Abstract
Osteichthyans (bony fishes and tetrapods) encompass 98% of modern vertebrate species. However, our understanding of the sequence of character evolution among stem osteichthyans has been substantially limited by the fragmentary nature of known stem osteichthyan fossils1,2,3,4. Here we investigate newly discovered articulated head and trunk material of Megamastax amblyodus5, which yields previously unseen morphological details of a Silurian stem osteichthyan. Megamastax—previously interpreted as a lobe-finned fish5—exhibits distinct osteichthyan traits in the dermatocranium, such as resorptive tooth shedding and the presence of extrascapular bones. However, the arrangement of its dorsal aortae is reminiscent of crown-group chondrichthyans. The premaxilla with extensive palatal lamina and the elongated post-hypophyseal region of the braincase recall the condition in maxillate placoderms6,7,8. Crucially, the discovery of an inner dental arcade of discrete tooth cushions on individual attachment bases aligns Megamastax with the fragmentary genera Lophosteus and Andreolepis2,3,4, corroborating the previous interpretation of isolated tooth cushions as part of the jaw dentition3,9 and verifying their identity as stem osteichthyans. Phylogenetic analysis places Megamastax within the osteichthyan stem, near the osteichthyan crown-group node, and provides a framework for exploring the sequence of character acquisition along the osteichthyan stem. Together, these new findings help to bridge the morphological gap between stem gnathostomes and modern osteichthyans, offering insights into the sequence of early evolutionary steps that shaped the osteichthyan lineage.
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Data availability
All data analysed in this paper, including the phylogenetic datasets, are available as part of the Article, Extended Data Figs. 1–10 and the Supplementary Information. Supplementary data are available at Figshare (https://figshare.com/s/af44487f1dab980642a0)40.
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Acknowledgements
We thank X.-F. Lu, J.-H. Xiong, C.-H. Xiong, C.-Y. Xiong, J. Zhang and Q. Deng for fieldwork assistance and J.-H. Xiong and C.-H. Xiong for fossil preparation. We thank Y.-M. Hou and P.-F. Yin for CT scanning, NICE Paleovislab, IVPP for three-dimensional life reconstruction. This research was supported by the Open Research Programme of the International Research Center of Big Data for Sustainable Development Goals (CBAS2023ORP01), the National Science Foundation of China (42130209, 92255301). P.A. acknowledges the support of a Wallenberg Scholarship from the Knut and Alice Wallenberg Foundation.
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M.Z. designed the project. M.Z., W.Z., L.J., J.L., T.Q. Z.P. and Q.L. conducted the fieldwork, fossil preparation and curation. J.L., X.C., M.Z., P.A. and D.C. conducted CT scanning and segmenting; M.Z., J.L. and Y.Z. compiled the character matrix and performed the phylogenetic analyses; Y.Y. helped with the Bayesian analysis using the supercomputing platform; J.L., M.Z., P.A. and B.C. contributed to fossil interpretation and wrote the manuscript; J.L., B.C. and X.L. produced the figures. All authors contributed to the interpretation of the results, discussion and manuscript writing.
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Extended data figures and tables
Extended Data Fig. 1 Articulated specimen of M. amblyodus (IVPP V18499.4) in dorsal view.
a, Photo. b, Illustrative drawing. Scale bar, 2 cm. Abbreviations as in Fig. 2.
Extended Data Fig. 2 Articulated specimen of M. amblyodus (IVPP V18499.4) in ventral view.
a, Photo. b, Illustrative drawing. Scale bar, 2 cm.
Extended Data Fig. 3 Comparison of premaxilla in selected gnathostomes.
a, Maxillate placoderm Entelognathus. b, Stem sarcopterygian Achoania. c-h, Stem osteichthyan Megamastax. 3D model in anteroverntral (d), lateral (e) and ventral (g) views. f, Virtural coronal section. h, Virtural sagittal section. i, Crown sarcopterygian Eusthenopteron23. Not to scale. Diagram in a adapted from ref. 7, Springer Nature Limited.
