Paleorhinus angustifrons, (KUHN, 1936)

Butler, Richard J., Rauhut, Oliver W. M., Stocker, Michelle R. & Bronowicz, Robert, 2014, Redescription of the phytosaurs Paleorhinus (‘ Francosuchus’) angustifrons and Ebrachosuchus neukami from Germany, with implications for Late Triassic biochronology, Zoological Journal of the Linnean Society 170 (1), pp. 155-208 : 162-180

PALEORHINUS ANGUSTIFRONS ( KUHN, 1936)

‘ Francosuchus angustifrons n. sp. ’; Kuhn, 1936: 69, textfig. 1–3, pl. 9, fig. 2a, b, pl. 10, figs 3, 5 ‘ Francosuchus angustifrons Kuhn’; Kuhn, 1938: 318 ‘ Francosuchus angustifrons ’; Huene, 1939: 142 ‘ Ebrachosuchus angustifrons ’; Huene, 1939: fig. 2 ‘ P. broilii ( Kuhn, 1933) ’; Gregory, 1962: 670 ‘ P. (F.) angustifrons O. Kuhn, 1936 ’; Westphal, 1976: 109, fig. 7c ‘ P. neukami Kuhn, 1936 ’; Hunt & Lucas, 1991: 489, 494 ‘ Paleorhinus sp. ’; Long & Murry 1995: 36

Holotype: BSPG 1931 X 502, skull missing the anterior portions of the premaxillae ( Kuhn, 1936: textfig. 1–3, pl. 9, fig. 2a, b, pl. 10, figs 3, 5; Figs 3–8 ).

Locality and horizon: Bed 9 ( Kuhn, 1933, 1936) of the Ebrach quarry, Bamberg district, Upper Franconia region of northern Bavaria, Germany. Blasensandstein of the Sandsteinkeuper, laterally equivalent to the Hassberge Formation of the Middle Keuper (Late Triassic: late Carnian).

Diagnosis: Species of Paleorhinus characterized by the following autapomorphic features: (1) stepped lateral rim of external naris that is strongly swollen and rugose at posterior end; (2) paired depressions on the anterior portions of the nasals (immediately posterior to the external nares) and anterior portions of the frontals; (3) foramen in ectopterygoid enlarged and subcircular in outline; (4) suborbital foramen elongate and boomerang shaped; (5) large postparietal foramen at junction between supraoccipital and parietal.

Comments: Several of the features identified here as autapomorphic for Paleorhinus angustifrons (characters 1, 2, 5) are also known in phytosaur material from Krasiejów in southern Poland (e.g. ZPAL Ab III 111, 200; Dzik, 2001) that is currently under study by R.B. If the Krasiejów phytosaur material proves to represent a species distinct from P. angustifrons then the status of these character states will probably change, to synapomorphies supporting a sistergroup relationship between P. angustifrons and the Krasiejów phytosaur.

DESCRIPTION OF HOLOTYPE OF PALEORHINUS ANGUSTIFRONS

General: Although generally well preserved, the surface of the skull ( Figs 3–8 ) has been reconstructed in some places (e.g. left ectopterygoid, pterygoid wing of right quadrate, much of the surface of the skull roof adjacent to the left orbit). Most of the cranial sutures are tightly closed or even fused. The skull as a whole is dorsoventrally shallow ( Figs 3D , 4D ), as is typical of early phytosaurs generally (e.g. Langston, 1949; Stocker, 2010), although this has undoubtedly been exaggerated by substantial dorsoventral crushing (for example, the occipital condyle appears to be strongly flattened dorsoventrally). As preserved, the skull measures approximately 320 mm from the broken anterior tip of the rostrum to the most posteriorly positioned part of the squamosals, and 184 mm transversely at the lateral extents of the quadrate condyles. The rostrum is broken a short distance anterior to the external nares, and most of the premaxilla has been lost. It is not possible to accurately reconstruct the missing length of the premaxilla, although com- parison with the Krasiejów phytosaur specimen ZPAL Ab III 200 suggests a skull length of approximately 500 mm. Most external skull elements have a weakly sculptured surface, although the rather smooth appearance might at least partially be due to preservation and/or preparation.

Fenestral morphology: The dorsally directed external nares are teardrop shaped in dorsal view ( Figs 3A , 4A , 5A : ‘en’), being rounded posteriorly and tapering anteriorly (each is approximately 40 mm in total length, and 13 mm wide at the widest point). Their

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posterior margins are set substantially anterior (c. 25 mm) to the anterior corners of the internal antorbital fenestrae (sensu Witmer, 1997a), and to the anterior corners of the antorbital fossae (separated by c. 20 mm). This is similar to the condition in Paleorhinus bransoni (FMNH UC 632; Lees, 1907; Stocker, 2010), Parasuchus hislopi ( Chatterjee, 1978) , Wannia scurriensis (TTU P-00539; Langston, 1949; Stocker, 2010, 2013), and in phytosaur specimens from Krasiejów (e.g. ZPAL Ab III 200, 1943, Dzik & Sulej, 2007). In all of these taxa the nares are also set distinctly anterior to the fossa. By contrast, in Ebrachosuchus neukami , the anterior margins of the internal antorbital fenestrae are positioned very close to the posterior rims of the external nares (separated by only c. 7 mm; BSPG 1931 X 501). In phytosaurid phytosaurs the posterior rims of the external nares are positioned posterior to the anterior corners of the internal antorbital fenestrae ( Stocker, 2010). In lateral view ( Figs 3D , 4D ), the dorsal rims of the external nares of Paleorhinus angustifrons are positioned below the level of the frontoparietal skull table, although they expand dorsally slightly above the level of the surface of the nasals.

The internal antorbital fenestra ( Figs 3A, D , 4A, D , 5C : ‘afen’) faces dorsolaterally and has a lenticular or eye-shaped outline (c. 58 mm long, 21 mm deep). In dorsal view ( Figs 3A , 4A ), the long axis of the fenestra extends subparallel to the skull midline. It is surrounded along nearly its entire margin by an antorbital fossa ( Figs 3A, D , 4A, D , 5C : ‘afos’), which is bounded by an external antorbital fenestra (sensu Witmer, 1997a). The antorbital fossa is excavated into the maxilla anterodorsally and anteroventrally, and into the lacrimal posterodorsally and posteroventrally. The fossa has a maximum length of 80 mm, and its posterior expansion onto the lacrimal is more than twice as long as the anterior expansion onto the maxilla ( Fig. 5C ). Ventrally, the fossa is discontinuous at the anteroventral border of the internal antorbital fenestra (i.e. there is a short section of the border of the internal antorbital fenestra that does not have an adjacent fossa), but dorsally, the anterior and posterior expansions of the fossa are connected by a broad depressed area on the ascending process of the maxilla and the anterior process of the lacrimal ( Fig. 5C ). The nasal suture forms the dorsal rim of the antorbital fossa. As far as can be determined, the jugal reaches the posteroventral rim of the antorbital fossa, but not the rim of the internal antorbital fenestra, so that the external rim of the internal antorbital fenestra appears to be formed only by the maxilla and lacrimal ( Fig. 4D ). The antorbital fossa is similarly well developed in Paleorhinus bransoni (TMM 31100- 101; Stocker, 2010), Parasuchus hislopi ( Chatterjee, 1978) , and Wannia scurriensis (TTU P-00539; Langston, 1949; Stocker, 2010, 2013), but is strongly reduced in Ebrachosuchus neukami , in which it is almost limited to the lacrimal at the posterior margin of the internal antorbital fenestra (BSPG 1931 X 501; see below). The antorbital fossa is reduced or absent in ‘ Paleorhinus ’ sawini and Phytosauridae ( Stocker, 2010) .

The dorsolaterally facing orbit ( Figs 3A, D , 4A, D : ‘orb’) is ovoid in outline, being moderately longer than deep (left orbit measured at widest points: anteroposterior length = 43 mm; dorsoventral depth = 33 mm) and with rounded anterior and posterior margins. As in other non-phytosaurid phytosaurs ( Ebrachosuchus neukami, BSPG 1931 X 501; Paleorhinus bransoni, TMM 31100-101, Stocker, 2010; Parasuchus hislopi, Chatterjee, 1978 ; the Krasiejów phytosaur specimens, ZPAL Ab III 200, Dzik & Sulej, 2007) the ventral margin of the orbit is slightly ventral to the dorsal margin of the internal antorbital fenestra ( Figs 3D , 4D ), whereas the orbit is distinctly elevated in leptosuchomorph phytosaurs such as Pravusuchus (AMNH FR 30646) and Mystriosuchus (GPIT 261/001; Stocker, 2010: fig. 7), probably as a result of the proportionally deeper skulls of these taxa.

