Osteohistology of sutural fusion in the skulls of Archosaurs: implications for maturity assessment in non-avian dinosaurs and for the evolution of skeletal tissues

Abstract

Sutures are fibrous tissues that unite the skull bones of vertebrates. The degree of sutural closure is often used in paleontology to assess maturity in mammals and dinosaurs. Surprisingly, little is known about the biology of sutures in the closest evolutionary groups to non-avian dinosaurs: birds and crocodilians (extant archosaurs). The purpose of this dissertation is to assess, by means of morphological observations, if the degree of sutural closure is indeed an accurate method for maturity assessment in non-avian dinosaurs, and to gain a better understanding of archosaurian sutures at the microscopic scale. The order in which sutures fuse in the skulls of emus (n=24) and American alligators (n=50) reveals that sutural closure is a useful proxy for maturity in the former species but not in the latter. As growth progresses in alligators, sutures become relatively wider and more open in larger, older individuals compared to smaller, younger specimens. This pattern is previously unreported in alligators and it likely reflects skull mechanics related to feeding and not exclusively ontogeny. This indicates sutural closure is not a robust proxy for maturity in non-avian dinosaurs. Next, the histology of craniofacial sutures in these same extant species and in some non-avian dinosaurs is studied. Comparisons with mammalian sutural histology from the literature are also made. Emus and mammals possess a sutural periosteum, but it disappears rapidly during ontogeny in American alligators. The histology of the sutural mineralized tissues of non-avian dinosaurs suggest that they also lack a sutural periosteum and that their primary mode of ossification involves the direct mineralization of the fibrous soft-tissues of their sutures. The microstructural differences between sutures in archosaurs and mammals are undeniable, indicating that extant mammals are a poor analogue for investigating the growth of non-avian dinosaurs. Finally, the sutural borders of embryonic and nestling hadrosaurs are investigated. They are composed of chondroid bone, a tissue that allows rapid sutural growth in some extant species, but these sutural mineralized fronts lack secondary cartilage. Instead, secondary cartilage was only observed at jaw articulations. This tissue is found exclusively in birds within extant sauropsids, reflecting the dinosaurian origin of birds.