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Morphology of Palaeospondylus shows affinity to tetrapod ancestors

Abstract

Palaeospondylus gunni, from the Middle Devonian period, is one of the most enigmatic fossil vertebrates, and its phylogenetic position has remained unclear since its discovery in Scotland in 1890 (ref. 1). The fossil’s strange set of morphological features has made comparisons with known vertebrate morphotype diversity difficult. Here we use synchrotron radiation X-ray micro-computed tomography to show that Palaeospondylus was a sarcopterygian, and most probably a stem-tetrapod. The skeleton of Palaeospondylus consisted solely of endoskeletal elements in which hypertrophied chondrocyte cell lacunae, osteoids and a small fraction of perichondral bones developed. Despite the complete lack of teeth and dermal bones, the neurocranium of Palaeospondylus resembles those of stem-tetrapod Eusthenopteron2 and Panderichthys3, and phylogenetic analyses place Palaeospondylus in between them. Because the unique features of Palaeospondylus, such as the cartilaginous skeleton and the absence of paired appendages, are present in the larva of crown tetrapods, our study highlights an unanticipated heterochronic evolution at the root of tetrapods.

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Fig. 1: Cranial skeleton of P. gunni, NSMPV 24679.
Fig. 2: Skeletal histology of P. gunni (NSMPV 24679) in comparison with the extant sarcopterygian Neoceratodus forsteri.
Fig. 3: Neurocranium of P. gunni, NSMPV 24679.
Fig. 4: P. gunni and early tetrapodomorphs.

Data availability

Series of CT images of NSMPV 24679 with voxel sizes of 6.63, 2.74 and 1.46 µm (in 8-bit TIFF format; media IDs: 000435207, 000435203 and 000434543, respectively) and surface files of the same specimen with a voxel size of 6.63 µm (in STL format) have been deposited at MorphoSource at https://www.morphosource.org/projects/000433576. A series of CT images of NSMPV 24,678 with a voxel size of 6.63 µm (in 8-bit TIFF format; media ID: 000435213) and surface files of the same specimen with a voxel size of 6.63 µm (in STL format) have been deposited at MorphoSource at https://www.morphosource.org/projects/000435209. MrBayes and PAUP* executable files (in NEXUS format) are presented in the Supplementary Information (Supplementary Data 14).

Code availability

The filtered back-projection algorithm26 used for reconstitutions of SRXµCT data is available online at http://www-bl20.spring8.or.jp/xct/index-e.html.

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Acknowledgements

We thank G. van der Brugghen and D. van der Brugghen for the acquisition of the fossils; T. Sasaki and C. Sakata for assistance with fossil preparation; P. E. Ahlberg, C. Burrow, G. C. Young, J. Lu, Y. Zhu, J. Long, L. Knuefing, Y. Iba, Y. Nakajima and H. Higashiyama for discussions; C. Cupello, P. M. Brito, Y. Yabumoto and S. Isogai for the acquisition of the SRXµCT datasets for N. forsteri; and A. Limaye for developing Drishti and optimizing the software for biological visualization. This work was supported by JSPS KAKENHI grant numbers JP17K18354 (to T.H.) and JP17H06385 (to S.K.).

Author information

Authors and Affiliations

Authors

Contributions

T.H. and S.K. conceived the project. T.H. and M.M. conducted the conventional µCT analyses of preservational status. T.H., K.U. and M.H. conducted the SRXµCT scanning. T.H. and Y.H. segmented the CT data. T.H. and Y.H. conducted histological and phylogenetic analyses. T.H., Y.H. and S.K. wrote the paper. All authors discussed the data and approved the final version of the manuscript.

Corresponding author

Correspondence to Tatsuya Hirasawa.

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Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature thanks Russell Garwood, Philippe Janvier 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 Cranial skeleton of P. gunni, NSMPV 24678.

a, Position of the cranial skeleton embedded intact within the matrix. b, Dorsal view. c, Ventral view. arp, arcual plate; aup1, autopalatinum 1; aup2, autopalatinum 2; bspp, basipterygoid process; epp, epipterygoid; hym, hyomandibula; ltc, lateral commissure; mc, Meckel's cartilage; mtp, metapterygoid; nac, nasal capsule; nas, nasal septum; ocpa, occipital arches (basioccipital portion of the neurocranium); prop, prootic process; qua, quadrate; vt1–7, vertebral segments 1–7. Scale bar, 1 mm.

Extended Data Fig. 2 Volume visualizations of P. gunni, NSMPV 24679, showing the distribution of cell lacunae within the skeletal tissues.

a–l, Serial transverse sections in ventral view, from ventral (a) to dorsal (l). Scale bar, 1 mm.

Extended Data Fig. 3 Phylogenetic position of Palaeospondylus.

a, Undated Bayesian analysis using MrBayes18. 50% majority-rule consensus of 27,777 trees (mean log-likelohood of 1,761.99). Numbers at nodes refer to posterior probabilities. b, Parsimony analysis using PAUP*19. Strict consensus of all 792 most-parsimonious trees (448 steps). Numbers at nodes refer to bootstrap percentages, based on 200 replicates. Taxonomic names in green, brown, blue, and reds indicate the dipnomorphs, rhizodonts, 'elpistostegalians', and limb-bearing tetrapods, respectively.

Supplementary information

Supplementary Information

This file contains details regarding the historical background, a detailed description of Palaeospondylus chondrocranium and the phylogenetic analysis, Supplementary Figs. 1–9 and references.

Reporting Summary

Supplementary Data 1

MrBayes executable file with a conservative interpretation regarding character 108 (Extended Data Fig. 3a).

Supplementary Data 2

PAUP* executable file with a conservative interpretation regarding character 108 (Extended Data Fig. 3b).

Supplementary Data 3

MrBayes executable file with an alternative interpretation regarding character 108 (Supplementary Fig. 9a).

Supplementary Data 4

PAUP* executable file with an alternative interpretation regarding character 108 (Supplementary Fig. 9b).

Supplementary Video 1

Cranial skeleton of P. gunni, NSMPV 24679.

Supplementary Video 2

Neurocranium of P. gunni, NSMPV 24679.

Supplementary Video 3

Volume visualizations of P. gunni, NSMPV 24679, showing the distribution of cell lacunae within the skeletal tissues, in dorsal view.

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Hirasawa, T., Hu, Y., Uesugi, K. et al. Morphology of Palaeospondylus shows affinity to tetrapod ancestors. Nature 606, 109–112 (2022). https://doi.org/10.1038/s41586-022-04781-3

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