Apatosuchus, Sues & Schoch, 2013

Sues, Hans-Dieter & Schoch, Rainer R., 2013, Reassessment of cf. Halticosaurus orbitoangulatus from the Upper Triassic (Norian) of Germany - a pseudosuchian, not a dinosaur, Zoological Journal of the Linnean Society 168 (4), pp. 859-872 : 861-870

APATOSUCHUS GEN. NOV.

Type species

cf. Halticosaurus orbitoangulatus Huene, 1932 (by monotypy).

Etymology

The generic nomen is derived from the Greek apate, deceit, deception, and the Greek name soukhos for the Egyptian crocodile-headed deity Sobek or Sobk. Apate refers to the fact that its original discoverer was deceived regarding the affinities and cranial structure of the unique specimen. The Latinized version of soukhos, suchus, is a commonly used suffix for generic names of reptiles.

Holotype

SMNS 12353b, incomplete and macerated but associated skull ( Figs 2–6 ). Identifiable preserved elements comprise parts of both maxillae, nasals, frontals (mainly as impression in matrix), right jugal, complete right parietal, ventral process of right postorbital, nearly complete left squamosal, complete right quadrate and right pterygoid, part of possible vomer, incomplete basioccipital and parabasisphenoid, complete exoccipitals and opisthotics (otoccipitals), complete supraoccipital, incomplete dentaries and splenials, nearly complete left angular, partial left surangular and right prearticular, and two incomplete ceratobranchials.

Type horizon and locality

Upper Fäule, middle (second) Stubensandstein, Löwenstein Formation. Weisser Steinbruch, south of Pfaffenhofen, Heilbronn district, Baden-Württemberg, Germany. Age: Late Triassic (Norian).

Diagnosis

Distinguished by the following combination of character states: squamosal with dorsolateral ridge but not forming dorsolateral overhang over temporal region; dorsal surface of squamosal bounding supratemporal fenestra flat; ventral process of squamosal with faint lateral ridge; maxilla lacking distinct antorbital fossa and medial bony lamina bordering the antorbital fenestra; ventral margin of antorbital fenestra forming thickened rim; lateral surface of dentary with prominent anteroposteriorly extending ridge.

The presence of the prominent lateral ridge on the dentary and the flat dorsal surface of the squamosal bounding the supratemporal fenestra may be autapomorphic for Apatosuchus orbitoangulatus .

Description

Cranial bones

Both maxillae preserve only the more posterior portions of the bones ( Figs 2A , 3A ). They are narrow mediolaterally and converge anteriorly. The dorsal (ascending) processes of both maxillae are only incompletely preserved, and thus the relative extent of the facial portion of the maxilla anterior to the antorbital fenestra cannot be ascertained. The right process is more complete but is crushed and distorted. The ascending process of the maxilla extends posterodorsally at an angle of about 45° to the long axis of the main body of the bone. Together with the toothbearing portion, it delimits the pointed anterior end of the antorbital fenestra. Elsewhere, a pointed anterior end of the antorbital opening is present in Postosuchus , Riojasuchus , Saurosuchus , and basal Crocodylomorpha such as Saltoposuchus ( Nesbitt, 2011) . The dorsal edge of the tooth-bearing ramus of the maxilla is thickened along the ventral margin of the antorbital fenestra. There is no trace of a distinct antorbital fossa on the preserved portion of either maxilla nor is there a thin median bony lamina, unlike in Saltoposuchus ( SMNS 12352; SMNS 12591a; SMNS 12596). The tooth-bearing portion of the maxilla decreases in height posteriorly (starting at the level of the last preserved alveolus on the left side), as in many archosauriforms ( Nesbitt, 2011), and curves posterolaterally along its dorsal contact with the jugal. The thick ventrolateral edge of the toothbearing ramus bears small neurovascular foramina, which, more posteriorly, form a narrow groove extending just lateral to the teeth. The lateral surface of the tooth-bearing ramus of the maxilla is smooth and gently concave dorsoventrally. It does not bear the lateral ridge present in some loricatans (e.g. Postosuchus ) and in coelophysoid theropods ( Nesbitt, 2011). It bears only a few larger neurovascular foramina. On the medial surface of the tooth-bearing ramus, there are distinct, triangular interdental plates that are bounded dorsally by a more or less continuous, longitudinal groove that presumably housed the dental lamina. Dorsal to this groove, a longitudinal ridge together with a second, more dorsally situated ridge encloses a depressed area that probably represents the surface for contact with the palatine. Just below the ventral margin of the antorbital fenestra the medial surface of the tooth-bearing ramus of the maxilla bears a large foramen. This opening, clearly evident on the right maxilla, probably served for passage of the maxillary nerve and accompanying vessels; corresponding foramina are present in Batrachotomus ( Gower, 1999) and Postosuchus ( Weinbaum, 2011) . There is no trace of medial (palatal) processes on the preserved portions of the maxillae.

