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Session Schedule & Abstracts
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|Sunday 3rd July, 2016|
|Moderator(s): Crofts SB, Sellers KC|
FED3-1 11:30 am Functional diversity and evolution of dicynodont (Therapsida: Anomodontia) jaw mechanics. Angielczyk K. D.*, Field Museum of Natural History; Nabavizadeh A., University of Chicago; Krentzel D., University of Chicago firstname.lastname@example.org |
Abstract: Dicynodont therapsids are one of the most successful synapsid clades aside from mammals. The dicynodont skull and mandible are highly modified relative to other therapsids to allow a palinal movement of the mandible. The basic operation of the dicynodont feeding system has been reconstructed for three taxa: Pristerodon, a basal dicynodont that retains 'postcanine' teeth, the highly autapomorphic Lystrosaurus, and the Triassic Kannemeyeria. Dicynodont skulls show high morphological disparity, much of which relates to the detailed morphology of jaw muscle attachment areas, but its functional implications are unexplored. We reconstructed m. adductor mandibulae externus medialis (= m. temporalis in mammals) and m. adductor mandibulae externus lateralis (a masseter analogue) in 32 dicynodonts and basal anomodonts by mapping origins and insertions in lateral view. We identified and connected centroids of each origin and insertion to create muscle vectors, and calculated mechanical advantage (MA) for each muscle in relation to a mesial bite point at the beak. Dicynodonts show a large range of mandibular MA values. Muscular MA values in dicynodonts are higher than in basal anomodonts, suggesting adaptive evolution of a more efficient herbivorous feeding mechanism. Among dicynodonts, MA ranges varied broadly with skull and jaw proportions. Genera with larger attachment sites for both muscles and deeper mandibles, such as Daptocephalus, Lystrosaurus, and Angonisaurus, show high MA values. In contrast, most kannemeyeriiforms present lower MA values caused by rostral displacement of the mAMEM and mAMEL insertions and low adductor angles. These notable transitions to lower adductor angles and MA values suggest a likely recruitment of m. pterygoideus musculature acting to induce a more orthal feeding stroke in the derived kannemeyeriiforms. Our results demonstrate that the radiation of dicynodonts was accompanied by a functional diversification of their feeding system.
FED3-2 11:45 am Modelling microscopic tooth wear using finite elements: indicating how abrasive particles interact with enamel surfaces . Berthaume MA*, Max Planck Weizmann Center for Integrative Archaeology and Anthropology; Kupczik K, Max Planck Weizmann Center for Integrative Archaeology and Anthropology; Schulz-Kornas E, Max Planck Weizmann Center for Integrative Archaeology and Anthropology email@example.com |
Abstract: Recent research on the etiology of microscopic dental wear has focused primarily on the agents causing wear and has not addressed the processes by which wear is formed. Here, we use finite element analysis (FEA) to investigate the process by which phyotliths cause dental wear to form on the micro scale, and how phytolith size, attack angle, and force with which it impacts the enamel can change the wear signature (e.g. microwear). A parametric FE model consisting of two enamel blocks with a spherical phytolith was constructed. Non-linear, elastic, contact simulations were run where ingesta characteristics and masticatory were taken into account. The upper enamel block was displaced the following manner: downwards towards the lower enamel block, compressing the phytolith(s), laterally, shearing the phytolith across the enamel’s surface, upwards to its original height, and laterally to its original starting position. The lower enamel block remained stationary, and the phytolith was constrained at its center with weak spring elements. The generated wear surfaces were quantified using 3D surface texture method following ISO 25178. Results indicate that changes in phytolith size affect the size of the pit formed on the enamel’s surface, but cannot make a pit look like a scratch. Decreases in attack angle cause pits to become elongated and if elongated enough, become scratches. Increases in phytolith concentration and indentation force cause higher levels of enamel wear, resulting in deeper, wider pits. These results show how factors unrelated to diet such as masticatory kinematics and bite force may play as important a role in pit and scratch formation as diet itself. In addition, the intensity of the dental microscopic wear signature (depth and number of pits, length and number of scratches) is sensitive to attack angle, particle size, particle concentration, and bite force. This research is funded by the Max-Planck-Society.
FED3-3 12:00 pm Tooth form and function of extinct durophagous reptiles, the Placodontia. Crofts S B*, New Jersey Institute of Technology; Summers A P, University of Washington, Friday Harbor Labs firstname.lastname@example.org |
Abstract: Dental morphology is one of the key characters for identifying the Placodontia, a group of extinct marine reptiles from the mid to late Triassic. Based on tooth morphology most placodonts were durophagous - predators of hard-shelled prey. The teeth are typically described as “low” or “blunted” in groups at the root of the tree and “hemispherical” or “flatter” in more nested groups. In addition to these qualitative descriptions of changes across the phylogeny, changes in tooth size, position, and replacement rate have also been documented. We are interested in quantifying differences in occlusal morphology over time and in understanding how these changes might have affected feeding. Variation in occlusal morphology can have an effect on tooth function, and we can predict a functionally ‘optimal’ shape for crushing teeth based on functional trade-offs: a flat or shallowly convex tooth which can both crush prey items and resisting tooth failure. Teeth that vary from the predicted morphology may suggest a different diet or indicate that some other aspect natural history, apart from pure function, is at play. To quantify tooth morphology we measured the functional radius of curvature (RoC) of the occlusal surface by fitting spheres to 3D surface scans. We found that palatine teeth tend to be flatter (large RoC) than maxillary teeth (small RoC). We also observed that stem taxa, which have faster, less organized replacement rates, have pointier teeth more prone to failure, than nested taxa which had teeth closer to the ‘optimal’ morphology and slower, more organized replacement rates. Within one well-nested clade, the placochelyids, the rear-most palatine teeth have a more complex morphology than predicted, with an overall concave occlusal surface and a small, medial cusp. These findings are in keeping with the theory that placodonts were specialized durophagous predators, with teeth modified to break hard prey items while resisting tooth failure.
