Osteoderms are dermal bone structures that support the upper layer of skin and serve as protection against the elements in a large variety of extinct and extant organisms, especially reptiles.[1] This structure is commonly called "dermal armor" and serves to protect the organism, while also helping with temperature regulation. Osteoderms represent hard tissue components of the integument, making them easy to identify in fossil examination.[2] This dermal armor is found prominently in many lizards. Some early amphibians have this armor, but it is lost in modern species with the exception a ventral plate, called the gastralia.[3][4]
Osteoderm demonstrates a slightly delayed development compared with the rest of the skeleton, as it does not appear until after hatching has occurred. Osteodermic bone develops via the transformation of the preexisting irregular, connective tissue. This mode of bone formation is identified as metaplasia. Osteoderms are not historically uniform but include a mix of tissues, including irregular calcified and un-calcified connective tissue.[5] There is a pattern of development and modification through fusion, deletions, and sinking bones. This pattern is determined by the appearance of the ossification centers. Similarities in these centers and their sequences help to show trends in development between species.[6] Between taxa, not all osteodermic tissue develop by homologous processes. It is agreed upon that all osteoderms may share a deep homology, connected by the similar properties of their dermis.[5]