Five teeth or complete alveoli are preserved in each partial maxilla; part of a sixth alveolus is exposed at the broken anterior end of either bone. The teeth have long and robust roots and are fairly widely spaced. Several have slipped out of their alveoli postmortem. In the left maxilla, a tooth is partially erupted in the second complete alveolus from the front, and another has already further erupted in the fourth complete alveolus from the front. The tooth crowns are gently recurved apicobasally. They exhibit only slight labiolingual flattening, unlike in many other archosaurian reptiles ( Nesbitt, 2011), and are lenticular in transverse section at midheight. There is no constriction between the crown and the long root. The root and crown of the upper teeth together have a distinctive, slightly sigmoidal curvature in side view. The labial surfaces of the tooth crowns are distinctly convex whereas the lingual surfaces are flat. Rauhut & Hungerbühler (2000) reported that the teeth of SMNS 12353b lack well-defined cutting edges and serrations. However, all well-preserved teeth in this specimen do have distinct mesial and distal cutting edges (carinae) ( Fig. 3C ). The mesial carina is restricted to the more apical portion of the crown and bears serrations, which are worn and barely visible on the preserved crowns. In contrast, the concave distal carina extends from the apex to the base of the crown and is serrated (with about six serrations per mm). The same condition is present on the second preserved tooth in the right dentary. Rauhut & Hungerbühler (2000) reported that the teeth of SMNS 12353b have longitudinal striations, and Knoll (2010) claimed the presence of apicobasal ‘fissures’. However, although marked by numerous dark-stained vertical fracture lines, the thin enamel covering of the crowns lacks genuine apicobasal striations or grooves on any of the preserved teeth.

The nasals are poorly preserved, and few details are evident ( Fig. 2B ). The left nasal is represented by bone and impression in the matrix whereas little of the right element remains. Medially the nasals form a furrow that lies below the level of the convex lateral margin of the left nasal; however, this feature may have been accentuated by postmortem crushing. A narrow, slightly oblique surface delimited by a dorsal ridge extending more or less anteroposteriorly near the lateral margin of the nasal possibly contacted the ascending process of the maxilla. The ridge resembles a similar feature on the dorsolateral edge of the nasal in Batrachotomus ( Gower, 1999) , Postosuchus ( Weinbaum, 2011) , and Rauisuchus ( Nesbitt, 2011) but the latter is more prominent, rugose, and placed along the dorsolateral edge of the nasal.

Huene (1932: 43) described what he considered to be part of the possibly right frontal. Bone fragments and impressions preserved in the matrix behind the nasals probably represent the frontals but preserve no noteworthy details ( Fig. 2B ). A small, triangular piece of bone at the back of the preserved portion of the skull roof represents either the posterolateral portion of the frontal or the postfrontal.

