Micro and Nanostructural Diversity of Lizard Osteoderm Capping Tissue in Relation to Mechanical Performance

Abstract Hard tissues in biology are typically hierarchical composites. Osteoderms are mineralized dermal structures, widespread in lizards, in which a hyper‐mineralized superficial layer, the capping tissue, has recently attracted attention for its unusual structural and mechanical features, with, e.g., moduli reaching those of enamel. Here, a comparative study of osteoderms from six lizard species whose osteoderms bear capping tissue ( Heloderma horridum , Pseudopus apodus , Broadleysaurus major , Corucia zebrata , Tiliqua scincoides , and Tiliqua rugosa ) are presented using X‐ray computed tomography, nanoindentation mapping, synchrotron X‐ray diffraction/fluorescence imaging, and finite element modeling. The capping tissue is consistently more mineralized than the underlying bone across all species. Mechanical testing shows that the capping tissue is stiffer and harder than bone, but its mechanical properties range widely, from values only slightly exceeding those of bone to enamel‐like levels. Two extreme architectures are observed: H. horridum and B. major exhibit unusually large, near‐isotropically arranged crystals and exceptional stiffness, while the other species display smaller crystals, are more textured than bone, and have less extreme stiffness. This demonstrates that capping tissue is a morphologically and functionally diverse specialization, highlighting its potential role in the evolutionary adaptation of osteoderms.