4.1. Premaxilla

The premaxillae are incomplete; they are broken transversely at the lateral exposure of the premaxilla-maxilla suture (Figs 3, 4). I infer from the preserved portions that the premaxillae are narrow mediolaterally and possibly tube-like. The presence or absence of a premaxillary crest cannot be confirmed. The contact between the premaxilla and maxilla is a zig-zag suture on the lateral surface of the rostrum, with an anterior process of the maxilla inserted between a smaller, lower process and a larger, posteriorly-directed ascending process of the premaxilla. That posterior process does not appear to be as extensive on the right side as on the left (Fig. 3A); on the left side, the laterally-directed ascending process of the premaxilla extends posteriorly almost to the posterior margin of the nares (Fig. 3B). Narrow processes of the premaxillae comprise approximately three-fourths of the anteroposterior length of the internarial septum; the nasals contribute the remaining one-fourth of the septum. The premaxillae are separated from the medial margins of the nares by thin extensions of both the ‘septomaxillae’ and nasals.

On the palatal surface, alveolar ridges extend anteriorly as sharp crests on either side of a wide, rounded interpremaxillary groove. Palatal processes of the premaxillae posteriorly extend to the choanae and contribute to their anteromedial margins; the choanae are located slightly more posterior than the nares. The premaxilla does not contact the palatine in Wannia scurriensis, whereas the premaxilla and palatine contact in all other North American phytosaurs I examined. A premaxilla–palatine contact previously was cited as a synapomorphy of Phytosauria (Sereno Reference Sereno1991). However, Paleorhinus angustifrons, Ebrachosuchus neukami, and the Krasiejów phytosaur also lack a premaxilla-palatine contact (Stocker pers. obs. 2011). The anteriormost alveolus present on the right side, and possibly the anteriormost on the left, are preserved in small, posterior fragments of the premaxillae that remain articulated with the maxillae. Those alveoli are small and rounded. No teeth appear to be preserved in any of the premaxillary alveoli.

4.2. Maxilla

The maxilla is bordered dorsally by the lateral ascending process of the premaxilla and by the nasal posterior to the posterior tip of the premaxilla. The maxilla surrounds the anterior border of the antorbital fenestra; an antorbital fossa is present on the maxilla anterodorsally (Fig. 4A) but not on the maxilla at the ventral border of the fenestra (Fig. 3). The lateral surfaces of the maxillae are oriented dorsoventrally and are marked by multiple small foramina randomly distributed anterior to the antorbital fenestra. The maxillae gently widen posterior to the anterior borders of the antorbital fenestrae. There they become much wider mediolaterally, though the posterior processes do not increase in dorsoventral thickness. The suture with the jugal is directed posteroventrally from the posterior edge of the antorbital fenestra and is clearly visible on the right side.

Ventrally, the maxillae have narrow palatal processes. The alveolar ridges continue posteriorly from the premaxillae, but the ridges remain low and are not visible in lateral view. The maxillae gently widen posteriorly until approximately the eighth alveolus from the posterior end of the tooth row. There, the maxillae widen much more dramatically and have laterally convex lateral margins around the last five alveoli. The posterior tip of the maxilla contacts the ectopterygoid and the jugal in ventral view. The maxilla forms the anterolateral margin of the suborbital fenestra. Just anterior to the suborbital fenestra, at the lateral apex of the palatine, is another, smaller fenestra. This smaller fenestra also is bordered laterally by the maxilla but is cut medially into the palatine. Lateral to the posterior tips of the palatal processes of the premaxillae, the palatal processes of the maxillae form rounded margins of the choanae.

All preserved alveoli that were mechanically prepared are separated by bony septa. The lateral edges of the alveoli are not more ventrally expanded than their medial edges. There are 16 alveoli preserved in the right maxilla, and at least five of those alveoli preserve broken teeth. All alveoli are circular in outline, though their diameters vary. TTU P-00539 exhibits anterior maxillary alveoli that are smaller in diameter than more posterior maxillary alveoli and smaller than the few preserved premaxillary alveoli; Nicrosaurus kapffi also features a reduction in size of the anteriormost maxillary alveoli posterior to the maxilla-premaxilla suture (Hungerbühler Reference Hungerbühler2000). The lack of a mediolateral constriction of the rostrum at the premaxilla-maxilla suture indicates the presence of a bipartite dentition (Hungerbühler Reference Hungerbühler2000). The teeth preserved in the right maxilla are too fragmentary to make detailed observations; however, the two that are visible in the posterior portion of the maxilla appear to have long axes oriented mesiodistally with labial surfaces that are more laterally convex than their lingual surfaces.