Huene (1932: 43) tentatively identified a partially exposed bone above the right maxilla as the left jugal ( Figs 2B , 3A ). We interpret this element as the right jugal, exposed in medial view ( Fig. 4D ). Preparation has now exposed the element. Its anterior portion is not preserved. The jugal is mediolaterally narrow and has two large processes, which Huene plausibly interpreted as the posterior and dorsal (ascending) process, respectively. The apex of the shorter of the two processes is obliquely truncated for contact with the ventral process of the postorbital, of which the corresponding distal portion is preserved on the right side of the skull. The dorsal process extends posterodorsally at a distinct angle to the presumed posterior (infratemporal) process of the jugal. The posterior process does not form a slot for the reception of the anterior process of the quadratojugal, unlike the condition in saurischian dinosaurs ( Rauhut, 2003; Nesbitt, 2011). The outline of the jugal differs from the inverted T-shape of the homologous element in basal crocodylomorphs such as Saltoposuchus ( SMNS 12597) and Dromicosuchus ( Sues et al., 2003) . Although the anterior portion of the bone is not preserved, the configuration of the dorsal and posterior processes more closely resembles that in Batrachotomus ( Gower, 1999) and Postosuchus ( Weinbaum, 2011) .

A fragment of bone on the right side is the distal end of the ventral process of the postorbital ( Fig. 3A ). The length and inclination of its oblique distal edge precisely match those of the dorsal margin of the ascending process of the jugal. Based on the length and shape of the two processes, we infer that the orbit of SMNS 12353b was tall dorsoventrally and probably not rounded as in many other archosaurian taxa.

Huene (1932: 43) identified a bone partially exposed posterodorsal to the left maxilla ( Fig. 2A, B ) as the right prefrontal. Preparation has now exposed much of this element and established that it is a nearly complete left squamosal ( Fig. 4A, B ). The squamosal is more or less L-shaped in dorsal view, with a well-defined posterolateral corner, rather than arcuate as in Saltoposuchus ( SMNS 12596; SMNS 12597) and Terrestrisuchus ( Crush, 1984; Sereno & Wild, 1992). Its transversely wide anterior process forms the posterolateral corner of the skull roof and a nearly flat dorsal surface. This surface is separated from the nearly vertical, anteroposteriorly convex lateral portion of the bone by a slightly thickened dorsolateral ridge, similar to the condition in Arizonasaurus ( Nesbitt, 2005) and Saurosuchus ( Alcober, 2000) . In Batrachotomus ( Gower, 1999) , Postosuchus ( Weinbaum, 2011) , and Crocodylomorpha ( Sues et al., 2003; Nesbitt, 2011) the element forms a prominent dorsolateral overhang or ‘brow’ over the lateral surface of the temporal region. In Saltoposuchus ( SMNS 12596; SMNS 12597), the squamosal forms an extensive, thin, and convex overhang lateral to the ridge. The squamosal of SMNS 12353b has a flat dorsal surface bounding the supratemporal fenestra posterolaterally. Its dorsal surface bears a shallow depression at the posterolateral corner of the squamosal. More posteriorly, the lateral portion of the squamosal faces posterolaterally and forms a short, anteroventrally extending, and triangular ventral process (that is somewhat incomplete along its distal edge). The latter, together with the anteromedially extending anterior process, bounds the infratemporal fenestra posteriorly and dorsally. The ventral process bears a faint lateral ridge, unlike the pronounced ridge in Batrachotomus ( Gower, 1999) . Posterolaterally, the squamosal forms a thin and vertical bony lappet behind the socket for the reception of the head of the quadrate. This feature resembles that in Arizonasaurus ( Nesbitt, 2005) but differs from the ventrally ‘kinked’ process in Batrachotomus ( Gower, 1999) and Postosuchus ( Weinbaum, 2011) . Huene (1932: 43) mistook the notch between this posterior process and the ventral process of the squamosal (‘otic notch’) for a peculiarly angled anterodorsal margin of the orbit, the basis for his choice of the specific epithet in the Linnean binomen cf. Halticosaurus orbitoangulatus for SMNS 12353b. The posterior (occipital) surface of the medial process of the squamosal is distinctly concave transversely and vertically and received the distal end of the paroccipital process, but there is no posterior descending process and thus no interlocking contact between the squamosal and the paroccipital process as in basal crocodylomorphs such as Saltoposuchus ( SMNS 12596; Sereno & Wild, 1992: fig. 10A) and Sphenosuchus ( Walker, 1990) . The occipital surface of the squamosal is set off from the dorsal surface by a pronounced, transversely S-shaped ridge in dorsal view. At its medial end, a clearly delineated ventral facet probably contacted the posterolateral process of the parietal. The posterior edge of the squamosal is slightly damaged but it apparently extended anteromedially toward the midline. At the junction of its various major processes, the squamosal bears a distinct ventral recess for the reception of the dorsal (proximal) head of the quadrate. In the articulated cranium, the head of the quadrate would have been fully concealed, as in Crocodylomorpha, Gracilisuchus , Postosuchus , and Rauisuchus ( Nesbitt, 2011) .

