Shinisaurus, AHL, 1929

Conrad, Jack L., 2004, Skull, mandible, and hyoid of Shinisaurus crocodilurus Ahl (Squamata, Anguimorpha), Zoological Journal of the Linnean Society 141 (3), pp. 399-434 : 401-422

SHINISAURUS AHL, 1929

SHINISAURUS CROCODILURUS AHL, 1929

Skull and mandible

Skull form: The skull may be divided into facial and cranial portions, divided at the maxilla- ectopterygoid contact. It may also be divided into the antorbital snout, orbital region, and temporal area.

The skull of Shinisaurus bears an unusual combination of proportions. Most anguimorphs possess a facial portion that is elongate relative to the cranial portion with a similarly proportioned antorbital snout. Although the snout is particularly short in Shinisaurus , the facial portion of the skull is proportionately much longer than in even the long-snouted Varanus .

McDowell & Bogert (1954) reported that the temporal portion of the skull was much shorter than the snout and orbital portions. Similarly, Hu et al. (1984) stated that the frontal is longer than the parietal. Whereas these observations hold true for juvenile specimens, allometric growth in the skull produces a relatively more elongate temporal skull as the animal matures ( Fig. 11C- F ). In the subadults and adults ( Figs 1 , 2 ), the temporal portion is subequal to or even greater in length than the rest of the skull.

The skull is deep with a box-like temporal region and a relatively short, blunt, snout ( Figs 1 , 2 ). It is widest at the posteroventral extreme of the jugal. The skull of Xenosaurus ( Fig. 3B ) is also widest at the posteroventral process of the jugal, but the snout tapers more sharply and is rounded at its anterior end. Carusia ( Figs 3A , 4A , 5A ) is similar in skull form (Borsuk- Bialynicka, 1985; Gao & Norell, 1998, 2000) to Shinisaurus .

Typically, most of the contacts of the quadrate suspension are hidden in dorsal view. The anterior part of the quadrate head in Shinisaurus projects slightly lateral to the squamosal and the tympanic crest is visible in dorsal view. However, this crest is much less pronounced than in Xenosaurus , particularly in X. platyceps King and Thompson. In the latter, the squamosals flare posterolaterally for contact with the head of the quadrate such that the quadrate lies mostly lateral to the rest of the skull.

The occipital condyle of Shinisaurus is a single posterior convexity of bone that is dorsally concave in occipital view ( Figs 1E , 2F , 3B , 15D ).

The external nares are small and face strongly anteriorly because of the reduced length of the snout, a condition seemingly also present in Carusia . The orbit is about one-third of the length of the skull and approximately twice as long as tall at its maximum respective diameters on the external surface of the skull. A palpebral cuts across the orbit at approximately the dorsal one-third ( Fig. 8 ).

The supratemporal fenestra is teardrop-shaped with a tapering posterior end ( Figs 1B , 2B ). It is approximately three times as long as wide at its greatest respective diameters. The greatest length is approximately one-half of the length of the skull, but the fenestra extends posteriorly beyond the occipital condyle in adults ( Fig. 11F ). In Xenosaurus , the supratemporal fenestra is more oval and does not extend posterior to the occiput ( Fig. 3B ) as is the case in most anguimorphs retaining a complete supratemporal arch. The infratemporal vacuity is rhomboid and bordered anteriorly by the jugal, dorsally by the postorbitofrontal and squamosal, and posteriorly by the quadrate.

The suborbital fenestra (mistakenly referred to as the internal naris by Zhang, 1991) is an elongate opening posterolateral to the internal nares. Lying ventral to the orbit, this fenestra is about one-half the greatest orbital diameter and approximately twice as long as wide.

The interpterygoid vacuity (sensu Romer, 1956; pyriform recess of Oelrich, 1956) is very narrow for its entire length ( Figs 1C , 2C ) when compared with some extant anguimorphs ( Fig. 4 ) Mosasauroids and many anguids possess a narrow interpterygoid vacuity ( Romer, 1956; Russell, 1967) At its widest, just anterior to the basipterygoid articulation, the interpterygoid vacuity is subequal to the greatest width of the external naris. Anteriorly, it extends between the palatines and even the vomers to the level of the internal nares (fenestrae exochoanalis of Oelrich, 1956), but terminates posterior to the fenestrae vomeronasalis externa (opening to the chamber housing Jacobson’s organ). Forward extension of the interpterygoid vacuity is seen in most platynotan lizards, although not in Gobiderma pulchrum Borsuk-Bialynicka ( Borsuk-Bialynicka, 1984) .

The post-temporal fenestra is an almond-shaped opening on the posterior face of the skull ( Figs 1E , 2F ). The medial border is formed by the contact of the supratemporal with the parietal and the lateral bor- der is formed by the paroccipital contact with the supratemporal process of the parietal. The foramen magnum is subcircular with its greatest diameter occurring transversely, ventral to its midpoint.

The palate is palaeochoanate. The internal nares are elongate openings separated from the vomeronasal chambers only by the posterior ends of the septomaxillae, a point that has been misinterpreted in the past ( Gao & Norell, 1998). This condition is shared with Xenosaurus (contra Gao & Norell, 1998), Heloderma , and Abronia , Elgaria , and Gerrhonotus ( Lee, 1997) . Among fossil forms, this condition seems to be present in the platynotans Gobiderma , helodermatids ( Norell et al., 1992; Lee, 1997; Gao & Norell, 1998, 2000), Saniwa ensidens Leidy , and Saniwides mongoliensis Borsuk-Bialynicka ( Gao & Norell, 1998) . The preservation of Carusia does not allow the exact condition to be determined, although it is certainly not of the neochoanate condition ( Gao & Norell, 1998).

