Dermal armour in lizards: osteoderms more common than presumed

Skin and its derivatives form the boundary with the external environment for most animals. Despite the vital character of its function, some of its features remain poorly understood. This is particularly the case for osteoderms, which are bone plates in the skin that occur widely but inconsistently throughout the tetrapod tree of life. Elucidating their function and evolutionary history requires systematic approaches, but this is hampered by the lack of a consistent and comprehensive catalogue of osteoderm expression. Squamate reptiles (lizards and snakes) harbour the greatest diversity of osteoderms in terms of shape, distribution, and expression among living tetrapods, and thus represent an ideal study system. We conducted the first qualitative investigation of osteoderm expression in squamate reptiles. Based on 1339 micro-computed tomography samples and 584 mentions in the literature, we report here osteoderms in 46% of lizard genera, making them 85% more common than prior literature suggests. We found substantial discrepancies with prior literature in the Varanidae and the Lacertidae, where osteoderms are common or ubiquitous, respectively. In squamate reptiles as a whole, osteoderm prevalence amounts to 25%. Our findings provide the groundwork for future systematic investigations, thus potentially providing new insight into evolutionary trajectories and constraints that shaped present-day biodiversity.