The dorsolaterally and slightly anteriorly (because of the transverse expansion of the skull across the ventral margin of the quadrates) facing infratemporal fenestra ( Figs 3A, D , 4A, D , 6A : ‘itf ’) has a trapezoidal outline, being slightly longer anteroposteriorly at its ventral margin than at its dorsal margin (anteroposterior length along ventral edge = 41 mm; anteroposterior length along dorsal edge = 34 mm; dorsoventral height at midpoint of fenestra, measured at 90° to the jugal/quadratojugal bar = 41 mm). Its margins are formed by the quadratojugal, jugal, postorbital, and squamosal. The dorsal margin of the infratemporal fenestra is positioned approximately at the level of the midpoint of the orbit (i.e. the rear part of the skull is slightly downturned relative to the orbital region); at its anteroventral corner the infratemporal fenestra extends beneath the most posterior part of the orbit. The shape of the infratemporal fenestra is similar to that of Paleorhinus bransoni (TMM 31100-101; Stocker, 2010), ‘ Paleorhinus ’ sawini (TMM 31213-16; Long & Murry, 1995), the Krasiejów phytosaurs (ZPAL Ab III 200, Dzik & Sulej, 2007), and Parasuchus hislopi ( Chatterjee, 1978) , but it is markedly different from the autapomorphic, highly anteroposteriorly elongated infratemporal fenestra of Ebrachosuchus neukami (BSPG 1931 X 501; see below), and also differs from the dorsoventrally elongated fenestra seen in many phytosaurid phytosaurs (e.g. Hungerbühler, 2002; Stocker, 2010).

The dorsally facing supratemporal fenestra has a subtriangular outline that tapers to an apex posterolaterally ( Figs 3A , 4A , 5F : ‘stf ’). As is usual in archosaurs, the supratemporal fenestra is bordered anteriorly and anterolaterally by the postorbital, anteromedially, medially and posteriorly by the parietal, and posterolaterally by the squamosal. As in other non-phytosaurid phytosaurs, as well as Angistorhinus (TMM 31098-1) and Brachysuchus megalodon (UMMP 10336), the parietal–squamosal bars are in the same horizontal plane as the skull roof ( Stocker, 2010). Moreover, the supratemporal fenestrae are completely open dorsally, unlike the condition in most leptosuchomorph taxa in which they are partially or entirely obscured by expansions of the postorbital and/or squamosal (e.g. Pravusuchus, AMNH FR 30646; Stocker, 2010). However, a supratemporal fossa anterior to the fenestra, as present in, for example, Euparkeria (SAM-PK-5867), is absent ( Fig. 5F ), with the posterior and medial edges of the postorbital, the anterolateral, lateral and posterolateral edges of the parietal, and the anteromedial edge of the squamosal forming a sharp rim. This sharply defined rim of the supratemporal fenestra is only interrupted in its posterolateral corner, where the supratemporal fenestra opens onto a slight dorsal depression on the dorsal surface of the squamosal.

The posttemporal fenestra ( Figs 3C , 4C , 6C : ‘ptf’) is present on the occipital surface approximately halfway between the foramen magnum and the lateral tip of the paroccipital process, being placed entirely dorsal to the level of the foramen magnum. It is elongate and slit like, with margins formed by the otooccipital ventrally, and the squamosal and supraoccipital dorsally, with the parietal being excluded from its margin by a small contact between the latter bones along its dorsal rim.

A small postparietal foramen ( Figs 3C , 4C , 6C : ‘ppfor’) is present at the dorsal extremity of the midline of the occipital surface, and is bordered by the parietals (dorsally and laterally) and the supraoccipital (ventrally). Whereas the ventral margin of this fenestra is straight, its dorsal margin is strongly arched. The foramen connects to what appears to be a sinus positioned above the posterior part of the endocranial cavity (visible in CT data). A similar foramen is also present in one of the Krasiejów phytosaur specimens (ZPAL Ab III 200), but has not been reported to our knowledge for any other phytosaur, and it is considered here as an autapomorphy of Paleorhinus angustifrons . Harris (2006) and Balanoff, Bever & Ikejiri (2010) discussed a postparietal foramen in a similar position in several sauropods that is proposed to be associated with the dural venous sinus ( Balanoff et al., 2010). Openings in a similar position have also been described in the small ornithopod Dysalotosaurus ; in this species, a single, rather large foramen in juveniles becomes subdivided into two smaller foramina by a posterior process of the parietals during ontogeny ( Pompeckj, 1920; Janensch, 1955). The same variation might also be present in phytosaurs, because a second skull from Krasiejów (ZPAL Ab III 111) appears to show a subdivided foramen.

The foramen magnum ( Figs 3C , 4C , 6C : ‘fm’) is a large, circular opening that is mostly bordered by the otooccipital, with only a small portion of the dorsal margin made up by the supraoccipital, and a small part of the floor possibly being contributed by the basioccipital.

The subtemporal fenestrae ( Fig. 3B , 4B : ‘subtf ’) are large and posterolaterally placed. They are elongate and ovoid in outline, being slightly transversely wider anteriorly than posteriorly. The margin of each subtemporal fenestra is formed by the ectopterygoid anteriorly, the pterygoid anteromedially and medially, the quadrate posteromedially and posteriorly, and the quadratojugal and jugal laterally. The lateral process of the pterygoid and the main body of the ectopterygoid ventrally underlie the anterior end of the fenestra.

The suborbital fenestra (= postpalatine fenestra of Sereno, 1991; = suborbital foramen of Stocker, 2010) is elongate ( Figs 3B , 4B , 7C, D : ‘sub’) and is similar to that of the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007), but it is curved more strongly along its length in BSPG 1931 X 502, with a boomerang shape. This boomerang shape appears to be autapomorphic for Paleorhinus angistifrons . Its margins are formed by the ectopterygoid and maxilla laterally, and the palatine medially. The choanae ( Figs 3B , 4B , 7C : ‘cho’) are larger but also slit-like, with rounded anterior and tapering posterior ends (although the posterior margins are not completely preserved), and they diverge slightly from one another anteriorly. Their anterolateral margins are formed by the maxillae, their lateral margins by the palatines, and their medial and posteriormost margins by the posteriorly expanding vomers. The choanae are positioned medial to the antorbital fenestra and posterior to the external nares.

Premaxilla: Only the most posterior parts of the premaxillae are preserved ( Figs 3 , 4 : ‘pm’); on the right side there appear to be no parts of the premaxillary tooth row preserved, whereas on the left the most anterior three of the preserved alveoli appear to be in the premaxilla. On the dorsal surface of the skull the premaxilla contacts the maxilla, the ‘septomaxilla’, and the nasal. Externally, the premaxillae form the midline of the rostrum anterior to the external nares, and each premaxilla possesses a short posteromedial process that projects between the anterior parts of the ‘septomaxillae’ ( Fig. 5A : ‘pmp’), but is separated by the latter and an anterolateral process of the nasal from the margins of the external nares. Laterally, a short posterolateral process of the premaxilla appears to project between the nasal and the maxilla, ventral to the anterior part of the external naris, as also occurs in Paleorhinus bransoni (TMM 31100-101; Stocker, 2010) and Parasuchus hislopi ( Chatterjee, 1978) . The premaxillae are dorsoventrally compressed, although they have probably been slightly crushed, and their dorsal surface is gently convex transversely. The external surfaces of the premaxillae are only weakly ornamented, with faint longitudinal striations.

On the ventral surface ( Figs 3B , 4B , 7B, C ), the transversely flat-to-gently-concave premaxillary palate is formed by medially extending shelves (one contributed by each premaxilla) that form a broad interpremaxillary fossa. At their lateral margins, adjacent to the alveoli, these shelves are thickened into distinct alveolar or palatal ridges (approximately 6 mm in width at the most anterior preserved point) that continue posteriorly onto the maxillae ( Figs 3B , 4B , 7B : ‘alvr’). These alveolar ridges are visible in lateral view, projecting ventrally just below the external margin of the snout ( Fig. 3D , 4D ). The premaxillary palate tapers in transverse width towards its posterior termination, which is placed slightly posterior to the level of the posterior margin of the external nares. Posteriorly, the premaxillae contact the vomers ( Fig. 7C ).

The few premaxillary alveoli preserved are round in outline, much smaller than the mid-maxillary alveoli and closely spaced. There is no notable change in size, spacing or shape of the alveoli at the premaxilla– maxilla boundary ( Fig. 7B : ‘pm-max’).