Mandibular form: The lower jaw ( Figs 1 , 2 , 16) of Shinisaurus is robust and ventrally arched. It is relatively rigid with broad overlap of the dentary and postdentary bones. Consequently, Shinisaurus lacks specializations of the intramandibular joint, as extensively discussed in varanoids and in snakes (see recent examinations by Lee, Bell & Caldwell, 1999; Rieppel & Zaher, 2000, 2001; Lee & Scanlon, 2001). Additionally, a well-developed syndesmotic joint joins the mandibles anteriorly.

The dentary makes up more than one-half the length of the mandible, possibly linked to the extension of the maxillary tooth row beneath the orbit. Meckel’s groove is enclosed by the robust, tall, splenial for more than one-half the length of the dentary. The dorsomedially orientated adductor fossa and mandibular foramen are large, as in Xenosaurus and varanoids. The retroarticular process angles slightly posteromedially.

Meckel’s cartilage passes anteroventrally from the articular to form the floor of the adductor fossa and extends anteriorly with slight medial exposure between the anterior and posterior medial arms of the coronoid. Here, it is enclosed by an overlap of the angular, surangular, splenial, and dentary. Anterior to the splenial, Meckel’s cartilage emerges and runs along the ventromedial surface of the dentary.

Dermal skull roof

Premaxilla: The premaxillae ( Figs 1 , 2 , 7A, B ) are fused, composing a single element even in neonates. The premaxilla contacts the nasal and maxilla to form the anteromedial border of the external naris. The premaxilla also contacts the vomer on the palate and the septomaxilla within the nasal capsule. The supposed premaxillary contact with the palatine ( Zhang, 1991) is absent from available specimens and is considered an erroneous report of the vomerine contact.

The only persistent indication of the midline suture occurs at the facet for contact with the vomers. This residual suture line is very short and terminates at the anterior border of the incisive process at its apex

( Fig. 7B ). The premaxilla has a relatively small external exposure along the alveolar margin and is nearly hidden in lateral view ( Figs 1A , 2A ) as in some anguids and Heloderma ( Fig. 4D ). The nasal process is small, extending slightly more than half the height of the medial border of the external nares.

The alveolar margin is approximately as long as its dorsoventral height at the nasal process. Because of its reduced size and the broadness of the snout, the premaxilla has little anteroposterior depth, but the basal plate is narrowly visible in lateral view.

The nasal process is approximately three tooth positions wide, or about one-half the total width of the alveolar margin. The dorsal extremity has two ventrally concave excavations to receive the anterior margins of the nasals in a butt joint. The posterior surface of the nasal process bears a slight ridge similar to that of Xenosaurus , but less pronounced. This ridge only extends for about the top one-half of the nasal process in Shinisaurus whereas it nearly reaches the dental shelf in X. platyceps .

The maxillary suture appears unusually complex because the reduction of the premaxilla allows more of the contact to be visible in external view. The premaxillary process of the maxilla extends onto the anterior surface of the premaxilla, but the dorsal portion of the maxillary process of the premaxilla does not lie inside the narial border as in other squamates. Instead, it is contiguous with the more ventral part of the basal plate. Thus, a dorsolateral wing of the basal plate extends along the anterodorsal surface of the maxilla for a short distance at the front of the naris. When

A B

A

B

articulated, this presents a complex outline to the premaxilla- maxilla suture such that the exposed portion of the premaxilla at the alveolar border is subequal to the width of the nasal process, but flares to nearly twice this width at the narial border ( Fig. 2A, B ). This led McDowell & Bogert (1954) and Zhang (1991) to suggest that the premaxilla is cruciform, but this is only the apparent morphology in the articulated skull.

There is an ethmoidal foramen lying just below each nostril. This seems to be plesiomorphic among anguimorphs and is seen in Xenosaurus , although in the latter the foramen is placed further down on the basal plate. The ethmoidal foramina are absent from the dental shelf of the premaxilla. There is no premaxillary- maxillary aperture (sensu Gao & Norell, 1998) like that of anguines, some diploglossines, and some varanids. Hu et al. (1984) were probably attempting to communicate this character state rather than suggesting the absence of ethmoidal foramina when they reported that there were no premaxillary foramina.

The dental shelf bears a facet extending more than one-half of its length to receive the premaxillary process of the maxilla. The two sides of the dental shelf form an obtuse angle here and receive the vomers. The bilobed incisive process looks like a ventromedial fold in the dental shelf behind the nasal process.

In most anguimorphs, the premaxilla is more anteriorly rounded than the anteroposteriorly shallow element seen in Shinisaurus . Broad snouted forms such as Heloderma , Lanthanotus , some species of Varanus , and some anguines have similarly anteroposteriorly flat premaxillae, usually with a broader premaxillary dental shelf. In Xenosaurus , the premaxilla is more anteriorly convex and the ethmoidal foramina are more ventrally placed.

Maxilla: The tall, laterally flattened, maxilla ( Figs 1 , 2 , 7C, D ) contacts the premaxilla, prefrontal, lacrimal, jugal, vomer, palatine, ectopterygoid, and septomaxilla. The mediolaterally broadest point occurs on the dental shelf. The maxilla is subtriangular with an anteriorly placed, vertical nasal process and a flat alveolar surface. The alveolar surface extends from near the anterior margin of the skull to beyond the midpoint of the orbit. Dorsal to the labial margin, the maxilla exhibits dermal sculpturing generally orientated posterodorsally toward the contacts with the prefrontal, lacrimal, and jugal ( Figs 1A , 2A ).

