Abstract from Description of a Cranial Endocast from a Fossil Platypus, Obdurodon dicksoni (Monotremata, Ornithorhynchidae), and the Relevance of Endocranial Characters to Monotreme Monophyly, by T. E. Macrini, T. Rowe and M. Archer (Journal of Morphology 267:1000-1015):

       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.

This specimen was collected from the middle Miocene Ringtail Site on the Riversleigh World Heritage property in Queensland, Australia. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Michael Archer of the University of New South Wales. Funding for scanning was provided by Dr. Archer.

The specimen was scanned by Richard Ketcham on 12-13 November 1998 along the coronal axis for a total of 612 512x512 pixel slices. Each slice is 0.26 mm thick, with an interslice spacing of 0.22 mm (for a slice overlap of 0.04 mm) and a field of reconstruction of 53 mm.

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Click on the thumbnail to the left for a pitch animation (1.6 mb) of the Obdurodon cranial endocast highlighted in red within the skull, which is rendered semi-transparent.

Click on the thumbnail to the left for a pitch animation (2.2 mb) of the isolated Obdurodon cranial endocast.

Click on the thumbnail to the left for a roll animation (2.1 mb) of the Obdurodon cranial endocast highlighted in red within the skull, which is rendered semi-transparent.

Click on the thumbnail to the left for a roll animation (2.1 mb) of the isolated Obdurodon cranial endocast.