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Scleromochlus and the early evolution of Pterosauromorpha

Abstract

Pterosaurs, the first vertebrates to evolve powered flight, were key components of Mesozoic terrestrial ecosystems from their sudden appearance in the Late Triassic until their demise at the end of the Cretaceous1,2,3,4,5,6. However, the origin and early evolution of pterosaurs are poorly understood owing to a substantial stratigraphic and morphological gap between these reptiles and their closest relatives6, Lagerpetidae7. Scleromochlus taylori, a tiny reptile from the early Late Triassic of Scotland discovered over a century ago, was hypothesized to be a key taxon closely related to pterosaurs8, but its poor preservation has limited previous studies and resulted in controversy over its phylogenetic position, with some even doubting its identification as an archosaur9. Here we use microcomputed tomographic scans to provide the first accurate whole-skeletal reconstruction and a revised diagnosis of Scleromochlus, revealing new anatomical details that conclusively identify it as a close pterosaur relative1 within Pterosauromorpha (the lagerpetid + pterosaur clade). Scleromochlus is anatomically more similar to lagerpetids than to pterosaurs and retains numerous features that were probably present in very early diverging members of Avemetatarsalia (bird-line archosaurs). These results support the hypothesis that the first flying reptiles evolved from tiny, probably facultatively bipedal, cursorial ancestors1.

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Fig. 1: Newly revealed anatomical features of S. taylori.
Fig. 2: Comparisons of selected features of S. taylori and pterosauromorphs.
Fig. 3: Time-calibrated strict consensus tree focused on Pterosauromorpha and different positions of S. taylori based on interpretations of the phylogenetic scores for the ankle.
Fig. 4: Digital 3D life reconstruction of S. taylori.

Data availability

The taxon–character data matrices for the phylogenetic analyses for TNT and MrBayes are available in Nexus and TNT formats in the Supplementary Information and in MorphoBank at https://morphobank.org/index.php/Projects/ProjectOverview/project_id/4327. The µCT datasets and videos of the six specimens of S. taylori are available in MorphoSource at https://www.morphosource.org/projects/000414456/?locale=en (the videos are also available as Supplementary Information files).

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Acknowledgements

We thank V. Fernandez, T. G. Davies and E. G. Martin-Silverstone for scanning the specimens; A. A. Chiarenza for discussion and assistance; G. Ugueto for creating the artwork that accompanies this paper; M. Humpage for the 3D reconstruction of the skeleton; A. Fitch for sharing the photograph of Raeticodactylus used in Fig. 2; and S. Hartman for permission to use silhouettes from phylopic.org. This study was supported by the Royal Commission for the Exhibition of 1851–Science Fellowship awarded to D.F. R.J.B., E.M.D., A.F., D.J.L. and P.J.V. were supported by a Leverhulme Research Project Grant (RPG-2019-365).

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Authors and Affiliations

Authors

Contributions

D.F. designed the project with input from N.C.F., S.W., R.J.B., S.L.B. and P.M.B. D.F. processed the μCT data and described the material. D.F., with the assistance of S.J.N. and P.M.B., scored the phylogenetic matrices and conducted the phylogenetic analyses. D.F. wrote the bulk of the manuscript and created the figures. P.M.B. conducted sedimentological tests on the specimens. All authors contributed to the writing, discussions and conclusions.

Corresponding author

Correspondence to Davide Foffa.

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Nature thanks Hans Dieter-Sues, Martin Ezcurra, Lawrence Tanner and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Life reconstruction of Scleromochlus taylori.

Artwork by Gabriel Ugueto (high-resolution version).

Extended Data Fig. 2 Digital rendering of Scleromochlus taylori specimens from µCT scans.

Holotype NHMUK PV R3556, dorsal view (top left); NHMUK PV R3557, ventral view (right); NHMUK PV R3914, ventral view (bottom left). Red shading highlights the skeleton traces on the digital peels, while solid red rendering indicates previously unknown body parts.

Extended Data Fig. 3 Strict consensus phylogenetic tree of analysis including indeterminate ankle scores.

Absolute and present/contradicted group bootstrap frequencies (respectively left and right above the branches) and Bremer support values (below the branches). Note that in ~95% of the most parsimonious trees Scleromochlus is found as the earliest-diverging lagerpetid (it is alternatively found as the earliest-diverging member of a lagerpetid clade also composed of Ixalerpeton, Kongonaphon and Lagerpeton).

Extended Data Fig. 4 Strict consensus phylogenetic tree of analysis using scores for an advanced fused mesotarsal ankle.

Absolute and present/contradicted group bootstrap frequencies (respectively left and right above the branches) and Bremer support values (below the branches). Note that in ~95% of the most parsimonious trees Scleromochlus is found as the earliest-diverging lagerpetid (it is alternatively found as the earliest-diverging member of a lagerpetid clade also composed of Ixalerpeton, Kongonaphon and Lagerpeton).

Extended Data Fig. 5 Strict consensus phylogenetic tree of analysis using scores for an “intermediate” mesotarsal ankle.

Absolute and present/contradicted group bootstrap frequencies (respectively left and right above the branches) and Bremer support values (below the branches).

Extended Data Fig. 6 Strict consensus phylogenetic tree of analysis based on scores for a crurotarsal ankle.

Absolute and present/contradicted group bootstrap frequencies (respectively left and right above the branches) and Bremer support values (below the branches).

Extended Data Fig. 7 Bayesian inference convergence topology trees.

The position of Scleromochlus taylori remains the same regardless of the scoring strategy of the ankle. The alternative topology within Pterosauria is found only when using the ‘crurotarsal ankle’ settings.

Extended Data Table 1 Table of measurements

Supplementary information

Supplementary Information

This file contains Supplementary Table of Contents; a URL containing a link to access Supplementary Datasets; Historical background; legends for Supplementary Videos and Supplementary References

Reporting Summary

Supplementary Video 1

Supplementary Video 2

Supplementary Video 3

Supplementary Video 4

Supplementary Video 5

Supplementary Video 6

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Foffa, D., Dunne, E.M., Nesbitt, S.J. et al. Scleromochlus and the early evolution of Pterosauromorpha. Nature 610, 313–318 (2022). https://doi.org/10.1038/s41586-022-05284-x

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