4.3. ‘Septomaxilla’

The triangular ‘septomaxillae’ (see Stocker (Reference Stocker2010) for discussion of the homology of this element in phytosaurs) form the slightly tapered anterior borders of the nares. The anterior tips of the ‘septomaxillae’ do not extend as far anteriorly as the anterior processes of the maxillae. The dorsal surfaces of the ‘septomaxillae’ are faintly roughened, unlike the anteroposteriorly-grooved dorsal surfaces of the ‘septomaxillae’ of Paleorhinus angustifrons (BSPG 1931 X 502), Ebrachosuchus neukami (BSPG 1931 X 501), Brachysuchus megalodon (UMMP 10336), Angistorhinus grandis (FMNH UC 631), some Angistorhinus-like specimens from the Otis Chalk localities (e.g. TMM 31098-1, TMM 31100-1332), and Angistorhinus talainti (MNHN TAL 1). Medially, the ‘septomaxillae’ in TTU P-00539 are separated from each other completely by the ascending processes of the premaxillae (Fig. 4A), rather than sutured to each other along the midline and forming most or all of the internarial septum as Sereno stated was characteristic for phytosaurs (Sereno Reference Sereno1991, p. 16). Thin laminae continue posteriorly to form the medial margins of the nares. Laterally, the ‘septomaxillae’ contact the nasals to form part of the lateral borders of the nares.

4.4. Nasal

The anterior process of the nasal articulates with the ‘septomaxilla’ dorsally and the premaxilla ventrally. Medially, thin laminae project anteriorly lateral to the premaxillae to form the medial margins of the nares, and those laminae contact the ‘septomaxillae’, approximately 2 cm anteriorly along the anteroposterior length of the internarial septum. The nasals form the majority of the lateral borders of the nares, which are inclined anterodorsally. The lateral edges of the nares project more dorsally than the internarial septum, and this projection is more prominent posteriorly. At the squared-off posterior borders of the nares, the nasals form a dorsal prominence. However, it can be inferred that the nares would not have been dorsal to the level of the skull roof. A slight constriction in mediolateral width is present just ventral to the narial margins, and this exaggerates the posterior widening of the nares in dorsal view. Laterally, the nasals are sculptured slightly by faint grooves and ridges that are oriented dorsoventrally. The posterolateral portions of the nasals are broken away and, as currently preserved, the nasals do not form any part of the inset antorbital fossae. The dorsal margins of the antorbital fenestrae are too damaged to determine whether the nasals contributed to those margins or whether the maxillae excluded the nasals. Posterior to the nares, the nasals possess a concave morphology that is ornamented more heavily. The articulations with the frontals are not preserved.

4.5. Frontal

The frontals are broken across their midsections, and the anterior portions are missing. The posterior portions of the frontals are preserved in articulation with the parietals. The anteroposteriorly oriented interdigitating suture between the left and right frontal is raised on a low ridge along the preserved length. At the posterior termination of that suture, an irregular depression is present at the junction of the frontals and the parietals. The distance between the orbits was hypothesised to have been narrower in TTU P-00539 than in Paleorhinus bransoni (Langston Reference Langston1949). The amount of breakage across the frontals requires a cautious assessment of interorbital width in TTU P-00539, and the incomplete rostrum makes estimation of skull length difficult. The frontals were at least 25 mm wide versus a length of approximately 340 mm from the premaxilla-maxilla suture to the occipital condyle in Wannia scurriensis, whereas the measured widths in two specimens of Paleorhinus bransoni are approximately 38 mm wide versus an approximate length of 342 mm in TMM 31100-101, and approximately 27 mm wide versus an approximate length of 345 mm in TMM 31025-172.

4.6. Lacrimal

The lacrimals form a large antorbital fossa around the dorsal and posterior borders of the antorbital fenestra (Fig. 4). Dorsally, the lacrimal portion of the antorbital fossa contacts a contribution from the maxilla, thus appearing to exclude the nasal from the margin of the antorbital fenestra. Though both right and left lacrimals are incomplete posterior to their portions of the antorbital fossae, the left lacrimal is less distorted than the right. The smooth fossa surrounding the posterior corner of the antorbital fenestra is widest at the posteroventral portion. Along the ventral margin of the antorbital fenestra, the lacrimal is reduced to a thin, anteriorly-directed process that excludes the jugal from participation in the antorbital fenestra.