CT data show that, anterior to the external nares, an anteroposteriorly extending, centrally positioned, sediment-infilled premaxillary cavity (pneumatic paranasal sinus) with a sub-quadrate outline is present above the premaxillary and maxillary palates ( Fig. 8A: ‘mcv’). It is likely that this cavity housed diverticula of the antorbital air sinuses, possibly as an adaptation to resisting torsion ( Witmer, 1997a). Lateral to this median sinus and adjacent to the alveoli is an anteroposteriorly extending alveolar neurovascular canal ( Fig. 8A–C: ‘nvc’) that would have contained the maxillary branch of the trigeminal nerve, and associated arteries and veins ( Witmer, 1997a). The canals connect via foramina with each of the thecodont alveoli, and also have connections with the median sinus. More posteriorly, the median sinus is continuous with the airway, external nares, and antorbital cavity ( Fig. 8B, C).

Maxilla: The maxilla ( Figs 3 , 4 : ‘max’) is an anteroposteriorly elongate and dorsoventrally shallow bone. Seventeen (empty) alveoli are present in the left maxilla (assuming that the most anterior three alveoli belong to the premaxilla), and 16 are present in the right maxilla. In lateral view, the left maxilla has a gently convex ventral margin. On the right maxilla the ventral margin appears to be nearly straight along its entire length, although this is probably due to deformation. The ventral margin of the maxilla is strongly convex in ‘ Paleorhinus ’ sawini (TMM 31213-16; Long & Murry, 1995).

Externally, the maxilla of Paleorhinus angustifrons contacts the premaxilla and nasal dorsally, the lacrimal posteriorly (both dorsal and ventral to the internal antorbital fenestra), and the jugal posteriorly. The bone has a slender main body with a posterodorsally extending ascending process ( Fig. 5C : ‘asc’) that is placed at about mid-length of the bone. The part of the main body placed anterior to the ascending process is subequal in height to that part placed posterior to it. The ascending process is strongly inclined posteriorly, forming an angle of more than 45° from the vertical. It is very slightly curved and meets the anterior process of the lacrimal above the antorbital fenestra at about two-thirds of the length of the fenestra.

The major feature of the lateral surface of the maxilla is the large internal antorbital fenestra and surrounding antorbital fossa. The antorbital fossa extends some 9 mm anterior to the antorbital fenestra onto the base of the ascending process ( Fig. 5C : afos), where it is shallow and demarcated by low ridges that define the external antorbital fenestra. At the anterior end of the fossa there is a small foramen that connects anteriorly to the alveolar canal, as shown by CT data. On the left side, this foramen also appears to have a connection to the central airway. Dorsally, the antorbital fossa covers the entire height of the majority of the ascending process of the maxilla, and is bordered at its dorsal extremity by a distinct step formed by the nasal (which represents the margin of the external antorbital fenestra). Ventrally, the maxillary antorbital fossa rapidly narrows in dorsoventral height posteriorly and disappears at about one-third of the length of the antorbital fenestra. At approximately mid-length of the ventral margin of the internal antorbital fenestra a posteriorly expanding antorbital fossa appears again, but this is probably entirely placed on the lacrimal. The external surface of the maxilla is strongly convex below the posterior end of the antorbital fossa (due to the anterior extension of a longitudinally orientated ridge that is present on the jugal; see below), and becomes flat to gently concave anteriorly.

On the ventral surface ( Figs 3B , 4B , 7B, C ), the alveoli generally have a subcircular outline. The preserved alveoli are small anteriorly (the fourth most anterior alveolus preserved on the left side, probably representing the most anterior alveolus of the maxilla, has an anteroposterior diameter of 4 mm) and increase in size posteriorly, reaching a maximum diameter in the region lateral to the choanae (the 13th alveolus preserved on the left side, which is probably the 10th maxillary alveolus, has an anteroposterior diameter of 10 mm). Alveolus size decreases again slightly at the posterior end of the tooth row, which is placed just anterior to the anterior rim of the orbit. The alveoli are closely spaced in the anterior and posterior ends of the maxilla, but become more widely spaced towards its mid-length, as evidenced by the mesiodistally expanded interalveolar septa. Medial to the tooth row, the alveolar/palatal ridges are continuous with those of the premaxillae, but become narrower and less pronounced posteriorly although the ridges are present as a distinct thickening along the entire tooth row. The maxilla ends just posterior to the tooth row and lacks an elongate posterior process.

The maxilla is contacted by the ectopterygoid immediately posterior and medial to the last alveolus ( Fig. 7D ). The maxilla forms the anterolateral margin of the slit-like suborbital fenestra; anterior to this fenestra the maxilla has a medially extending shelf that forms the anterolateral and anterior margins of the choana ( Figs 3B , 4B , 7C ), and which prevents contact between the premaxilla and the palatine. Contact between the premaxilla and the palatine previously was cited as a synapomorphy of Phytosauria ( Sereno, 1991) but is absent in many taxa including Wannia scurriensis ( Stocker, 2010, 2013), the Krasiejów phytosaur specimens (e.g. ZPAL Ab III 200), and Nicrosaurus kapffi and N. meyeri ( Hungerbühler, 1998) .

‘Septomaxilla’: Kuhn (1936) reconstructed the ‘septomaxillae’ (for discussion of the homology of this element see Sereno, 1991; Hungerbühler, 2002; Stocker, 2010; Nesbitt, 2011) as forming the entire internarial bar in Paleorhinus angustifrons ; such a situation would be unusual, because the internarial bar is typically formed by both the ‘septomaxilla’ and the nasal or predominantly by the nasal in phytosaurs (e.g. Stocker, 2010). However, a clear ‘septomaxilla’–nasal contact cannot be identified. Given that a ‘septomaxilla’ is generally present in phytosaurs, we assume that this element is present ( Figs 3A , 4A , 5A : ‘sm’), but that the ‘septomaxillae’ and nasals are indistinguishably fused. In comparison with other phytosaurs, it seems most likely that the contact occurs at approximately the midpoint of the internarial bar, where the bar narrows in transverse width and the dorsal margin is slightly concave. In lateral view, the internarial bar is raised along the anterior half of its length to a position more dorsal than the lateral borders of the external nares ( Figs 3D , 4D , 5B ).

The ‘septomaxillae’ articulate with one another along the midline posteriorly and slightly flare mediolaterally anteriorly, where they terminate a short distance anterior to the anterior corner of the external nares ( Figs 3A , 4A ), as indicated by Kuhn (1936: fig. 1). However, differing from the reconstruction of Kuhn, the anterior processes of the ‘septomaxillae’ are separated from one another by posteromedial processes of the premaxillae ( Fig. 5A ), as in the Krasiejów phytosaur specimens (e.g. ZPAL Ab III 200; Dzik & Sulej, 2007) and Paleorhinus bransoni (TMM 31100-101; Stocker, 2010). Anterolaterally, the ‘septomaxillae’ are separated from the nasals by a groove ( Fig. 5A : ‘gr’) that extends anteriorly from the anterior corner of the external naris. CT data show that this groove continues anteriorly as a foramen for a short distance, apparently within the premaxilla. On the right side, a small connection of this foramen with the median paranasal sinus is visible in CT data. The ‘septomaxillae’ differ from those of Wannia scurriensis (TTU P-00539), which do not articulate with one another on the midline or form any part of the internarial septum ( Stocker, 2010, 2013).

Nasal: The nasals ( Figs 3A, D , 4A, D : ‘na’) are anteroposteriorly elongate elements that form the posterior and lateral margins of the external nares, and probably the posterior half of the internarial septum (although the ‘septomaxilla’–nasal contact is unclear: see above). They contact the premaxillae and maxillae laterally, the prefrontals posterolaterally, and the frontals posteriorly. The nasals are flat to slightly convex transversely, and are strongly rugose along their lateral margins, dorsal to the antorbital fossa and along the raised lateral rim of the external naris. The part of the nasal forming the rim of the external naris is distinctly raised and thickened, most prominently along the posterolateral and posterior margins, where it also flares laterally ( Fig. 5A : ‘fl’), contributing to the teardrop-shape of the nares. In lateral view, the rim of the external naris is therefore stepped, being low and concave anteriorly, but raised into a prominent knob-like convexity posteriorly, with a much smaller knob anteriorly ( Fig. 5B : ‘convx’). A similar condition is absent in Parasuchus hislopi ( Chatterjee, 1978) , Wannia scurriensis (Stocker, 2013) and Paleorhinus bransoni (TMM 31100-101; Stocker, 2010), but does appear to be present in the Krasiejów phytosaur specimens (ZPAL Ab III 200). The anterior processes of the nasals forming the lateral margins of the nares continue anteriorly some 13 mm anterior to the external nares. Here, the processes are slightly expanded mediolaterally and form the lateral borders of the grooves anterior to the nares mentioned above, and thus together with the ‘septomaxillae’ exclude the premaxillae from the external nares. There is a small depression on the nasal, immediately posterior to the rim of the external naris ( Figs 3A , 4A , 5A : ‘ndp’); a similar depression is present in the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007: fig. 10).

