Goniopholididae Cope, 1875

Halliday, Thomas J. D., Andrade, Marco Brandalise De, Benton, Michael J. & Efimov, Mikhail B., 2015, A re-evaluation of goniopholidid crocodylomorph material from Central Asia: Biogeographic and phylogenetic implications, Acta Palaeontologica Polonica 60 (2), pp. 291-312 : 292-299

Family Goniopholididae Cope, 1875 Genus Sunosuchus Young, 1948

Type species: Sunosuchus miaoi Young, 1948 . Holotype specimen from the Late Jurassic of Gansu, China.

“ Sunosuchus ” thailandicus Buffetaut and Ingavat, 1980

Figs. 1–3 , 4A .

1988 Sunosuchus shartegensis sp. nov.; Efimov 1988: 54, fig. 8.

Holotype: PIN 4174-1. The holotype is a fragmented skull, comprising the rostrum, the preorbital region of the skull table, the quadrates and parts of the quadratojugal, the occipital condyle and near-complete mandibles ( Fig. 1 ). The holotype is the only specimen known of this species. There has been some discrepancy in the museum number recorded in the literature, namely PIN 4174-1 (e.g., Efimov 1988a) and PIN 4171-1 (e.g., Efimov 1988b). The correct number on the specimen label is PIN 4174-1.

Type locality: The specimen was found in the Ulan Malgait beds, in the Shar Teeg locality, of the Gobi-Altai region of Outer Mongolia, embedded in grey clay. The Ulan Malgait Beds are situated 2200 m east-southeast from Ulan Malgait Mountain, and are described in Gubin and Sinitza (1996), who indicate that PIN 4174-1 was extracted from “Layer 2” .

Type horizon: The age generally ascribed to this section is Upper Jurassic (Tithonian). Sedimentological profiles indicate that the Ulan Malgait beds were formed in a temporary lacustrine environment with seasonal outwashes of shore sediments and drying of lakes ( Gubin and Sinitza 1996; Watabe et al. 2007). This gives the specimen a similar age to Sunosuchus miaoi from north-west China, geographically near to Shar Teeg, and well within the time during which the goniopholidids were most diverse. Shar Teeg has since yielded a diverse array of species, including insects such as lacewings ( Khramov 2011), fishes, turtles, crocodyliforms, and temnospondyl amphibians ( Gubin and Sinitza 1996).

Emended diagnosis.— “ Sunosuchus ” thailandicus differs from all other goniopholidids except Kansajsuchus in lacking neurovascular foramina on the dorsal surface of the rostrum and in possessing a relatively broad quadrate with an expanded medial hemicondyle. The ventral margin of the neurovascular foramina is very close to the teeth, along the alveolar margin, compared with other goniopholidids, but the maxillary depression is elevated from the tooth row higher than in other goniopholidids. Unlike other goniopholidids, the mandibular symphysis is inclined dorsally. Differs from all goniopholidids except Calsoyasuchus in the presence of an anteroposteriorly elongate antorbital cavity. The extent to which the premaxillo-maxillary notch surrounds the tooth is limited, and the lat-

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eral margins of the nasal are convex, as in other Asian goniopholidids, but unlike the European forms. Differs from S. miaoi in having a tooth row lower than the quadrate condyle. Differs from S. junggarensis in the festooning of the jaw, which has a double rather than single sinusoid. Differs from both S. miaoi and S. junggarensis in lacking a maxillo-palatine fenestra and dental ornamentation.

Description.— General features: The holotype comprises a highly fragmented and slightly distorted skull ( Figs. 1–3 ). Several sutures cannot be discerned reliably because the material is incomplete and fractured.

