Letter | Published:

A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes

Nature volume 541, pages 208211 (12 January 2017) | Download Citation

Abstract

Chimaeroid fishes (Holocephali) are one of the four principal divisions of modern gnathostomes (jawed vertebrates). Despite only 47 described living species1, chimaeroids are the focus of resurgent interest as potential archives of genomic data2 and for the unique perspective they provide on chondrichthyan and gnathostome ancestral conditions. Chimaeroids are also noteworthy for their highly derived body plan1,3,4. However, like other living groups with distinctive anatomies5, fossils have been of limited use in unravelling their evolutionary origin, as the earliest recognized examples already exhibit many of the specializations present in modern forms6,7. Here we report the results of a computed tomography analysis of Dwykaselachus, an enigmatic chondrichthyan braincase from the ~280 million year old Karoo sediments of South Africa8. Externally, the braincase is that of a symmoriid shark9,10,11,12,13 and is by far the most complete uncrushed example yet discovered. Internally, the morphology exhibits otherwise characteristically chimaeroid specializations, including the otic labyrinth arrangement and the brain space configuration relative to exceptionally large orbits. These results have important implications for our view of modern chondrichthyan origins, add robust structure to the phylogeny of early crown group gnathostomes, reveal preconditions that suggest an initial morpho-functional basis for the derived chimaeroid cranium, and shed new light on the chondrichthyan response to the extinction at the end of the Devonian period.

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Acknowledgements

We thank the Evolutionary Studies Institute micro-CT scanning unit at the University of the Witwatersrand in Johannesburg, South Africa, for provision of scanning facilities and K. Carlson and K. Jakata for technical assistance. We thank R. Smith and the staff of the South African Museum, Cape Town, for access and loan of specimen. We thank A. Gillis for the donation of Callorhinchus milii embryos and hatchlings. This work was supported by grants DEB-0917922 and DEB-1541491 from the National Science Foundation (to M.I.C.), and a research grant from the National Research Foundation/Department of Science and Technology South African Centre of Excellence in Palaeosciences (to R.W.G.).

Author information

Affiliations

  1. Department of Organismal Biology and Anatomy, University of Chicago, Chicago, Illinois 60637-1508, USA

    • Michael I. Coates
    • , Katharine E. Criswell
    •  & Kristen Tietjen
  2. Geology Department and Albany Museum, Rhodes University, Grahamstown 6139, South Africa

    • Robert W. Gess
  3. UCD School of Biology and Environmental Science, UCD Science Education and Research Centre, University College Dublin, Belfield, Dublin 4, Ireland

    • John A. Finarelli
  4. UCD Earth Institute, University College Dublin, Belfield, Dublin 4, Ireland

    • John A. Finarelli

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Contributions

This project was conceived by M.I.C. and R.W.G. and developed by M.I.C., R.W.G., J.A.F. and K.E.C. CT scanning of Moythomasia durgaringa was conducted by K.T.; renderings and animations of CT scans and graphics were completed by K.T. with input from M.I.C., K.E.C. and J.A.F. C. milii preparation and staining were completed by K.E.C. Phylogenetic data were collected by M.I.C., K.E.C. and J.A.F. Phylogenetic analyses were completed by J.A.F. and M.I.C. Morphometric data were collected by M.I.C. and K.E.C.; analysis was completed by M.I.C., K.E.C. and J.A.F. Manuscript and supplementary text preparation was undertaken by M.I.C., R.W.G. and J.A.F. with input from K.E.C.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Michael I. Coates.

Reviewer Information Nature thanks M. Friedman, P. Janvier and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data

Supplementary information

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    Supplementary Information

    This file contains Supplementary Notes, Morphometric and Phylogenetic Analysis and Supplementary references.

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    Supplementary Data

    This zipped file contains the data matrix nexus file.

Videos

  1. 1.

    Dwykaselachus braincase rotate and reveal endocast

    Dwykaselachus braincase rotate and reveal endocast.

  2. 2.

    Callorhinchus subadult skull rotate and internal morphology reveal

    Callorhinchus subadult skull rotate and internal morphology reveal, illustrating palatal fusion to braincase, large orbits positioned mostly dorsal to forebrain, superficial ophthalmic nerve paths and associated ethmoid canal, brain and otic labyrinth configuration.

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https://doi.org/10.1038/nature20806

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