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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Symposium: Mechanisms of whole dentition patterning in extant and extinct amniotes 3

Room: Salon B   2:30 pm–4:00 pm

Moderator(s): J. Richman, L. Hlusko, & T. Grieco
DEN3-1  2:30 pm  Embryonic tooth development in an Early Jurassic dinosaur. Reisz RR*, University of Toronto Mississauga; LeBlanc ARH, University of Toronto Mississauga; Maddin H, Carleton University
Abstract: The rare occurrences of dinosaurian embryos are punctuated by even rarer preservation of their teeth. Some aspects of dinosaur embryonic development have been studied recently, but no such opportunities have presented themselves for the study of embryonic dental development. Here we report on the discovery of the oldest known embryonic jaw materials with teeth in a terrestrial vertebrate, the Early Jurassic sauropodomorph dinosaur Lufengosaurus from China. The preservation of maxillary and dentary teeth at various developmental stages permit the first detailed investigation of dental embryology in a dinosaur. Our results show that tooth development occurred early in sauropodomorph embryogenesis, resulting in several tooth replacement events prior to hatching with none of the teeth erupting. High-resolution micro-computed tomography and histology show that none of the embryonic teeth attach to the jaw, and successive generations were located within a common, large crypt. We propose that this pattern of tooth development and early replacement permitted the evolution of complex dental batteries in the giant herbivorous diplodocoid sauropods through paedomorphosis, the retention of early ontogenetic features in the adult. Similarly, ontogenetic changes in the morphology of successive generations of embryonic teeth of Lufengosaurus indicate that the pencil-like teeth within a single large crypt seen in large sauropods also evolved via paedomorphosis, suggesting that these were essential events leading to the success of the largest land dwelling animals of all time.

DEN3-2  3:00 pm  The simplification of sauropod teeth as an adaptation to herbivory. Whitlock JA*, Mount Aloysius College
Abstract: In the vast majority of vertebrates, a simple correlation can be drawn between tooth complexity, either in terms of overall shape or in terms of cusp number, and diet. As reliance on carnivory increases, tooth complexity diminishes: consider the carnassial teeth of felids and canids relative to the complex, multicusped molars of ungulates. The drivers behind this pattern of morphological evolution are perhaps obvious—animal tissue requires less oral processing due to its ease of digestion, and so teeth are largely used as prey capture devices and as tools to slice off boli of flesh small enough to be swallowed, whereas plant tissue typically locks its nutrients behind a tough-to-digest cellulose wall which must be mechanically broken down before digestion can occur. However, in the largest herbivorous animals to ever walk the earth, sauropodomorph dinosaurs, we see a refutation of this trend through evolutionary time. The earliest sauropodomorphs were small and likely omnivorous with teeth of a general archosaur shape—leaf shaped, with many marginal cusps—good for slicing both plant and animal tissue as needed. Through time, these organisms became larger and more reliant on herbivory, and the initial trend in tooth morphology appears to reflect the expected patterns. Sauropod teeth became more massive, with larger slicing and (potentially) grinding surfaces and distinct wear patterns consistent with direct occlusion. Starting in the Mid- to Late Jurassic, however, multiple clades of sauropod dinosaurs began to sharply reduce tooth size and complexity, resulting in small, peg-like teeth without evidence of direct occlusion. Although this appears to have little adaptive value for an animal requiring massive quantities of difficult to digest forage daily, studies indicate that this may have been an adaptation to increase the replacement rate of teeth in an effort to combat extreme tooth wear caused by equally extreme rates of plant ingestion.

DEN3-3  3:30 pm  The evolution of dental batteries: new insights from extinct reptiles. LeBlanc A/RH*, University of Toronto Mississauga; Reisz R/R, University of Toronto Mississauga
Abstract: Most non-mammalian amniotes exhibit very limited oral processing and have simple conical teeth. Some groups have, however, evolved dentitions that allowed them to access new food resources. The most dramatic dentition-level changes are seen in taxa possessing dental batteries, which incorporate multiple generations of teeth at a single position into a larger grinding or shearing surface. The developmental processes that underlie such dramatic changes in dental organization are poorly understood. Here we present comparative histological studies of the teeth and jaws of the Permo-Carboniferous captorhinid reptiles and Late Cretaceous hadrosaurid dinosaurs to highlight the development of two types of dental batteries. Many captorhinid taxa possess multiple rows of teeth on the dentaries and maxillae. Studying single and multiple-rowed taxa reveals that an asymmetrical pattern of jaw growth and a delay in tooth replacement are responsible for the observed diversity of dentitions in Captorhinidae. In hadrosaurids, teeth are stacked and interlocked vertically and mesiodistally into a mass of up to 300 teeth. Thin sections of complete dental batteries reveal accelerated development in the formation of tooth attachment tissues and extensive deposition of dentine. The former prevented each tooth from being replaced by its successor, and the latter allowed each tooth to contribute to the integrity of the battery and continuously erupt. These results show that in order to develop a dental battery, tooth replacement must be delayed so that older teeth can be retained. Studying these unique dentitions shows that tooth development and the replacement are actually two independent events that can be spatially separated by jaw growth or by heterochronic acceleration in tooth development. The lack of modern amniotes with dental batteries highlights the importance of studying fossils to gain insight into conserved and plastic processes of dental development and evolution.

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