The largest preserved part is composed mostly of maxillae and nasals, clearly distinguishable from one another in dorsal view ( Fig. 1 ). The premaxillae are present, though the tip is missing, and the nares cannot be fully delineat- ed. It appears that the anterior portions of the frontals, as well as all of the prefrontals, are preserved towards the rear of this portion. The teeth extend laterally along the entire length of this fragment ( Figs. 1 , 2 ), and no maxillojugal suture can be seen. The skull is reconstructed as being between 40 and 50 cm long in total, and about 20 cm wide at the quadrates, which agrees broadly with the conclusions of Efimov (1988a), whose estimate of length was slightly shorter, perhaps following his interpretation of the antorbital fenestra-like structures as orbits (see below). The increase in width at the orbital region of the skull is dramatic, and the skull has an overall medium-length, but narrow rostrum. The rostrum is nonetheless broader than it is high, and has a slightly concave appearance when viewed laterally ( Fig. 1B ). It seems to make up a significant proportion of the length of the skull—PIN 4174-1 is therefore considered meso- to longirostrine, as is “ S. ” thailandicus . The dermal bones of the skull are ornamented with a pitted pattern for the majority of their length ( Fig. 1A ), though the more posterior parts of the maxilla do not have preserved ornamentation. When the mandible is reconstructed from the several fragments, the quadrate condyle is clearly not level with the tooth row.

Premaxilla: The rostrum is broken at the premaxillo-maxillary suture, where there is a constriction and the skull is narrower. The suture is just present on the anteriormost end of the specimen ( Fig. 1B ). Because of this, the shape of the premaxillae is not clear, but given that there appears to be a strong constriction, as in Goniopholis , Eutretauranosuchus , and other goniopholidids, it is highly probable that there is a lateral expansion anterior to the constriction. The nares do not appear to be preserved in this specimen; there is an area lacking bone in the centre of the rostrum ( Fig. 1A ), but this is too far from the expected end of the snout, and seems to represent taphonomic loss. The nasals do not extend to the end of the specimen, and are therefore certainly excluded from the narial cavity, as in other Sunosuchus species, Goniopholis , and Eutretauranosuchus .

Only the posterior end of the palatal region of the premaxilla is preserved ( Fig. 2 ); this part is unornamented and not perforated by any fenestrae. The palatal rami meet in the middle of the palate and contribute significantly to the region. The suture with the maxillary region of the palate is straight rather than concave or convex.

Maxilla: The maxillae are festooned in a sinusoidal pattern both in dorsal and lateral view ( Fig. 1 ), with the lateral expansions coinciding with ventral expansions and increases in tooth size ( Fig. 2 ). The entire dorsal surfaces of the maxillae are ornamented with a series of pits, except for a small, smooth depression towards the posteriormost end of the specimen (maxillary depression in Fig. 1B ), which contains two neurovascular foramina, the larger of which is large enough and positioned in such a way that it could have housed the maxillary branch of the trigeminal nerve. This oval depression appears to be entirely enclosed by the maxilla, though its posterior end is missing, and there are no sutures apparent between any of the lacrimal, maxillary or jugal bones. As the posterior section is missing, the proportions of the depression are unknown. The depression has a raised anterior rim through which the largest of the neurovascular foramina passes. Although maxillary depressions are synapomorphic for goniopholidids, this specimen possesses notable differences from the standard goniopholidid pattern. Primarily, the position of the depression is considerably dorsally displaced relative to the alveolar margin and the main lines of the neurovascular openings. Goniopholidid maxillary depressions are thought to derive from the neurovascular foramina ( Andrade 2009), and hence this displacement indicates a lack of homology, despite the structural similarity to that of Eutretauranosuchus ( Smith et al. 2010) and other goniopholidids.

There is a possible antorbital fenestra between the maxilla and the prefrontal, taking the form of an elliptical opening, with the main axis oriented roughly anteroposteriorly, as in Calsoyasuchus ( Fig. 1A ). This feature is unreported in all other specimens of Sunosuchus , and indeed is unique among goniopholidids in Calsoyasuchus . The maxillary depression is positioned near to the anterolateral edge of this antorbital fenestra, which penetrates through the skull to the palate.