On the palate, the maxilla contacts the vomer very narrowly in front of the fenestra vomeronasalis externa, just lateral to the contact with the dental shelf of the premaxilla ( Figs 1C , 2C ). It also contacts the palatine and the ectopterygoid on the palate. The infraorbital canal passes through the palatine and into the maxilla at the contact between the two bones ( Fig. 7D ). Within the nasal capsule, the maxilla supports the major contact of the septomaxilla on the dorsal surface of the premaxillary process and contacts the ventromedial process of the prefrontal at the ventromedial corner of the posterior lacrimal foramen

A B

(mistaken for the ophthalmic nerve foramen by Zhang, 1991).

There are usually five or six labial foramina, though this number is variable even between sides of a specimen. Posterior foramina are larger than anterior ones. Those located anterior to the level of the common infraorbital canal are orientated almost directly outward. The penultimate and final labial foramina occur at the level of, or just anterior to, the palatine- maxilla contact. The penultimate foramen is angled slightly anteriorly and the final angled posteriorly.

The premaxillary process is forked and ventromedially orientated. The external ramus of the premaxillary process is more robust and overlies the premaxilla. The septomaxillary ramus is laterally flat-

E pojpr tened and meets the dental shelf to form an inverted V-shape that caps the dental shelf of the premaxilla. A facet for the basal plate of the premaxilla lies between the septomaxillary and external rami. Posterior to the premaxillary process, the nasal process slopes posterodorsally to form the margin of the naris, curving dorsally to meet the lateral facial exposure of the prefrontal.

The posterior border of the nasal process is more complex than in most anguimorphs ( Figs 1 , 2 , 7C, D ). Previous illustrations of this area have been inaccurate, misrepresenting the contacts between the maxilla, prefrontal, and lacrimal. The dorsal margin of the nasal process slopes posterodorsally to form a posterodorsally orientated peg just anterior to the midpoint of the maxilla. The posterior border is unusual in that it possesses two semilunate excavations. A small excavation just ventral to the peak of the nasal process borders a subpalpebral fossa on the prefrontal anteriorly, dorsally, and ventrally ( Figs 2A , 7G ). Immediately ventral to the prefrontal recess is a similar, but larger and less-pronounced, lacrimal recess. A small point of the maxilla narrowly enters the prefrontal- lacrimal contact on the face and separates these two recesses. The nasal process flattens to form the ventral limit of the lacrimal recess before sloping posteroventrally again to meet the main body of the maxilla and eventually terminate at the alveolar margin.

The nasal process extensively overlies the prefrontal laterally, contributing to the dermal covering of the nasal capsule. Whereas this contact occupies about the dorsal one-half of the nasal process, the ventral portion borders the nasal capsule. The medial surface of the maxilla bears a narrow dental shelf that meets the palatine ( Figs 1C , 2C ).

The infraorbital canal branches at the contact between the maxilla and palatine. One branch passes through a foramen carrying the maxillary artery and the superior alveolar nerve into the maxillary infraorbital canal ( Oelrich, 1956). This canal passes anteriorly through the maxilla and branches off to give rise to the labial foramina on the lateral surface of the maxilla. The ethmoidal nerve exits through a small foramen on the medial surface near the anterior end of the nasal process and courses to the premaxilla. Medial to the maxillary infraorbital canal, the maxilla is slightly concave, forming the ventrolateral margin of the second branch from the infraorbital canal, the intermediate palatine nerve ( Fig. 7D ). Thus, the intermediate palatine nerve is housed dorsally by the lacrimal, and ventromedially by the palatine ventrolaterally by the palatal shelf of the maxilla.

Posterior to the infraorbital foramen is a shallow, narrow groove for the maxillary lamina on the suborbital process of the jugal. This groove expands posteriorly to form a shelf supporting the jugal. This shelf is formed, in part, by a lateral expansion of the maxilla contiguous with the orbital ridge. At the ventrolateral border of this expansion is an emargination where the ectopterygoid is exposed on lateral skull surface.

Xenosaurus is similar to Shinisaurus in possessing the palaeochoanate condition (contra character scoring in Lee, 1997, 1998; Lee & Caldwell, 2000). The posteriorly emarginated nasal process in Shinisaurus is distinct from most anguimorphs. However, in some Varanus a dorsal peg on the nasal process of the maxilla fits into the prefrontal dorsal to prefrontal and lacrimal recesses ( Figs 3E , 5E ).

Nasal: The nasals ( Figs 1 , 2 , 7E, F ) are paired and in medial contact for about two-thirds of their total length (contra McDowell & Bogert, 1954: fig. 20; Rieppel, 1980: fig. 4; Zhang, 1991: fig. 4). Anteriorly, the premaxilla narrowly overlaps a shelf on each nasal and slightly invades the internasal suture. Posteriorly the anterior point of the frontal separates the nasals. The sculptured nasals are gently convex dorsally and are pierced by a variable number of foramina.

The nasal is triangular, although a slight overlap by the prefrontal at the narial border makes it appear subrhomboid when articulated. This prefrontal overlap covers a small corner of the nasal contributing to the posterolateral margin of the external naris. This process is absent in many anguimorphs ( Fig. 3 ), but is present in nonanguimorphs, some anguids, and the platynotans Proplatynotia longirostrata Borsuk- Bialynicka and Gobiderma ( Borsuk-Bialynicka, 1984) .

