The tetrapod fauna of the upper Permian Naobaogou Formation of China: 3. Jiufengia jiai gen. et sp. nov., a large akidnognathid therocephalian

Skull roof

A prominent feature is the long snout, which represents more than half (58%) the length of the skull (Fig. 1; see Table 1). The skull has a triangular outline in dorsal view, being slightly constricted behind the canine and wider posteriorly. The general morphology resembles that of Annatherapsidus, but the latter does not show constriction behind the canine (Ivakhnenko, 2011: fig. 22a).

The laterally wide premaxilla features an eroded anterior tip and ascending processes. The anterior surface of the right side is also eroded exposing the roots of some incisors (Fig. 2). The premaxilla has a narrow exposure on the lateral side, covered by the anterior lamina of the maxilla (Fig. 1). The alveolar margin of the premaxilla is upturned anteriorly. The dorsal surface of the bone forms the ventral rim of the external nares with overlapping septomaxilla, only preserved on the left side. On the palate, the premaxilla meets the vomer with a short, trapezoidal, posteriorly-directed vomerine process (Fig. 3).

There are five upper incisors (Fig. 2). Although the lateral surfaces of the alveoli are eroded, the crowns of the left five incisors are still preserved, whereas the crowns have nearly vanished on the right side. The incisors are narrow, sharply conical, slightly curved lingually and somewhat anteriorly directed from alveoli. In lateral view, the alveolar margin is nearly straight. The anterior four incisors are similar in the crown height (ca. two cm), the fourth has the largest diameter, and the fifth is smaller.

The maxillary dorsal margin is incomplete, and most of the suture with the nasal is untraceable (Fig. 1). The maxilla contacts the prefrontal posterodorsally and the lacrimal and jugal posteriorly. It has a triangular posterior process which contacts the also triangular anterior process of the jugal. The maxilla is very high (height is close to 40% of the bone length) and long, forming the majority of the lateral region of the snout. It extends anteriorly to the position of the fourth incisor and contacts both the septomaxilla and premaxilla. Its height increases posteriorly, and the margins of the maxillae from both sides are separated a short distance on the dorsal surface (Fig. 4), suggesting the presence of a very narrow nasal at that point of the skull. The maxilla also bears numerous longitudinal grooves and pits, but not foramina, on the external surface of the facial plate. In lateral view, the ventral margin of the maxilla is slightly convex and directed anterodorsally in front of the level of the canine, while the posterior part is nearly straight.

In palatal view the maxilla has a broad exposure medial to the canine alveoli, approaching the wide anterior margin of the vomer. The maxillary exposure on the palate is constricted behind the canine, and its alveolar buccal margin is laterally concave. The maxilla houses two precanines, one canine, and four postcanines. The two small precanines are conical; the canine is cylindrical, with a diameter of 10 mm at the base. The apex of the canine is lost, and the preserved crown has more than three cm in height. The canine extends below the ventral margin of the lower jaw (Fig. 1). The canine is slightly curved posteriorly, and its crown surface is covered by regular longitudinal striations. A wide diastema of 8 mm is present posterior to the canine. The postcanines are conical, with a massive base and posteriorly curved crown, which is slightly flattened labiolingually. The two more posterior postcanines have bigger crowns than the first two. The third postcanine is more strongly curved posteriorly when compared with the other postcanines.

Only the posterior portion of the nasals is preserved, close to the prefrontals. The dorsal surface of the nasal on the left side is relatively well-preserved. The suture between the nasal and the frontal is unclear. If the posterior extension of the nasal is level with the anterior margin of the orbit, as in Annatherapsidus, the length of this bone is almost half of the skull length. No mid-sagittal crest is observed on the nasal.

Only the left orbit is preserved (Figs. 1 and 4). The orbit is rounded in dorsal view, and has a 44 mm anteroposterior length. The anterior wall of the orbit is formed by the prefrontal and lacrimal; the relatively deep suborbital bar is formed by the jugal, and maxilla only anteriorly; and the moderately slender postorbital arch is formed by the jugal and postorbital. The dorsal roof of the left orbit is incomplete. The lacrimal sutures with the prefrontal dorsally and the jugal ventrally. It is a rectangular bone in lateral view (Fig. 1). The triradiate jugal has a long anterior process, which reaches nearly the level of the anterior margin of the lacrimal. The jugal also extends anteriorly beyond the anterior margin of the orbit in Akidnognathus and Annatherapsidus, but it does not approach the anterior margin of the lacrimal (Ivakhnenko, 2011). Posteriorly, it contacts the anterior process of the squamosal approximately midway beneath the temporal fenestra, forming the zygomatic arch.