There is a wide and relatively shallow constriction at the premaxillo-maxillary suture ( Fig. 1 ). This broadly agrees with Efimov (1988a), who stated that “the festooning at the premaxilla is located for the insertion of the mandibular tooth”.Although the “festooning” is present, no large caniniform mandibular tooth is preserved, and the constriction is shallower and wider than would be expected for such a tooth. Only the posterior end of the constriction is preserved, leading to an impression of simple narrowing. The constriction can be distinguished from an anterior narrowing of the jaw, as the rapid decrease in tooth size would suggest a diastema rather than the end of the jaw, where no significant reduction in tooth size would be expected.

The maxilla forms a large proportion of the preserved secondary palate ( Fig. 2 ), though many of the sutures are fused or destroyed. The maxillary portion is entirely unornamented, and extends as far back as the antorbital fenestra, where is appears to meet the palatine bone. Because of the high fragmentation of the palate, the identification of any fenestrae is next to impossible, but from what is preserved, the “anterior palatal openings” previously described ( Efimov 1988a) are not evident. The presence of anterior fenestrae in the maxillary palate is one of the supposed synapomorphies of Sunosuchus , and the absence of such a feature here is notable. Even though it is fragmented, the palate seems to form a continuous surface in the area in which such fenestrae would be expected ( Fig. 2 ). The edges of the secondary palate, as mentioned by Efimov (1988a), are bounded by a groove, and his interpretation that this held the palatine artery is followed here.

Nasal: The nasal bones are incomplete along their length. The sutures with the maxillae are nonetheless clear, as the maxillae are well preserved. The nasals are rectangular, with no lateral concavity or convexity along their length ( Fig. 1A ). They are of constant width along the rostrum, and they do not taper towards either end. No midline suture between the nasals has been preserved. The nasals are slightly concave when viewed in lateral aspect, curving with the whole rostrum ( Fig. 1B, C ). The anterior limit of the nasal bones is not clear, but it appears that they are excluded from the nares by the premaxillae. The nasals are ornamented with the same pattern of pits as the other dermal bones of the skull.

Posteriorly, the nasals are limited by the nasofrontal suture, which occurs at the same level as the large elliptical antorbital fenestra ( Fig. 1A ). This suture is narrow and straight, unlike other specimens of Sunosuchus . The morphology of this suture is used to distinguish Sunosuchus from other goniopholidids such as Calsoyasuchus and Eutretauranosuchus , which possess a W-shaped naso-frontal suture, although Anteophthalmosuchus also possesses a V-shaped suture. The lack of a nasal process between the frontal and prefrontals relates this specimen to Sunosuchus , although the area is heavily damaged, and interpretations of the positions of sutures are tentative.

Frontal: The unpaired frontal is partially preserved, with the anterior portion that contacts the prefrontal and nasal bones relatively well preserved. In PIN 4174-1, the frontal and prefrontal bones are very closely associated ( Fig. 1A ), and distinction between these elements is difficult. This region, where the frontal meets the prefrontal and nasal, lies directly between the antorbital fenestrae, and is flat to slightly concave ( Fig. 1B, C ). The bone is covered in pitted ornamentation. The frontal tapers significantly anteriorly and extends far further forward than the orbital region, which contrasts with Efimov’s (1988b) interpretation, which suggested that previous interpretations of the forward position of the frontal, such as Young’s (1948) description of S. miaoi , were wrong. This interpretation possibly arose from misinterpretation of the antorbital fenestra as an orbit. In Calsoyasuchus and in S. miaoi (see Young 1948; Tykoski et al. 2002), the frontals extend anterior to the orbits, and in Calsoyasuchus the former are level with the antorbital fenestra (the latter taxon does not possess an antorbital fenestra). There is neither a transverse nor a longitudinal ridge on the frontal bone ( Fig. 1A ), though the level of the frontal is slightly below the surrounding bones, giving the appearance of ridges surrounding the frontal.

Prefrontal: The paired prefrontals are both preserved in their entirety, and are positioned on the medial edge of the antorbital fenestra. They are wedge-shaped, tapering to a point anteriorly and probably contacting the nasals, though the bone is broken here, and they may have been excluded from contact by a nasofrontal suture. They are covered in heavy pitted ornamentation ( Fig. 1A ).