When articulated, the short sides of the nasal face anterolaterally to contact the naris and posteromedially to form a scarf joint with the frontal ( Figs 1B , 2B ). The long sides are butt joints with the prefrontal posterolaterally and medially between the nasals. The medial and posterior surfaces are relatively thick, but only a thin flange of bone joins them ( Fig. 7F ).

Palpebral: The L-shaped palpebral ( Fig. 8 ) contacts the prefrontal and forms the anterolateral border of the orbit. It is dorsoventrally thin and dorsally sculptured. The only articular surface on the palpebral occurs anteriorly, where there is a broad facet for contact with the prefrontal. The prefrontal facet is broadest medially within the orbit and only slightly narrower laterally. A flange on the lateral surface of the prefrontal supports the palpebral dorsal to the subpalpebral fossa.

The recent discovery of a palpebral in Lanthanotus shows that palpebrals are present in all extant anguimorphs except, perhaps, Heloderma ( Maisano et al. 2002) . Among fossil anguimorphs, palpebrals are known for Carusia , Bainguis , Parophisaurus pawneensis Gilmore , Necrosaurus , Parviderma , Aiolosaurus oriens Gao & Norell , Saniwa , and Estesia ( Sullivan, 1987; Norell et al., 1992; Gao & Norell, 1998).

Sclerotic ring: The sclerotic ring ( Figs 1A, B , 2B , 9G ) is of typical design for lizards. There are 14 overlapping ossicles in the eye.

Prefrontal: The robust prefrontals ( Figs 1 , 2 , 7G, H ) are distinctive in Shinisaurus . Each prefrontal is teardrop-shaped in dorsal view, and T-shaped in lateral view. It contacts the nasal, frontal, maxilla, lacrimal, and palatine, and supports the palpebral. A jugal contact is variably present (see below). The prefrontal forms the anteromedial margin of the orbit, the medial border of the lacrimal foramen, and the posterodorsal margin of the external naris. When articulated, the exposed dorsal surface shows extensive sculpturing with raised ridges. The anterodorsal surface of the skull is developed into a strong tuberosity.

A very thin narial lamina extends forward from the main body of the prefrontal to form the posterodorsal rim of the external naris ( Figs 1B , 2B ). Foramina similar to those present on the nasal pierce the narial lamina. Pregill et al. (1986) reported that there is extensive connective tissue joining the nasal, maxilla and prefrontal in Heloderma . That this tissue is weakly calcified may affect separation of the prefrontal from the nasal with desiccation ( Pregill et al., 1986). The reduced anterior nasal and prefrontal laminae in Shinisaurus may represent a similar state, but with more calcification.

The prefrontal lies in a groove on the lateral surface of the frontal. This frontal process extends for about one-third of the orbit and does not approach the postorbitofrontal.

The prefrontal forms a strong dorsolateral edge to the skull roof with an angle of approximately 90 degrees between the dorsal and descending surfaces. The dorsal one-half of the maxillary nasal process overlies the lateral surface in a lap joint, but is interrupted by the raised rim of a subpalpebral fossa that telescopes through it ( Fig. 7G ). The subpalpebral fossa, located at the anterodorsal corner of the orbit is a circular depression on the prefrontal ventral to the palpebral.

Ventral to the subpalpebral fossa, the prefrontal laterally contacts the lacrimal, enters the orbit, and descends to meet the palatine and sometimes the jugal within the orbit. The lacrimal contacts are straight butt joints both above and below the lacrimal foramen Fig. 10A, D ). The ventral contact with the palatine is a broad scarf joint, orientated mediolaterally and posteroventrally. Dorsomedial to this, the prefrontal forms the medial wall of the orbitonasal fenestra. Anteriorly, the prefrontal is concave, forming the posterodorsal and much of the posterolateral walls of the olfactory chamber ( Fig. 7H ).

In contrast to Shinisaurus , the narial lamina is absent in all other described anguimorphs. In Xenosaurus and anguids, the prefrontal is excluded from the narial border by the maxilla- nasal contact and the lateral border of the prefrontal is weakly developed and rounded. Although dermal sculpturing is present in many anguimorphs, there is never a raised rugosity as in Shinisaurus . Whereas the emargination for the lacrimal foramen is completely within the lacrimal in Shinisaurus , the descending process is deeply emarginated for the lacrimal foramen in Xenosaurus ( Fig. 3B ). In Xenosaurus the ventral contact with the palatine is angled ventrolaterally and the posteroventral tip of the prefrontal contacts the jugal, a condition variably present in Shinisaurus ( Fig. 10A, D ).

Lacrimal: Narrow both anteroposteriorly and mediolaterally, the lacrimal ( Figs 1 , 2 , 7I, J ) is a semilunate element contacting the prefrontal, maxilla, jugal, and palatine. It forms the anterior border of the orbit and three sides of the lacrimal foramen. The anterior portion of the orbital ridge extends onto the lacrimal from the jugal. Although the lacrimal is reduced in size, external dermal sculpturing is present.

Dorsally, a small facet on the lacrimal abuts the ventral rim of the subpalpebral fossa on the prefrontal. From there, a narrow anterior lappet underlies the posterior margin of the nasal process of the maxilla ( Fig. 7I ). The ventral portion of this lappet has a facet articulating with the jugal. The suture with the jugal may be mediolaterally or posterolaterally orientated. In the latter case, the contact is longer and the lacrimal is sometimes cut off from the palatine ( Fig. 10A- C ).

Emargination for the lacrimal foramen is more clearly visible in posterior view ( Fig. 10A, D ). Located dorsal to this emargination is the facet for articulation with the subpalpebral fossa. Ventral to the emargination is an oval surface for articulation with the prefrontal. A similarly shaped facet for contact with the palatine and sometimes one for the jugal are present on the ventral and ventromedial surfaces.

