in Ecology

Evolution of terrestrial anatomy revealed by a derived stem tetrapod

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Abstract

Tetrapods (limbed vertebrates) include lissamphibians and amniotes, and now comprise half of all vertebrate species, with particular importance on land. Limbs first appeared in stem tetrapods by the Late Devonian, 365 million years ago1,2,3,4,5. However, many taxa either have limbs with fin-like functional anatomy or have secondarily lost them, raising questions about the ecological context and early evolution of the tetrapod limb6. Here we provide considerable new data on the enigmatic Early Carboniferous Westlothiana lizziae, widely cited as the earliest terrestrially adapted stem amniote7,8. We find unexpected plesiomorphies of the skull roof, palate, braincase and mandible, alongside aquatic adaptations, including an ossified internal gill skeleton and extensive denticles on the palate and mandible. These traits return Westlothiana as a stem tetrapod in our phylogenetic analyses, demonstrating that terrestrial adaptations such as an amniote-like pedal formula, loss of fin-like forelimb function and superficially claw-like terminal phalanges evolved in a mosaic fashion, and that these traits evolved in a transitional, amphibious context before the origin of the tetrapod crown group. Our findings reinforce recent suggestions of a taxonomically inclusive tetrapod-stem lineage, but also illuminate substantial gaps in anatomical knowledge, especially for small-bodied taxa, that obstruct our understanding of early tetrapod evolution.

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Fig. 1: W. lizziae, NMS G.1990.72.1, specimen, renders of skull, foot and vertebrae.
Fig. 2: Skull and mandible of W. lizziae.
Fig. 3: Time-calibrated phylogeny of early tetrapods.

Explore related subjects

Discover the latest articles and news in related subjects.

  • Evolution

  • Palaeoecology

  • Palaeontology

  • Phylogenetics

  • X-ray tomography

  • Geometric Morphometrics in Phylogenetic Analysis

Data availability

Files related to the phylogenetic dataset are available as supplementary files and stored on OSF at https://osf.io.xnm6t. Reconstructions and anatomical information of NMS G.1990.72.1 Computed tomography data and 3D models are available on Morphosource: individual elements, https://www.morphosource.org/concern/media/000672184; skull reconstruction, https://www.morphosource.org/concern/media/000736020.

Code availability

Nexus scripts for Bayesian phylogenetic analyses and R code for conducting ancestral state reconstruction are stored with OSF at https://osf.io/xnm6t.

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Acknowledgements

We acknowledge the European Synchrotron Radiation Facility for providing beam time on ID19 for proposal ls2687. We thank S. Walsh and the National Museums of Scotland for promoting the study of Westlothiana, and M. Lowe, U. Göhlich, C. Bird, M. Day and B. Simpson for allowing us to study the specimens under their care. We thank M. Ellison for assistance with the photo-mosaic of the skull (Extended Data Fig. 2). We thank J. Schick for the artistic reconstruction of Westlothiana used in this study.

Funding

B.I. was supported by a Natural Environment Research Council Doctoral Training Partnership award no. NE/S007474/1. This work was supported by ESRF award no. ls2687 to R.B. We also thank the National Science Foundation of the United States (NSF EAR-PF award no. 2518154 to X.J.). Our work was further supported by the Natural Environment Research Council grant nos. NE/S000739/1 and NE/W007576/1 to J.H. and NE/L002507/1 to L.R., as well as the Biotechnology and Biological Sciences Research Council under grant no. BB/X014819/1 to L.R.

Author information

Authors and Affiliations

  1. Department of Earth Sciences, University of Oxford, Oxford, UK

    Ben Igielman

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

    Xavier Jenkins & Roger Benson

  3. Department of Zoology, University of Cambridge, Cambridge, UK

    Jason Head

  4. University Museum of Zoology, Cambridge, UK

    Jason Head & Timothy Smithson

  5. European Synchrotron Radiation Facility, Grenoble, France

    Vincent Fernandez

  6. Natural History Museum, London, UK

    Lucy Roberts

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  1. Ben Igielman
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  2. Xavier Jenkins
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Contributions

B.I., X.J., R.B. and T.S. planned the research. R.B. and V.F. scanned the specimen. B.I. and X.J. segmented the data and wrote the description. B.I., L.R., X.J. and R.B. conducted the phylogenetic analyses. B.I., X.J., R.B., T.S. and J.H. wrote the paper, with comments from all authors.

Corresponding authors

Correspondence to
Ben Igielman or Xavier Jenkins.

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The authors declare no competing interests.

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Nature thanks Claudia Marsicano 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 The holotype specimen of Westlothiana, NMS G.1990.72.1.

a, counterpart; b, part. Photographs by Roger Benson. Scale bar, 10 mm.

Extended Data Fig. 2 Cranial material of Westlothiana lizziae, based on NMS G.1990.72.1.

a, Part, b, Counterpart, photographed by Roger Benson, with assistance from Mick Ellison. Renderings of the right and left maxillae; c, right maxilla in lateral view, d, right maxilla in medial view, e, left maxilla in lateral view, f, left maxilla in medial view. g, tomographic image generated using synchrotron X-ray computed micro-tomography (see Methods) of the maxillary dentition, showing the infolding at the base of the teeth; Right lacrimal; h, lateral view, i, posteromedial view. Skull in; j, dorsal view. k, ventral view. Labels for panel j are provided in Fig. 1. Right quadrate; l, lateral view; m, medial view. n, Temporal region in postero-dorso-lateral view (i.e. above and behind the back, right corner of the skull), demonstrating the relationship of the different bones of this area, o, reconstructed braincase of NMS G.1990.72.1 in left lateral view; p, exoccipital and basioccipital in posterior view; q, frontals in ventral view. Anterior is oriented towards the right. Scale bars, a,b, j, k, 5 mm, o, 3 mm. Abbreviations: Anterior coronoid (Ac), Angular (Ang), Articular (Art), Cranial Nerves V and II (CnV, CnII), Dentary (De), Ectopterygoid (Ect), Exoccipital (ex), Jugal (Ju), Maxilla (Max), Middle Coronoid (Mc), Orbitosphenoid (Obs), Parietal (P), Prearticular (Pa), Palatine (Pal), Posterior coronoid (Pc), Postorbital (Po), Postfrontal (Pof), Premaxilla (Pmx), Postparietal (Pp), Parasphenoid (Ps), Pterygoid (Pt), Quadrate (Q), Surangular (Sa), Supratemporal (St), Tabular (Tab), Vomer (Vom).