Huene (1932: 43) identified a partially visible bone behind the right maxilla and below the prearticular and surangular as a possible left squamosal. This bone has now been more fully exposed by preparation ( Fig. 4A ). It does not resemble the left squamosal but probably represents the right parietal. A low, anteroposteriorly extending ridge divides the dorsal surface of the for the most part horizontal portion into a transversely slightly concave region with a thin medial margin and a narrower, slightly oblique, and flat lateral region, the edge of which probably formed part of the margin of the supratemporal fenestra. The posterior margin of the bone is thickened. The posterolateral corner of the element forms a long, distally tapering process, which extends nearly laterally (rather than posterolaterally as in many other archosaurs) and is preserved ventral to the posterior end of the right maxilla.

Huene (1932: 42) identified the distal end of the right quadrate ( Fig. 2C ) and preparation has now completely exposed the medial surface of this bone ( Fig. 4C ). The quadrate has a concave, robust posterior edge, which terminates in a rounded dorsal (proximal) head situated anterior to the distal articular end. Its extensive, anteromedially directed flange for contact with the pterygoid is triangular in medial view. A low ridge extends from the distal end of the anterior margin of the pterygoid flange toward the lateral condyle of the quadrate, resulting in a slightly concave anterior surface just above the distal condyles. A narrow lateral ridge extends ventrally from the proximal head of the quadrate but its full extent is not evident. The structure of the distal articular end of the quadrate indicates that the quadratojugal did not enter into the craniomandibular joint as in crocodyliform archosaurs. The medial distal condyle of the quadrate is more robust than the lateral one. The quadrate lacks pneumatic openings, unlike in Crocodyliformes ( Nesbitt, 2011), and was vertically aligned rather than anterodorsally inclined. There is no indication that it contacted the prootic, unlike in Crocodylomorpha ( Nesbitt, 2011).

Huene (1932: 44) interpreted a small piece of a median, vertically aligned, plate-like bone between the maxillae as part of the ‘prevomer’. The featureless fragment ( Fig. 3B ) could represent a vomer but does not permit definite identification.

The right pterygoid lacks only the quadrate flange. [ Huene (1932: 44) identified it as the left element.] The bone is now almost completely exposed in ventromedial view ( Fig. 3B ). The flat facet for contact with the basipterygoid process faces medioventrally and posteriorly. A short medial projection of the ventromedial edge of the pterygoid at the junction of the various rami of this bone slightly extends over the facet in ventral view. The medial margin of the pterygoid anterior to the contact with the basipterygoid process forms a thick, posteriorly rounded ridge. The extensive palatal flange of the pterygoid is transversely concave in ventral view, forming a long trough extending anteroposteriorly. The ventral surface of the lateral or transverse flange curves slightly downward and has a concave posterior margin.