Depending upon the length and orientation of the contact between the lacrimal and jugal, a nonoverlapping contact is sometimes formed with the palatine at the junction of the prefrontal, palatine, and jugal inside the orbit ( Fig. 10A ). Specimens with the prefrontal- jugal contact exhibit a posterolaterally orientated lacrimal- jugal suture allowing contact with the anteromedial process of the jugal. Where these contacts are absent, the lacrimal contact with the jugal is more mediolaterally orientated ( Fig. 10D ). Of the eight articulated skulls available for observation, the contact between the prefrontal and jugal is present in three, absent in four, and obscured in one. Three of these skulls represent fully mature individuals, three are subadults, one is a juvenile, and one is a neonate. The individual for which the condition could not be seen was a subadult. Of the three adults, only the largest possessed the prefrontal- jugal contact. One subadult and the juvenile possessed the contact whereas the neonate did not. This variation therefore does not appear to be dependent upon size or ontogenetic state.

Xenosaurus platyceps , Heloderma ( Norell et al., 1992) , most anguids, and some Varanus possess the prefrontal- jugal contact ( Fig. 10B, C ). Xenosaurus platyceps and those anguids with the contact possess a narrow posteroventral process of the prefrontal separating the lacrimal and palatine to reach the jugal ( Fig. 10B ). Heloderma has a greatly reduced lacrimal ( Figs 3D , 5D , 10C ), thus creating a relatively broad jugal- prefrontal contact. Estesia ( Fig. 10E ) and Lanthanotus exhibit a morphology of the posteroventral part of the prefrontal similar to that of Heloderma , but the lacrimal blocks contact with the jugal ( Norell et al., 1992). This is also the case in Xenosaurus grandis Gray ( Fig. 3B ).

Jugal: The jugal ( Figs 1 , 2 , 9A, B ) is L-shaped in lateral view. It contacts the maxilla, lacrimal, postorbitofrontal, ectopterygoid, and sometimes the prefrontal. It forms the ventral and most of the posterior border of the orbit and bears a large portion of the orbital ridge. This ridge flattens on the body of the jugal posterior to the midpoint of the orbit. The jugal does not extend antorbitally but terminates in the anterior quarter of the orbit. Dermal sculpturing is present on the suborbital process and extends onto the postorbital process in some specimens.

The jugal is triangular in cross section, with a flat external side and a long medial ridge. The medial ridge runs along the axis of the bone and travels from the suborbital to the postorbital process. A thin orbital lamina extends dorsolaterally along the anterior suborbital portion and flattens onto the postorbital process. A series of small suborbital foramina pierce the jugal at the medial angle ( Fig. 9A ).

Anteriorly, the suborbital process tapers to a point in external view ( Figs 1A , 2A , 9A, B ). It is broadened anteriorly in dorsal view and the squared dorsal surface bears a lacrimal articular facet. This articulation extends somewhat posterolaterally, but does not reach the anteromedial margin of the jugal. The anterolateral point is exposed inside the orbit and sometimes narrowly contacts the prefrontal ( Fig. 10A ).

A double tongue-in-groove joint constitutes the jugal- maxilla contact. The ventral surface and the medial ridge of the jugal together form a maxillary groove for reception of the posterior portion of the maxillary nasal process. In turn, the medial ridge fits into a shallow groove on the maxilla, just medial to the posterior descending flange of the nasal process ( Fig. 7D ). Confluent with the maxillary groove is a facet for the lateral expansion of the ectopterygoid ( Fig. 9F ). The maxillary groove is bordered posteriorly by a short ventral extension of the medial ridge at the level of the postorbital process, which forms a sharp point. This point is variably developed in length ( Figs 1A , 2A , 9E ).

The postorbital process is narrower than the suborbital process and lacks anterior or posterior laminae. It extends posterodorsally and tapers to a point lying in a facet of the postorbitofrontal ( Fig. 9C, G ). The jugal does not contact the squamosal ( Figs 1A , 2A , 9C, G ; contra Romer, 1956; Estes et al., 1988; Gao & Norell, 1998; Caldwell, 1999). Originating at the dorsal margin of the postorbital process, a postorbital groove runs to nearly the midpoint of the process along the anterior side of the medial ridge.

Xenosaurus differs significantly from Shinisaurus in the morphology of the jugal ( Fig. 5B ). Whereas the suborbital process of Shinisaurus is deepened by the orbital lamina and ventral flange, the suborbital process of the jugal in Xenosaurus is dorsoventrally narrow at its anterior end. The contact with the maxilla is a lap joint rather than a tongue-in-groove articulation. Xenosaurus platyceps possesses a more extensive contact between the jugal and prefrontal, similar to the condition in Heloderma wherein the jugal overlaps the prefrontal laterally near the posterior lacrimal foramen.

The posteroventral process is present in Xenosaurus ( Fig. 5B ), Carusia ( Borsuk-Bialynicka, 1985; Gao & Norell, 1998, 2000) ( Fig. 5A ), Heloderma ( Fig. 3D ), Restes ( Gilmore, 1942; Gauthier, 1982), and many members of Anguidae ( Sullivan, 1979; Estes, 1983). Anniella , Heloderma , and Carusia ( Gao & Norell, 1998, 2000) exhibit a similar condition with this process pointing mostly ventrally rather than posteriorly.

