The occipital, otic, basicranial and pterygoid regions of the Gorgonopsia

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AbstractThis paper is based on information derived from a sectioned skull of Cynariops robustus Broom, a prepared skull of Cynariops longiceps Olson, prepared and sectioned specimens of Captorhinus, Dimetrodon and various cynodonts, and the literature dealing with these forms.In the otic region, it is shown that the internal auditory meatus, the fallopian aqueduct and the semicircular canals change but little in the series from Captorhinus to the cynodonts. Certain structures, the fenestra ovalis, which becomes progressively smaller, the vestibule, which lies progressively more ventral to the floor of the brain case, the recessus lagenacochlea, which increases in size, and the prootic, which enlarges, show progressive change. The fenestra rotundum is discussed at length. It is absent in Captorhinus, just back of the fenestra ovalis in the Gorgonopsia and Dimetrodon, and confluent with the foramen jugularis in the Cynodontia.The length and relationships of the parasphenoid in the Gorgonopsia are offered as evidence supporting the theory of the homology of the reptilian parasphenoid and the mammalian vomer.The sphenethmoid complex in Cynariops is divided into two elements, a basal member, the presphenoid, and a dorsal member, identified as the orbitosphenoid but possibly incorporating a portion of ossified mesethmoid as well.The comparative studies point out certain basic trends of evolution in the Synapsida, and indicate the level of evolutionary development of the groups studied.