Lacrimal: The lacrimals do not appear to be preserved. They are expected to bound the rim of the antorbital fenestra on the lateral edge, but they cannot be located because they have either been destroyed or the sutures are not preserved.

Palate: The secondary palate of “ S. ” thailandicus is preserved nearly complete ( Fig. 2A ), except under the premaxilla, and it extends back to the limit of the internal nares. The maxillae comprise the majority of the palate; the medial suture is not visible. The palatine bones are just visible at the posterior end, also perforated by several neurovascular foramina of varying size. The largest of these is on a region of the palate that could be the right palatine wing ( Fig. 2A ). The suture between maxillae and premaxillae is not obvious, since this area is damaged.

The palate was originally described as possessing two distinctive openings at the posterior end of the maxilla, but no evidence of palatal openings warranting the description “distinctive” was found. These are understood to refer to the anterior palatal openings, which in Sunosuchus miaoi are positioned between the maxilla and palatine. Among neosuchians, such openings are only known in Sunosuchus and Eutretauranosuchus ( Buffetaut 1986) .

Quadrate: The articular heads of both quadrates are well preserved ( Fig. 1A ), and the right quadrate retains its connection to the quadratojugal. The condylar heads of the quadrate are not equal in size, with the medial head being considerably smaller but more ventrally directed than the lateral head. The heads are separated by a well-defined groove on the ventral surface. The quadrates are held horizontally, as in all other goniopholidids and pholidosaurids. There are no identifiable large foramina on the preserved surface of the quadrate, and Efimov’s (1988a) claim that the air cavity connecting the middle ear to the maxillary sinuses can be seen opening in the quadrate cannot be substantiated. The bone is not complete, however. The quadrate is an entirely unornamented bone, in contrast to those that surround it ( Fig. 1A ), and is non-pneumatic. The posterior edge of the quadrate expands laterally and shows a weaker concavity than that of S. junggarensis or S. miaoi .

Basioccipital: The basioccipital is almost complete, with everything ventral to the occipital condyle present ( Fig. 3 ). The bone is extremely spongy, and is perforated by a variety of foramina for nerves, blood vessels, and also sinusal channels. Efimov (1988a) devoted considerable space to identifying the paths of the different sinuses, but little can be seen of the sinuses on the exterior surface. It is possible that the specimen has degraded since 1988, but the detail in Efimov’s description cannot now be confirmed; only a tomographic scan could reveal the pneumatic structure. The occipital condyle is subcircular, with two wing-like structures on the lateral edges, which give it an overall heart shape in posterior view.

When viewed laterally ( Fig. 3B ), the hypoglossal nerve cranial nerve XII) canal can be seen clearly, passing through the occipital bone, surrounded as it is by the highly perforat- ed and spongy structure. The path of the hypoglossal nerve was interpreted by Efimov (1988a) to be a primitive feature in an otherwise highly derived occipital region. As described by Efimov (1988a), the braincase floor is verticalised ( Fig. 3A ), a trait characteristic of more derived members of Eusuchia , which would suggest, in conjunction with the overall body size and single frontal that this is a mature and derived crocodylomorph.

Dentary: The mandible is shallow and straight throughout its length ( Fig. 1B, C ), and Y-shaped in dorsal view ( Fig. 2B ), as it has an extensive mandibular symphysis, similar to that in Sunosuchus miaoi and “ S. ” junggarensis . The tooth rows run parallel along the entire length of the preserved symphyseal region, of which the anterior section is missing. The bone of the symphyseal region is highly pneumatic and spongy, and the section is inclined dorsally by approximately 5° ( Fig. 1B, C ). The dorsal surface of the symphysis is flat, with no depressions or ridges, and is estimated to have been about twice as long as it is wide. As in the maxilla, the teeth are isolated in their own alveoli, at least in the symphyseal region, where the bases of the teeth are preserved in greatest detail.

Splenial: The splenial contributes substantially to the mandibular symphysis, entering as a wedge-shaped projection into the dentary portion of the symphysis ( Fig. 2B ). In most other Sunosuchus species the splenial plays a small part in the mandibular symphysis, but none as strongly as in “ S. ” thailandicus . The splenial peg in the symphysis is present on both dorsal and ventral surfaces, and the splenial bone is large and robust throughout.