In contrast to the narrow postorbital process in Shinisaurus , that of Xenosaurus is broadened by anterior and posterior laminae ( Fig. 5B ). Dorsally, the postorbital process has a posterodorsal angle facilitating the contact with the squamosal. A postorbital groove is completely absent and the contact between the postorbitofrontal and the jugal is a butt contact on the dorsal surface of the skull table.

Postorbitofrontal: A compound bone composed of the postfrontal and postorbital, the postorbitofrontal ( Figs 1 , 2 , 9C, D, G ), forms the anterolateral corner of the skull table. It contacts the frontal, parietal, and jugal, and forms the posterior and posteromedial margins of the orbit. In lateral view, the postorbitofrontal is subtriangular, and in dorsal view it is tetraradiate. Dermal sculpturing is present along its dorsal and lateral surfaces.

The posterior orbital margin is J-shaped in dorsal view ( Figs 1B , 2B ). The frontal process forms the long arm of the ‘J.’ From its anterior point, the frontal process extends posterolaterally and then abruptly turns anteriorly, tapering as a postorbital prong. The surface of the curved margin is smooth and anteriorly concave in dorsal view, but anteriorly convex when viewed sagittally.

The postorbital process extends ventrally from the dorsal surface of the postorbitofrontal for a distance equal to about two-thirds the maximum width of the element. The ventral tip is V-shaped and a narrow jugal facet is present on its posterior edge ( Fig. 9C ). This facet overlaps a very narrow lamina running along the ventral edge of the postorbitofrontal. The squamosal facet also lies on this lamina and extends for almost half the total length of the bone. The area between the jugal and squamosal facets forms the only external contribution of the postorbitofrontal to the infratemporal vacuity ( Figs 1B , 2B , 9C ). A small foramen is present posteriorly in the jugal facet and a similar one is present in the squamosal facet.

The parietal process of the postorbitofrontal contributes to the medial border of the supratemporal fenestra. The frontal and parietal processes are set at a slightly acute angle ( Figs 1B , 2B ) spanning the frontoparietal suture laterally. The frontal process is more pronounced than the parietal process and bears a ventral flange underlying the frontoparietal suture and the anterolateral portion of the parietal.

Xenosaurus differs from Shinisaurus in the morphology of the postorbitofrontal with only the frontal and parietal processes showing significant similarities. Xenosaurus lacks a descending postorbital process ( Fig. 5B ) such that the postorbitofrontal is developed only as a thin plate. No anterior postorbital prong is present, the anterior margin of the postorbitofrontal contacts the jugal laterally, and the contacts with the jugal and squamosal are butt joints rather than tongue-in-groove joints. Xenosaurus possesses a thin lamina joining the parietal and squamosal processes and roofing a portion of the supratemporal fenestra.

Although apparently lacking the lamina roofing the supratemporal fenestra, the postorbitofrontal in Carusia is generally very similar to that in Xenosaurus ( Fig. 3A, 3B , 5A, B ). Varanus and anguids ( Fig. 3C, E ) generally resemble Shinisaurus in the form of the postorbital process, but lack the postorbital prong. Heloderma and Lanthanotus have lost the postorbital altogether and Heloderma exhibits a linear medial contact with the frontal and parietal ( Fig. 3D ).

Squamosal: The squamosal ( Figs 1 , 2 , 9E, F ) delimits the skull posterolaterally. In lateral view, it hooks posteroventrally with a short process descending to the quadrate. The squamosal contacts the postorbitofrontal, supratemporal, and quadrate. It forms the posterolateral margin of the supratemporal fenestra and the posterodorsal margin of the infratemporal vacuity.

Anteriorly, the squamosal tapers to a point, articulating with the postorbitofrontal. The postorbitofrontal facet is located on the medial surface and extends posteriorly for about one-quarter of the squamosal length. A tiny foramen at the posterior extremity of this facet communicates with a similar foramen on the squamosal facet of the postorbitofrontal.

The posterior quarter of the bone curves ventrally. Along its posteromedial margin, an elongate facet articulates in a butt joint with the supratemporal. At the extreme posteroventral tip is a narrow facet forming another butt joint with the dorsolateral edge of the quadrate.

The squamosal of Xenosaurus broadly resembles that of Shinisaurus . However, in Xenosaurus , Carusia ( Gao & Norell, 1998) , and Paravaranus angustifrons Borsuk-Bialynicka ( Borsuk-Bialynicka, 1984) a dorsal process is present ( Fig. 3A, B ). This feature is unusual among anguimorphs, although it is present in Iguania, Teiidae ( Estes et al., 1988; Caldwell, 1999) and the fossil Meyasaurus diazromerali Evans and Barbadillo ( Evans & Barbadillo, 1997, 1998). In Xenosaurus a thin lamina joins the postorbitofrontal and dorsal processes, but this is absent in other lizards.

Supratemporal: The Y-shaped supratemporal ( Figs 1 , 2 ) contacts the squamosal laterally, the parietal and otooccipital medially, and the quadrate ventrally. It is narrow mediolaterally and moderately long anteroposteriorly. McDowell & Bogert (1954) referred to this bone as the tabular.

Dorsally, the supratemporal is bifurcate with a medial process contacting the lateral side of the supratemporal process of the parietal and a lateral process with an elongate contact with the squamosal ( Figs 1B , 2B ). The former articulation surface covers the dorsal side of the supratemporal for most of its length, from the tip of the parietal process to the main body posterior to the bifurcation. Ventral to this is an otooccipital facet, which extends to the posteroventral tip of the supratemporal. The much smaller squamosal process of the supratemporal projects dorsolaterally. The squamosal facet extends from the dorsal tip of the process and onto the main body of the supratemporal almost to the quadrate articulation. The main body of the supratemporal turns slightly laterally at its ventral edge helping to support this articulation.

