55 - The Anterior Braincase of Cotylorhynchus Romeri (Synapsida: Caseidae) Shows Similarities to Therapsida

Poster Board Number: 55
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Co-authors:

Daniel Barta - Anatomy and Cell Biology - Oklahoma State University College of Osteopathic Medicine at the Cherokee Nation

Abstract Body : The orbitosphenoid is a bone that contributes to the anterior braincase of amniotes and carries the olfactory bulbs. Studies of both extant and fossil taxa have attempted to clarify the evolutionary and developmental history of this element, particularly in mammals and other synapsids. In extant mammals, both the sphenethmoid and orbitosphenoid are present and contribute to the interorbital septum. However, in fossil taxa, there are discrepancies in the identification of these elements. In the tetrapod Diadectes and in early amniotes, such as the parareptile Delorhynchus and the “pelycosaur” Oedaleops, the anterior braincase element is the sphenethmoid, which is a broad Y-shaped sheet of bone in the orbital region that contacts the frontals and the cultriform process. An ossified orbitosphenoid is present in the stem-amniote Seymouria and later diverging synapsids, such as therapsids and cynodonts. In the cynodonts Thrinaxodon and Procynosuchus, the orbitosphenoid is a hemicylinder that lacks an ossified ventral process. In contrast to these taxa, the endocranial anatomy of many early diverging “pelycosaur”-grade synapsids is not well understood. Most descriptions of these taxa were published before the widespread use of computed tomography (CT), which is needed to fully visualize these internal elements. Detailed descriptions of anterior braincase elements in early diverging synapsid clades may contribute to understanding the homology relationships and evolutionary history of these elements across Synapsida.




To this end, we obtained CT scans of a complete Cotylorhynchus romeri skull (OMNH 4329) and used Avizo 2020.1 to manually segment and reconstruct aspects of internal anatomy not documented in past descriptions. C. romeri is a lower Permian caseid synapsid, a clade that arose soon after the divergence of synapsids from the amniote common ancestor.




The orbitosphenoid of C. romeri is a vertical midline bone in the interorbital region. We identify this element as an orbitosphenoid instead of a sphenethmoid because it is restricted to the interorbital region. The sphenethmoid in temnospondyls is more anteroposteriorly expansive and spans between the nasal and the sphenoid areas. The orbitosphenoid of C. romeri has thin dorsal wings that flare laterally and form a cylinder. Within the cylinder, a median vertical process, a feature absent in cynodonts, projects dorsally to divide the olfactory cavity. The dorsal margin of the C. romeri orbitosphenoid forms a broad horizontal plate that likely contacted the frontals. Ventrally, a process projects to contact the cultriform process of the parabasisphenoid. These features are similar to those of dicynodonts and gorgonopsians, suggesting an earlier evolution of this orbitosphenoid morphology within Synapsida.