Description of a cranial endocast from a fossil platypus, Obdurodon dicksoni (Monotremata, Ornithorhynchidae), and the relevance of endocranial characters to monotreme monophyly

Authors

  • Thomas E. Macrini,

    Corresponding author
    1. Jackson School of Geosciences and University of Texas High-Resolution X-ray Computed Tomography Facility, University of Texas at Austin, Austin, Texas 78712, USA
    • University of Texas at Austin, Geoscience Department, 1 University Station C1100, Austin, TX 78712-0254
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  • Timothy Rowe,

    1. Jackson School of Geosciences and University of Texas High-Resolution X-ray Computed Tomography Facility, University of Texas at Austin, Austin, Texas 78712, USA
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  • Michael Archer

    1. School of Biological Science, University of New South Wales, Sydney, New South Wales 2052, Australia
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Abstract

A digital cranial endocast of the Miocene platypus Obdurodon dicksoni was extracted from high-resolution X-ray computed tomography scans. This endocast represents the oldest from an unequivocal member of either extant monotreme lineage and is therefore important for inferring character support for Monotremata, a clade that is not well diagnosed. We describe the Obdurodon endocast with reference to endocasts extracted from skulls of the three species of extant monotremes, particularly Ornithorhynchus anatinus, the duckbill platypus. We consulted published descriptions and illustrations of whole and sectioned brains of monotremes to determine which external features of the nervous system are represented on the endocasts. Similar to Ornithorhynchus, well-developed parafloccular casts and reduced olfactory bulb casts are present in the Obdurodon endocast. Reduction of the olfactory bulbs in comparison with tachyglossids and therian mammals is a potential apomorphy for Ornithorhynchidae. The trigeminal nuclei, ganglia, and nerves (i.e., trigeminal complex) are enlarged in Obdurodon, as evidenced by their casts on the endocast, as is the case in the extant platypus. The visibility of enlarged trigeminal nucleus casts on the endocasts of Obdurodon and Ornithorhynchus is a possible synapomorphy of Ornithorhynchidae. Electroreception and enlargement of the trigeminal complex are possible synapomorphies for Monotremata. J. Morphol. © 2006 Wiley-Liss, Inc.

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