The mystery of a missing bone: revealing the orbitosphenoid in basal Epicynodontia (Cynodontia, Therapsida) through computed tomography

Abstract

The basal non-mammaliaform cynodonts from the late Permian (Lopingian) and Early Triassic are a major source of information for the understanding of the evolutionary origin of mammals. Detailed knowledge of their anatomy is critical for understanding the phylogenetic transition toward mammalness and the paleobiological reconstruction of mammalian precursors. Using micro-computed tomography (μCT), we describe the internal morphology of the interorbital region that includes the rarely fossilized orbitosphenoid elements in four basal cynodonts. These paired bones, which are positioned relatively dorsally in the skull, contribute to the wall of the anterior part of the braincase and form the floor for the olfactory lobes. Unlike procynosuchids and the more basal therapsids in which the orbitosphenoids are well developed, dense, and bear a ventral keel, the basal epicynodonts Cynosaurus, Galesaurus, and Thrinaxodon display cancellous, reduced, and loosely articulated orbitosphenoids, a condition shared with many eucynodonts. The hemi-cylindrical orbitosphenoid from which the mammalian condition is derived re-evolved convergently in traversodontid and some probainognathian cynodonts.

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Acknowledgements

We thank K. Jakata (ESI, University of the Witwatersrand, ZA), P. Keanly (X-Sight, ZA), C. Churms (DebTech, DeBeers, ZA), G. Rougier (University of Louisville), and the European Synchrotron Radiation Facility for the provision of synchrotron radiation facilities and P. Tafforeau for the assistance in using beamline ID17. We also thank ESI preparators T. Nemavhundi and C. Dube. This research was conducted with the financial support from PAST and its Scatterlings projects, the National Research Foundation of South Africa, the Claude Leon Foundation, and the DST-NRF Centre of Excellence in Palaeosciences (CoE in Palaeosciences). Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily to be attributed to the CoE in Paleosciences.

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Communicated by: Robert R. Reisz

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Benoit, J., Jasinoski, S.C., Fernandez, V. et al. The mystery of a missing bone: revealing the orbitosphenoid in basal Epicynodontia (Cynodontia, Therapsida) through computed tomography. Sci Nat 104, 66 (2017). https://doi.org/10.1007/s00114-017-1487-z

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Keywords

  • Cynodontia
  • Orbitosphenoid
  • Braincase
  • μCT scan
  • Thrinaxodon