Laterally the nasals form the dorsal margin of the external antorbital fenestrae, thus bordering the antorbital fossae, and they have clearly defined sutures with the prefrontals. Kuhn (1936: fig. 1) identified the nasal–frontal suture in a posterior position, level with the anterior corner of the orbit. The iden- tification of the nasal–frontal suture in this area was probably based upon paired depressions on the skull roof between the prefrontals ( Figs 3A , 4A , 5E : ‘fdp’), which are also seen in some other other basal phytosaurs (M. R. Stocker, pers. obs. of TMM 31025- 172, Paleorhinus bransoni ; the Krasiejów phytosaurs, ZPAL Ab III 200). However, the actual nasal–frontal suture is placed more anteriorly ( Figs 3A , 4A , 5E : ‘na-fr’), nearly level with the posterior margin of the antorbital fenestra, as also occurs in the Krasiejów phytosaurs (ZPAL Ab III 200, Dzik & Sulej, 2007), Parasuchus hislopi ( Chatterjee, 1978) , and Paleorhinus bransoni ( Stocker, 2010) . Thus, the paired posterior depressions are positioned on the anterior part of the frontals, rather than the nasals, and each nasal forms a sharply pointed posterior process separated from the other element by the anterior processes of the frontals. The posteriormost extension of the nasal reaches approximately level with the mid-point between the antorbital fenestra and the orbit.

Prefrontal: The prefrontal forms the anteromedial and most of the anterior margin of the orbit ( Figs 3A , 4A : ‘prf ’), and is a substantially larger element than that identified by Kuhn (1936). It is approximately triangular in dorsal view, tapering to points anteriorly, posteriorly, and laterally. Its dorsal and lateral surfaces are strongly rugose and pitted (it is one of the most strongly ornamented of the skull roof elements), and it is thickened at its orbital margin. This thickening is offset from the dorsal surface of the prefrontal by a marked dorsal step. A shallow, crescentic fossa is present on the dorsal surface just anterior to, and curving parallel to, this thickening of the orbital margin ( Fig. 5E : ‘prod’; Fig. 8F). A similar feature was described by Hungerbühler (2002) in Mystriosuchus as the ‘pre-orbital depression’, and its strong development in Mystriosuchus was considered by him to be autapomorphic for that genus. Laterally, the exact position of the prefrontal–lacrimal suture is difficult to determine, but it probably occurs close to the sharp junction between the nearly horizontal skull roof and the nearly vertical lateral surface of the skull anteroventral to the orbit. The prefrontal–nasal suture is clearly defined ( Fig. 5E : ‘na-prf’), whereas that with the frontal is clearly defined anteriorly, but becomes difficult to trace more posteriorly. The prefrontal clearly forms the anteromedial and anterior parts of the orbital rim.

Frontal: The surface of the left frontal is heavily reconstructed ( Fig. 8F, G) and the definition of the margins of the frontals generally is poor: their sutures with the nasals and the prefrontals are clear anteriorly, but posteriorly the sutures with the postfrontals and parietals are difficult to recognize. The dorsal surfaces of the frontals ( Figs 3A , 4A : ‘fr’) are covered with rugose and pitted ornamentation similar to that of the prefrontal and other skull roof bones. The frontals are gently concave transversely (judging from the surface of the right frontal), and towards their anterior ends they each bear a small depression on their dorsal surfaces (see above). These depressions ( Figs 3A , 4A , 5E : ‘fdp’) are mainly marked by raised posterior and posterolateral rims and are continuous anteriorly with the rather smooth dorsal surface of the nasals, whereas the surfaces of the frontals posterior to and the prefrontals lateral to the depressions are notably rugose. The frontals contribute to the medial rims of the orbits, and they are slightly thickened at their lateral margins (seen clearly on the right side and in CT images: Fig. 8G) as also occurs in many other phytosaurs ( Stocker, 2010). On the right side the ventral surface of the frontal is partially exposed. The contribution of the frontal to the medial margin of the orbit curves medioventrally towards the contact with the laterosphenoid (at the anteriormost extent of the latter bone). CT data reveal a marked transverse concavity on the medial parts of the ventral surfaces of the frontals for the reception of the olfactory bulbs ( Fig. 8G). This concavity is narrow between the orbits but widens anteriorly ( Fig. 8F, G).

Lacrimal: The lacrimal forms the anteroventral rim of the orbit and the posterior rim of the antorbital fenestra ( Figs 3A, D , 4A, D : ‘lc’). It contacts the maxilla anteriorly, both dorsal and ventral to the internal antorbital fenestra (although the exact position of the lacrimal–maxilla suture ventral to the internal antorbital fenestra is uncertain), the jugal ventrally, and the prefrontal and nasal dorsally. Most of the anterior portion of the lacrimal is emarginated laterally by the antorbital fossa ( Fig. 5C ), which is continuous from the anterodorsal process to the anterior expansion of the ventral end of the main body. At its posteriormost point, the antorbital fossa is separated from the orbit by a raised surface only 7.7 mm wide. Posterodorsally, there is a marked step on the lateral surface of the element along the orbital margin; the prefrontal–lacrimal suture appears to be situated just medial to this step. Whereas the orbital margin and skull roof dorsal to this step are strongly rugose, the orbital margin and surface of the lacrimal ventral to the step are smooth. The nasolacrimal foramen is visible in CT data on the internal surface of the orbital margin, at about the mid-height of the orbit, and is probably positioned on the boundary between the lacrimal and prefrontal as in other phytosaurs ( Witmer, 1997a), although the suture is not visible. Anteriorly, the nasolacrimal canal extends dorsomedial to the antorbital fenestra, but becomes very narrow and difficult to trace in CT data. In contrast to the situation in Machaeroprosopus pristinus , where the canal divides into two branches, one leading anteriorly and one ventrally ( Camp, 1930), only a single canal is evident in Paleorhinus angustifrons .

Jugal: The jugal is a triradiate element whose contacts with surrounding elements are relatively clearly visible on the left side of the skull ( Figs 3A, D , 4A, D : ‘jg’). The dorsal process of the jugal articulates with the postorbital along an almost horizontal suture level with the ventral border of the orbit, and forms part of the ventral rim of the orbit and the ventral part of the anterior rim of the infratemporal fenestra. The jugal–postorbital suture appears to be overlapping, with the dorsal process of the jugal extending dorsally along the medial surface of the posterior margin of the jugal–postorbital bar. There is a small and very shallow fossa on the dorsal process of the jugal ( Figs 3A , 4A , 6A : ‘fo’), adjacent to the anteroventral corner of the infratemporal fenestra (most clearly visible on the left side), the anterior margin of which is defined by a low break-in-slope. A similar fossa is present in some other phytosaurs (e.g. Mystriosuchus planirostris, Hungerbühler, 2002 : fig. 12C; Pravusuchus hortus, Stocker, 2010 : fig. 6; Krasiejów phytosaur specimen ZPAL Ab III 200).

The transversely compressed and dorsoventrally shallow posterior process of the jugal forms the anterior part of the ventral border of the infratemporal fenestra. This posterior process tapers in dorsoventral height towards its posterior termination, and it is extensively overlapped along its dorsal edge by the anterior corner of the quadratojugal. The posterior process of the jugal extends close to the quadrate on the lateral surface, but the clearly visible suture with the quadratojugal on the medial surface of the left jugal demonstrates that it does not contact the quadrate.

The dorsoventrally deep anterior process of the jugal contacts the maxilla laterally along an anterodorsallyto-posteroventrally trending suture, which begins immediately ventral to the posterior corner of the internal antorbital fenestra and ends at the posterior end of the maxillary tooth row. The anterior process also forms an elongate suture with the lacrimal that coincides with the ventral margin of the antorbital fossa. The jugal apparently thus forms part of the posteroventral rim of the antorbital fossa (margin of the external antorbital fenestra), but is excluded from entering the rim of the internal antorbital fenestra by the contact between the maxilla and the lacrimal (contra Kuhn, 1936). Medially, the jugal is contacted by the ectopterygoid adjacent to the jugal–maxilla contact ( Fig. 7D ).