The forked morphology of the supratemporal in Shinisaurus is unusual but is seen also in gerrhonotines ( Criley, 1968). In most anguimorphs ( Fig. 3 ) and other squamates, the supratemporal is a narrow splint lying between the squamosal and the parietal, as previously described and illustrated in error for Shinisaurus ( McDowell & Bogert, 1954; Rieppel, 1980; Wu & Huang, 1986; Zhang, 1991). The squamosal is very small in Heloderma and Lanthanotus , increasing the contact between the parietal and supratemporal. Xenosaurus and some anguids ( Fig. 3B, C ) have a smaller supratemporal fenestra with more contact of the squamosal and the supratemporal process of the parietal, and the supratemporal inserted in between.

Frontal: The frontals ( Figs 1 , 2 , 11A, B ) are fused into a single element contacting the nasals, prefrontals, postorbitofrontals, and parietal. The frontal forms the dorsalmost margin of the orbit. It is subtriangular in dorsal view and usually slightly arched in lateral view. Constriction between the orbits is absent and the interorbital margins of the frontal are parallel, though the posterior part of the frontal dramatically expands posterior to the orbit for the parietal contact ( Fig. 11A, B ). The entire dorsal surface is sculptured, most prominently posterior to the midpoint of the orbit ( Figs 1B , 2B ).

The exposed anterodorsal surface of the articulated frontal tapers to a point and invades the posterior third of the internasal suture. Previously, the frontal was figured with a triradiate anterior margin ( McDowell & Bogert, 1954; Wu & Huang, 1986; Zhang, 1991), probably due to poor preservation of the described skulls and some shifting of the bones. A lamina narrowly underlies the prefrontals and the posterior tips of the nasals. This lamina originates at the point where the frontal begins its anterior tapering and extends for two-thirds the distance to the tip, to the level of the junction of the nasals, prefrontals, and frontal ( Fig. 11A ). The lateral portions of this lamina extend beyond its main body as tiny points underlying the prefrontal contact with the nasal.

A supraorbital foramen is sometimes present behind the prefrontal articulation. Examination of multiple specimens shows that this foramen may be present on both sides, on one side only, or absent altogether.

When articulated, the exposed posterolateral edges of the frontal form sharp angles. Small tabs are present at each posterior corner of the frontal. Just medial to these is another pair of tabs that are slightly broader posterolaterally and are contiguous with the cristae cranii (subolfactory processes of some authors) ( Fig. 11B ). These two sets of tabs meet matching flanges at the anterolateral corners of the parietal, the more lateral tabs fitting onto the ventral surface and the more medial ones overlaying it. The combined width of these parietal tabs reaches medially to the level of the interorbital width of the frontal. Medial to this, there is very slight interdigitation of the frontal and parietal.

The cristae cranii are well developed, but extend only ventrally without medial projections and do not significantly underlie the olfactory tracts. They originate at the level of the prefrontal lamina and are deepest just posterior to the midpoint of the orbit. They flatten some posteriorly, but remain discernible as they progress to the ventrolateral parietal tabs. Laterally, the dorsal surface of the frontal slightly overhangs the cristae cranii ( Fig. 11B ).

Although fused in all three taxa, the frontal of Xenosaurus and Carusia differs significantly from that of Shinisaurus . In Xenosaurus the frontal lacks a subsurface lamina and is markedly constricted between the orbits ( Fig. 3A, B ). In Carusia , lateral extensions of the frontal contact the maxillae anteriorly, completely separating the prefrontals from the nasals ( Fig. 3A ), as seen in many scincomorphs and gekkotans. In Xenosaurus , the posterior border of the frontal is linear without any indication of an interdigitating suture. Parietal tabs are absent and the cristae cranii flatten and disappear in the posterior part of the orbit. In Carusia , the crista cranii approach each other, but do not contact ( Fig. 4 ) ( Borsuk-Bialynicka, 1985; Gao & Norell, 1998, 2000). Small parietal tabs of the frontal are present in the xenosaurid Restes , although only the ventrolateral pair seems to be present ( Estes, 1975: fig. 7A; Gauthier, 1982: fig. 5). Parietal tabs are more variably developed in mosasauroids ( Russell, 1967; Bell, 1997). They are larger with broader parietal contacts in many mosasauroids, but others (e.g. Clidastes and Platecarpus ) are proportionately similar to Shinisaurus (see Russell, 1967: text fig. 4).

Parietal: The parietals ( Figs 1 , 2 , 11 ) are fused into a single element contacting the frontal, postorbitofrontals, supratemporals, quadrates, epipterygoids, and prootics. Contact with the squamosal is blocked by the supratemporal. The parietal houses the pineal foramen and forms the medial and posteromedial margins of the supratemporal fenestrae. It is quadrangular in dorsal view with concave posterior and lateral margins. The supratemporal processes curve posteroventrally in lateral view. Dermal sculpturing covers the entire surface of the parietal table ( Figs 1B , 2B , 11E, F ).

Frontal tabs are present anterolaterally and contact the frontal process of the postorbitofrontal ( Figs 1B , 2B ). The posterolateral surface of this process bears a postorbitofrontal facet ( Fig. 11C ). The lateral margins of the parietal are medially concave and the edges are downturned to form the medial margin of the supratemporal fossae where the adductor musculature originates ( Haas, 1960; Rieppel, 1980). The supratemporal fossa extends from the parietal- postorbitofrontal contact to the supratemporal. The downturned lateral margin of the parietal is straight and slightly inclined posteroventrally where it merges with the decensus parietalis. The decensus parietalis contacts the dorsal head of the epipterygoid and the crista alaris of the prootic ala, although this contact may be lost with desiccation ( Figs 1E , 2F ). Posterior to the decensus parietalis, the margin is a dorsally convex arch that is much sharper anteriorly.