It is not clear what Huene (1932: 44) identified as the epipterygoid ‘a short distance anterior to the [basicranial] joint’ (our translation). He was possibly referring to a portion of the then only partially exposed pterygoid flange of the right quadrate.

The supraoccipital ( Figs 2A, B , 5A ) appears to have been identified by Huene (1932: 43) as a probably right frontal. It is plate-like and anterodorsally inclined. The posterior (occipital) surface of the supraoccipital is concave dorsoventrally and transversely. A low median ridge marks the attachment of the nuchal ligament. Ventrally, the supraoccipital is notched and presumably formed the dorsal margin of the foramen magnum as in most archosaurs in which this condition can be determined ( Nesbitt, 2011). Its dorsal and lateral margins have distinct surfaces for contact with the parietals.

As in most archosaurian reptiles, the exoccipital and opisthotic are indistinguishably fused to each other to form an otoccipital ( Fig. 5B, C ). The right compound element is more completely preserved than the left. Crushing has separated and displaced both otoccipitals, exposing the considerably expanded facets on the exoccipital portions for contact with the basioccipital. Based on the size and medial expansion of the latter, it is likely that they met along the midline and excluded the basioccipital from the floor of the endocranial cavity and the ventral margin of the foramen magnum, the plesiomorphic condition amongst Archosauriformes ( Nesbitt, 2011). The paroccipital process (which was presumably formed primarily by the opisthotic) projects posterolaterally ( Fig. 5C ) and is flattened anteroposteriorly. It is somewhat deeper dorsoventrally towards its distal end. Although the lateral margins of the distal ends are damaged, there is no indication of a pronounced dorsoventral expansion of the distal end of the process as in various derived paracrocodylomorph taxa (e.g. Postosuchus ; Weinbaum, 2011). The thick dorsal edge near the base of the process formed the ventral margin of the post-temporal fenestra. On the right side, a groove along the medial edge of the opening probably carried the external occipital vein as inferred in Batrachotomus ( Gower, 2002) . The ventral surface of the paroccipital process is grooved for the reception of the stapes. The anterior face of the right process is flat and striated, serving for contact with the prootic more medially and with the occipital portion of the squamosal more laterally. The thick posterior part of the exoccipital portion of the otoccipital above the facet for contact with the basioccipital forms the lateral margin of the foramen magnum. A small medial flange of the compound bone forms the dorsolateral corner of this opening; thus the two otoccipitals formed a slight shelf over the foramen magnum. The posterior portion of the exoccipital forms a lateral recess, which apparently contains only a single foramen for passage of cranial nerve XII (N. hypoglossus). This differs from the presence of two exits for the hypoglossal nerve in most other archosaurs ( Nesbitt, 2011), and it is possible that the smaller anterior opening is obscured by preservation. The lateral recess is bounded anterolaterally by a bony flange (subcapsular buttress; Walker, 1990), which separates it from the fenestra ovalis and the ‘metotic foramen’ for passage of cranial nerves IX- XI and associated vessels at the base of the paroccipital process. None of these openings is visible because of crushing and displacement of the otoccipitals.

Large fragments of curved, plate-like bone crushed against the anterior portions of the otoccipitals probably represent portions of the basisphenoid, based on comparison with Sphenosuchus ( Walker, 1990) .

A fragment of abraded bone on the ventral aspect of the basicranial fragment comprises the basioccipital posteriorly and possibly part of the basisphenoid more anteriorly.