4.7. Jugal

The anterior rami of both left and right jugals articulate along an anterodorsally–posteroventrally oriented suture with the maxillae. The anterior ramus of the jugal extends anterior to the posterior corner of the antorbital fenestra, but it does not appear to participate in any part of the margin of the fenestra or the fossa. At the ventral expression of the maxilla–jugal suture the mediolateral width of the skull decreases, enhancing the swollen appearance of the posterior portions of the maxillae in ventral view. Posteriorly, the ventral margin of the jugal curves posterodorsally so that the posterior ramus of the right jugal is raised relative to the ventral margin of the maxilla and the ectopterygoid is visible in lateral view (Fig. 3). The most posterior portion of the jugal is preserved as a rod-like process, and the contact with the quadratojugal is not preserved. On the lateral surface of the jugal, a continuous ridge slopes posteriorly from the posterior corner of the antorbital fossa and along the posterior ramus of the jugal. This continuous ridge differs from the row of raised nodes present on the lateral surface of the jugal in Paleorhinus bransoni (e.g. TMM 31100-101), Paleorhinus angustifrons (BSPG 1931 X 502), and the Krasiejów phytosaurs (Dzik & Sulej Reference Dzik and Sulej2007).

4.8. Parietal

The parietals articulate along the midline of the skull roof with an interdigitating suture. At approximately midlength along that suture, an elongate pit is present (Fig. 5). This pit was hypothesised to be separated from the endocranial cavity (Langston Reference Langston1949), but infilled matrix prevents further definitive assessment of Langston's interpretation. The parietals do not greatly overhang the dorsal portion of the supraoccipital, but they form a slight lip at the posterior margin of the skull roof. The parietals have long, mediolaterally narrow squamosal processes that are dorsally exposed and in the plane of the skull roof. Small depressions are present in the dorsal surface of the parietals at the base of those processes. Along the posterolateral extension of the squamosal processes, the parietals clearly form mediolaterally thin extensions. It is unclear how far dorsoventrally the parietals expand. In posterior view, it appears that the parietals could be only the thin dorsal expressions of the squamosal processes (Fig. 6) and separate ossifications may be in place between the parietals and supraoccipital. However, sutures in this area often are difficult to delimit, and the dorsal edges of the parietal–squamosal bars are slightly damaged.

4.9. Squamosal

Only the posterodorsal corners of the squamosals are preserved. What remains of the squamosals appears to be ornamented dorsally with rugose ridges and grooves. On the dorsal surface of the right squamosal, a trace of an anteroposteriorly trending groove is preserved. The squamosals posteriorly enclose large supratemporal fenestrae that have rounded posterior borders in dorsal view (Fig. 5). The squamosals lack posterior processes; there is no posterior expansion of the squamosals beyond the paroccipital processes of the otoocciptials (see section 4.19 for note on nomenclature). There appears to be a long descending process of the squamosal anterior to the quadratojugal, but preservation in that area of the skull is poor.

4.10. Quadratojugal and quadrate

Only the dorsal-most portion of the right quadratojugal is preserved, and no new observations can be made. The dorsal portion of the right quadrate also is preserved. A tab-like flange is visible laterally and forms part of the dorsal and lateral edges of a large, posteriorly-oriented quadrate foramen. On both the left and right sides, the large, flat quadrate flanges of the pterygoid–quadrate wings are preserved. Those wings are oriented with their flat surfaces directed anterolaterally (Fig. 6). A faint suture is distinguishable on the anterior surface of each pterygoid–quadrate wing indicating the overlapping contact of the quadrate on the pterygoid. Portions of the sutures are visible posteriorly, but poor preservation prevents observation of the entire suture.

4.11. Vomer

Portions of the vomers may be present along the midline of the palate, but that area is poorly preserved. The articulated vomers appear to comprise the choanal septum, though a fracture separates the vomers from the premaxillae.

