Kirby, Alexander Charles; (2020) A characterisation of the integumentary skeleton of lizards (Reptilia: Squamata). Doctoral thesis (Ph.D), UCL (University College London).

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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.

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