Abstract
A head region with specialised appendages, sclerotization, and segmentation of the trunk (arthrodization), and its appendages (arthropodization) represent three key innovations in arthropod evolution. Two scenarios have been proposed for acquisition of these innovations either in a synchronous or a sequential mode. Here we describe a new species Sunella dimorphismus sp. nov., from the Chengjiang biota (ca. 518 Ma), which displays a bivalved carapace, raptorial frontal appendages, and an arthrodized trunk with a series of biramous arthropodized appendages revealed by new specimens and new observations with assistance of computed tomography. Phylogenetic analyses placed Sunella as the earliest diverging deuteropods besides Erratus. Ancestral state reconstructions refine our understanding of the gain and loss of key characters in the euarthropod. The results demonstrate that trunk limb arthropodization preceded trunk arthrodization, with both prior to the evolution of a six-segmented functional head, while the trunk arthrodization is found to be lost in isoxyiids.
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Data availability
All source data in the paper are available in Figshare at https://figshare.com/s/12614553906408fe1a6469. The source data behind the description of carapace are available in Supplementary Data 1. The source data behind the Fig. 5 and Supplementary Figs. 5A and 6 are contained within Supplementary Data 2, while the data for the Supplementary Fig. 5B are compiled in Supplementary Data 3. The source data for the Fig. 6 and Supplementary Fig. 8 are available in Supplementary Data 4. The source data for the Supplementary Fig. 7 are available in Supplementary Data 5. The source data for the Supplementary Fig. 2A, B are available in Supplementary Data 6. Any further data required for reanalysis are available from the corresponding authors upon reasonable request.
Code availability
The measurement data for Sunella valves is included as EXCEL formatted file (Supplementary Data 1). The phylogenetic datasets necessary to run the MrBayes analyses, are included as EXCEL formatted file (Supplementary Data 2). The datasets necessary to run the convergence of Bayesian analysis are included as P formatted files (Supplementary Data 3). The datasets and the commands necessary to run the Ancestral state reconstruction analyses, are included as R formatted files (Supplementary Data 4, 5). The datasets and the commands necessary to run the Elliptical fourier analyses and linear discriminant analysis, are included as R formatted file (Supplementary Data 6). The outline of the Sunella valve from EFA analysis is provided as a JPG formatted file (Supplementary Data 6).
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Acknowledgements
We thank the comments of J. Esteve and of the anonymous reviewers, which greatly improved the work. Thanks to H. Zeng for supplying the photo of Kylinxia zhangi (YLSNHM 01124) in Supplementary Fig. 9. This work was supported by the National Key Research and Development Program of China (2025YFF0811701), National Natural Science Foundation of China (42502008 and 42372023), 111 Project (D17013), Natural Science Basic Research Plan of Shaanxi Province (2025JC–YBQN–396), and National Environment Research Council Independent Research Fellowship (NE/X017745/1).
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X. Zhang designed the research, and C. Liu performed analyses. Liu prepared the first draft, which was reedited by S. Pates, X. Zhang, D. Fu, and Y. Wu. M. Zhang and J. Ma deliberated on functional morphology. All authors interpreted specimens. Liu constructed all figures with critical insights from all authors. Liu and Pates updated and coded taxa in the character matrix. Liu led phylogenetic analyses, Pates performed elliptic Fourier analysis, and Y. Wu performed ancestral state reconstruction.
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Communications Biology thanks Jorge Esteve and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rupali Sathe.
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Liu, C., Pates, S., Zhang, M. et al. 3D morphology of the Cambrian bivalved arthropod Sunella informs about head segmentation, arthrodization, and arthropodization. Commun Biol (2026). https://doi.org/10.1038/s42003-026-09909-z
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DOI: https://doi.org/10.1038/s42003-026-09909-z