4.12. Palatine

The well-preserved palatines are triangular in ventral view. Their medial edges are slightly convex and diverge from each other posteriorly. They are approximately 3·5 cm apart anteriorly and approximately 7·5 cm apart at their posterior corners. Anteriorly, small points of the palatines insert into the maxillae at the widest points of the choanae. The interdigitating sutures between the anterior portions of the palatines and maxillae are well defined by matrix (Fig. 4B). Posteriorly, the connection between the palatines and maxillae becomes irregular with asymmetrical fenestrae present near the lateral apices of the palatines. The posterolateral edges of the palatines form the medial edges of the elongate suborbital fenestrae. Those fenestrae have long axes oriented posteromedially. On the left side the posterior-most portion of the palatine appears to be broken away, though the right is complete and articulates with the ectopyterygoid along a short straight suture. Portions of the dorsomedial flanges of the palatines are preserved and visible though it is difficult to distinguish palatine from pterygoid on the right side. A slight curved ridge is present along the length of the palatine on each side at the juncture between the palatal flange and the dorsomedial flange.

4.13. Pterygoid

The pterygoids are preserved on both parts of the skull as it is preserved currently. The right anterior process is articulated with the right palatine on the facial portion of the skull, and the posterior portions of both the left and right pterygoids are preserved in articulation with the quadrates and basipterygoid processes of the parabasisphenoid on the braincase portion of the skull. The anterior portion of the right pterygoid is difficult to observe because that area of the palate was not fully prepared; however, it does not appear that the pterygoids form the midline of the palate or the choanal septum.

Posteriorly, the pterygoids articulate with the quadrates by overlapping flanges on the dorsoventrally deep pterygoid-quadrate wings (Fig. 6). A sharp shelf is formed by the ventral edge of the pterygoids on their posterior surfaces. Those shelves become more pronounced toward the midline as they approach the basipterygoid processes, terminating by wrapping anteriorly around the basipterygoid processes with broken anterior edges.

4.14. Ectopterygoid

The base of the right ectopterygoid is preserved, articulated with the maxilla and jugal laterally, and the palatine and pterygoid medially and posteriorly (Figs 3, 4). The suborbital fenestra extends between the ectopterygoid and the palatine for approximately two-thirds of the length of the fenestra, and the posteriormost corner of the fenestra is incised into the ventral surface of the ectopterygoid. Along the edge of that fenestra, the ectopterygoid has a rough and undulating edge. In lateral view, the body of the ectopterygoid has a strong ventral curvature posterior to the articulation with the jugal and maxilla, so that the entirety of the element is visible in lateral view. The posterior process of the ectopterygoid is poorly preserved, and both the ectopterygoid foramen and the articulation with the pterygoid are broken away. Only the lateral portion of the left ectopterygoid is preserved, still articulated with the maxilla and a small section of the left jugal, but this side does not contribute additional information about the element.

4.15. Epipterygoid

Both left and right epipterygoids are preserved in articulation with the lateral surfaces of the endocranial region (Figs 7, 8). Each long, slender epipterygoid is angled approximately 45° anterodorsally from its ventral articulation with the pterygoid and basipterygoid process to its tapered dorsal point near the exit for the trigeminal nerve in the prootic. The epipterygoids do not appear to be rounded in cross-section but rather they have flat lateral surfaces.

4.16. Basioccipital and parabasisphenoid

The rounded occipital condyle is visible in dorsal view, protruding posteriorly beyond the edge of the supraoccipital, although the exoccipital portions of the otooccipitals meet to cover nearly the entire dorsal surface of the condyle and exclude the basioccipital from the floor of the foramen magnum. In posterior view, the basioccipital is a tall element (approximately 4·5 cm in dorsoventral height) with the basitubera completely visible ventral to the condyle. Ventrally, the basioccipital has a sharp edge separating the condyle from the anteroposteriorly short neck (Fig. 7). The basioccipital forms the posterior portions of the basitubera, and the suture with the parabasisphenoid appears to be exposed more posteriorly than ventrally. A deep anteroposterior cleft widely separates the tubera along the midline, and at the posteriormost expression of this cleft there appears to be a basioccipital recess (sensu Gower, Reference Gower2002). The basitubera extend further laterally than do the basipterygoid processes, unlike the morphology in Paleorhinus bransoni (TMM 31100-101) where the basipterygoid processes are wider than the basitubera.