External mandibular fenestra: The mandibular fenestra is preserved on both rami ( Fig. 1B, C ), each across multiple fragments. The fenestra is long and thin, with angular ends, and is oriented horizontally. Each fenestra occurs at the point where the articular rises to the condyle. It is in line with the tooth row, and slightly below the level of the quadrate-articular joint, and is bounded by the articular ventrally and surangular dorsally.

Angular: The angulars are heavily pitted on their external surfaces, like the dermal bones of the skull, but unlike the mandible itself ( Fig. 1B, C ). The posterior portion of the jaw shows no increase in depth or curvature.

Surangular: The surangular is in two sections on both sides, and the morphology of each end differs slightly. The posterior end, which extends onto the retroarticular process, and forms the posteriormost part of the preserved specimen, is more strongly pitted, while the region above the external mandibular fenestra is smooth and unornamented, contacting the angular smoothly ( Fig. 1B, C ).

Articular: Both articulars are complete. The condyle is extremely robust relative to the rest of the bone, which is entirely unornamented. The articular ventrally contacts the angular and laterally the surangular with simple sutures. The descending process of the articular on the medial side of the mandible is strongly grooved down the centre ( Fig. 1B, C ). The quadrate condyle is oriented horizontally, and has a deeper rim on the posterior than on the anterior edge. It is directly beneath the articular, on the posteroventral surface, where the ventral surface of the mandible is most curved. This feature of greatest curvature on the posteroventral surface rather than directly below the external mandibular fenestra is a character common to all goniopholidids.

Teeth: The teeth are similar in morphology throughout, being unornamented and cone-shaped ( Figs. 2 , 4A ). Along the maxilla, the tooth size changes ( Table 1), with the greatest diameter at the points of greatest lateral and ventral expansion ( Fig. 2A ), and least where the snout is narrowest. The teeth are circular in cross-section, and are conical to caniniform in morphology. This differs strongly from other Sunosuchus specimens, which possess ornamented, ridged posterior teeth that are slightly laterally compressed, and possess a clear keel ( Buffetaut and Ingavat 1984; Wu et al. 1996; Averianov 2000; Maisch et al. 2002). Though in PIN 4174-1 the teeth are not preserved save in cross section and for one relatively anterior dentary tooth ( Fig. 4A ), the teeth are clearly not compressed in cross section rather than subcircular, as in other Sunosuchus specimens.

Each tooth is vertical and set in a separate alveolus ( Fig. 2 ), isolated from other teeth and from both lateral and medial walls of the alveolar margin. Most previously described specimens of Sunosuchus also possess separate alveoli, though some had apparent grooves (Maisch et al. 2002). The preserved mandibular teeth are 10 mm in length, and 6 mm in diameter at the base ( Fig. 4A ), similar in size to the teeth of the fragment described by Maisch et al. (2002). They are slender, and taper to a point. All teeth are approximately the same size in the mandible, unlike in the maxilla.

The tooth rows are continuous in both upper and lower jaws ( Fig. 2 ), with no diastemata. Teeth are neither cusped nor faceted. Because few teeth are preserved in full, occlusion is difficult to determine, but it appears that the upper and lower dentitions interlocked—there is some suggestion of pits between the lower teeth, although these are not well preserved—and there is no overbite, as in Goniopholis . The maxillary tooth row extends far further back than the dentary tooth row, to almost the same level as the mandibular external fenestra.

Remarks.— The antorbital fenestra of PIN 4174-1 is seen among Goniopholididae otherwise only in the American Jurassic Calsoyasuchus ( Tykoski et al. 2002) , though it is known in more basal crocodylomorphs ( Osmólska et al. 2007) and many notosuchians ( Andrade and Bertini 2008; Kley et al. 2010). An antorbital fenestra is absent in other species of Sunosuchus . Since PIN 4174-1 and Calsoyasuchus do not form a monophyletic group in this study, the presence of the antorbital fenestra in these taxa is optimized as a convergence rather than a synapomorphy. Many outgroups to Goniopholididae possess antorbital fenestrae, and the loss of the trait may be a general neosuchian feature.