FED3-4 12:15 pm Quantifying textures of tooth wear for dietary analysis of fishes, sharks and whales. Purnell Mark A*, University of Leicester email@example.com |
Abstract: Changes in trophic niche and/or diet are central components of many evolutionary scenarios, yet good evidence of what extinct taxa ate can be hard to come by. Analysis of tooth wear and microwear has a proven track record in analyses of dietary change and for testing hypotheses of trophic differences between vertebrate taxa, but microwear analysis has been applied almost exclusively to terrestrial mammals, particularly ruminants and primates. More recently, quantitative approaches, particularly 3D texture analysis, have been developed and applied more broadly to investigate trophic niche and test hypotheses of evolutionarily significant dietary change in a wider range of vertebrates. Recent and ongoing studies include early stem-mammals and a variety of aquatic vertebrates – sarcopterygian and actinopterygian fishes, chondrichthyans, and cetaceans. By calibrating differences in tooth wear and microtextures in fossils against extant taxa with known diets, these analyses are providing powerful new insights into aspects of niche partitioning, competition, and adaptive radiations in aquatic vertebrates that were previously beyond what the fossil record could reveal.
FED3-5 12:30 pm Ecomorphological relationships between tooth morphology and diet in varanid lizards (Squamata: Varanidae). Larson D.W.*, Philip J. Currie Dinosaur Museum; Evans D.C., Royal Ontario Museum firstname.lastname@example.org |
Abstract: The ecomorphological relationships between tooth shape and diet have not been quantitatively demonstrated in reptiles. Previously, ziphodont teeth, which are blade-like and often denticulate teeth, have been suggested to indicate a broad carnivorous diet. However, in the only group of extant ziphodont reptiles, varanid lizards, diet is highly variable and often species-specific. We predicted that variation in tooth morphology between varanid species would be reflected in dietary preferences, as dentition plays an important role in prey capture. Here, we used a quantitative multivariate redundancy analysis to find which morphological variables explained a significant portion of observed dietary data in varanids. We collected linear variables and geometric morphometric landmarks from over 150 specimens in 41 species of varanids, including members from every subgenus. Gut content abundances were collected from the literature. Results of our combined linear and geometric morphometric analysis indicate that 4 significant ecomorphological axes are present in the data governed by the relationships between 4 significant morphological variables. Using these results, varanid species can be categorized as belonging to one of 8 ecomorphs based on diet and morphology. Identified ecomorphs include 1) specialists on vertebrates, 2) terrestrial invertebrates and reptiles, 3) both terrestrial invertebrates and vertebrates, 4) general aquatic prey and terrestrial invertebrates, 5) molluscs, 6) fish, and 7) crustaceans, as well as 8) generalists that do not have a strong prey preference. Separate analysis of only the linear dataset also indicates that the preference for larger vertebrate prey is related to larger tooth denticle size. Using these ecomorphological axes, we can reconstruct the likely diets of species with unknown dietary preference data. The strength of these results suggests that species-specific ecological data can be recovered from morphological datasets.
FED3-6 12:45 pm A high-fidelity, 3D model of the skull of Alligator mississippiensis (Archosauria: Crocodylia) and its significance for vertebrate feeding biomechanics. Sellers KC*, University of Missouri; Davis JL, University of Southern Indiana; Middleton KM, University of Missouri; Holliday CM, University of Missouri email@example.com |
Abstract: Accurate modeling of cranial morphology, muscular conformation, and feeding performance is critical to understanding the functional morphology and evolution of the vertebrate skull. Although the primary components of jaw muscle, bite, and joint forces are oriented dorsoventrally during biting, the oft-overlooked mediolateral and rostrocaudal components of muscle forces are particularly important for platyrostral animals such as crocodylians. Accurate three-dimensional anatomical data are therefore crucial to understanding cranial function of not only crocodyliforms but also most other taxa with complex cranial musculoskeletal systems. This study uses CT-derived biomechanical models to characterize the ontogeny of muscle forces, moments about joint axes, and joint forces in the American Alligator. We made finite element and free body models to estimate how individual muscles contribute to bite force as well as joint forces. Muscle attachments and physiologies were modeled according to dissections and previous work. Despite challenges in modeling muscle wrapping and soft-tissue attachments, modeled bite forces still approach those measured in vivo, supporting this approach. Thus, we can use these hi-fi models to explore the functional environment of alligator, crocodyliform, and vertebrate skulls in ways not previously possible. For example, we show quantitative support for the ontogenetic shifts in jaw muscle orientation as alligator skulls flatten with age. We found the pterygomandibular joint is loaded with a greater magnitude than the primary jaw joint, suggesting crocodylians may employ a dual craniomandibular joint system. Finally, contrary to the loading of mammalian TMJs, we found the working-side jaw joint is loaded in tension during unilateral biting. These robust, three-dimensional methods now allow us to accurately test hypotheses about the loading environment of the skull, its joints, and skeletal tissues during gape cycles, ontogeny and macroevolution.
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