In lateral view, the jugal is proportionally shallower dorsoventrally and longer anteroposteriorly than the deep and short element shown in Parasuchus hislopi ( Chatterjee, 1978) , and the very dorsoventrally deep jugal of ‘ Paleorhinus ’ sawini ( Long & Murry, 1995: fig. 24A), but similar to that of some of the Krasiejów phytosaur specimens (e.g. ZPAL Ab III 200, Dzik & Sulej, 2007) and Paleorhinus bransoni ( Long & Murry, 1995: fig. 24D). The ventral margin of the jugal is only gently concave in lateral view in Paleorhinus angustifrons , rather than strongly concave as occurs in Wannia scurriensis (TTU P-00539), and Parasuchus hislopi ( Chatterjee, 1978) . As a result, only a small part of the ectopterygoid is visible in lateral view in Paleorhinus angustifrons . However, it is possible that these features of the jugal may have been affected by dorsoventral crushing of the skull.

Approximately 10 mm ventral to the ventral rim of the orbit (at approximately two-thirds of the dorsoventral height of the jugal), the ornamentation of the lateral surface of the jugal takes the form of a linear row of six to seven slightly raised nodes, extending anteroposteriorly from the posteroventral rim of the antorbital fossa to the anterior margin of the small fossa adjacent to the infratemporal fenestra ( Figs 3A, D , 4A, D , 5D , 6A : ‘jgb’). A similar nodular row is present in the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007) and referred specimens of Paleorhinus bransoni (TMM 31100-101; M. R. Stocker, pers. obs), but this row is absent in other phytosaur taxa. The lateral surface of the jugal immediately dorsal and ventral to this row of nodes is smooth. At approximately 10 mm (one-third of the dorsoventral height of the element) from the ventral margin of the jugal there is a marked ridge that extends anteriorly from the base of the posterior process of the jugal onto the maxilla, fading out beneath the antorbital fenestra ( Fig. 5D : ‘jmri’). This ridge separates the row of nodes and associated smooth surface texture (above) and a more typical rugose surface texture (below, and on the posterior process).

On the medial surface of the jugal there is a large, concave area ventral to the orbit, which is separated from the medial surface of the posterior process by a pronounced internal ridge that extends posterodorsally from the contact with the ectopterygoid onto the dorsal process of the jugal. The medial surface of the dorsal process of the jugal is slightly bevelled along the margin of the infratemporal fenestra.

In CT cross sections ( Fig. 8F) an anteroposteriorly extending canal is visible within the anterior part of the jugal. Anteriorly, this canal enters the bone dorsomedially approximately along the ectopterygoid– jugal contact, posterior to the maxillary tooth row and lateral to the suborbital fenestra (at approximately 60% of the anteroposterior length of the fenestra). It extends posterolaterally and slightly dorsally into the jugal for approximately 24 mm, and exits the jugal on its medial surface immediately posterior to the termination of its contact with the ectopterygoid. Posteriorly the course of this canal continues as a narrow trough visible for around 35 mm on the medial surface of the base of the posterior process of the jugal. Anteriorly, a much smaller canal appears to branch off from the main canal within the jugal and opens dorsomedially through a foramen on the dorsal surface of the ectopterygoid lateral to the suborbital fenestra. A foramen piercing anteriorly into the jugal body on its medial surface is also present in Pravusuchus (AMNH FR 30646), ‘ Machaeroprosopus ’ zunii (UCMP 27159), and ‘ Phytosaurus ’ doughtyi (ANMH FR 4919) ( Stocker, 2010). Stocker (2010) suggested that this canal may have housed the maxillary branch of the trigeminal nerve.

Postorbital: The postorbital is a triradiate element ( Figs 3A, D , 4A, D : ‘po’), the entire external surface of which is weakly rugose. The central body and ventral process of the postorbital form most of the posterior rim of the orbit and the dorsal half of the anterior rim of the infratemporal fenestra. The posterior process of the postorbital forms the anterior half of the dorsal rim of the infratemporal fenestra, whereas the posterior and medial processes of the postorbital together form the anterolateral rim of the supratemporal fenestra. The exact positions of the sutures between the postorbital and the parietal and postfrontal are not clear. The posterior process of the postorbital terminates dorsal to the midpoint of the infratemporal fenestra (contra Kuhn, 1936, who reconstructed this as a substantially longer process), and forms the anterior half of the transversely narrow postorbital–squamosal bar. As in Paleorhinus bransoni ( Stocker, 2010) and the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007), the posterior process of the postorbital tapers to a point in dorsal view in Paleorhinus angustifrons , and this point fits into a slot on the anterior process of the squamosal ( Figs 3A , 4A ). Stocker (2010: character 22) identified this as the plesiomorphic condition for Phytosauria , and described derived conditions present in many phytosaurid phytosaurs. The postorbital– squamosal bar of Paleorhinus angustifrons is strongly compressed dorsoventrally.

On the medial surface of the postorbital, the ridge extending from the dorsal process of the jugal continues dorsally, ending at approximately the postorbital–postfrontal contact, and marks a sharp boundary between the orbital facet (facing anteromedially) and a broad medially and slightly posteriorly facing surface. Based on CT data, there does not seem to be a contact between the postorbital and the laterosphenoid.

Postfrontal: The margins of the postfrontal are not clear, but the bone presumably forms the posterodorsal corner of the orbital rim, and contacts the frontal anteriorly and medially, the parietal posteromedially, and the postorbital laterally and posterolaterally ( Figs 3A , 4A , 5F : ‘pof’). The dorsal surface of the postfrontal is weakly rugose, with an ornamentation similar to that of the other skull roof elements. The postfrontal is slightly thickened at its orbital rim. Its posteromedial margin may contact the laterosphenoid ventrally. On the right side, a pronounced, oval groove is present in the ventral surface of the postfrontal (and possibly also on the adjacent portions of the frontal and parietal) just lateral to the laterosphenoid contact.

Parietal: The contacts of the parietals ( Figs 3A , 4A , 5F : ‘pa’) with the frontals, postfrontals, and postorbitals are poorly demarcated as a result of fusion of the skull roof bones, combined with the presence of strongly ornamented bone surfaces and some reconstruction of the bone surface. The dorsal surfaces of the parietals bear a slight midline convexity at the interparietal suture that is most prominent at approximately midlength but that continues posteriorly as far as the posterior termination of the main bodies of the parietals: a similar feature was described for the holotype of Pravusuchus (AMNH FR 30646; Stocker, 2010). There are shallow concavities within the parietals on either side of this convexity. The main bodies of the parietals expand laterally anteriorly to form the anterolateral corners of the supratemporal fenestrae. The transversely orientated frontoparietal suture seems to be slightly posterior to the posterior margin of the orbits. Posteriorly, the posterolateral corners of the parietals are drawn out posterolaterally ( Figs 5F , 6C : ‘lpw’) to form the parietal component of the anterolaterally-toposteromedially compressed parietal–squamosal bar. This bar, and the parietal as a whole, is not depressed ventrally relative to the infraorbital portion of the skull roof (see above). The almost vertical lateral parietal wing forms essentially the entire dorsal part of the posterior margin of the supratemporal fenestra in dorsal view, dorsally overlapping a medial process of the squamosal and tapering to a point posterolaterally. In posterior view, the posterolateral parietal wings form a substantial component of the dorsal margin of the occiput ( Figs 3C , 4C , 6C : ‘lpw’), extending laterally to slightly beyond the lateral margins of the posttemporal fenestrae. The posterolateral wing dorsally overlaps a medial process of the squamosal on the occiput, with the parietal being excluded from the dorsal margin of the posttemporal fenestra by a small contact between the supraoccipital and the squamosal. Medially, the parietals form the entire dorsal and lateral margins of the small postparietal foramen.

Squamosal: Four processes extend from the main body of the squamosal ( Figs 3 , 4 : ‘sq’): an anteriorly (and slightly medially) directed postorbital process forming the posterior half of the postorbital– squamosal bar; a medially (and slightly anteriorly) directed parietal process forming the lateralmost and ventral parts of the parietal–squamosal bar; an anteroventral process extending along the lateral surface of the quadrate and contacting the quadratojugal ( Fig. 6 : ‘avpsq’); and a short ventrally directed opisthotic process ( Fig. 6B, D : ‘opsq’). As in non-phytosaurid phytosaurs and Angistorhinus ( Stocker, 2010: character 24), the squamosal of Paleorhinus angustifrons lacks a distinct posterior process that extends posteriorly beyond the paroccipital process of the opisthotic.