PIN 4174-1 exhibits several features consistent with placement in the Goniopholididae , including a highly festooned rostrum, a strong constriction at the premaxilla-maxilla suture, and the pattern of ornamentation ( Fig. 1 ). The maxillary depression is present in a highly unusual form. This structure is traditionally a key synapomorphy of Goniopholididae , but Martin and Buffetaut (2012) consider it homologous to that in some pholidosaurs. This structure is ontogenetically related to the line of neurovascular foramina that runs along the alveolar margin ( Andrade 2009), and in all goniopholidids possessing the maxillary depression, the structure is situated in this region (e.g., Schwarz 2002; Andrade 2009, Andrade et al. 2011).

The maxillary depression in S. miaoi , as described by Buffetaut (1986) is a “deep elongated depression subdivid- ed by faint transversal ridges”, and is a feature unique to Goniopholididae , in which it is most usually bordered by the maxilla, close to the lacrimal and jugal sutures. There is a depression of sorts in PIN 4174-1; it lacks the posteri- or end, but appears not to be elongated, and possesses no transverse ridges. There is also no evidence that the lacrimal was involved in this depression; no sutures are seen

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in close proximity to the bone, and the position is far too dorsal with respect to the alveolar neurovascular network to be considered homologous to a true goniopholidid maxillary depression.

In summary, PIN 4174-1 possesses many goniopholidid synapomorphies, and it shares some features with other species of Sunosuchus , and yet the lack of other definitive synapomorphies suggests it might belong to a different genus, or, if the derivation of the maxillary depression from the alveolar neurovascular region is considered a universally held goniopholidid synapomorphy, it might even lie outside that clade. There is no diagnostic feature that separates it from the extremely fragmentary S. thailandicus , suggesting that PIN 4174-1 could be considered a synonym of S. thailandicus . These ideas are tested further in the cladistic analysis.

Figures

Fig. 1. Adult crocodylomorph “Sunosuchus” thailandicus Buffetaut and Ingavat, 1980, PIN 4174-1, from the Tithonian (Upper Jurassic) of Shar Teeg, Mongolia. A. Rostrum, quadrates, and occipital condyle in dorsal aspect. B, C. Rostrum and mandible in left (B) and right (C) views. Photographs (A 1 –C 1) and explanatory drawings (A –C ).

Fig. 2. Crocodylomorph “Sunosuchus” thailandicus Buffetaut and Ingavat, 1980 dentition, PIN 4174-1, from the Tithonian (Upper Jurassic) of Shar Teeg, Mongolia. A. Rostrum in palatal aspect. Photograph (A1) and explanatory drawing (A2). B. Anterior mandible in dorsal view, from symphysis to the anterior edge of the external mandibular fenestra.

Fig. 3. Crocodylomorph “Sunosuchus” thailandicus Buffetaut and Ingavat, 1980, PIN 4174-1, from the Tithonian (Upper Jurassic) of Shar Teeg, Mongolia. Occipital condyle in lateral (A) and posterior (B) views.

Fig. 4. Teeth of crocodylomorphs from the Cretaceous of Central Asia. A. “Sunosuchus” thailandicus Buffetaut and Ingavat, 1980, left mandibular tooth of PIN 4174-1. Tooth is from mid-mandible and is similar in size to all of the other mandibular teeth, which show little size variation. B, C. Kansajsuchus extensus Efimov, 1975. B. Caniniform tooth of PIN 2399-313 in anterior (B1), posterior (B2), and lateral (B3) views. The tooth is characterized by prominent ridges, including a distinctive “double keel” on the posterior side of the tooth. The tooth is from the anterior part of the jaw. C. Molariform tooth of PIN 2399-314 in posterior (C1), anterior (C2), and lateral (C3) views. The tooth shares the ridges characteristic of Kansajsuchus, including a less prominent version of the double keel, but is blunter, and derived from the posterior part of the jaw.