The prefrontal is roughly triangular in dorsal view (Fig. 4). It contacts medially with the nasal and frontal. The frontal is poorly preserved. The postorbital has a long ventral process that covers the lateral surface of the jugal and partially forms the suborbital bar. The anterodorsal portion of the postorbital is missing, and the posterior process is short, forming the anterior lateral surface of the temporal fenestra, below the parietal crest (Fig. 5).

The temporal fenestra is roughly quadrangular (Figs. 1 and 4). It is only slightly longer than the orbit anteroposteriorly.

The parietal is short, forming most of the narrow intertemporal region (Fig. 4). It forms a parietal crest which is more than half of the temporal fenestra length. There is no evidence of the parietal foramen due to poor preservation. The parietal forms the central portion of the lambdoidal crest, which is mostly eroded in the specimen, and also extends ventrally to contact the interparietal on the occiput (Fig. 6).

The squamosal extends medially to contact the parietal and posteriorly the tabular. In occipital view, it reaches the post-temporal fossa and forms its dorsolateral corner (Fig. 6). The squamosal extends ventrally to encase the lateral surface of the paroccipital process of the opisthotic, forming a rudimentary mastoid process (Fig. 3). The posteroventral margin of the squamosal forms a vertical notch to accommodate the quadrate and quadratojugal. Anteromedially, it has a long ventral process which contacts the quadrate process of the pterygoid and the paroccipital process. The squamosal sends an anteroventral process to contact the prootic medially and the epipterygoid anteroventrally (Figs. 4 and 5). This process forms the bar anterior to the pterygo-paroccipital foramen.

Palate

The choana is confluent with the fossa for the lower canine, and it extends anteriorly to the level of the last incisor and posteriorly to the level of the third postcanine (Fig. 3). The choana is bordered laterally by the maxilla and palatine, anteriorly by the premaxilla, medially by the vomer, and posteriorly by the vomer and palatine.

The unpaired vomer is wide, anteriorly contacting the vomerine process of the premaxilla. The suture between these bones lies posterior to the level of the anterior margin of the choanae. Its anterior width is greater than half of the vomer length between choanae. The vomer is narrow at the level of the posterior margin of the choana, but it is much wider than in most akidnognathids, with the exception of Moschorhinus (Liu & Abdala, 2017a). The greatest width of the vomer is at the level of the posterior margin of the choanae, and progressively reduces its width posteriorly until it contacts the pterygoid, which is posterior to the level of the anterior margin of the suborbital vacuity, as in Olivierosuchus parringtoni (Botha-Brink & Modesto, 2011). The posterior plate of the vomer is ventrally concave, lacking a ventromedian crest. In most akidnognathids, the ventromedian crest is developed on both anterior and posterior plates of the vomer, and only on the posterior plate in Olivierosuchus (Botha-Brink & Modesto, 2011; Liu & Abdala, 2017a: fig. 6).

The palatine has a slightly curved medial suture with the vomer. Its lateral suture with the maxilla is almost parallel to the lateral margin of the skull (Fig. 3). The posterolateral process of the palatine extends to the middle of the lateral margin of the suborbital vacuity. The posteromedial side covers the pterygoid. The medial portion of the palatine almost lies on the same plane as the nearly flat lateral portion, and the posterior part of the crista choanalis is a crest on a nearly flat surface. The crista choanalis turns laterally behind the posterior margin of the choanae, and extends to the notch on the anterior margin of the suborbital vacuity. On the lateral portion, the palatine bifurcates in two short anterior processes located lateral and medial to the maxillo-palatine foramen. The maxillo-palatine foramen lies at the level of the anterior margin of the first postcanine.

The suborbital vacuity is large, slightly longer than it is wide (Fig. 3). The vacuities in the specimen have a different size, but the left one seems to be less distorted and closer to the natural size. The vacuity is formed anteriorly by the palatine, medially and posteriorly by the pterygoid, and posterolaterally by the ectopterygoid. On the medial margin of the vacuity the palatine and the pterygoid form two parasagittal crests, which become two tuberosities. Between them, a prominent ventromedial tuber lies anterior to the interpterygoid fossa, similar to the morphology observed in Promoschorhynchus (Mendrez, 1974b). The lateral lamina of the pterygoid is poorly developed.

The transverse process extends laterally and terminates in a swollen tuberosity (Fig. 3) as in Annatherapsidus (Ivakhnenko, 2011) and Oliverosuchus (Botha-Brink & Modesto, 2011). The transverse process is more or less horizontal on the medial side, but it turns nearly vertical laterally.

The ectopterygoid is contacted ventrally and anteriorly by the palatine, and it expands dorsoventrally at the posterior portion. The ectopterygoid forms a fossa on the posterolateral corner of the suborbital vacuity.

