Using avian bone histology to trace back the evolution of flight-related locomotor ontogeny in the dinosaur–bird transition

Informations projet

HISTOLOC

N° de convention de subvention: 882758

DOI

10.3030/882758

Projet clôturé

Date de signature de la CE 21 Avril 2020

Date de début 1 Octobre 2020

Date de fin 30 Septembre 2022

Financé au titre de

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Coût total

€ 212 933,76

Contribution de l’UE

€ 212 933,76

Coordonné par

THE UNIVERSITY OF BIRMINGHAM
United Kingdom

Periodic Reporting for period 1 - HISTOLOC (Using avian bone histology to trace back the evolution of flight-related locomotor ontogeny in the dinosaur–bird transition)

Période du rapport: 2020-10-01 au 2022-09-30

Résumé du contexte et des objectifs généraux du projet

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.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

Altogether, 327 limb elements of 12 species (7 modern and 5 fossil taxa) were investigated of which 130 were prepared by the fellow in the course of this project (Table 1). Sections were examined under polarized light microscopes and digitally photographed for histomorphometric measurements.

I developed a new method and quantitative osteohistological parameter: radial porosity profile (RPP). As primary cortical vascularity is known to reflect growth dynamics, RPPs were designed to capture the relative porosity changes from the innermost to the outermost cortex in the mid-shaft of limb bones and are quantified as four-point trajectories (Fig. 1).

To identify RPP types that may correlate with the known avian locomotor developmental strategies, we used multivariate exploratory methods and trajectory analyses (Fig. 2). We found that the different RPP types correlate with the level of ontogenetic functional limb performance.
Most importantly, as functional performance increases, we observed a progressive intraskeletal alignment of RPPs as a result of similar cortical compaction patterns (Fig. 3). We referred to this pattern as RPP channelization and quantified it by the standard deviation of RPPs within and across limb bones. This provided additional support that RPPs are powerful correlates of locomotor ontogeny and standardisable in interspecific comparisons. The strongest RPP channelization coincides with fledging; the most dramatic locomotor transition in the life history of volant birds (Fig. 4).

RPPs were generated for the limb bones of five fossil bird-like dinosaurs representing crucial clades at the dawn of avian evolution (Fig. 5). As these specimens passed their fastest growth phase, they showed strong RPP channelization making detection of developmental differences difficult (Fig. 6).

Unfortunately, new fossil data could not be included which compromised evolutionary character mapping.

I presented my work at UoB in two seminars and at the virtual meeting of The Palaeontological Association’s Annual Meeting (2021, Manchester, UK) as a live-stream online talk.
This project resulted in two research articles, one published in the Royal Society Open Science (Prondvai et al. 2022), and one in press in the Journal of Morphology (Prondvai & Butler, in press).

The webpage created for this project also increases its visibility.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

RPP is the first quantitative osteohistological method verified on extant birds to yield reliable information on locomotor ontogeny. We demonstrated that RPP channelization is a powerful correlate of functional maturation of the locomotor system. Using RPPs can generate breakthroughs in the study of ontogeny and evolution of flight in fossil birds and pterosaurs.

Figure 3. RPP channelization through ontogeny in the limb bones of A, pheasants and B, pigeons

Figure 1. Measurement method for defining radial porosity profiles (RPP)

Figure 5. Phylogenetic interrelationships among the studied fossil and extant maniraptoran taxa

Figure 2. GBTM analysis results of the duck RPP dataset

Figure 6. Clustering of RPPs of the fossil paravian dataset into 3 groups by different methods

Figure 4. Comparative ontogenetic changes in the RPPs of limb bones in pheasants, ducks and pigeons

Table 1. Complete list of maniraptoran species used in this project