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Synchrotron tomography of a stem lizard elucidates early squamate anatomy

Abstract

Squamates (lizards and snakes) include more than 10,000 living species, descended from an ancestor that diverged more than 240 million years ago from that of their closest living relative, Sphenodon. However, a deficiency of fossil evidence1,2,3,4,5,6,7, combined with serious conflicts between molecular and morphological accounts of squamate phylogeny8,9,10,11,12,13 (but see ref. 14), has caused uncertainty about the origins and evolutionary assembly of squamate anatomy. Here we report the near-complete skeleton of a stem squamate, Bellairsia gracilis, from the Middle Jurassic epoch of Scotland, documented using high-resolution synchrotron phase-contrast tomography. Bellairsia shares numerous features of the crown group, including traits related to cranial kinesis (an important functional feature of many extant squamates) and those of the braincase and shoulder girdle. Alongside these derived traits, Bellairsia also retains inferred ancestral features including a pterygoid–vomer contact and the presence of both cervical and dorsal intercentra. Phylogenetic analyses return strong support for Bellairsia as a stem squamate, suggesting that several features that it shares with extant gekkotans are plesiomorphies, consistent with the molecular phylogenetic hypothesis that gekkotans are early-diverging squamates. We also provide confident support of stem squamate affinities for the enigmatic Oculudentavis. Our findings indicate that squamate-like functional features of the suspensorium, braincase and shoulder girdle preceded the origin of their palatal and vertebral traits and indicate the presence of advanced stem squamates as persistent components of terrestrial assemblages up to at least the middle of the Cretaceous period.

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Fig. 1: Skeleton of B. gracilis with bones digitally segmented.
Fig. 2: Cervical part of the vertebral column of Bellairsia.
Fig. 3: Right pes and pelvis of Bellairsia.
Fig. 4: Linear reconstruction of the skull of Bellairsia and its phylogenetic position.

Data availability

All computed tomography data and three-dimensional models reported in this paper are available at MorphoSource for open download at https://www.morphosource.org/projects/00000C672.

Code availability

Our phylogenetic scripts, including full analytical settings, are available at https://doi.org/10.17605/OSF.IO/WHJT7.

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Acknowledgements

The work of M.T. at UCL was financed by the Mobility Plus programme (1608/MOB/V/2017/0) from the Ministry of Science and Higher Education, Poland. We thank the John Muir Trust and NatureScot for permission to carry out fieldwork on the Elgol coast Site of Special Scientific Interest under permit. We thank S. Walsh, A. Wolniewicz and R. Butler for support and assistance with fieldwork, and the Oxford University John Fell Fund and National Museums of Scotland for funding fieldwork. We thank S. Moore-Faye for partial preparation of the specimen, and L. Parry for assistance with running MrBayes on a computer cluster. Synchrotron tomography was carried out at the European Synchrotron Radiation Facility.

Author information

Authors and Affiliations

Authors

Contributions

M.T., R.J.B. and S.E.E. planned the research. R.J.B. and E.P. collected new specimens. R.J.B. and V.F. scanned the specimens. M.T. segmented the computed tomography data, wrote the description and drafted the manuscript with R.J.B. and S.E.E. M.T. and E.P. constructed the figures. M.T., R.J.B. and S.E.E. conducted the phylogenetic analysis. All authors provided feedback on the manuscript.

Corresponding authors

Correspondence to Mateusz Tałanda or Roger J. Benson.

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Competing interests

The authors declare no competing interests.

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Peer review information

Nature thanks Juan Daza, Hans-Dieter Sues and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Preserved skeleton.

A slab with visualized preserved skeleton of Bellairsia gracilis Evans, 1998 from the Middle Jurassic, Kilmaluag Formation, Skye. a, Dorsal. b, Ventral. c, Close-up of the visualized skeleton in the rock from anteroventral view.

Extended Data Fig. 2 Orbital bones.

Bones surrounding the orbit of Bellairsia. a, Right orbit in dorsal view. b, Right orbit in ventral view. c, Left orbit in dorsal view. d, Left orbit in ventral view.

Extended Data Fig. 3 Posterior skull.

Posterior part of the skull of Bellairsia. a, Dorsal view. b, Ventral view. c, left pterygoid in dorsal view. d, left quadrate in medial view. e, left quadrate in lateral view. f, left pterygoid in ventral view.

Extended Data Fig. 4 Mandible.

Right mandible (ELGOL2016 021), dentary and premaxilla (NMS G1992.47.10) from Kilmaluag Formation, Skye; frontal, left premaxilla, left dentary, maxilla from Kirtlington. a, ELGOL2016 021 mandible in medial view. b, ELGOL2016 021 in lateral view. c, ELGOL2016 021 in dorsal view. dh, NMS G1992.47.10 preserving right dentary and premaxilla, with dentary in (d) occlusal, (f) lingual, and (h) buccal views, and premaxilla in (e) lingual and (g) buccal views. i, NHMUK PV R16331 frontal from Kirtlington in dorsal and ventral views. j, NHMUK PV R12680 left premaxilla from Kirtlington, in dorsal, lingual, and buccal views. k, NHMUK PV R12678 left dentary from Kirtlington, in posterior, lingual, and buccal views. l, NHMUK PV R12679 anterior tip of right maxilla from Kirtlington in buccal and lingual views.

Extended Data Fig. 5 Forelimb.

Left forelimb of Bellairsia. a, Whole preserved limb. bg, Humerus in various views. hj, Bones of the manus.

Extended Data Fig. 6 Molecular backbone constraint trees.

a, Constraint tree for Dataset 1 (dataset modified from ref. 24). b, Constraint tree for Dataset 2 (dataset modified from from ref. 28).

Extended Data Fig. 7 Bayesian analysis of Dataset 1 with taxa omissions.

Majority rule consensus tree from Bayesian analysis of Dataset 1 (modified from ref. 24) including molecular backbone constraint and omitting taxa with unstable phylogenetic positions that limit resolution of the consensus tree (Scandensia ciervensis and Vellbergia bartholomaei). a, Majority rule consensus including nodes with posterior probability < 0.5; b, Majority rule consensus excluding nodes with posterior probability < 0.5.

Extended Data Fig. 8 Bayesian analysis of Dataset 1 without taxa omissions.

Majority rule consensus tree from Bayesian analysis of Dataset 1 (modified from ref. 24), not omitting any taxa. a, Tree from analysis including molecular backbone constraint; b, tree from analysis with minimal backbone constraint (constraining monophyly of extant squamates relative to Sphenodon).

Extended Data Fig. 9 Bayesian analysis of Dataset 2 with molecular constraint.

Majority rule consensus tree from Bayesian analysis of Dataset 2 (modified from ref. 28) including molecular backbone constraint.

Extended Data Fig. 10 Bayesian analysis of Dataset 2 without molecular constraint.

Majority rule consensus tree from Bayesian analysis of Dataset 2 (modified from ref. 28) without molecular backbone constraint.

Supplementary information

Supplementary Information

The file includes a section on systematic palaeontology, an extended osteological description, comments on the phylogenetic analysis and results, supplementary references, and a list of synapomorphies of particular clades.

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Tałanda, M., Fernandez, V., Panciroli, E. et al. Synchrotron tomography of a stem lizard elucidates early squamate anatomy. Nature 611, 99–104 (2022). https://doi.org/10.1038/s41586-022-05332-6

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