A Silurian placoderm with osteichthyan-like marginal jaw bones

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Abstract

The gnathostome (jawed vertebrate) crown group comprises two extant clades with contrasting character complements. Notably, Chondrichthyes (cartilaginous fish) lack the large dermal bones that characterize Osteichthyes (bony fish and tetrapods). The polarities of these differences, and the morphology of the last common ancestor of crown gnathostomes, are the subject of continuing debate. Here we describe a three-dimensionally preserved 419-million-year-old placoderm fish from the Silurian of China that represents the first stem gnathostome with dermal marginal jaw bones (premaxilla, maxilla and dentary), features previously restricted to Osteichthyes. A phylogenetic analysis places the new form near the top of the gnathostome stem group but does not fully resolve its relationships to other placoderms. The analysis also assigns all acanthodians to the chondrichthyan stem group. These results suggest that the last common ancestor of Chondrichthyes and Osteichthyes had a macromeric dermal skeleton, and provide a new framework for studying crown gnathostome divergence.

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Figure 1: Competing hypotheses of dermal skeleton condition at the crown gnathostome node.
Figure 2: Entelognathus primordialis gen. et sp. nov., a 419-million-year-old jawed fish from the Kuanti Formation (Late Ludlow, Silurian), Qujing, Yunnan.
Figure 3: Entelognathus primordialis gen. et sp. nov.
Figure 4: Entelognathus primordialis gen. et sp. nov.
Figure 5: Skull roof and braincase of Entelognathus primordialis gen. et sp. nov.
Figure 6: Results of phylogenetic analysis and palatoquadrate conditions among major gnathostome groups.

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Acknowledgements

We thank M.-M. Chang, J. Long, G. Young, D. Goujet and J.-Q. Wang for discussions, X.-F. Lu, J. Zhang and C.-H. Xiong for specimen preparation, Y.-M. Hou for X-ray micro-computerized tomography. This work was supported by the Major State Basic Research Projects (2012CB821902) of MST of China, and the National Natural Science Foundation of China (40930208). P.E.A. and Q.Q. were supported by ERC Advanced Investigator Grant 233111 and a Wallenberg Scholarship from the Knut and Alice Wallenberg Foundation, both awarded to P.E.A.

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Contributions

M.Z. conceived the project. M.Z., W.Z., L.J., Y.Z., J.L. and T.Q. did the fieldwork. M.Z., P.E.A., T.Q., J.L., Q.Q., B.C., X.Y. and H.B. conducted the phylogenetic analyses. J.L, P.E.A. and M.Z. performed computerized tomography restorations. M.Z., P.E.A., X.Y. and B.C. discussed the results and prepared the manuscript.

Corresponding author

Correspondence to Min Zhu.

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The authors declare no competing financial interests.

Additional information

The LSIDs urn:lsid:zoobank.org:pub:1CE67C34-52CC-4CEF-B731-DF1A4E85DE4B (article), urn:lsid:zoobank.org:act:D88DD67F-1159-4E7E-AE4C-814249337351(genus), and urn:lsid:zoobank.org:act:77F76728-FBC1-407D-B2C1-1C5D5BF9B59C (species) have been deposited in ZooBank.

Supplementary information

Supplementary Information

This file contains Phylogenetic analyses, Supplementary Table 1, Supplementary Figures 1-27 and Supplementary References (see Table of Contents for full list). (PDF 12048 kb)

Supplementary Data

This zipped file contains the Nexus file of the dataset with 253 morphological characters for 75 taxa. (ZIP 11 kb)

41586_2013_BFnature12617_MOESM74_ESM.mov

Digital dynamic restoration of specimen IVPP V18620 (holotype of Entelognathus primordialis gen. et sp. nov.) showing the cheek complex, sclerotic ring, mandible, submandibular and gular series in external and internal views. (MOV 8467 kb)

An exceptionally-preserved placoderm from the Silurian of China

Digital dynamic restoration of specimen IVPP V18620 (holotype of Entelognathus primordialis gen. et sp. nov.) showing the cheek complex, sclerotic ring, mandible, submandibular and gular series in external and internal views. (MOV 8467 kb)

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Zhu, M., Yu, X., Ahlberg, P. et al. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature 502, 188–193 (2013). https://doi.org/10.1038/nature12617

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