Using ossification patterns and timing to determine the homology of the unique human pisiform

The pisiform and calcaneus are paralogous bones of the wrist and ankle. Both are elongated and typically form from two ossification centers with an associated growth plate in mammals. While the human calcaneus retains two ossification centers and a growth plate, the truncated human pisiform develops from a single ossification center without a growth plate. Pisiforms are highly evolvable across mammals; however, extremely short pisiforms are rare. This raises the question of whether the short human pisiform is homologous to the primary ossification centers or epiphyses in other species. To address this question, we performed an ontogenetic study of pisiform and calcaneus ossification patterns and timing, with respect to dental eruption stage, in macaques, apes, and humans (n=907). Timing of pisiform primary and secondary center ossification and fusion typically corresponds to that of the calcaneus in non-human primates, while the single human pisiform ossification corresponds to the late forming calcaneal epiphysis. The human pisiform ossifies at the same dental stages as pisiform and calcaneal epiphyses in other hominoids. Additionally, the human pisiform ossifies irregularly, much like the epiphyses of other species, and it lacks the characteristic articular surface for the triquetral seen during early ossification of the pisiform primary centers in non-human primates and mice. These data indicate that the human pisiform is homologous to the pisiform epiphysis of other species, and that humans have lost a primary ossification center and associated growth plate. This represents a substantial morphological and developmental change not only among primates, but among mammals. This project was funded by the National Science Foundation Doctoral Dissertation Improvement Grant (NSF BCS-1540418) and by the Hill Fellowship, Department of Anthropology, Pennsylvania State University.