Extended Data Fig. 4 The segmented braincase of M. amblyodus (IVPP V18499.4), based on HRCT.
a and b, In ventral view (a), and the braincase with cranial nerves are shown in yellow (b). c, Illustrative drawing of the ventral view of the braincase. The path of the lateral dorsal aorta is shown in red. d and e, In lateral view (d), and the braincase with cranial nerves (in yellow) and veins (in green) (e). f, Showing the path of the lateral dorsal aorta (in red). g, Virtural coronal section through the posterior part of braincase, showing the exits for lateral dorsal aortae. h and I, In dorsal view with the skull roof rendered transparent (h), and with segmented cranial nerves (in yellow), veins (in green) and the lateral dorsal aorta (in red) (i). Scale bar, 2 cm.
Extended Data Fig. 5 3D restoration of the articulated head of M. amblyodus (IVPP V18499.4), showing the palatoquadrate, cheek and hyomandibula.
a, The restored braincase and the palatoquadrate in ventral view. b, The maxilla in outer (upper) and inner (lower) views. c-e, The hyomandibula in dorsal (c), lateral (d), and ventral (e) views. f and h, 3D assembling of the palatoquadrate, braincase and lower jaw in outer (f) and inner (h, with the braincase invisible) views. g, The interpretation drawing of the palatoquadrate. Scale bars, 2 cm.
Extended Data Fig. 6 Transformations of jaw apparatus among osteichthyans.
a, Megamastax. b, Mimipiscis, Devonian actinopterygian. c, Eusthenopteron, Devonian sarcopterygian. In stem osteichthyans, the coronoids are formed by the separate attachment bases of tooth-cushion, each with tooth cushions on them. In crown osteichthyans, the coronoids are continuous and connected, bearing miniature teeth (b, actinopterygians), or miniature teeth with paired fangs (c, sarcopterygians).
Extended Data Fig. 7 Morphological variation of tooth cushion of M. amblyodus (IVPP V18499.4).
a–d, i–m, external views; e–h, n–r, internal views. Short arrows indicate the origin of initial teeth and the direction of median tooth files. Not to scale.
Extended Data Fig. 8 Histological section and HRCT virtual slice of M. amblyodus, showing the pore-canal network.
a and b, Photos of the lower jaw of M. amblyodus (IVPP V18499.2), a showing the scanned area. c, Virtural transverse section through the anterior part. d, Virtural horizontal section through the anterior part. e, Vertical transverse section, showing the horizontal canal, pore-canal and pore openings. c, cosmine; hc, horizontal canal; pc, pore canal; po, pore opening; vc, vertical canal. Scale bars, a and b, 5 mm; e, 1 mm; c and d are not to scale.
Extended Data Fig. 9 Articulated scales of M. amblyodus (IVPP V18499.9).
a, Photo in lateral view. b, Part of segmented articulated scales based on HRCT in lateral view. c, Selected typical scales in basal view. Scale bar, 2 cm. b and c are not to scale.
Extended Data Fig. 10 Full cladogram of the phylogenetic analyses showing the phylogenetic position of M. amblyodus.
a, Strict consensus tree of the parsimony result, Numbers above branches denote Bremer decay indices larger than zero. b, Majority rule consensus tree of the Bayesian inference.
Supplementary information
Supplementary Information (download PDF )
This Supplementary File contains a list of taxa, geological time, and references, a list of characters, Supplementary Fig. 1, full descriptions of the Supplementary Data and Video files (provided separately) and Supplementary references. Supplementary Data 1 and 2 are surface-mesh files deposited on Figshare due to their large file sizes (https://figshare.com/s/af44487f1dab980642a0)40.
Supplementary Video 1 (download MP4 )
Three-dimensional restoration model of Megamastax. 360° rotation of the three-dimensional restoration of Megamastax.
Supplementary Video 2 (download MP4 )
Life reconstruction of Late Silurian Xiaoxiang fauna. Animation showing the life restoration of Megamastax, the largest Silurian vertebrate, from 416 million years ago.
Supplementary Data 3
Nexus file of the data matrix used in the parsimony and Bayesian phylogenetic analyses.
Supplementary Data 4 (download PDF )
A large-format PDF for a phylogenetic tree depicting all character state transformations on internal nodes.
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Lu, J., Choo, B., Zhao, W. et al. Largest Silurian fish illuminates the origin of osteichthyan characters. Nature 651, 122–127 (2026). https://doi.org/10.1038/s41586-025-10008-y
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DOI: https://doi.org/10.1038/s41586-025-10008-y
