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Was Ophiacodon (Synapsida, Eupelycosauria) a Swimmer? A Test Using Vertebral Dimensions

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Abstract

Ophiacodon, a Permian synapsid, has been hypothesized to be semi-aquatic. This interpretation is based on a range of evidence, including observations of histology, phalangeal morphology, dentition, and taphonomy. However, many of these data are inconclusive or have been reinterpreted. Here we investigate whether the morphology of the axial skeleton in Ophiacodon displays specializations for aquatic locomotion. Qualitative and quantitative comparisons of Ophiacodon to extant terrestrial and semi-aquatic tetrapods demonstrate that the distribution of centrum lengths in its vertebral column is similar in some ways to those of extant semi-aquatic reptiles. However, other basal synapsids that are widely regarded as terrestrial show comparable patterns, and the correlation between swimming style and vertebral morphology in extant semi-aquatic tetrapods may be weaker than previously thought. Therefore, vertebral proportions provide little support for a semi-aquatic lifestyle in Ophiacodon. Given that most lines of evidence are equivocal at best, we suggest that future studies that consider the ecology of Ophiacodon use a terrestrial lifestyle as a null hypothesis.

Keywords

Permian Carboniferous  Centrum length Limb length Aquatic tetrapods Ophiacodontidae 
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Notes

Acknowledgments

Data collection for this project was supported by a Field Museum Internship awarded to RNF. We thank P. Holroyd, A. Resetar, W. Simpson, and W. Stanley for assistance with specimens. K. Melstrom measured the terrestrial varanid specimens. J. Caruso, D. Heins, and R. Parsley provided feedback on a previous version of the manuscript. S. Pierce, S. Sumida, and an anonymous reviewer also provided helpful suggestions. Presentation of results from this study at the 2008 Annual Meeting of the Society of Vertebrate Paleontology was made possible by a grant from The Jackson School of Geosciences, University of Texas, Austin.

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Authors and Affiliations

  1. 1.Department of Biological SciencesOhio UniversityAthensUSA
  2. 2.Department of GeologyField Museum of Natural HistoryChicagoUSA

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