The postorbital–squamosal bar is strongly compressed dorsoventrally. As in Paleorhinus bransoni (TMM 31100-101; Stocker, 2010) and the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007), the dorsal surface of the squamosal has an anteroposteriorly extending concavity or furrow that begins at the most posterior corner of the squamosal and extends onto the postorbital–squamosal bar ( Figs 5F , 6D : ‘sqd’). This furrow becomes shallower anteriorly, fading out on the dorsal surface of the postorbital. The lateral edge of the postorbital– squamosal bar is a sharp ridge that extends along the lateral margins of both the postorbital and the squamosal to reach the most posterolateral corner of the squamosal. This ridge overhangs the dorsal margin of the laterotemporal fenestra and the base of the anteroventral process of the squamosal, and thus separates the strongly ornamented dorsal surface from the smooth lateral surface of the anteroventral process. Above the posterior edge of the quadrate this ridge bifurcates, sending off a short posteroventrally extending ridge along the lateral surface of the squamosal ( Figs 3D , 4D , 6A, B : ‘sqr’). These two ridges enclose a small, elongate, teardrop-shaped concavity. An identical arrangement of paired ridges enclosing a small slit-like concavity is present in referred specimens of Paleorhinus bransoni (TMM 31025-172; Stocker, 2010: fig. 9) and the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007) but is absent in other basal phytosaurs. It is here identified as autapomorphic for the genus Paleorhinus .

The main body of the squamosal is dorsoventrally shallow, as in other basal phytosaurs ( Stocker, 2010: character 25). In dorsal view, the squamosal tapers posteriorly to a posterolaterally projecting point, similar to the condition in the Krasiejów phytosaur specimens (ZPAL Ab III 200, Dzik & Sulej, 2007) and Paleorhinus bransoni ( Stocker, 2010: fig. 9). In lateral view, the squamosal extends beyond the quadrate head posteriorly, but only a short distance posterior to the quadrate condyles. The squamosal is drawn out posteroventrally into a short opisthotic process ( Fig. 6B, D : ‘opsq’), the posterior surface of which articulates with the paroccipital process.

The anteroventral process of the squamosal ( Fig. 6 : ‘avpsq’) is anteroposteriorly broad dorsally but tapers along its length and forms slightly more than 60% of the posterior margin of the infratemporal fenestra, articulating with the quadratojugal ventrally and the quadrate posteriorly. The anteroventral process is transversely compressed and smooth and unornamented on its lateral surface.

Medially, the parietal process of the squamosal forms the most posterolateral part of the parietal– squamosal bar and the lateral part of the dorsal margin of the slit-like posttemporal fenestra ( Figs 3A, C , 4A, C ). The process tapers medially, being overlapped dorsally by the lateral process of the parietal, and reaches almost to the medial margin of the supratemporal fenestra. On the occiput, the parietal process articulates medially with a thin lateral process of the supraoccipital above the posttemporal fenestra and ventrally with the dorsal margin of the paroccipital process.

Quadratojugal: The exact contacts of the subtriangular quadratojugal with surrounding elements are poorly defined, but it forms the posteroventral rim of the infratemporal fenestra ( Figs 3 , 4 , 6A : ‘qj’). The quadratojugal has a long, anteriorly tapering anterior process that articulates with the posterior process of the jugal along an elongate anterodorsalto-posteroventrally inclined suture beneath the infratemporal fenestra. The process thus forms approximately three-quarters of the ventral margin of the infratemporal fenestra. Posteriorly the quadratojugal overlaps the lateral edge of the quadrate laterally, and dorsally it articulates with the anteroventral process of the squamosal and a laterally extending tongue of the quadrate that forms the roof of the posteriorly facing quadrate foramen ( Fig. 6A ). The quadratojugal forms the lateral margin of this foramen. CT data suggest that a narrow and anteroposteriorly short longitudinal canal is present in the thickened main body of the quadratojugal, parallel to the quadrate foramen, but not connected to the latter, although the resolution of the data is not sufficiently high to resolve the details of this canal. This canal appears to open posteriorly a short distance lateroventral to the quadrate foramen and anterolaterally through a small opening onto the lateral side of the quadratojugal.

Quadrate: In posterior view ( Figs 3C , 4C , 6D : ‘qd’), the main body of the quadrate is transversely expanded at its ventral margin to form the condyles for articulation with the mandible ( Fig. 6D : ‘qdc’); dorsally it is tightly sutured to the squamosal and the paroccipital process of the opisthotic. The width of the skull across the quadrate condyles at their ventralmost part (c. 180 mm) is greater than that across the squamosals (134 mm), giving the skull a somewhat trapezoidal outline in posterior view, although this outline has probably been modified by post-mortem dorsoventral compression. In ventral view the quadrate condyles are anteroposteriorly narrow and the medial condyle is set slightly further posteriorly than the lateral condyle ( Figs 3B , 4B ). The articular surface of the condyles is flat-to-gently convex laterally and gently concave medially, although the morphology appears to have been distorted by post-mortem compression.

The quadrate articulates laterally with the quadratojugal, and the quadrate forms the ventral, medial, and dorsal margins of the large quadrate foramen ( Figs 3 , 4 , 6D : ‘qf’). The dorsal margin of the foramen is formed by a lateral flange of the quadrate ( Fig. 6B, D : ‘lfqd’), which articulates with the quadratojugal and the anteroventral process of the squamosal. Medial to the quadrate foramen, the main body of the quadrate is slightly raised into a low vertical ridge. Anteriorly, the quadrate is thickened in the area of the quadrate foramen, so that the latter has the appearance of a short channel between the quadrate and the quadratojugal. The foramen opens into a canal that runs almost directly anteriorly, exiting along the anterior surface of the quadrate/ quadratojugal ( Figs 3B , 4B ).

The pterygoid wing of the quadrate extends anteromedially ( Figs 3 , 4 , 6D , 7A : ‘ptqd’). It is robust, curves slightly laterally towards its anterior end, and meets the posterior wing of the pterygoid in an extensive suture whereby the anterior end of the pterygoid wing of the quadrate is medially overlapped by the posterior wing of the pteryoid. The ventral margin of the pterygoid wing is placed slightly above the quadrate condyles and is flexed medially, to form an expanded medial shelf that extends along the full length of the pterygoid–quadrate plate. Dorsal to the shelf, the pterygoid wing is strongly concave dorsoventrally. The lateral surface of the pterygoid wing is dorsoventrally convex and forms the posteromedial border of the subtemporal fenestra.

Pterygoid and epipterygoid: The pterygoid is a complex element consisting of several major processes ( Figs 3B, C , 4B, C : ‘pt’). The posterior wing or quadrate process curves posterolaterally away from the midline ( Fig. 7A : ‘qdpt’), and its lateral margin overlaps the pterygoid wing of the quadrate to form the pterygoid– quadrate plate. The posterior wing is strongly concave dorsoventrally in medial view and flexes medially at its ventral margin to form a ventral medial shelf continuous with that on the pterygoid wing of the quadrate. Posteriorly the posterior wing is forked, with the dorsal part of the wing reaching slightly further posteriorly (almost level with the posterior end of the occipital condyle) than the ventral process ( Figs 3C , 4C ). At the proximal end of the posterior wing of the pterygoid, the ventral surface of the pterygoid is drawn out medially into a small medially extending plate of bone ( Fig. 7A : ‘mppt’). This plate has a broadly subrectangular outline, but its posteromedial corner is drawn out posteriorly such that the posterior margin of the plate is concave in ventral view. The posterior part of this medial plate forms the articular surface for the basipterygoid process of the basisphenoid. The left and right articular surfaces for the basipterygoid processes are broadly separated from one another, so that an interpterygoid vacuity is present at this point, through which the cultriform process of the parabasisphenoid can be seen ( Fig. 7A : ‘clt’).

There is a low thickening of the medial margin of the base of the posterior process of the pterygoid, immediately lateral to the basipterygoid articulation. Dorsal to this thickening, CT data show that the pterygoid begins to be drawn out dorsally to form the dorsoventrally expanded posterior wing ( Fig. 8H). CT data indicate the presence on the right side of a dorsoventrally extending epipterygoid bone at this point ( Fig. 8H). Basally, the epipterygoid is strongly expanded anteroposteriorly and slightly transversely expanded, forming a sheet that laterally overlies the dorsal expansion of the pterygoid. The epipterygoid tapers dorsally, articulating with a cup-like surface on the ventrolateral margin of the laterosphenoid ( Fig. 8H).

Anterior to the basipterygoid articulation, a lateral flange of the pterygoid extends anterolaterally and ventrally to contact the ectopterygoid and palatine ( Fig. 7A, D : ‘lfpt’). The lateral flange is rather short and the articulation with the ectopterygoid appears to be sutural in contact, with only a slight dorsal overlap of the pterygoid onto the ectopterygoid, whereas the palatine has a posteriorly orientated process that appears to articulate with a small cup-shaped fossa on the anterior margin of the lateral flange of the pterygoid ( Fig. 7D ).

