Skull Skull roof The anterodorsal-most tip of the premaxilla forms a rostral process, which overhangs the incisors, like in most eutherocephalians. The external nares are relatively large and face anterolaterally. The rostrum is relatively long and about as high as it is broad. A constriction of the snout exists directly behind the caniniforms, causing the snout to look relatively narrow. The posterior region of the maxillary facial plate is folded inward onto the palatal region, like in most non-akidnognathid whaitsioids. It extends from the jugal onto the posterior end of the maxillary facial plate, creating orbital convergence and appearing more triangular than circular in dorsal view. The orbit is positioned relatively high in the skull. There is a postorbital process on the jugal. The zygomatic arch is very slender. The parietal borders the temporal fenestra dorsally and is expanded posteriorly on the midline of the parietal foramen. The parietal crest is usually quite long., another trait shared with non-akidnognathid whaitsioids. The quadrate and quadratojugal are reduced in height, and situated in the depression on the anterior face of the squamosal. There is no nasal-lacrimal contact and no prefrontal-postorbital contact in adults.
Palate The palatal fenestra of the lower caniniform merges with the internal naris. A portion of the vomer separates the choanae, and bears specialized transverse processes just anterior to the contact with the premaxilla. The vomers are either fused anteriorly or completely fused. No palatine teeth have ever been found on specimens of
Theriognathus. The pterygoid flange expansion is severe, and looks like posteriorly protruding wings with a slight posterolateral tilt, a character shared with most therocephalians. The interpterygoid vacuity of
Theriognathus adults is either absent or extremely reduced.
Theriognathus shares this character with
Moschowhaitsia, and is convergent in scylacosaurids.
Overal size Considering the morphology of therocephalians, and the basal skull length of
Theriognathus (roughly 25 cm in adult individuals), the approximate size and appearance of
Theriognathus is that of a slender to stout animal about 1 m long.
Bone histology Ricqlès (1969) suggested differential rates of growth between a basal therocephalian from the
Middle Permian of South Africa and the Late Permian whaitsiid
Notosollasia (now synonymous with
Theriognathus). According to Ricqlès,
Theriognathus’ radius midshaft bone wall is extremely thick and has a reduced central cavity without cancellous structures. The vascular motif is primarily longitudinal and radial. Ricqlès suggested that therocephalians might have exhibited accelerated growth rates later in their evolutionary history due to the comparatively more vascularized cortical bone in the radius of the whaitsiid. The femur bears a relatively thick wall as well, and could be related to impact loading, due to its orientation. The thick cortical bone is made many primary osteons in a parallel and woven fibered matrix. There are numerous osteocyte lacunae that have a spherical shape within the growth zones, but more lenticular and ordered near growth marks. The cortex has a moderate amount of vascularization. The pattern and number of the growth marks indicated some amount of plasticity in the growth style of
Theriognathus. ==Classification==