The eggshell of reptiles is essential for not only protecting the embryo, but can also serve as source of calcium for embryonic skeletal development. Whereas embryonic lepidosaurs and chelonians rely on their yolk sac for calcium during development, embryonic archosaurs mobilise eggshell calcium supply to both the embryo and the yolk sac. By the time archosaurs hatch, their residual yolk sacs have a calcium content equal or greater than at time of oviposition, which is used to support post-hatching growth. To date, no study has looked into how removal of the calcareous eggshell affects embryonic development in archosaurs. I tested how the removal of the calcareous eggshell affects embryonic and hatchling growth and biomechanic function of the skeleton in embryos and hatchlings of the American alligator (Alligator mississippiensis). Experimental eggs had their eggshell manually peeled, while control eggs were sham handled but otherwise not altered. Sampling of eggs occurred on a weekly basis until the end of incubation. Embryos, yolk sacs, and eggshells were removed and analyzed for morphological, histological and biomechanical parameters. Results show that at the time of eggshell peeling yolk sac calcium reserves were sufficient for experimental embryos to develop, but animals hatched in diminutive state. Serial clearing and staining of embryos revealed that onset of bone mineralization was similar for both treatment groups. Growth trajectory of experimental hatchlings parallelled that of control animals, without compensatory growth. Experimental hatchlings were observed to have flexible lower jaws and produced a weaker bite force than control hatchlings. Cross-sections of the mandible and femoral mid-diaphysis had a significantly reduced cross-sectional area in experimental hatchlings. I conclude that loss of the calcareous eggshell during incubation leads to severe constraint on growth and biomechanics of the alligator skeleton.

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Questions as to the tempo and mode of changes in faunal composition at the Triassic/Jurassic boundary remain unanswered. Inquiries are hampered by a general paucity of sections containing correlatable Late Triassic/Early Jurassic strata. Vertebrate fossil material from Cromhall Quarry, recovered as part of an extended project centered on Late Triassic fissure infillings in southwestern Britain, was organized into a biostratigraphic scheme in 1993. This scheme could prove important in correlating Late Triassic continental strata. The Cromhall data consists of taxonomic counts from individual Stratigraphic levels within a series of fissures. Level counts were sorted into a series of sequential sedimentary and faunal associations that rest on the assumption that significant differences in faunal composition occur between individual fissures, but do not exist between individual sedimentary strata within any one fissure. Before the biostratigraphic framework could be utilized, the actual degree of similarity between the fissure faunas had to be quantified and tested. The original database, and counts from newly processed material, were subjected to Principle Components and Cluster Analyses. The analyses suggest that the majority of stratigraphic levels within any one fissure contain faunas with very similar composition. Levels from fissures that are temporally sequential contain faunas that are somewhat similar in composition. Overlap, in faunas found in strata at the top and bottom of sequential fissures, can be detected. In addition, there is a trend toward increased presence of certain taxa in the younger levels. These results indicate that the biostratigraphic framework may be used to enhance Triassic-Jurassic boundary research. / Master of Science

Les archosaures sont un clade de vertébrés comprenant les oiseaux, les crocodiliens, ainsi que de nombreux groupes fossiles. Ce groupe fait depuis plusieurs décennies l'objet d'un important débat parmi les paléontologues quant à l'évolution du thermométabolisme au sein de ses différentes lignées. L'hypothèse classique considère que seuls les oiseaux modernes sont endothermes, tandis que tous les autres archosaures sont ectothermes. L'histologie osseuse permet d'étudier plusieurs traits relatifs au thermométabolisme impossibles à mesurer sur des spécimens fossiles ; c'est pourquoi nous avons utilisé des caractères mesurés sur des coupes histologiques d'os longs.Nous nous sommes consacrés dans une première partie à une revue détaillée de la mesure du signal phylogénétique pour des caractères ostéohistologiques dans deux clades de vertébrés, ce qui nous a permis de mieux définir l'approche à suivre dans la construction de nos modèles prédictifs.Après une étude préliminaire consacrée à l'élaboration d'un modèle prédictif du taux de croissance osseuse, nous avons construit un modèle global capable de prédire directement le taux métabolique de nos spécimens fossiles. Nos résultats montrent que la majorité des archosaures de notre échantillonnage étaient endothermes. Cela implique que le dernier ancêtre commun des archosaures était probablement endotherme, et que les crocodiliens sont donc devenus secondairement ectothermes, probablement en réponse aux contraintes du milieu aquatique. Des études plus spécifiques sur la lignée des pseudosuchiens devraient permettre de déterminer à quel niveau de l'arbre phylogénétique s'est effectué le retour à cet état ectotherme. / Archosaurs are a clade of vertebrates that includes birds, crocodiles, and numerous fossil groups. This clade has been a matter of debate among paleontologists for decades concerning the evolution of thermometabolism in its different lineages. The classical hypothesis considers that only modern birds are true endotherms, whereas all other archosaurs are ectotherms. Bone histology allows to study several traits linked to bone growth rate and thermometabolism, otherwise impossible to estimate on fossil specimens; for this reason, we used characters measured on long bone histological sections.In the first section, we extensively reviewed the measure of phylogenetic signal for osteohistological features in two clades of vertebrates, which was then used to define the methodology for building our predictive models.After a preliminary study during which we built a predictive model for bone growth rate, we built a global model to predict the metabolic rate of our fossil specimens, using both histological features and phylogenetic information for each specimen. Our results show that a majority of archosaurs in our sample were endotherms. This implies that the last common ancestor of archosaurs was likely an endotherm, and that modern crocodiles became secondarily ectothermic, probably in response to their aquatic environment. More specific studies on pseudosuchians should allow to precisely identify the level of the phylogenetic tree at which the ectothermic state was acquired, as well as adaptive constraints behind this acquisition.

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