The main objective of HISTOLOC was to identify quantitative osteohistological parameters in the limb bone shafts of birds that correlate with their different locomotor developmental strategies along the precocial – altricial spectrum, including the ontogenetic onset of powered flight. These correlates of ontogenetic locomotor strategies (OLS) were then to be used as the first standardisable osteohistological tool tested on modern birds that can be applied to extinct bird-like dinosaurs in an ontogenetic evolutionary context to better understand the evolution of flight at the dinosaur-bird transition. These aims were successfully achieved, as demonstrated by the two research papers resulting from this project (one published, one in review), and as detailed below.
We set out four specific research objectives:
RO1: To select, quantify and test osteohistological parameters that best reflect OLS-related intraskeletal growth and functional development in avian wing and leg bone shafts.
The fellow collected, sampled and/or analysed the bones of ontogenetic series of three bird species representing three distinct OLS strategies: (1) pheasants with precocial wing and leg development; (2) ducks with altricial wing and precocial leg development; and (3) pigeons with altricial wing and leg development. This dataset was complemented with opportunistically salvaged specimens of various bird species, including young specimens of hoatzins. Reviewing well-established concepts and literature data on bone growth and function combined with new observations made on this newly acquired dataset of birds, the fellow developed a new methodology and a novel osteohistological parameter referred to as radial porosity profiles (RPP). RPPs are trajectories designed to describe the dynamic changes in the primary porosity of limb bones through ontogeny which have a tight link with skeletal growth and functional development.
RO2: To assess how the identified osteohistological correlates of OLS change through ontogeny, with particular focus on the onset of powered flight, and whether they persist into adulthood.
Applying the RPP method developed by the fellow (see RO1), we identified important and consistent ontogenetic patterns in the RPPs of wing and leg bones in the studied modern birds that reflect their respective level of limb precocity through development. More specifically, RPP channelization – described as the intraskeletal alignment of RPPs across different limb bones resulting from similar radial cortical compaction patterns – indicates increasing locomotor performance of the developing limbs. The ontogenetic onset of flight (i.e. fledging) results in the most drastic and striking level of RPP channelization making it the strongest osteohistological correlate of the OLS-specific fledging time. In the investigated taxa, RPP differences generated by differing OLS largely disappear by adulthood due to strong RPP channelization related to completed functional maturation combined with the formation of an avascular outer circumferential bone layer and extensive medullary cavity expansion resulting in thin long bone cortices characteristic of birds.
RO3: To test whether avian osteohistological correlates of OLS can be identified in fossil species at the dinosaur-bird transition.
We re-examined the limb bone histology of five fossil maniraptoran dinosaurs (representatives of feathered dinosaur lineages leading to birds), and by generating and analysing their RPPs in the OLS-RPP context established in modern birds. In sum, RPP analysis captured the active growth in the late juvenile specimen of Eosinopteryx; however, all other studied fossil specimens were closer to or have reached adulthood, and hence showed high level of RPP channelization erasing the bone tissue archive of potential OLS differences in earlier ontogenetic stages.
RO4: To map osteohistological correlates of OLS onto phylogenetic trees leading to birds and thereby gaining further insight into the ontogeny and evolution of flapping flight.
Apart from the RPP analyses of the limb bones of the five maniraptorans mentioned above, these objectives could not be addressed in-depth due to the lack of access to further fossil specimens related to COVID-19 restrictions in 2020 and 2021 worldwide.