The interpterygoid fossa is quite small, as in Annatherapsidus (Ivakhnenko, 2011). It lies at the base of the transverse processes. The very short edges of the interpterygoid fossa meet to form a posteromedial crest, which connects with the high and sharp parasphenoid keel. The basisphenoid process of the pterygoid extends posteriorly to form the lateral margin of the external opening of the internal carotid canal, together with the parabasisphenoid. The lamina between the quadrate process and the basisphenoid process is strongly concave ventrally, bordered laterally by the strong crest of the quadrate process.

The dorsal portion, and possible partial ventral portion, of the right quadrate is preserved. It is an expanded lamina located adjacent to the anterior surface of the squamosal. There is no evidence of the quadratojugal.

Braincase

The parabasisphenoid features an anterior parasphenoid keel and a wide, deep groove located between the strong spheno-occipital tubercles (Fig. 3). The parabasisphenoid forms the majority of the anterior margin of the fenestra ovalis, while the basioccipital forms the medial margin of the fenestra ovalis.

The posterior portion of the basicranium is incomplete with part of the basioccipital and exoccipitals eroded (Fig. 3). Anteriorly, the basioccipital participates in the formation of the spheno-occipital tubercle; laterally it contacts the opisthotic and posterolaterally it sutures with two exoccipitals, forming the ventral margin of the foramen magnum (Fig. 6).

The exoccipital forms the lateral portion of the occipital condyle and the foramen magnum (Fig. 3). The bone is nearly triangular in occipital view (Fig. 6). Its medial side forms the lateral wall of the foramen magnum, and its ventral side contributes to the roof of the large jugular foramen. The exoccipitals do not meet along the midline.

The occiput is inclined posteriorly (Fig. 3). In the occiput, the supraoccipital is fairly broad. It participates in the formation of the upper margins of the foramen magnum, separating the exoccipitals (Fig. 6). Anteriorly, it is also exposed in the temporal fossa (Fig. 5). From the margin of the foramen magnum, the supraoccipital has a ridge extending dorsolaterally that continues on the tabular, as in Moschorhinus (Durand, 1991). The area between these ridges has a deep indentation, mainly on the interparietal and the supraoccipital, for the attachment of the supravertebral cervical muscles. The supraoccipital has a short lateral extension between the opisthotic and the tabular.

The rectangular interparietal is a broad and low bone, with a short median ridge (Fig. 6). The high tabular forms a substantial portion of the lambdoid crest. It extends ventrally to contact the dorsolateral corner of the supraoccipital. Its ventral tip forms part of the dorsal border of the post-temporal fenestra.

The opisthotic consists of a robust paroccipital process, which forms the ventral border of the post-temporal fenestra in the occiput (Fig. 6). A shallow notch divides the mastoid process from the longer quadrate process (ventral flange). The mastoid process has only a rudimentary extension posteriorly, along with the posterior flange of the squamosal. A well-developed posterodorsal process of the opistothic contacts the tabular dorsolaterally and the supraoccipital dorsomedially, and forms half of the dorsal margin of the post-temporal fenestra. The internal process forms a lamina between the fenestra ovalis and the jugular foramen.

The left prootic is well preserved. Its basal area is in contact with the parabasisphenoid posteriorly and it forms the lateral side of the anterior border of the fenestra ovalis (Fig. 3). Its anteroventral and anterodorsal processes are not well exposed, being partially covered by the epipterygoid (Fig. 5). Dorsally, the prootic contacts the supraoccipital, and posterior to the epipterygoid there is a triangular incisure, interpreted as a venous foramen. The long posterodorsal and posteroventral processes contact the squamosal and form almost the entire dorsal and ventral margins of the posttemporal fenestra. There is no central process as described in Promoschorhynchus (Mendrez, 1974b). In ventral view, the suture between the prootic and the opisthotic runs medially towards the fenestra ovalis and continues on the dorsal border of the fenestra.

Both epipterygoids are preserved, but only the left one is complete (Figs. 3–5). It is a flat, blade-like bone (Fig. 5). It expands dorsally to contact the parietal, and its posterodorsal process contacts the supraoccipital posteriorly. It has a posterior apophysis that is posterior to the posterior foramen, which overlaps the anterodorsal process of the prootic. The basal part expands anteroposteriorly to cover nearly half the length of the quadrate ramus of the pterygoid. The posteroventral process contacts the squamosal, and a foramen (f in Fig. 4) is separated from the large cavum epiptericum.

Mandible

Both mandibles are preserved, but the right one is more complete (Figs. 1 and 7). The dentary is long and deep, and the well-developed coronoid process has a straight terminal end. The horizontal ramus is constricted in height behind the canine, and distinctly expands posterior to the last postcanine (Fig. 7). A tall unfused symphysis forms a deep anterior chin. It is smoothly convex anteriorly in lateral view, forming an angulation of 110° with the dentary ventral margin. The dentary lateral surface is convex outwards, lacking any fossa. The dentary angle, anterolateral to the reflected lamina, is rounded and slightly convex.

