system. Such work has been carried out on a number of extinct and extant taxa within the vertebrate realm, including mammals, fish, non-avian dinosaurs, and conodonts (e.g. Purnell, 1995; Purnell et al., 2006; Williams et al., 2009; Green et al., 2012). The aim of this study was to evaluate the applicability of methods previously only used for vertebrates on the claws of crabs. Dental microwear methods were used to replicate and analyze the functional surfaces of the claws of the Florida stone crab, Menippe mercenaria (Say, 1818). The claw surfaces were imaged using scanning electron microscopy (SEM) to find any microwear features. These features were statistically analyzed for significance between major and minor claw types to test for a difference in use of the two claw types present on Menippe mercenaria. The ecological role of the crab claw has been the subject of much study. A crab’s claw is an appendage used for many purposes, all of which may be categorized into three groups; foraging, agonistic behavior, and behavior associated with mating (Lee and 3 4 Seed, 1992). It is these three behaviors which apply the greatest selection pressures during the evolution of crab claws (Lee, 1995). Foraging style appears to have an effect on heterochely in crabs, with more predatory crabs typically having a larger, more robust right claw and a more gracile left claw (Abby-Kailo and Warner, 1989), while non-predatory crabs typically exhibit isochely (Lee, 1995). Lee (1995) also suggested that presence or absence of symmetry in prey items (e.g. bivalves and gastropods, respectively) may also have an influence on claw structure. The selection pressures applied by agonistic behaviors are important when competition for habitat or food occurs. These pressures appear to favor larger chelipeds, as is evidenced by the increased use of the “meral spread” display, in which the crab extends the chelipeds laterally in a show of aggression (Smith et al. 1994 in Lee, 1995). Sexual selection plays a strong role in the evolution of the crab claw. In crabs, as in many other arthropods, the claw is often used to attract mates, and is modified to do so, sometimes at the energetic expense of the crab (Lee, 1995). Some crabs take this use of the claw to the next level, using it solely for mating purposes, as in the male fiddler crab (Uca, spp. Leach, 1814), the large claw is of no use for foraging (Lee, 1995). However, in most crabs a larger claw also adds the benefit of allowing the animal to capture larger prey, so the selective pressures do not always conflict (Lee, 1995). Evidence for the importance of the claw in sexual selection comes from the obvious difference in size between males and females of crabs, with the males possessing larger claws for use in mating displays (Lee, 1995). 5 The importance of phenotypic plasticity, or the ability for the morphology of an organism to change in response to