Ontogeny of macroscopic morphology of limb bones in modern aquatic birds and their implications for ontogenetic ageing

Abstract

Although the assessment of ontogenetic stage of fossil and skeletal specimens (ontogenetic ageing) plays fundamental roles in avian paleontology, detailed information of postnatal ontogeny of macroscopic morphology, which is essential for ontogenetic ageing, has been lacking for the avian skeleton. In this study, the postnatal skeletal development was described for six major limb bones (humerus, ulna, carpometacarpus, femur, tibiotarsus, and tarsometatarsus) in four species of modern aquatic birds (Calonectris leucomelas, Phalacrocorax capillatus, Larus crassirostris, and Cerorhinca monocerata) from 14–28 individuals of known ontogenetic stages for each species, with emphasis on the macroscopic morphology and timing of skeletal maturity. The presence of the epiphysial ossification center at the proximal tibiotarsus was confirmed in the all species except Larus crassirostris where old chicks were poorly sampled. Epiphysial ossification centers may be variably observed in other limb bones. A distinct ossification center is observed in crista fibularis of the tibiotarsus in Phalacrocorax capillatus. Foramen nutriens is generally larger in non-adults than in adults, and in some cases there may even be multiple foramina. Longitudinal growth of bones generally ceases around the time of fledging, by which epiphyses are ossified in most cases. Limb bones of fledglings/juveniles were generally as large as those of adults, but tend to be more slender, indicating that circumferential bone growth is still in progress in these stages. Although the overall pattern of morphological variation is consistent among the species examined, one should be aware of interspecific variation in the timing of skeletal maturity, as well as inter-elemental variation in the timing of ossification of epiphyses.