Comparative population histovariability within the Archosauria
Ballard, HollyNoelle Woodward
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The microstructure of bone can be used to infer growth rate, physiology, and age of a taxon. Because of this, bone histology is frequently employed in studies of non-avian dinosaurs to better understand how they grew. Unfortunately, the potential for individual skeletal variability is rarely accounted for in extant studies, making interpreting the growth histories of dinosaurs somewhat ambiguous. This dissertation addresses the question of individual variability in the bone microstructure of an extant dinosaur relative, the American alligator, thus aiding in establishing a baseline from which to draw conclusions about the growth of dinosaurs. Appendicular elements of thirty three alligators demonstrate growth rate variability within elements of the same individual, and also across individuals. Regardless, no element skipped yearly growth mark deposition. Additionally, some alligators possessed an external fundamental system (EFS), revealing that skeletal maturity was achieved. This suggests that dinosaurs likely also had determinate growth. Next, the results of the alligator studies were used to reassess the interpretations of previous dinosaur paleohistology studies. Reanalysis of polar dinosaur histology using seventeen samples falsifies the hypothesis that growth marks are related to hibernation, demonstrating the danger of drawing conclusions about behavior from only two samples. The histology of Raptorex was similarly reassessed, with the microstructure here interpreted as that of an immature individual rather than a subadult or early adult. Finally, a new histological study on the dinosaur Maiasaura demonstrates the growth rate variability within a taxon that often goes unaccounted for in smaller sample sets. With forty eight tibia specimens, this represents the largest single-element dinosaur histology study to date. In general, the first growth mark, indicating yearlings, appears in animals that would have been between 2.5 m and 3.5 m in body length. The largest individuals have not only LAGs but also EFS. Growth appears to plateau between nine and ten years, supporting the findings of the original Maiasaura growth study. However, this study also shows there is a considerable degree of individual variation in growth rates, and cautions against constructing generalized growth curves using only a handful of specimens.