Identifying life history traits in modern and extinct archosaurs utilizing osteohistology

Abstract

Interpreting dinosaur life history traits is reliant upon data from modern taxa and large, ontogenetically variable dinosaur samples. Recent advances in the study of osteohistology have increased our understanding of dinosaur life histories, although aspects of dinosaur biology remain unresolved due to an absence of foundational studies and low dinosaur taxon sample sizes. The purpose of this dissertation is to investigate bone cortical signals of life history traits and behavior in modern and extinct archosaurs. Osteohistological data can then be used to further bolster examinations of bone response to changes in loading regimes in dinosaur long bones.

Bone cortices of the North Island Brown Kiwi (Apteryx mantelli) and American Alligator (Alligator mississippiensis) are examined for cortical signals of sexual dimorphism. Aves and some non-avian dinosaurs form medullary tissue as a calcium reservoir for egg laying, while alligators mobilize calcium from cortical bone. Medullary tissue is found in the female kiwi but not in the male kiwi or alligator sample. Intracortical signals of egg laying are absent in both kiwi and alligators. Extinct archosaur sex cannot be determined from intracortical signals and, instead, continue to rely on the presence of medullary tissue.

Next, bone growth in Maiasaura (Dinosauria: Hadrosauridae) humeri is examined and compared to growth in tibiae. Cortical growth marks are counted in each humerus (n=47) and used to construct growth curves and survivorship curves. Humerus growth is best represented by a monomolecular growth curve, similar to tibiae. Likewise, survivorship data shows high mortality in first year Maiasaura and mature adults, both mortality trends are similar to those derived from tibiae. Maiasaura humeri are reliable for interpreting life history traits in Maiasaura and, likely, hadrosaur dinosaurs in general.

Finally, Maiasaura age data and humerus bone geometry is used to infer ontogenetic changes in forelimb loading regimes. Regressions of humerus micro- and macro-morphology do not support changes in loading regimes associated with locomotor habit shifts. Age data is used to test separate age groupings, and juvenile regressions also do not support shifts in forelimb function. The results further suggest locomotor shifts did not occur in hadrosaurs.