The parabasisphenoid forms the anterior portions of the basitubera and has a much smoother surface than the basioccipital portions of the basitubera. The basitubera and basipterygoid processes in TTU P-00539 are anteroposteriorly separated from each other by no more than 1 cm; most of the separation between these structures is in a dorsoventral direction because the basipterygoid processes extend farther ventrally than do the basitubera. At the midline between the basitubera and the basipterygoid processes is a rounded, ventrally concave depression that may be homologous with the median pharyngeal recess of other archosaurs (Witmer Reference Witmer, Currie and Padian1997; Gower Reference Gower2002). The basipterygoid processes are surrounded laterally and anteriorly by portions of the pterygoids. The exits for the internal carotid arteries are not visible ventrally; the foramina should exit the parabasisphenoid laterally in phytosaurs (Gower Reference Gower2002), and in this specimen they are most likely blocked from view by the epipterygoids. Anteriorly, the cultriform process of the parabasisphenoid is in the same transverse plane as the basioccipital and a small raised tubercle is present on its anteroventral surface. The preserved portion of the cultriform process is broken just anterior to its base, although a portion of it may continue into the broken posterior portion of the palate in the other part of the specimen (Fig. 4B).

4.17. Laterosphenoid

The left and right laterosphenoids are articulated with the ventral surfaces of the frontals and parietals, and with the prootics posteriorly (Fig. 8). These complex elements enclose two anteriorly directed foramina for the passage of cranial nerves I and II (Fig. 8). Two additional small foramina are visible on the lateral surface of the left laterosphenoid; these may be for other cranial nerves (possibly CN III and IV), but it is unclear which nerves would pass through each foramen or if they are actually blind pits that do not communicate with the endocranium. A faint groove is present that crosses the posteriormost foramen on the lateral surface of the laterosphenoid, and this may have held a branch of cranial nerve V that exited dorsally from the prootic.

4.18. Prootic

The prootic is a large element that is exposed on the lateral surface of the braincase (Fig. 8). An extensive articulation with the parietal is present at the dorsal margin of the prootic, and a foramen (possibly for the dorsal head vein) sits in a deep pocket at the posterodorsal contact between the prootic and the parietal. Posteroventrally, the prootic forms the crista prootica, which dorsally covers cranial nerve VII and forms the anteromedial border of the pteroccipital fenestra in this specimen. The trigeminal foramen is oriented anterodorsally near the anteroventral edge of the prootic, but it is difficult to discern whether the prootic forms the entire foramen in this specimen.

4.19. Otooccipital

The opisthotic and exoccipital appear to be indistinguishably fused to one another; therefore, the two elements are described together here as the otooccipital (sensu Evans Reference Evans, Gans, Gaunt and Adler2008). The exoccipital pillars are angled posteroventrally to meet posteriorly on the occipital condyle and nearly cover the basioccipital in dorsal view. Posteriorly, the otooccipitals form nearly the entire border of the circular foramen magnum (Fig. 6). Small depressions are present on the ventrolateral surfaces of the pillars, and these are interpreted as the exits for cranial nerve XII. The anterior edges of the pillars form the posterior borders of the metotic foramina, and those foramina are each separated from the fenestra ovale on each side by the crista interfenestralis (=ventral ramus of the opisthotic; Gower Reference Gower2002; Nesbitt Reference Nesbitt2011).

The nearly unexpanded paroccipital processes extend posterolaterally from their bases and form the ventral borders of small, almond-shaped posttemporal fenestrae. Proximally, the otooccipitals contact the supraoccipital in approximately horizontal sutures. Distally, the paddle-like paroccipital processes contact the squamosals and quadrates. That area is too damaged on the left side to discern sutures among these elements, though on the right side the paroccipital process appears to articulate against the posteriormost portion of the squamosal dorsomedially without covering the dorsal surface of the squamosal. The distal edges of the paroccipital processes may have been expanded more posteriorly at their ventral corners, but erosion of the tips of both processes prevents direct confirmation.

4.20. Supraoccipital

The wide supraoccipital is dorsally exposed and barely contacts the dorsal edge of the foramen magnum, which instead is bordered almost completely by the exoccipital processes of the otooccipitals. The suture with the otooccipitals is nearly horizontal, and the supraoccipital tapers to a point at the median corners of the posttemporal fenestrae. On the dorsally-exposed surface, an anteroposterior ridge trends along the midline of the element and terminates just slightly under the posterior edge of the parietals (Fig. 5). There may be two small fenestrae on either side of the midline at the anterior end of the supraoccipital ridge, but this is difficult to interpret without further preparation.