VICKARYOUS, M.K.*; HALL, B.K.; Dalhousie University, Halifax; Dalhousie University, Halifax: Osteoderm development in the American Alligator

A wide variety of modern amniotes develop localizations of skeletal material within the integument. Referred to as osteoderms, they are typically considered to represent remnants of the once extensive exoskeleton (dermoskeleton) of early vertebrates. Unusual among skeletal elements, osteoderms do not form until late in ontogeny, well after hatching/parturition. In most groups their development remains undocumented. This investigation offers preliminary information on the early ontogeny of osteoderms in the American Alligator. Each presumptive osteoderm begins as a dome of loose mesenchyme, which is gradually replaced by very dense and thick collagen bundles. These bundles are particularly well organized within the basal two-thirds of the presumptive osteoderm, forming stratified horizons that demonstrate strong positive reactions for various connective tissue stains, albeit not Alcian blue. Structurally, the stacks of collagen are arranged into either orthogonal or �herring bone� lattice-works. Towards the external surface and keel of the presumptive osteoderm the collagen bundles are thinner, poorly stratified, and stain less intensely. The first signs of ossification occur in the keels of cervical osteoderms immediately adjacent to the sagittal plane. In section, these incipient osteoderms develop within the poorly stratified, less intensely staining horizons. Incipient osteoderms do not appear to demonstrate a regular periosteum, nor is there any sign of cartilage. The unusual pattern of osteoderm development, as compared to other elements of the exoskeleton, is discussed in an evolutionary context. A possible role for metaplasia is evaluated. This work was supported by a Jurassic Foundation Grant to MKV and an NSERC Operating Grant to BKH.