The Function and Evolution of Osteoderms in Squamate Reptiles

Abstract

Osteoderms, bone plates in the skin, occur widely but discontinuously throughout the tetrapod tree of life. Their morphological disparity has inspired many hypotheses on possible functions. Yet, the true prevalence of osteoderms on a lower taxonomic level has remained elusive, their evolutionary history is poorly understood, and most proposed functions have never been systematically tested. A central obstacle has been the lack of a comprehensive catalogue of osteoderm expression and of quantitative comparative analyses in ecologically diverse groups. This thesis addresses these gaps through four interlinked studies in squamate reptiles (lizards and snakes), which harbour the greatest osteoderm diversity among living tetrapods. In Chapter 1, I review the literature on osteoderm function. The proposed hypotheses span protection, lifestyle and locomotion, physiology, and visual functions, yet the vast majority remain anecdotal. I identify gaps and propose future research directions, with a focus on extant non-avian reptiles and the fossil record. In Chapter 2, I present a comprehensive catalogue of phenotypic osteoderm expression in squamate reptiles based on 1339 micro-computed tomography (micro-CT) samples and 584 literature reports. Osteoderms were found in 46% of lizard genera, making them 85% more common than previously assumed. Substantial discrepancies with prior reports were identified in the Varanidae and Lacertidae, where osteoderms are common or ubiquitous, respectively. In Chapter 3, I build on this catalogue to reconstruct the evolutionary history of osteoderm expression across 320 million years of reptile evolution, covering 643 taxa including 70 extinct and outgroup representatives. I demonstrate that squamate osteoderms are the product of at least 13 independent acquisitions. These occurred largely during the Late Jurassic and Early Cretaceous, followed by rate deceleration and absence of reversions producing a state of evolutionary stability. As an exception, osteoderms were lost in the varanid stem-lineage and secondarily re-acquired in Australopapuan varanids during the Miocene. In Chapter 4, I test whether the structure of the osteoderm cover carries a lifestyle signal. Using micro-CT-based microanatomical quantification, I conduct a phylogenetically informed comparative analysis of the cephalic and cervical osteoderm cover in the Australian Egernia group. The analysis accounts for shared ancestry, allometric scaling, and competing lifestyle effects. Taxa in dry habitats possess thicker osteoderms, consistent with a role in water retention. Fossorial taxa exhibit a tighter association of their osteoderms around the neck, which implies reinforcement against mechanical strain during head-first burrowing. These results represent the first systematic support for lifestyle signals in structural traits of the osteoderm cover, suggesting functions beyond the traditionally assumed role as body armour. In the Appendix, I report on a Higher Degree Research internship with Museums Victoria. Based on an inventory of ~45 TB of existing micro-CT scan data, I proposed a strategy towards a functioning digital micro-CT library to increase the scientific impact and public outreach of museum collections. With this thesis, I chart new territory in digital collection management, science communication, and micro-CT-based quantification techniques. My comparative analysis improves our understanding of osteoderm functions and may inform future studies of the underlying mechanisms. The evolutionary reconstruction puts future discussions of the evolutionary processes underlying phenotypic osteoderm expression on a firmer footing, and the identified microanatomical correlates may serve to reconstruct past lifestyles and habitats from the fossil record. Together, these studies advance osteoderms from a poorly catalogued curiosity to a viable system linking morphological evolution with ecological function. Show more