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Enlargement of sternum traits facilitated the evolution of powered flight in birds

Nature Ecology & Evolution volume 9, pages 1705–1718 (2025)Cite this article

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Abstract

An enlarged sternum with a prominent keel is a central feature of the flight apparatus of modern birds. However, sterna of near-bird dinosaurs (Pennaraptora) and early avialans are either substantially different from those of living birds or absent altogether, raising questions about how specialized sternal structures evolved in birds and how they are related to function. This remains poorly understood because of the fragmentary nature of the fossil record, and the challenges in inferring form and function from crushed fossils. We use ancestral character estimations to trace sternal trait acquisition through the bird stem group, and multivariate phylogenetic regressions to analyse relationships between sternum morphology, body mass and flight capabilities. We find that sternum evolution was episodic: basal members of Pennaraptora had proportionally small sterna, which became larger and more craniocaudally elongated in Avialae. This enlargement precedes the appearance of a midline ridge, a possible precursor of the sternal keel, in Pygostylia. Sternum size increased again in crownward Ornithuromorpha, alongside a fully formed sternal keel and enlarged caudal projections, both critical areas of flight muscle attachment. Sternal experimentation in relation to flight characteristics occurs several times throughout Pennaraptora, including within Paraves and Enantiornithes, indicating that powered flight may have evolved several times before proliferating in crown-group birds.

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Fig. 1: Fossil sterna from representative taxa included in the study.
Fig. 2: Phylogenetic generalized least squares analysis of relative sternum plate area (n = 112 extant taxa; n = 28 fossil taxa).
Fig. 3: Ancestral character estimation of relative sternum plate area.
Fig. 4: Phylogenetic generalized least squares analysis of sternum length relative to width (n = 112 extant taxa; n = 28 fossil taxa).
Fig. 5: Ancestral state estimation of discrete sternal traits (n = 112 extant taxa; n = 31 fossil taxa).
Fig. 6: The evolution of the sternum across the dinosaur–bird transition.

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Data availability

Data generated and analysed in this study are available via Dryad at https://doi.org/10.5061/dryad.jsxksn0ng (ref. 135).

Code availability

R code for analyses performed here are available via Dryad at https://doi.org/10.5061/dryad.jsxksn0ng (ref. 135).

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Acknowledgements

We would like to thank A. Bailleul, M. Wang and Z. Zhonghe at the Institute of Vertebrate Palaeontology and Palaeoanthropology, as well as H. Gibbins, C. Coy and P. Currie at the University of Alberta Laboratory for Vertebrate Palaeontology for assistance and access to fossil specimens. J. Benito and D. Field (University of Cambridge) provided the reconstruction of Ichthyornis used in this study. Useful discussions, guidance and methodological assistance were provided by G. Funston, L. Mahler, J. Weir, D. McLennan and H. Larsson. This work was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) postgraduate scholarship (PGSD3-547147-2020) to T.M.L.-M., as well as NSERC Discovery Grants to D.C.E. (RGPIN-2018-06788) and S.C. (RGPIN-2018-06747). R.B. acknowledges support from the European Union’s Horizon 2020 research and innovation programme 2014–2018 under grant agreement no. 677774 (European Research Council starting grant: TEMPO). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.

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Authors and Affiliations

  1. Department of Anatomy, New York Institute of Technology, College of Osteopathic Medicine, Old Westbury, NY, USA

    Talia M. Lowi-Merri

  2. Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada

    Talia M. Lowi-Merri, Santiago Claramunt & David C. Evans

  3. Department of Natural History, Royal Ontario Museum, Toronto, Ontario, Canada

    Talia M. Lowi-Merri, Santiago Claramunt & David C. Evans

  4. Division of Paleontology, American Museum of Natural History, New York, NY, USA

    Roger Benson

  5. Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China

    Han Hu

  6. Negaunee Integrative Research Center, Field Museum of Natural History, Chicago, IL, USA

    Jingmai O’Connor

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  1. Talia M. Lowi-Merri
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Contributions

T.M.L.-M., R.B., S.C. and D.C.E. conceived and designed the project. R.B., H.H. and J.O. provided substantial data and verified data quality. T.M.L.-M. collected morphological and stratigraphic data, conducted all analyses and interpretations and wrote the initial paper draft. T.M.L.-M., R.B., H.H., J.O., S.C. and D.C.E. edited the paper.

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Correspondence to Talia M. Lowi-Merri.

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Extended data

Extended Data Fig. 1 Discriminant analysis results.

Results from 100 iterations of the phylogenetic flexible discriminant analysis (pFDA) for the presence of forelimb propulsion ability in each of the fossil species, based on relative footprint area to body mass (N = 112 extant taxa; N = 28 fossil taxa). Blue values are from the pFDA that incorporates minimum body mass estimates, and orange values are maximum body mass estimates; corresponding dotted lines represent first quartile, median, and third quartile of estimates over 100 replicates of the analysis. X-axes represent probabilities for the presence of forelimb propulsion over 100 iterations of the analysis, and y-axes represent frequency of probabilities across 100 iterations. The red dotted line in the middle represents the 50% probability for flight presence, and the grey rectangle demarcates 0.33 to 0.67 probability, or the zone of uncertainty. Median probabilities (‘Mdn’) for flight presence are given for both body mass estimates. Values that are greater than 0.67 indicate that flight is likely present in the taxon, while less than 0.33 indicates that flight is likely absent; median probabilities in between fall in the uncertainty zone. Panels A-Z correspond with analyses performed for each fossil taxon.

Extended Data Fig. 2 Ancestral character estimation from results of discriminant analysis.

Ancestral character estimation of median probability of flight presence across the dinosaur-bird transition from the discriminant analysis based on relative sternum plate area using minimum body mass estimates (left) and maximum body mass estimates (right) (N = 28 fossil taxa). Darker colours indicate a likely presence of flight ability in the taxon, lighter colours indicate likely absence of flight ability. The extant portion of the tree was condensed into a single branch labelled ‘Neornithes’, setting the tip value at 0, which represents the relative value for the ancestor of the living bird crown group, Neornithes.

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Supplementary Information

Supplementary Results, Figs. 1–9 and Tables 1–10.

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Lowi-Merri, T.M., Benson, R., Hu, H. et al. Enlargement of sternum traits facilitated the evolution of powered flight in birds. Nat Ecol Evol 9, 1705–1718 (2025). https://doi.org/10.1038/s41559-025-02795-4

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