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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Hard-Tissue Biology 2

Room: Salon G   11:30 am–1:00 pm

Moderator(s): Canoville A, Kuhn-Hendricks SM
HRD2-1  11:30 am  Environmental change, resource availability and the evolution of dental eruption patterns in artiodactyls (Mammalia: Artiodactyla). Monson TA*, Museum of Vertebrate Zoology, UC Berkeley; Hlusko LJ, Department of Integrative Biology, UC Berkeley
Abstract: Environmental change, habitat structure and resource availability have been influential in the evolution of craniodental morphology in mammals. Our work explores variation in dental eruption patterns across Artiodactyla and investigates the ecological and environmental factors driving these patterns. Terrestrial artiodactyls are globally distributed, almost exclusively herbivorous, and play significant roles in their local ecosystems. Additionally, many species are habitat specialists that respond strongly to environmental change. We examined postcanine eruption patterns in 80 genera spanning 10 families of Artiodactyla. We visually examined specimens across ontogenetic stages using earlier and/or more complete eruption of either the fourth premolar or the third molar to classify dental eruption patterns. Our ancestral state reconstruction supports that the third molar erupted last in the ancestor of Artiodactyla with a 93.5% likelihood, and that the fourth premolar erupted last in the ancestor of ruminants with 100% likelihood. Eruption of the third molar last evolved secondarily in Subfamily Caprinae, likely sometime in the Miocene. The ruminants are characterized by an unfused lower jaw and forward placement of the masseter muscle, and the ruminant dental eruption pattern may be associated with diet, jaw morphology and/or the biomechanics of chewing. The derived dental eruption pattern of the caprines is not correlated with lifespan, litter size, or body size, but is significantly associated with the habitation of mountainous ecosystems. Caprines occupy such high-elevation habitats, fairly uniquely among artiodactyls. We hypothesize that evolution of the unique dental eruption pattern in caprines is associated with limited resource availability in high-elevation mountain systems and the necessity to process a wide range of vegetation types. This work was funded by the Museum of Vertebrate Zoology and the Department of Integrative Biology at UC Berkeley.

HRD2-2  11:45 am  Convergent dental dynamics between extinct rodent-like mammals and rodents . Gomes Rodrigues H*, CR2P, MECADEV, Muséum National d’Histoire Naturelle, PARIS; Billet G, CR2P, Muséum National d’Histoire Naturelle, PARIS
Abstract: Understanding the ontogenetic dynamics of the dentition in extinct and extant mammal species is crucial to better understand the evolutionary mechanisms driving dental diversification patterns. The recently extinct group of South American notoungulates precociously (i.e. 35 Ma) acquired a range of cranio-dental innovations, some of which strikingly resemble the masticatory apparatus of rodents. For instance, they repeatedly acquired enlarged and ever-growing incisors and cheek teeth combined with a reduction of the number of teeth. Questions concerning the exact sequence of ontogenetic and morphological changes that guided the evolution toward these convergent specializations remain, however. This is difficult to assess due to the rapid and extreme simplification of mesial cheek teeth and the lack of data concerning dental generations for the most specialized taxa. The study of undescribed juvenile specimens coupled with X-Ray microtomographic data on dental replacement in diverse groups of notoungulates allowed a more accurate definition of their dental homologies. Interestingly, rodent-like notoungulates (e.g. Mesotheriidae) show dental ontogenetic dynamics convergent with some rodents in having a strong reduction of the dentition related to the gradual enlargement of distal teeth during growth, leading to mesial drift and loss of mesial teeth. Such a dental drift also occurs in some other mammals, including the closest extant relatives of notoungulates, the Perissodactyla, but to a lesser extent. These results on the dental dynamics of notoungulates combined with recent data on enamel microstructure stress the interest of studying and comparing the masticatory apparatus of notoungulates with extant rodents to provide more robust functional and paleoecological inferences. This study benefited from LabEx BCDiv fundings.

HRD2-3  12:00 pm  The effects of dental wear on hard object food breakdown . Fitton LC*, Hull York Medical School; Swan KR, Hull York Medical School; Cobb SN, Hull York Medical School
Abstract: Tooth occlusal morphology is important in food breakdown as it typically forms the initial point of contact between forces generated by the masticatory muscles and a food item. Differences in dental topography have previously been associated with differences in mechanical performance, therefore changes in dental form due to wear during an individual’s lifetime may affect function. Cercocebus atys a specialist hard object feeding primate, erupts molar teeth with high sharp cusps that wear down to form an enamel ridge surrounding a dentine pool. Individuals of all ages have been observed feeding habitually on the stress resistant seed casing of Sacoglottis gabonensis, as such it is predicted that differences in dental topography due to tooth wear are functionally neutral. In order to test this, stainless steel M1 dental models representing hypothetical wear stages in C. atys were compressed onto 3D printed hard brittle hemispheres (hollow and solid domes, representing a seed case and seed respectively) using a universal tester, and force required to initiate fracture was recorded. For the hollow dome force at initial fracture was comparable across all dental wear stages, with the exception of a decrease in force in the intermediate wear stages. For the solid dome the results were similar to the hollow but the enamel ridge model had an increase in force required (thus a decreased performance). Results suggest adult C. atys teeth with extreme wear are functionally neutral for feeding on hollow hard food items. This is beneficial given their diet but also suggests that dental wear in C. atys is not producing a dental ‘secondary morphology’ as is found in many terrestrial herbivores. Interestingly results also suggest that the dental topography at intermediate wear stages may compensate for reduced overall muscle capabilities of younger C. atys by decreasing the force required to initiate failure.