Medial and anterior to the lateral flange, sheet-like anterior flanges of the pterygoid extend dorsomedially, and approach one another very closely along the ventral midline ( Fig. 7A, C, D : ‘afpt’). These anterior flanges are badly damaged, possibly from the original preparation, and much of the bone surface is missing. Posteriorly, they are offset dorsally from the basipterygoid articulation and, to a lesser extent, from the lateral flange by a distinct step. They articulate with the palatines at their lateral margins and with the vomers anteriorly. At their medial margins they are markedly thickened into sharp ridges that arise from the medial edges of the medial rectangular sheets that form the basicranial articulation, and the ridges are near continuous anteriorly with the ridges of the vomers ( Fig. 7C, D ). CT cross sections show that the anterior flanges are drawn out at their medial margins into vertical sheets, between which extends the cultriform process of the parabasisphenoid ( Fig. 8F). These flanges continue anteriorly dorsal to the vomers ( Fig. 8E), which underlie them ventrolaterally, terminating a short distance anterior to the posterior margins of the choanae.

Ectopterygoid: The left ectopteryoid is damaged along its posterior margin and has been partially reconstructed with plaster, but the right ectopterygoid is relatively well preserved ( Figs 3 , 4 ; ‘ect’). It contacts the pterygoid posteromedially and the palatine anteromedially. The ectopterygoid extends anterolaterally as a broad bar of bone from the contact with the pterygoid and the palatine to its contact with the posterior end of the maxilla and the adjacent anterior part of the jugal, where the bone is expanded posteriorly ( Fig. 7D ). It forms the posterolateral margin of the suborbital fenestra. A large elliptical foramen is present on the ectopterygoid adjacent to the medial contact with the pterygoid ( Fig. 7D : ‘ectf ’). A foramen is also present in the ectopterygoid in other phytosaurs (often associated with a groove), but the very rounded nature of the opening and the lack of an associated groove appear to be unique to Paleorhinus angustifrons . In the Krasiejów phytosaurs this foramen is elongated and slit-like rather than oval (ZPAL Ab III 200, Dzik & Sulej, 2007: fig. 10; R. Bronowicz, pers. obs.). CT data show that the foramen opens into an anterolaterally extending cavity that terminates within the bone ( Fig. 8G). Doyle & Sues (1995) suggested that this opening was related to the antorbital sinus system, and Witmer (1997a) discussed the possibility that somewhat similar ‘ectopterygoid recesses’ in theropods represented cranial pneumaticity.

Palatine: The palatine is an elongate, horizontally orientated element ( Figs 3B , 4B : ‘pal’), the main body of which comprises a horizontal shelf that forms most of the lateral region of the posterior portion of the palate. At its posterior end the palatine articulates with the anteromedial corner of the ectopterygoid; medially it articulates with the pterygoid along much of its length, although large parts of both bones, which obviously were very thin in this region, are missing. The palatine forms the medial margin of the slit-like and curved suborbital fenestra, although on both sides the palatine has an unfinished edge along the margin of this fenestra ( Fig. 7C, D ). Anterior to the suborbital fenestra the horizontal main body of the palatine tapers in mediolateral width as it articulates with a medially extending shelf of the maxilla ( Figs 3B , 4B , 7C, D ); this medially extending shelf excludes contact between the palatine and premaxilla ( Fig. 7C : ‘atpal’). Medial to the main body of the palatine, there is a dorsomedially extending and vaulted flange ( Fig. 7C, D : ‘dmfpal’) that articulates with the pterygoid posteriorly and with the vomer anteriorly. These two portions of the palatine are separated by a marked step, which is continuous posteriorly with the step separating the anterior pterygoid flanges from the basipterygoid articulations (as described above).

Vomer: The vomers form the entirety of the septum separating the choanae ( Figs 3B , 4B , 7C : ‘v’); they are separated slightly from one another, and are transversely compressed with their ventral margins forming sharp ridges ( Fig. 5D ). At their anterior end the vomers maintain their sharp median ridges, but also become dorsoventrally flattened and expand transversely (visible in CT data) to form a plate that appears to underlie the posteriormost parts of the premaxillae and laterally contact the maxillae, although the exact position of the contacts between the vomers and the premaxillae are unclear. At the posterior end of the choanae, the vomers are also expanded transversely (and form the posteriormost corners of the margins of the choanae), and underlie the anterior extensions of the pterygoids ( Fig. 7C : ‘pexv’; Fig. 8E). The sharp median ridges on the vomers are near continuous posteriorly with those of the pterygoids but disappear anteriorly at about the contact with the premaxillae and do not continue anteriorly onto the ventral surface of the latter bones.

Supraoccipital: The external surface of the posterodorsally facing supraoccipital ( Figs 3B, C , 4B, C , 6C : ‘so’) is exposed broadly in dorsal view and slightly raised at its midline, although without forming a sharp median crest. The surface of the supraoccipital is more dorsally than posteriorly directed, although this might be slightly exaggerated by dorsoventral compression. The supraoccipital forms only a very small part of the median dorsal margin of the foramen magnum. Laterally, the supraoccipital con- tacts the otooccipital ventrally and forms the medial margin of the posttemporal fenestra ( Fig. 6C ). Dorsally, it sends a slender lateral process above the posttemporal fenestra, forming the dorsomedial margin of the fenestra, contacting the squamosal laterally, and excluding the parietal from the margin of this fenestra. CT data show that a small foramen enters the supraoccipital from the endocranial cavity and extends posterolaterally and slightly ventrally through this bone, until it exits at about the supraoccipital–parietal suture; this foramen most probably marks the course of the mid-cerebral vein towards its posterior exit ( Sedlmayr, 2002).

Otooccipital: The opisthotic and exoccipital appear to be indistinguishably fused to one another; therefore, the two elements are described together here as the otooccipital ( Figs 3C , 4C , 6C : ‘oto’). The exoccipital portion forms the lateral and dorsolateral margin, as well as part of the ventrolateral margin, of the foramen magnum. A small, oval facet for the contact with the proatlas is present on the otooccipital along the dorsolateral margin of the foramen magnum, adjacent to the suture with the supraoccipital. The contacts of the otooccipital with the basioccipital are unclear, so it is uncertain how much of the occipital condyle it forms, although it seems that the basioccipital forms a small portion of the ventral margin of the foramen magnum. On the lateral surface of the opisthotic portion of the left otooccipital, the metotic foramen and fenestra ovalis are visible as two deep recesses separated by the descending ramus of the opisthotic (= crista interfenestralis). The metotic foramen is slightly more ventrally placed than the more anteriorly positioned fenestra ovalis, so that the crista interfenestralis runs slightly obliquely posterodorsally. The anteroventral base of the crista is expanded, but the structure rapidly narrows towards its posterodorsal end. Only a very small and shallow stapedial groove ( Fig. 7A : ‘stgr’) extends from the fenestra ovalis posterolaterally onto the base of the paroccipital process and disappears at about the posterolateral edge of the crista interfenestralis.

The paroccipital processes of the opisthotic portions of the otooccipitals extend posterolaterally, and their external surfaces face posterodorsally ( Figs 3C , 4C , 6C, D : ‘par’). The processes are anteroposteriorly expanded and dorsoventrally shallow at their bases, such that they form a flattened, elongate, triangular platform in ventral view. They become more compressed anteroposteriorly and gently expanded dorsoventrally towards their distal ends, although the distal tips of the processes appear slightly incomplete on both sides. Nevertheless, it can be established that the paroccipital processes did not reach the posterolateral edge of the squamosal. The paroccipital process forms the ventral margin of the elongate, slit-like posttemporal fenestra. Laterally, the paroccipital process shares a tight and immobile suture with the quadrate along the anterior margin of the process. Distally, the paroccipital process is buttressed by the opisthotic process of the squamosal ( Fig. 6D ).

Basioccipital and parabasisphenoid: The basioccipital ( Figs 3B , 4B : ‘bo’) forms most of the apparently heavily crushed occipital condyle ( Figs 3B, C , 4B, C : ‘occ’), which is strongly compressed dorsoventrally and has a transversely convex ventral surface. There is no distinct transverse constriction or neck separating the occipital condyle from the body of the basioccipital. The surface of the basioccipital is more ventrally than posteriorly directed, although this is probably exaggerated by compression. In ventral view the basioccipital is flared laterally towards its anterior end, forming the posterior parts of the basitubera ( Figs 3B, C , 4B, C , 7A : ‘bt’). The basitubera are separated from one another by a narrow, concave, anteroposteriorly extending slot ( Fig. 7A : ‘slt’). This slot extends posteriorly as a shallow depression to the rim of the basioccipital condyle, and is separated from small longitudinal depressions on either side of the basioccipital (positioned close to the suture with the exoccipital) by low, flattened ridges. Lateral to the narrow groove separating the basitubera, a small posteroventrally directed depression ( Fig. 7A : ‘dep’) is present on each side on the posteromedial surface of the tubera, adjacent to the suture with the basisphenoid.

