Largest Silurian fish illuminates the origin of osteichthyan characters
Article Date: 04 March 2026
Article URL: https://www.nature.com/articles/s41586-025-10008-y
Article Image: https://www.nature.com/articles/s41586-025-10008-y/figures/7
Summary
New material and high-resolution CT scans of Megamastax (notably M. amblyodus) — the largest known Silurian vertebrate — reveal detailed cranial, jaw and scale anatomy that bear key osteichthyan (bony-fish) characters. The study presents 3D restorations of the skull, braincase and jaw apparatus, documents tooth cushions and coronoid structure, pore-canal networks in the dermal skeleton, and articulated scales. Phylogenetic analyses (parsimony and Bayesian) place Megamastax on the stem of Osteichthyes, indicating several hallmark bony-fish features were already present in the Late Silurian. Extensive supplementary data, 3D surface meshes and videos are provided (Supplementary Information; Figshare deposit).
Key Points
- Megamastax amblyodus is confirmed as the largest Silurian vertebrate and shows unexpectedly derived osteichthyan-like anatomy.
- HRCT scanning and 3D segmentation reveal detailed braincase, palatoquadrate, hyomandibula and marginal jaw bones.
- The lower jaw bears tooth cushions/coronoids and a pore-canal network reminiscent of early bony fishes.
- Articulated scales and dermal skeleton histology support comparisons with early osteichthyan scale types.
- Phylogenetic analyses place Megamastax as a stem osteichthyan, implying key bony-fish characters evolved earlier than previously thought.
- Data transparency: full matrices, surface meshes and videos are available in the Supplementary Information and on Figshare.
- Authors used a combination of field collection, fossil preparation, CT segmentation and both parsimony and Bayesian methods to test relationships robustly.
Content summary
The paper describes new specimens of Megamastax and uses high-resolution CT to reconstruct its skull and jaw elements in three dimensions. The authors identify osteichthyan-like marginal jaw bones (premaxilla/maxilla), composite coronoid structures with tooth cushions, and a complex dermal pore-canal system. These morphological details are integrated into a comprehensive character matrix and analysed under parsimony and Bayesian frameworks; the resulting trees place Megamastax on the osteichthyan stem. The combination of anatomical detail and rigorous phylogenetics leads the authors to argue that several defining bony-fish characters originated by the Late Silurian. Supplementary meshes and videos enable independent inspection and 3D visualisation of the material.
The study is heavily supported by CT-based visualisations (Extended Data figures) and supplemental surface-mesh files on Figshare. The work is a multi-institution collaboration, led from Chinese institutions with contributions from Uppsala and Flinders, among others. All primary data and the nexus file for phylogenetic analyses are provided.
Context and relevance
This finding matters because it pushes the appearance of several osteichthyan characters deeper into the Silurian, narrowing the gap between early jawed vertebrates and the later radiation of true bony fishes. It interacts directly with ongoing debates about jaw evolution, the sequence of acquisition of dermal and marginal jaw bones, and the timing of major vertebrate divergences. For researchers tracking the deep history of vertebrate anatomy, evolution and macroevolutionary timing, the detailed CT datasets and phylogenetic matrices are a rich resource.
Author’s take
Punchy: this paper is a proper game-changer for early vertebrate studies — big fossil, big implications. If you work on jaw evolution, early fish anatomy or macroevolutionary timing, the 3D data and phylogenetic argument here deserve careful reading; the work substantially tightens where key bony-fish traits first appear.
Why should I read this?
Short answer: because a gigantic Silurian fish with CT-backed anatomy rewrites when important bony-fish traits showed up. The write-up is dense but the authors include meshes and videos so you can eyeball the evidence yourself — saves you the grind of hunting through supplementary folders.