Osteoderms, also called dermal armour, often play a role in predator defence. The presence of osteoderms is highly irregularly distributed across the squamate phylogeny and they have not been found in snakes. In this study, we searched for candidate snake species that would benefit from such armour to protect their body, focusing primarily on fossorial species with defensive tail displays. We examined the tail morphology of 27 snake species from different families using micro-computed tomography (µCT) and micro- radiography. We discovered dermal armour in four species of sand boas (Erycidae) that also feature enlarged and highly modified caudal vertebrae. This is the first description of dermal armour in snakes. Ancestral state reconstructions revealed that osteoderms likely evolved once or multiple times in Erycidae. We have not found osteoderms in any other examined snake species. Nevertheless, similar structures are known from unrelated squamate clades, such as gerrhosaurids and geckos. This supports the idea of underlying deep developmental homology. We propose the hypothesis that osteoderms protect sand boas like the "brigandine armour" of medieval warriors. We interpret it as another component of the sand boas' rich defence strategy.

Department of Anatomy 3rd Faculty of Medicine Charles University 100 00 Prague Czech Republic

Department of Histology and Embryology 3rd Faculty of Medicine Charles University 100 00 Prague Czech Republic

Department of Zoology Faculty of Science Charles University 128 43 Prague Czech Republic

Institute of Experimental and Applied Physics Czech Technical University Prague 110 00 Prague Czech Republic

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Hidden Armour: The Passive Protective Function of Caudal Osteoderms in Snakes

Hooding cobras can get ahead of other snakes in the ability to evoke human fear

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