Radial porosity profiles are a powerful tool for tracing locomotor maturation in developing limb bones

Radial porosity profiles (RPP) are a new quantitative osteohistological parameter designed to capture the dynamic changes in the primary porosity of limb bones through ontogeny, providing insights into skeletal growth and functional development of extant and extinct vertebrates. Previous work hypothesized that RPP channelization-the intraskeletal alignment of RPPs across different bones resulting from similar cortical compaction patterns-indicates increasing locomotor performance of the developing limbs. By investigating RPPs in ontogenetic series of pheasants, pigeons and ducks representing distinct locomotor developmental strategies, we test this hypothesis here and show that RPPs are indeed powerful osteohistological correlates of locomotor ontogeny. Qualitative and quantitative analyses reveal strong association between RPP channelization and fledging, the most drastic locomotor transition in the life history of volant birds. The channelization signal is less clear in precocial leg function; however, when additional intraskeletal and intercohort RPP characteristics are considered, patterns related to leg precocity can also be identified. Thus, we demonstrate that RPPs can be used in future by palaeobiologists to generate breakthroughs in the study of the ontogeny and evolution of flight in fossil birds and pterosaurs. With further baseline data collection from modern terrestrial vertebrates, RPPs could also test hypotheses regarding ontogenetic postural shifts in dinosaurs and other terrestrial archosaurs.

Radial porosity profiles: a new bone histological method for comparative developmental analysis of diametric limb bone growth

In fossil tetrapods, limb bone histology is considered the most reliable tool not only for inferring skeletal maturity-a crucial assessment in palaeobiological and evolutionary studies-but also for evaluating the growth dynamics within the ontogenetic window represented by the primary bone cortex. Due to its complex relationship with bone growth and functional maturation, primary cortical vascularity is an indispensable osteohistological character for reconstructing growth dynamics, especially in the context of various developmental strategies along the precocial-altricial spectrum. Using this concept as our working hypothesis, we developed a new quantitative osteohistological parameter, radial porosity profile (RPP), that captures relative cortical porosity changes in limb bones as trajectories. We built a proof-of-concept RPP dataset on extant birds, then added fossil paravian dinosaurs and performed a set of trajectory-grouping analyses to identify potential RPP categories and evaluate them in the context of our ontogeny-developmental strategy working hypothesis. We found that RPPs, indeed, reflect important developmental features within and across elements, specimens and taxa, supporting their analytical power. Our RPPs also revealed unexpected potential osteohistological correlates of growth and functional development of limb bones. The diverse potential applications of RPPs open up new research directions in the evolution of locomotor ontogeny.