The basitubera extend further laterally than the lateral edges of the occipital condyle in ventral view, and they extend a short distance ventral to the occipital condyle in posterior view. Their ventral surfaces are flattened, although this might at least partially be due to dorsoventral compression.

The basisphenoid is expanded transversely at its anterior and posterior ends. Posteriorly the basisphenoid forms the anterior three-quarters of the basitubera, whereas anteriorly it forms the basipterygoid processes ( Fig. 7A : ‘bpt’) that extend laterally and ventrally to articulate with the pterygoids. The transverse width across the basitubera is slightly greater than the transverse width across the basipterygoid processes. There is a deep, cone-shaped concavity that covers the ventral midline of the basisphenoid just anterior to the contact with the basioccipital ( Fig. 7A : ‘conc’); this concavity has a subcircular outline and is delimited posteriorly by a low transversely extending ridge that separates it from the narrow concave slot on the basioccipital described above. Anteriorly, there is no clearly defined ridge between the basipterygoid processes, such that the concavity opens anteriorly.

Anterior to the main body of the basisphenoid, the cultriform process ( Fig. 7A : ‘clt’) is poorly exposed but extends anteriorly as a mediolaterally narrow process, the anterior termination of which is hidden by the pterygoids. CT data show that the cultriform process has a ‘V’-shaped cross section ( Fig. 8G: ‘clt’) that leads posteriorly into the sella turcica, and extends anteriorly as a slender process terminating approximately level with the posterior ends of the maxillary tooth rows. There is no subsellar recess at the base of the cultriform process, unlike the condition in many dinosaurs ( Witmer, 1997b).

Laterosphenoid: The right laterosphenoid is visible through the orbit and temporal fenestrae. It is an anteroposteriorly elongate bone that is triangular in outline in lateral view. The laterosphenoid has a long, pointed anterior process that articulates with the frontal dorsally and reaches almost to half the length of the orbit. The lateral process of the laterosphenoid is short and anteroposteriorly broad and appears to contact the postfrontal but not the postorbital. On the lateral surface of the bone, the anterior and lateral processes are separated by a rounded break-in-slope that extends dorsally and slightly anteriorly from the ventrolateral margin of the laterosphenoid. Posterior to this break-in-slope, the lateral surface of the bone is dorsoventrally concave, whereas anterior to it the lateral surface is slightly concave anteroposteriorly and slightly convex dorsoventrally. The posterior suture with the parietal cannot be clearly distinguished, and the posteroventral contact with the prootic is covered by matrix.

Figures

Figure 3. Paleorhinus angustifrons (Kuhn, 1936), BSPG 1931 X 502. Photographs of holotype skull in dorsal (A), ventral (B), posterior (C), and left lateral (D) views. Asterisks show positions of transverse sections shown in Figure 8.

Figure 4. Paleorhinus angustifrons (Kuhn, 1936), BSPG 1931 X 502. Interpretive line drawings of holotype skull in dorsal (A), ventral (B), posterior (C), and left lateral (D) views. Dark grey areas represent openings or sediment-infilled fenestrae. Light grey areas represent recessed areas of bone (antorbital fossa, fossa on squamosal adjacent to supratemporal fenestra). Cross-hatched areas are broken or restored surfaces. Abbreviations: afen, antorbital fenestra; afos, antorbital fossa; alvr, alveolar ridge; bo, basioccipital; bt, basal tubera; cho, choana; ect, ectopterygoid; en, external naris; fdp, depression on anterior end of frontal; fm, foramen magnum; fo, fossa; fr, frontal; itf, infratemporal fenestra; jg, jugal; jgb, series of bosses on lateral surface of jugal; lc, lacrimal; max, maxilla; na, nasal; ndp, depression on anterior end of nasal; occ, occipital condyle; orb, orbit; pa, parietal; pal, palatine; par, paroccipital process; pm, premaxilla; po, postorbital; pof, postfrontal; ppfor, postparietal foramen; prf, prefrontal; pt, pterygoid; ptf, posttemporal foramen; qd, quadrate; qf, quadrate foramen; q j, quadratojugal; sm, ‘septomaxilla’; so, supraoccipital; sq, squamosal; sqr, paired ridges on lateral surface of squamosal; stf, supratemporal fenestra; sub, suborbital fenestra; subtf, subtemporal fenestra; v, vomer.

Figure 5. Paleorhinus angustifrons (Kuhn, 1936), BSPG 1931 X 502. Close-up photographs of holotype skull. A, external nares in dorsal view (anterior of specimen to right). B, lateral margin of left external naris in lateral view (anterior of specimen to left). C, left antorbital fenestra in lateral view. D, main body of right jugal in lateral view, showing nodular row ventral to the orbit. E, nasofrontal region of skull roof in dorsal view. F, right lateral part of dorsal skull roof in dorsal view. Abbreviations: afen, antorbital fenestra; afos, antorbital fossa; alvr, alveolar ridge; asc, ascending process of maxilla; convx, convexities along lateral rim of the external naris; en, external naris; fdp, depression on anterior end of frontal; fl, laterally flared posterolateral margin of external naris; fr, frontal; gr, groove; itf, infratemporal fenestra; jg, jugal; jgb, series of bosses on lateral surface of jugal; jmri, ridge on jugal and maxilla; lc, lacrimal; lpw, lateral parietal wing; max, maxilla; na, nasal; na-fr, nasal–frontal suture; na-prf, nasal-prefrontal suture; ndp, depression on anterior end of nasal; orb, orbit; pa, parietal; pmp, posteromedial process of the premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; prod, pre-orbital depression; rec, reconstructed skull surface; sm?, probable position of septomaxilla; sq, squamosal; sqd, depression on dorsal surface of the squamosal; stf, supratemporal fenestra; thor, thickened orbital rim; tp.pmp, tip of the posteromedial process of the premaxilla.

Figure 6. Paleorhinus angustifrons (Kuhn, 1936), BSPG 1931 X 502. Close-up photographs of holotype skull. A, left infratemporal region in lateral view. B, left squamosal in lateral view. C, foramen magnum and supraoccipital in posterior view. D, quadrate and distal end of paroccipital process in posterior view. Abbreviations: avpsq, anteroventral process of the squamosal; bo, basioccipital; fm, foramen magnum; fo, fossa; jgb, series of bosses on lateral surface of jugal; itf, infratemporal fenestra; lfqd, lateral flange of the quadrate; lpw, lateral parietal wing; opsq, opisthotic process of the squamosal; orb, orbit; oto, otooccipital; par, paroccipital process; po, postorbital; ppfor, postparietal foramen; ptf, posttemporal foramen; ptqd, pterygoid wing of the quadrate; qd, quadrate; qdc, quadrate condyles; qf, quadrate foramen; qj, quadratojugal; sq, squamosal; sqd, depression on dorsal surface of the squamosal; sqr, paired ridges on lateral surface of squamosal; stf, supratemporal fenestra.

Figure 7. Paleorhinus angustifrons (Kuhn, 1936), BSPG 1931 X 502. Close-up photographs of holotype skull. A, braincase and posterior palate in ventral view. B, anterior end of skull in ventral view. C, choanae, vomers, palatines, and anterior parts of pterygoids in ventral view. D, left ectopterygoid, suborbital fenestra, and surrounding areas in ventral view. Abbreviations: alvr, alveolar ridge; afpt, anterior flange of the pterygoid; atpal, anterior termination of the palatine; bpt, basipterygoid process; bt, basal tuber; cho, choana; clt, cultriform process; conc, concavity on ventral surface of basisphenoid; dep, depression; dmfpal, dorsomedial flange of the palatine; ect, ectopterygoid; ectf, ectopterygoid foramen; jg, jugal; lfpt, lateral flange of pterygoid; max, maxilla; mppt, medial process of pterygoid; occ, occipital condyle; pal, palatine; par, paroccipital process; pm, premaxilla; pm-max, approximate position of the premaxilla–maxilla contact; pexv, posterior transverse expansion of the vomers; ptqd, pterygoid process of quadrate; qdpt, quadrate process of pterygoid; slt, median slot on basioccipital between basal tubera; stgr, stapedial groove; sub, suborbital fenestra; v, vomer.