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Galeaspid anatomy and the origin of vertebrate paired appendages

Abstract

Paired fins are a major innovation1,2 that evolved in the jawed vertebrate lineage after divergence from living jawless vertebrates3. Extinct jawless armoured stem gnathostomes show a diversity of paired body-wall extensions, ranging from skeletal processes to simple flaps4. By contrast, osteostracans (a sister group to jawed vertebrates) are interpreted to have the first true paired appendages in a pectoral position, with pelvic appendages evolving later in association with jaws5. Here we show, on the basis of articulated remains of Tujiaaspis vividus from the Silurian period of China, that galeaspids (a sister group to both osteostracans and jawed vertebrates) possessed three unpaired dorsal fins, an approximately symmetrical hypochordal tail and a pair of continuous, branchial-to-caudal ventrolateral fins. The ventrolateral fins are similar to paired fin flaps in other stem gnathostomes, and specifically to the ventrolateral ridges of cephalaspid osteostracans that also possess differentiated pectoral fins. The ventrolateral fins are compatible with aspects of the fin-fold hypothesis for the origin of vertebrate paired appendages6,7,8,9,10. Galeaspids have a precursor condition to osteostracans and jawed vertebrates in which paired fins arose initially as continuous pectoral–pelvic lateral fins that our computed fluid-dynamics experiments show passively generated lift. Only later in the stem lineage to osteostracans and jawed vertebrates did pectoral fins differentiate anteriorly. This later differentiation was followed by restriction of the remaining field of fin competence to a pelvic position, facilitating active propulsion and steering.

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Fig. 1: Holotype of T.vividus.
Fig. 2: The postcranial anatomy of galeaspids.
Fig. 3: Results of CFD simulations.
Fig. 4: Ancestral-state estimation analysis for the evolution of paired appendages in vertebrates.

Data availability

All data analysed in this paper are available in the Article, Extended Data Figs. 17 and Supplementary Data 15. The nomenclatural acts in this publication have been registered at ZooBank (LSID: urn:lsid:zoobank.org:pub: BD7A6929-33DE-4DDD-ADE2-51A67A489E1B).

Code availability

The R script used for the analyses is available as Supplementary Data 5.

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Acknowledgements

We thank R. Freitas for helpful discussion on mechanisms of fin development, R. Zhao, X. Shan, X. Lin, L. Peng, L. Jia, Q. Wang, Q. Wen, Q. Rao, Y. Zhao, Q. Xue, Z. Xian, X. Meng, Y. Luo, Y. Yan, H. Wang, Q. Deng, J. Xiong, C. H. Xiong, C. Y. Xiong, J. Zhang, Y. Chen, Z. Zhou and L. Nie for the fieldwork assistance, J. Xiong for the specimen preparation, J. Rong and Y. Wang for discussion on stratrigraphy, A. Shi for drawing the interpretive illustrations, Q. Zheng for drawing the artistic life restoration, D. Yang for generating the 3D reconstruction, L. Peng and L. Jia for photographing the fossil and X. Jin for scanning electron microscopy imaging. This work was supported by the National Natural Science Foundation of China (42130209, 41972006, 42072026), the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-DQC040), the Strategic Priority Research Program of CAS (XDA19050102, XDB26000000), the National Program for support of Topnotch Young Professionals and Mee-mann Chang Academician Workstation of Yunnan province. P.C.J.D. was funded by the Natural Environment Research Council (NE/G016623/1, NE/P013678/1), the Biotechnology and Biological Sciences Research Council (BB/T012773/1) and the Leverhulme Trust (RF-2022-167). H.G.F. was funded by the European Commission through a Marie Skłodowska-Curie Research Fellowship (H2020-MSCA-IF-2018-839636). J.N.K was funded by ERC grant no. 788203 (INNOVATION).

Author information

Authors and Affiliations

Authors

Contributions

M.Z. and P.C.J.D. conceived the project. M.Z., J.W., Z.G. and Q.L. conducted the fieldwork, fossil preparation and fossil curation. Z.G., P.C.J.D., M.Z. and H.G.F. contributed to fossil interpretation and wrote the manuscript. H.G.F. and P.C.J.D. conducted computational fluid-dynamics analyses. J.N.K. undertook the ancestral-state reconstruction analyses. All authors edited and approved the manuscript.

Corresponding authors

Correspondence to Philip C. J. Donoghue or Min Zhu.

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Nature thanks John Dabiri, Matt Friedman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Geological setting of Tujiaaspis vividus.

a, Maps of the two fossil localities in Xiangxi and Xiushan Tujia and Miao Autonomous Prefecture (County). b, Horizon of the fish-bearing Huixingshao Formation.

Extended Data Fig. 2 The postcranial anatomy of Tujiaaspis vividus.

uncoated counterpart (b) with an interpretative drawing (c), the paratype, IVPP V27410 (right, in dorsal view) and V27411 (left, in ventral view). d, Close-up of the tail magnified from the box region of (b), in lateral view. e, Close-up of the tail of the holotype, IVPP V26668, in lateral view. Abbreviations as in Figs. 1, 2.

Extended Data Fig. 3 The postcranial anatomy of another new form of Eugaleaspidiformes from the same locality and horizon with the holotype of Tujiaaspis vividus.

a, b. Photographs of specimen IVPP V26669 uncoated (a) and coated (b) by a layer of ammonium chloride sublimate, respectively, in ventral view, Abbreviations as in Figs. 1, 2.

Extended Data Fig. 4 Difference in lift force (N) between the models with and without ventrolateral fins (ΔLift) against the angle of attack (AoA).

Linear regression coefficient and significance of the slope is showed.

Extended Data Fig. 5 3D virtual restoration of Tujiaaspis vividus.

a, In dorsal view. b, In ventral view. c, Close-up of the anterior two dorsal fins. d, Close-up of the tail, (c,d), in lateral view.

Extended Data Fig. 6 Computed Fluid Dynamics analysis of Tujiaaspis vividus.

a, 3D model including ventrolateral ridges in dorsal (dr), ventral (vn), lateral (lt) and frontal (fr) views. b, c, d, Computational domain (b), mesh overlying the models without (upper) and with (lower) ventrolateral fins in dorsal (dr) and ventral (vn) views, and general mesh (c) employed in the CFD analysis noting the different boundary conditions (in, inlet; ou, outlet; ns, non-slip; ss, slip symmetry), refinement volume (rf) and inflation layers (if).

Extended Data Fig. 7 The artistic life restoration of Tujiaaspis vividus (Picture credit Qiuyang Zheng).

The ventral side of the body in Tujiaaspis vividus manifests a pair of continuous pectoral-pelvic lateral fins which our Computed Fluid Dynamic experiments demonstrate passively generate lift to escape from predators such as sea scorpions to escape from predators such as sea scorpions.

Supplementary information

Reporting Summary

Supplementary Data 1

Results of all CFD simulations performed with the models of Tujiaaspis with and without ventrolateral ridges (VL), including details about the mesh and the calculations of the Reynolds number, apparent weight, drag and lift coefficients and lift-to-drag ratios.

Supplementary Data 2

Results of independence tests for mesh size, domain size and refinement volume.

Supplementary Data 3

Character data and stratigraphic data used in the ancestral state estimation analyses.

Supplementary Data 4

Results of the ancestral state estimation analyses.

Supplementary Data 5

R script used in the ancestral state estimation analyses.

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Gai, Z., Li, Q., Ferrón, H.G. et al. Galeaspid anatomy and the origin of vertebrate paired appendages. Nature 609, 959–963 (2022). https://doi.org/10.1038/s41586-022-04897-6

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