Osteohistological Evidence for Determinate Growth in the American Alligator

Abstract

Abstract An external fundamental system (EFS) is a form of bone microstructure present in the outermost cortex of long bones in animals that have attained skeletal maturity. It indicates an effective cessation of any significant periosteal growth (i.e., growth in circumference or girth). Although an EFS has been noted in several reptile taxa, the idea that reptiles grow continually throughout their lives remains popular. Examination of femoral bone microstructure from captive American Alligators (Alligator mississippiensis) reveals parallel-fibered tissue terminating periosteally in an EFS, thus confirming determinate growth in another reptile taxon. The results of this study have several important implications for both modern and fossil tetrapods: first, because many birds, nonavian dinosaurs, pterosaurs, and basal pseudosuchians all produce an EFS, it can be concluded that determinate growth is a shared characteristic of Archosauria; second, because the captive alligators were not senescent, an EFS shou...