A characterisation of the integumentary skeleton of lizards (Reptilia: Squamata) Alexander Charles Kirby DEPARTMENT OF MEDICAL PHYSICS UNIVERSITY COLLEGE LONDON (U.C.L.) A thesis submitted in accordance with the requirements for the partial fulfilment of the Doctor of Philosophy degree 21/06/2020 London, United Kingdom 1 Signed Declaration of Originality “I, Alexander Charles Kirby confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis.” 2 Abstract Osteoderms (ODs) are present within the dermis of 14 families of squamates although snakes and Sphenodontidae lack ODs. The expression of ODs within squamates has been described as highly variable and diverse since they were first reported. Some examples of squamate OD expression are as compound structures, fused together into overlapping, imbricating plates (as in Scincidae); as distorted, bent cylinders, partially overlapping one another (as in Varanidae), or as discrete, regularly tessellated, non-overlapping, polygonal beads (as in Helodermatidae), but this is not an exhaustive list. Currently, our understanding of OD structure-to-function relationships, general anatomy and internal composition remains limited. In this study, using histological staining, computed tomography, polarised light microscopy and electron microscopy, the microstructure of materials comprising ODs from multiple families of lizards is revealed. The results show that ODs are comprised of different proportions of numerous biomaterials including osteodermine, a highly mineralised, dense capping tissue on the apical surface of the osteoderm; lamellar bone rich in secondary osteons (haversian bone tissue), Sharpey-fibred bone, woven bone and parallel-fibred bone. These results are an indication that ODs of closely related families can differ substantially in terms of their overall anatomy, the structural composition of material elements and consequently their function and ontogeny. Histological variation within the biomineralised tissues is shown to relate to differences in overall OD architecture; these data provide insights into the mechanism of formation of ODs from different families and afford a direct comparison of the histological properties between the ODs of the multiple species. Attempts are made to elucidate the relationship between OD expression and other variables, such as function and phylogeny. Finally, the varied assortment of biomaterials found within ODs manifesting in diverse hierarchical structures are shown to be valuable in future translational applications, including the creation of bioinspired materials. 3 Impact Statement This research presents data on the histology of the osteoderms of lizards, both extinct and extant, from multiple families. Benefits of this research to academia are that this work contributes towards our understanding of the micro- and macro-structure of the dermal skeleton of lizards and in turn, broadens our understanding of function and development of the reptilian skeletal system and integument. I used characterisation methods more modern than those been used previously, including novel techniques using polarised light. The areas in which the research is likely to have the clearest impact are in understanding the evolutionary history and relationships of squamates and the micro or nanofabrication of bioinspired materials and protective armours. The first two chapters present results on the osteoderms of a Gila monster (Heloderma suspectum). An unusual, non-osseous tissue, osteodermine, was described here for the first time as were implications for metaplastic development of the basal Sharpey-fibred bone region. These data formed the basis of a poster presentation at the international congress on vertebrate morphology (ICVM2019). The analysis presented could be used as leverage in reptile conservation efforts, as it highlights how natural biological materials are a source of both phylogenetic data and novel materials for material characterisation and bioinspiration. Conservation is very important as habitat destruction threatens populations of Heloderma throughout most of its range and currently helodermatid lizards inhabit a small fraction of what was once suitable habitat. During the last 150 years, thousands of square miles of desert and tropical dry forest habitats have been cleared for agriculture and/or residential development in their natural habitat of seasonally dry scrub in North America. This research therefore underscores the importance of the conservation of natural habitats for lizards to protect from climate change and/or habitat destruction. In subsequent chapters, similar materials, including osteodermine, are described in the osteoderms of other families of lizards, as well as in fossil osteoderms. Data concerning implications for osteoderm development in two separate lizards, Heloderma suspectum and Varanus komodoensis were published as an article in Journal of Anatomy (Kirby et al., 2020). Information on bone development in reptiles has been shown to be a useful model for human diseases, and thus the impact of knowing more about bone formation in lizards helps us to make models for medical applications as well. 4 This research will likely bring about impact through the various disseminating outputs used during the project, including publications made in scholarly journals, presentation of the research at international conferences, and collaboration with both academics and non-academics. 5 Acknowledgements The author would like to thank all the family and friends who have supported this project over the course of my graduate career, without whom this work would have proved impossible. The author would like to express sincere appreciation to his primary supervisor, Professor Susan Evans, whose guidance, knowledge, comments, support, supervision and encouragement were paramount to completing the project. Their secondary supervisor, Professor Alessandro Olivo, was also a fantastic help with any questions and gave much needed advice and direction. Acknowledgements are also extended to Dr. Shweta Agarwal for her assistance with electron microscopy analysis. Additional thanks to Dr. Matt Hayes for his assistance with standard polarised light microscopy, SEM and TEM analysis. Prof. Alan Boyde (Queen Mary University London) was indispensable for his assistance in implementing the novel technique of multi-rotation polarised light microscopy, and for all his knowledge surrounding mineralised tissues. Dr. Mehran Moazen was a great help for anything related to mechanical engineering and as a collaborator on many side projects and papers. The fossil osteoderms were provided by Dr Jerry Hooker (Natural History Museum, London) and sectioned courtesy of Professor Roger Benson (University of Oxford, Earth Sciences). I’d also like to thank all of the undergraduate and postgraduate students for their enthusiasm and contributing their knowledge towards the project. Additional acknowledgements are made to Dr. Sergio Bertazzo, who conceived the idea for the project in association with Prof. Evans and Dr. Moazen, and assisted a great deal for the first few years. This PhD research was funded by a studentship from the Engineering and Physical Sciences Research Council (EP/1789616) and the Human Frontier Science Program (RGP0039/2019). 6 Table of Contents Abstract ......................................................................................................................... 3 Impact Statement .......................................................................................................... 4 Acknowledgements ........................................................................................................ 6 Table of Contents ........................................................................................................... 7 List of Tables ................................................................................................................ 11 List of Figures ............................................................................................................... 12 List of Abbreviations .................................................................................................... 21 Glossary of bone terminology ....................................................................................... 23 Glossary References ..................................................................................................... 25 Copyright Declaration .................................................................................................. 26 1 CHAPTER 1 Introduction to mineralised tissues ..................................................... 27 1.1 Outline of thesis structure ............................................................................ 27 1.2 Introduction to mineralised tissues ............................................................... 28 1.3 The vertebrate integumentary skeleton ........................................................ 33 1.4 Osteoderms ................................................................................................. 35 1.4.1 Evolution .......................................................................................................... 37 1.4.2 Phylogenetic information ................................................................................. 39 1.4.3 Expression .......................................................................................................