Histological Diversity And Evolution Of Lizard Osteoderms

Abstract

Many reptiles reinforce the dermis with discrete mineralized organs known as osteoderms. Among lizards, osteoderms demonstrate species‐specific differences in size, shape, and distribution across the body. Whether osteoderms also vary in details of tissue composition, including the organization of the fibrillar matrix, remains unclear. Here, we investigate osteoderm histology in three species‐rich lizard groups: gekkotans (geckos), scincids (skinks), and anguimorphans (anguids, helodermatids, and related taxa). With one possible exception, all lizard osteoderms are dominated by bone tissue. Unexpectedly, representative members of all the major groups develop an enigmatic, collagen‐poor capping tissue. Gekkotan osteoderms are rare (<2% of species) and demonstrate considerable histological heterogeneity between species. This includes variation in bone matrices, and the development of additional (non‐osseous) skeletal tissues. In the gecko genus Geckolepis, putative osteoderms appear to lack bone and instead are composed of dense collagen plates capped by a vitreous collagen‐poor tissue. Unlike geckos, osteoderms are common to virtually all skinks. Most skink osteoderms are compound elements, composed of multiple conjoined smaller plates (referred to as osteodermites). Histologically, skink osteoderms are dominated by lamellar bone with well‐organized Sharpey’s fibres linking adjacent plates together and some woven and parallel‐fibred bone. Most species develop the collagen‐poor capping tissue, particularly where adjacent osteodermites articulate with one another. Osteoderms are also common to many anguimorphans, but the histology varies considerably between taxa. While anguid and helodermatid osteoderms demonstrate multiple bone matrices (woven‐fibred, parallel‐fibred, lamellar bone, and Sharpey‐fibred), Shinisaurus osteoderms are primarily woven‐fibred and Sharpey‐fibred bone, and those of Varanus are mostly parallel‐fibred bone. Expression of the capping tissue also varies, from thick and well‐defined (Heloderma) to discontinuous (e.g., several anguids) or entirely absent (Shinisaurus, Varanus). We propose that the histological diversity observed in lizard osteoderms is driven by factors including phylogeny, ecology, and function. The identity of the capping tissue remains unresolved, but our findings reveal it is far more widespread than previously considered. The results presented here provide important steps towards understanding the diversity and evolution of the skeleton in the skin.