HRD2-4  12:15 pm  A biomechanical explanation for the ampullae of tyrannosaurid teeth based upon fracture mechanics. Kuhn-Hendricks SM*, Department of Biological Science, Mechanical Engineering, Florida State University; Erickson GM, Department of Biological Science, Florida State University
Abstract: The teeth of tyrannosaurid dinosaurs possess ampullae, rounded or circular structures found in the dentin of serration interdenticle sulci. The function of these structures has been a mystery. Hypotheses include: devices to 1) anchor serrations or 2) maintain the structural integrity of the tooth during feeding. Considering engineering fracture mechanics, presumably they function to decrease stress concentrations at interdenticle sulci that would cause cracking. To test this hypothesis, we conducted a combined histological and FEM analysis of ampullae in tyrannosaur teeth. Longitudinal sections of serrations were described using comparative dental histology. Additionally, a finite element model of a tyrannosaur tooth was made. This incorporated our findings on tissue distribution and their inferred material properties. Meshes with and without ampullae were loaded along the distal carina. To evaluate crack resistance, the inhomogeneous J-integral was computed for a propagating crack. Histological analysis revealed that the ampullae are composed of globular mantle dentin surrounded by sclerotic dentin. FEM results revealed stress concentrations at each interdenticle sulcus as hypothesized, predisposing the tooth to cracking. Additionally, fractures within the sections do originate between serrations, propagating through the enamel and terminating within the ampullae. By plotting the J-integral, we find that computed values decrease at the globular mantle dentin-sclerotic dentin junction, facilitating crack arrest. Additionally, pores within the globular mantle dentin act to increase the radius of the crack tip, decreasing stress concentration. Lastly, multiple fractures in the globular mantle dentine cause shielding at neighboring crack tips, causing further toughening. However, these toughening mechanisms have only been evaluated numerically and we propose material property testing will be needed to further evaluate the likelihood of any of these mechanisms.

HRD2-5  12:30 pm  Microanatomical diversity of amniote ribs. Canoville A*, Steinmann Institute for Geology, Mineralogy and Paleontology, University of Bonn, Bonn, Germany; de Buffrénil V, CR2P, Centre de Recherches sur la Paléobiodiversité et les Paléoenvironnements, Sorbonne Universités, CNRS/MNHN/UPMC, Muséum National d’Histoire Naturelle, Paris, France; Laurin M, CR2P, Centre de Recherches sur la Paléobiodiversité et les Paléoenvironnements, Sorbonne Universités, CNRS/MNHN/UPMC, Muséum National d’Histoire Naturelle, Paris, France
Abstract: Bone microanatomical diversity in extant and extinct tetrapods has been extensively studied with increasingly sophisticated quantitative methods in order to assess its ecological, biomechanical and phylogenetic significance. The majority of previous works was conducted on the appendicular skeleton, and a strong relationship has been found between limb bone microanatomy and lifestyle (e.g., aquatic, amphibious, terrestrial). This relationship has been used for paleoecological reconstructions. Few comparative studies focused on the microanatomy of the axial skeleton and its ecological signal. Here, we propose the first exploratory study of the microanatomical diversity of amniote ribs. Our comparative sample comprises 155 species of extant amniotes and encompasses the taxonomic, ecological, and body size diversity of this group. We standardized our sampling location to the midshaft of mid-dorsal ribs. Transverse sections were obtained from classical petrographic methods, as well as from X-ray microtomography. Most of the microanatomical and size parameters of the ribs display a phylogenetic signal, an expected result also observed in amniote limb bones and vertebrae. We found a significant relationship between rib cortical thickness, global compactness and lifestyle. As for the vertebrae, the development of the spongiosa in the medullary region seems to be strongly correlated with size. Even though an ecological signal was found in the inner structure of the ribs sampled, additional work is needed to document the intra-individual variability of the rib microanatomy along the rib cage and within a single element.

HRD2-6  12:45 pm  Mechanical loading and lifestyle adaption response in secondary bone tissue: A quantitative assessment of secondary osteon geometry . Mitchell J*, University of Bonn; Sander P. M., University of Bonn
Abstract: Fatigue and micro-crack damage in bones has been shown in studies on a few species to be a major contributor to bone remodeling, stimulating secondary osteon formation along the main loading direction in compact bone. Secondary osteons have also been observed to be associated with muscle attachment sites, further indicating a response to mechanical loading. Although secondary osteons can be found in numerous taxa, both extinct and extant, very few quantitative comparative analyses have been performed to test the relationship of Haversian tissue to factors such as mechanical loading across different species. This study investigates whether secondary osteon geometry (area and aspect ratio) of long bones and ribs of several taxa within Aves, Dinosauria, Mammalia and Testudines can be quantitatively correlated with mass as well as lifestyle adaptation (e.g., terrestrial versus aquatic adaptation) in a phylogenetic framework. Results indicate that secondary osteon area increases with mass, whereas more semi-aquatic species have less round secondary osteons compared to terrestrial species. Moreover, the variance in area and aspect ratio is correlated with mass and lifestyle adaptation, respectively. The larger and more varied aspect ratios in semi-aquatic to aquatic species indicate secondary osteons form at different angles to the plane of section, whereas secondary osteons of terrestrial species are more constrained to one direction. This may result from reduced loading in aquatic environments, supporting the idea that the mechanical stress on bones is a controlling factor in secondary osteon formation.

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