Mandibular bones

The tooth-bearing rami of both dentaries in SMNS 12353b are missing their more anterior portions. Furthermore, the left element ( Fig. 6A ) is slightly crushed dorsoventrally. Posteriorly, the dentary bounds the pointed anterior end of the large lateral (external) mandibular fenestra with a posterodorsal and a posteroventral process. The long posteroventral process tapers posteriorly along the ventral margin of the fenestra and overlapped the anteroventral surface of the angular. The dorsal margin of the dentary ascends slightly posterodorsally whereas the ventral margin remains almost straight. Starting at the anterior end of the mandibular fenestra a pronounced ridge extends horizontally along the lateral surface of the dentary ( Figs 2A , 6A ). Just dorsal to the ridge a narrow groove is present, similar to lateral grooves on the dentaries of Fasolasuchus ( Bonaparte, 1981) and Saurosuchus ( Alcober, 2000) . Knoll (2010) explicitly compared this feature to the lateral ‘parapet’ on the dentary of the coelophysoid theropod Dilophosaurus but the latter appears to be an independent evolutionary acquisition. The preserved portion of the left dentary in SMNS 12353b has three complete posterior alveoli of which the anterior one contains the root of a tooth and the second an almost complete but damaged tooth; in addition there is the posterior wall of a fourth alveolus at the broken anterior end of the bone. The right dentary fragment also preserves the three posterior alveoli, the anterior two of which still contain complete teeth. The crown and partial root of an additional, dissociated tooth mentioned by Huene (1932: 45) adhered to the lateral surface of the dentary and were removed during preparation. It is not clear whether this tooth belonged to the dentary or to the maxilla. On the medial surface of the toothbearing ramus of the dentary, distinct, triangular interdental plates are bounded ventrally by a more or less continuous, longitudinal groove, which presumably housed the dental lamina.

The splenial is partially preserved on both mandibular rami. It conceals the dorsoventrally concave medial surface of the dentary ( Fig. 6B ) and forms the medial wall of the Meckelian canal. The left splenial was displaced and rotated upward during fossilization. Its more anterior portion is not preserved. Huene (1932: 45) identified this bone as the left angular. The splenial is gently convex dorsoventrally and increases in height posteriorly. Although the outline of its thin posterior portion is not preserved, on either side of the mandible the left surangular has a tapering posteroventral process. A ridge above the laterally inflected ventral margin of the splenial forms the dorsal edge of a long, dorsoventrally concave facet for contact with the medially inflected ventral margin of the dentary.

The disarticulated left angular ( Fig. 6 ) is preserved behind the left maxilla and only lacks its posterior tip. Huene (1932: 43) misidentified this bone as the left postorbital. The rather slender angular bears a well-defined ventrolateral surface for contact with the ventral process of the dentary for much of its length. Its ventral margin is inflected medially where it forms a medially directed surface for contact with the splenial. The posterior portion of the angular curves medially. A dorsolateral facet on the posterior end of the angular contacted the surangular.

The left surangular ( Figs 2B, C , 6A ) is missing its anterior portion and the posterior tip. Ventrally, it forms a nearly vertical plate with an anteroposteriorly convex ventral edge. The dorsal portion of the surangular has a prominent lateral ridge that overhangs the lateral surface of the element. A corresponding ridge is present in Arizonasaurus ( Nesbitt, 2005) , Batrachotomus ( Gower, 1999) , and Postosuchus ( Weinbaum, 2011) . A small foramen is present anterior to the articular facet of the craniomandibular joint where the dorsolateral ridge fades into the bone posteriorly. A surangular foramen is present in nearly all non-crocodylomorph suchians but absent in most known basal crocodylomorphs ( Nesbitt, 2011). Posteriorly, the surangular overlaps the articular laterally and forms the lateral edge of the mandibular facet for the jaw joint.

The nearly complete right prearticular is preserved alongside the left articular and surangular ( Fig. 2C ). The expanded, ascending posterior portion of the prearticular contacts the articular posteromedially. It rapidly decreases in height anteriorly. The strongly concave dorsal surface of the prearticular bordered the internal mandibular fenestra ventrally. The element becomes much more narrow anteriorly. Its broken and slightly displaced anterior process has a medioventral surface for contact with the angular.

