Abstract
The Main Karoo Basin of South Africa has provided one of the wealthiest and most complete fossil records ranging from the Late Carboniferous to the Early Jurassic, spanning approximately 110my. The Therocephalia is an extinct suborder of synapsids that shares a close relationship with the Cynodontia, the group widely known to have given rise to modern mammals. The exact relationship of the Therocephalia and the Cynodontia is still debated, and so new information is useful in providing a better understanding of the origins of mammals and the various characters used to define the group.
Hewittia albanensis was first described by Brink (1959) based on the specimen AMG 4208, a specimen consisting of only the snout and part of the anterior portion of the braincase, collected from the Chris Hani District Municipality (former Cradock District), Eastern Cape Province, South Africa. Since then, the taxon has been rarely included in studies.
Here, we provide a redescription of H. albanensis based on a new specimen recovered from the Chris Hani District Municipality within the Lystrosaurus maccaigi-Moschorhinus subzone of the Daptocephalus Assemblage Zone of the Karoo Basin in South Africa. The new specimen consists of an almost complete skull with some dorsoventral distortion. The new description used non-destructive techniques including illustrations, μ-CT scanning and digital visualization and created digital 3D models of the skull. The μ-CT scanning and the visualization of the scans were conducted at the RADTOM department of South African Nuclear Energy Corporation (NECSA) at Pelindaba and the visualizations were done on VG Studio.
A comparative study was conducted using other closely related therocephalian fossils from various institutions in order to give an accurate description of the skull. The new specimen was confirmed to be a new specimen of Hewittia albanensis based on the diagnosis by Brink with some proposed changes. The dentition was also described, and the lack of tooth replacement activity was discussed.
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The description of the endocranial morphology of CGP/1/2301, including that of the braincase and snout was then conducted. Subsequent comparisons with previously described relatives, including Olivierosuchus and Moschorhinus, allow for an important contribution to the body of knowledge of the evolution of the braincase and internal anatomy of the skull. The study showed that the internal anatomy of CGP/1/2301 conforms to the general aspects of that of closely related akidnognathids but shows some differences including the possible absence of a posterior apophysis of the epipterygoid. Along with the description of the skull, the maxillary canal was segmented and described along with an approximation of the endocast and inner ear.
Hewittia differs from other akidnognathids by presenting a concave ventral step on the maxillary facial plate, a zygomatic which is at its greatest width midway along its length, the upturning of the alveolar margin, a distinct mastoid process, the presence of a diastema between the maxillary canine and first maxillary postcanine, only three lower incisors, a premaxilla which hangs over the incisors forming a pointed snout, a vomer whose anterior extent does not extend beyond the anterior limit of the choanae, and the presence of a functional precanine. The results show that Hewittia is a valid taxon within the Akidnognathidae and forms a clade with Euchambersia, Cerdosuchoides, and Moschorhinus.