Extended Data Fig. 3 Palatal elements of Westlothiana, based on NMS G.1990.72.1.

Renderings of the premaxilla and vomer, a, premaxilla in anterior view. b, premaxilla in anteroventromedial view. c, premaxilla and vomer in palatal view, illustrating contact between them. Palatine, d, ventral view; e, posterolateral view, demonstrating the position of the canal that carries the maxillary artery and nerve. Renderings of the palatal fangs, f, fangs of the vomer. g, fangs of the palatine. Right pterygoid, h, posterior view i, ventral view, j, medial view in the region of the basal articulation. Parasphenoid, k, ventral view, l, posterior view, m, anterior view, n, dorsal view. Hyobranchial in o, ventral and p, dorsal views. Ceratobranchial in q, ventral and r, dorsal views. Scale bars, 1 mm. Abbreviations: Pmx, premaxilla, Vom, vomer.

Extended Data Fig. 4 The atlas-axis complex (a-j) and dorsal vertebrae (k-n) of Westlothiana, NMS G.1990.72.1, renderings and drawings.

a, b, dorsal view. c, d, right lateral view. e, f, Ventral view. g, h, anterior and posterior views respectively, renderings. i, j, anterior and posterior views, k, left lateral view of 9-11th dorsal vertebrae. l, 11th dorsal vertebrae in anterior and m, posterior views. n, ventral view of 9-11th dorsal vertebra, reconstruction. Scale bar = 1 mm. Abbreviations: Axc, axial centrum; Axn, axial neural arch; At, atlas; Ic, intercentrum; Na, neural arch; Nc, notochordal canal, Pc, pleurocentrum, Prat, Proatlas.

Extended Data Fig. 5 The forelimbs of Westlothiana, NMS G.1990.72.1.

As preserved, both forelimbs have been displaced relative to the trunk and are preserved in opposite orientations. a, left forelimb in dorsal view, right forelimb in ventral view. b, left forelimb in ventral view, right forelimb in dorsal view. Left humerus, renderings and drawn reconstructions: c, d, dorsal view. e, f, ventral view, g, proximal view, h, anterior view, i, posterior view, j, distal view. Right forelimb (right radius and right ulna), renderings and drawn reconstructions, k, l, dorsal view, m, n, ventral view. Right ulna, o, anterior view, p, posterior view. Right radius, q, anterior view; r, posterior view. Scale bars, 1 mm. Abbreviations: Rad, Radius, Ul, Ulna.

Extended Data Fig. 6 Pelvis and hindlimbs of Westlothiana, NMS G.1990.72.1.

Pelvis and hindlimbs as preserved in NMS G.1990.72.1, dorsal view, a, composite rendering of pelvis and hindlimb elements from both the slab and counterslab. b, drawn reconstruction. Dark grey indicates damage and missing bone. Claw-like digit IV terminal phalanx in c, dorsal, d, ventral, and e, lateral view. Renderings and reconstructions of the left femur, f, rendering in dorsal view, g, drawn reconstruction in dorsal view, h, rendering in ventral view, i, drawn reconstruction in ventral view, j, anterior view, k, posterior view, anterior is oriented towards the left, l, proximal view, m, distal view, dorsal is oriented upwards. Scale bars = 1 mm. Abbreviations: Femur (Fe), Fibula (Fi), Puboischiadic plate (Pui), Tibia (Ti).

Extended Data Fig. 7 Bayesian analyses of early tetrapods.

Majority rule consensus tree from Bayesian analyses a, this study, b, Mann et al. (2026), and c, Clack et al. (2019) using the time-calibrated FBD model including nodes with posterior probability < 0.50 labelled beneath the nodes. Placement of Westlothiana indicated by yellow star.

Extended Data Fig. 8 Ancestral character estimation of cranial traits associated with aquatic ecologies.

Reconstruction was performed under the best-supported MkV model (ER, or equal rates model). Probabilities of presence or absence of traits, indicated by numbers next to nodes, mapped onto summarized topology resulting from Bayesian phylogenetic analysis. a, sensory (or lateral line) canals; b, branchial arches with groove for afferent gill artery; c, prearticular shagreen and palatal fangs, and d, gill arch articulation with braincase.

Extended Data Fig. 9 Ancestral character estimation of postcranial traits associated with aquatic or transitional ecologies.

Reconstruction was performed under the best-supported Mk model (ER, or equal rates model). Probabilities of presence or absence of traits indicated by numbers next to nodes, mapped onto summarized topology resulting from our Bayesian analysis. a, L-shaped humeri. b, limb reduction and body elongation.

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This Supplementary Information file contains the following sections: (1) systematic palaeontology; (2) specimens and literature examined; (3) character list; (4) spreadsheet of taxon ages; (5) synapomorphies of selected clades; and (6) supplementary references.

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Igielman, B., Jenkins, X., Head, J. et al. Evolution of terrestrial anatomy revealed by a derived stem tetrapod.
Nature (2026). https://doi.org/10.1038/s41586-026-11090-6

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