Cortical bone remodeling and Haversian bone in amniotes: a deep-time and comparative perspective

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Bone tissue adapts and changes through the lifetime of an individual, best demonstrated by the process of bone remodeling. Bone remodeling and histological analysis of remodeled tissue are in the focus of modern medical and forensic science because of their importance in human bone development. The tissues that develop during remodeling are called secondary bone because they replace the primary bone laid down during the initial growth of the animal. In cortical bone, remodeling produces secondary osteons which are called Haversian bone if they completely replace the primary bone. Secondary osteons and Haversian bone have been observed and described in many amniotes, particularly non-avian dinosaurs, eutherian mammals, and birds. Haversian bone is present more in large, endothermic animals. However, no sufficient explanation has been offered as to the cause. Other factors that have been invoked to explain Haversian bone in extant mammals, particularly humans, such as fatigue damage, mechanical loading and physiology all play a role in bone remodeling. But the importance and influence on histology of each of these factors has yet to be explored in detail from a comparative perspective involving sauropsids, particularly dinosaurs, and birds. Studies in histology of fossil bone generally focus on primary bone histology, and thus, there is lack of detailed analyses and comparisons of secondary bone in extinct taxa. This project aims to improve the understanding of the formation of Haversian bone and the process of bone remodeling at the histological level by approaching the problem from a comparative and evolutionary perspective, using a broad range of amniotes, extinct and extant. Dinosaurs are particularly important in this comparison because they are the only large-bodied and presumably warm-blooded land animals available for comparison with mammals. By taking the different life styles, life histories, locomotory patterns, food sources and physiologies into account, we will attempt to answer how different factors influence Haversian bone development and explain the observed patterns in cortical bone remodeling. The strength of this project is the use of data from the fossil record to understand a pervasive feature of the bone biology of extant animals, including humans.