The dentary contains four incisors, one canine and five or six postcanines. The fourth incisor is smaller and located lateral to the others based on the CT image. Five posteriorly inclined postcanines are counted on the left, and six on the right. However, the first right postcanine could be a replacing canine. The first left postcanine is smaller than the others, whereas the last right postcanine (four mm) is less than half the size of the previous teeth.

The splenial is an elongated bone (Fig. 3). Anteriorly, it meets its counterpart and participates in the formation of the lower part of the symphysis. It reaches the highest point below the second postcanine where its dorsal border is close to the upper margin of the dentary, and decreases in height from there until the level of the last postcanine, from where it extends posteriorly as a slender rod. It contacts the angular posteriorly behind the level of the transverse process, but the suture is not clearly visible. The ventral margin of the splenial is nearly confluent with that of the dentary near the symphysis, but it gradually rises posteriorly.

The postdentary fossa is elongated and formed by the surangular, prearticular, and angular (Fig. 7). The surangular is a narrow strip, having a concave ventral margin as the dorsal border of the mandibular fenestra. The mandibular fenestra is exposed laterally posterior to the dentary. The angular forms the ventral border of the fenestra on the lateral side. The angular reflected lamina has a wide ‘U’-shaped notch posteriorly, which is more posteriorly than dorsally directed. The lateral surface of the reflected lamina is ornamented with ridges, grooves and corrugations. The prearticular is a narrow, splint-like bone, which extends anteriorly to the level of the transverse process (Fig. 3). It forms the ventral margin of the mandibular fenestra on the medial side. A triangular coronoid forms the anterior margin of the mandibular fenestra on the medial side. The articular is partially preserved and has no features to mention.

Postcranial skeleton

Some disarticulated postcranial bones were preserved with the skull. The following are the only identifiable elements.

Pectoral girdle. Only part of the ventral portion of the right scapula (Fig. 8A) was preserved. The right coracoid is complete, and a small piece of procoracoid is fused to it (Figs. 8B and 8C). The glenoid fossa seems to be formed by only the coracoid and scapula. The coracoid is expanded ventrally and curved inward toward the midline. The tuberosity for the coracoid head of the triceps lies posteroventral to the glenoid and far from it, as in Promoschorhynchus (Huttenlocker, Sidor & Smith, 2011) and other therocephalians (Kemp, 1986). A small fossa lies on the posterodorsal side of the medial surface of the coracoid, just below the articular surface. Some longitudinal striations are observed anterior to this fossa and might be for the attachment of the muscle subcoracoideus.

Forelimb. The right humerus lost most of its proximal and distal ends (Figs. 8D and 8E). The proximal half of the bone curves dorsally relative to the distal half. The proximal half is broad, with a smoothly concave ventral surface limited anteriorly by the deltopectoral crest and posteriorly by a ridge. The middle diaphysis is short and approximately circular in cross section, beyond which the bone expands to form the wide distal region. A large entepicondylar foramen lies in the posterior face and opens anteriorly into a deep trough on the ventral surface.

The proximal portions of the right ulna and radius are preserved. The ulna has no olecranon process, and the proximal side is rough (Figs. 8F and 8G). In anterior view, the proximal articular facet is lateromedially expanded. Ventral to the proximal end is a fossa for the origin of the extensor musculature (Fig. 8F). The radius has a thin shaft and expanded end. The proximal articulating facet is concave (Figs. 8H and 8I). A sharp ridge extends on the posterior side of the bone, below the facet for articulation to the ulna.

Elements of four digits of the right manus, identified as digits I–IV, are preserved together (Fig. 9). Their shape is similar to that of Oliverosuchus (Botha-Brink & Modesto, 2011). Distal carpals 1 and 2 are preserved. The medial surface of distal carpal 1 is strongly concave and the distal is articulated with the metacarpal I. A bone in contact with metacarpal II is interpreted as an out of place distal carpal 2. The bone is relatively large and transversely expanded on one side. Metacarpals I–IV are nearly complete, measuring 7, 22, 29, and 30 mm, respectively. The rectangular, wider than longer, metacarpal 1 is remarkably short and squat, differing from the quadrangular element of Olivierosuchus (Botha-Brink & Modesto, 2011: text-fig. 7). The other three metacarpals are long with expanded ends, and with similar length as the III and the IV, which are longer than the II. A similar pattern of metacarpal lengths is represented in Olivierosuchus (Fontanarrosa et al., in press: fig. 3B). The diaphysis of metacarpal II is remarkably wider than that of metacarpals III and IV (Fig. 9B). The phalanges preserved for each digit are 2-2-1-3, including three disarticulated phalanges from digit IV (Figs. 9C and 9D). The terminal (ungual) phalanx is narrow, pointed and claw-like, while the others are short, broad and squat.