Little of the left articular is exposed except for the mandibular facet. The posterior portion of the bone behind the facet is not preserved.

Hyoid apparatus

Two gently curved, slender, and rod-like bones adjacent to the right maxilla and pterygoid in SMNS 12353b represent the first ceratobranchials ( Figs 2C , 3A, B ). The preserved anterior end of the right ceratobranchial is slightly thickened. Both bones lack their posterior ends.

Figures

Figure 2. Skull of Apatosuchus orbitoangulatus (SMNS 12353b, holotype). Epoxy resin cast of the specimen prior to recent preparation in A, left lateral, B, dorsal, and C, ventral views. In each row, the photograph is on the left and an explanatory diagram on the right. In the diagrams, grey denotes exposed bone; arrow in B is directed to the front. Scale bars = 1 cm. Abbreviations: an, angular; ao.f, antorbital fenestra; bc.f, basicranial fragment; cb, ceratobranchial; d, dentary; d.r, lateral ridge on dentary; e.m.f, external mandibular fenestra; f?, fragment of frontal or postfrontal; f.d, facet for contact with dentary (on angular and splenial, respectively); m, maxilla; n, nasal; n.r, dorsolateral ridge on nasal; pa, prearticular; pt, pterygoid; q, quadrate; sa, surangular; so, supraoccipital; sp, splenial; sq, squamosal. ‘?’ denotes unidentified bone.

Figure 3. Cranial remains of Apatosuchus orbitoangulatus (SMNS 12353b, holotype) after preparation in A, right lateral, and B, ventral views. C, camera lucida drawing of the crown of the posterior tooth preserved in the left maxilla and detail of the apical region of the crown with mesial and distal serrations (extent marked by small arrows). Large arrow points anteriorly (mesially). Scale bars = 1 cm for A, B and 5 mm for C, respectively. Abbreviations: cb, ceratobranchial; j, jugal; m, maxilla; n, nasal; p, parietal; pa, prearticular; po, postorbital; pt, pterygoid; q, quadrate; sa, surangular; v?, possible vomer.

Figure 4. Details of cranial structure of Apatosuchus orbitoangulatus (SMNS 12353b, holotype). A, oblique posterodorsal view of the left squamosal and right parietal. B, lateral view of the left squamosal. C, left squamosal in posterior view and right quadrate in medial view. D, oblique dorsal view of the right side of the specimen, showing the right jugal in medial view and ventral process of the right postorbital (in cross-section). Scale bars = 1 cm. Abbreviations: a.pa, contact area for paroccipital process on squamosal; f.p, facet for contact with parietal; j.dp, dorsal process of jugal; j.pp, posterior process of jugal; m, maxilla; m.t, maxillary tooth; p, parietal; po, postorbital; q, quadrate; q.r, recess on squamosal for proximal head of quadrate; sa, surangular; sq.lr, lateral ridge on ventral process of squamosal; sq.pp, posterior process of squamosal; sq.r, dorsolateral ridge on squamosal; sq.vp, ventral process of squamosal.

Figure 5. Supraoccipital and basicranial fragment of Apatosuchus orbitoangulatus (SMNS 12353b, holotype). A, supraoccipital in posterior view. B, C, basicranial fragment in B, posterior and C, ventral views. Scale bars = 1 cm. Abbreviations: f.p, facet for parietal on supraoccipital; p.p, paroccipital process; po.f, posttemporal foramen; XII, foramen for cranial nerve XII (N. hypoglossus).

Figure 6. Apatosuchus orbitoangulatus (SMNS 12353b, holotype). Bones of the left mandibular ramus as preserved in A, lateral, and B, oblique dorsomedial views. Scale bar = 1 cm. Abbreviations: an, angular; d.r, lateral ridge on dentary; d.vp, posteroventral process of dentary; e.m.f, external mandibular fenestra; f.d, facet for contact with dentary (on angular and splenial, respectively); id, interdental plate; sp, splenial.