The supratemporal processes are located at the posterior portion of the parietal table. These are thin blades with their short axes orientated ventrolaterally. The processes are set at an angle of 90 degrees along their long axes in adults ( Fig. 11F ), whereas subadults ( Fig. 11E ) and juveniles exhibit progressively more obtuse angles. The length of the supratemporal processes increases relative to the main body of the parietal with age. Neonates possess only very short supratemporal processes, similar to the condition in Xenosaurus ( Fig. 3B ). The length of the supratemporal processes illustrated in the specimen by McDowell & Bogert (1954: fig. 4) shows that it was a juvenile. The posterior side of the supratemporal process bears muscle scars for the nuchal muscles. These muscle scars extend medially onto the extreme posterior margin of the parietal table in adult specimens.

The distal tip of each supratemporal process bears a lateral supratemporal facet, a ventral quadrate facet, and a medial otooccipital facet ( Fig. 11C, D ). The supratemporal facet extends anteriorly about twothirds the distance to the posterior parietal table. The quadrate and otooccipital facets are smaller, about one-third the size of the supratemporal facet.

Ventrally, a pair of low ridges, the postfoveal crests (cristae postfovealis of Klembara, 1979, 1981, 1986) joins the decensus parietalis to the posteromedial margin of the parietal. These ridges define the anteroventral margin of the supratemporal processes and are separated posteriorly by an extension of the parietal fossa. The anterior margin of the parietal fossa is much more sharply demarcated and has a gently raised anterior rim posterior to the pineal foramen.

Although Shinisaurus and Xenosaurus have been allied based on the morphology of the parietal, its morphology differs substantially between adults of these taxa. Xenosaurus possesses much shorter supratemporal processes than Shinisaurus ; it is a simple posterolaterally directed point at the posterior border of the parietal table ( Fig. 3B ). Similar to Shinisaurus in the posterior contacts of the supratemporal process, Xenosaurus is different in that there is a contact with the squamosal anterior to the supratemporal. This contact may also be present in Carusia ( Fig. 3A ). However, Carusia possesses narrower supratemporal processes than Xenosaurus , more similar to the condition seen in Shinisaurus . Xenosaurus lacks pronounced postfoveal crests and posterior elongation of the parietal fossa. The decensus parietalis is confluent with a ridge travelling the length of the supratemporal processes and the parietal fossa is a discrete oval depression. A single, median crest extends posteriorly from the parietal fossa to a semilunate emargination of the parietal at the posterior midline. The glyptosaurines Arpadosaurus gazinorum Meszoely , Glyptosaurus rugosus Marsh , and Melanosaurus maximus Gilmore ( Meszoely, 1970; Estes, 1983) share this morphology with Xenosaurus . Other glyptosaurines including Peltosaurus granulosus Cope and Odaxosaurus piger Gilmore (= Pancelosaurus piger of Meszoely, 1970) show a Shinisaurus - type condition ( Meszoely, 1970: fig. 16).

The dorsal origin of the jaw adductors on the parietal in Shinisaurus is the plesiomorphic condition seen in many anguimorphs. In anguids (except Anniella ), xenosaurids, helodermatids, Carusia ( Fig. 3 ), and Gobiderma , the jaw adductors originate from the ventral surface of the parietal.

Septomaxilla: The septomaxillae are paired, flat, elements forming the dorsal and posterior borders of the chamber for Jacobson’s organ. Each septomaxilla is a simple bone lacking dorsal and ventral processes, in contrast to most anguimorphs ( Fig. 3 ). It contacts the premaxilla, maxilla, vomer, and the cartilaginous internasal septum. Despite some suggestions to the contrary, the septomaxillae never meet at the midline in squamates (see Discussion in Rieppel & Zaher, 2000). The septomaxilla connects the vomer to the maxilla, partly separating the fenestra vomeronasalis externa from the internal naris ( Figs 1C , 2C ) in a modified palaeochoanate condition. It is D-shaped in dorsal view, with the rounded margin facing anteriorly and lying along the anteroventral margin of the external naris. It is pierced by a single ethmoid foramen.

Palate and quadrate

Vomer: The paired vomers ( Figs 1C , 2C , 12C ) lie at the anterior midline of the palate. Each vomer is anteriorly flattened dorsoventrally and posteriorly flattened mediolaterally. The vomer contacts the premaxilla, maxilla, septomaxilla, and palatine. It forms the medial and much of the posterior border of the fenestra vomeronasalis externa, the anteromedial border of the internal naris, and the anteromedial border of the interpterygoid vacuity. The vomers contact along the midline for about their anterior one-third to one-half, with individual variation.

The anterior margin of the vomer curves medially in a contact with the premaxilla on the premaxillary dental shelf at the midline. Lateral to this contact the vomer narrowly contacts the maxilla. The contacts with the premaxilla and maxilla are dorsoventrally thin. A ventrally projecting flange begins at the contact with the premaxilla and maxilla and curves to run parasagittally ( Figs 1C , 2C ). Posterior to the fenestra vomeronasalis externa, the crest flattens on the septomaxillary process. The lacrimal groove lies lateral to, and is partially floored by, this crest.

Although it does not reach the maxilla laterally, the septomaxillary process partly encloses the fenestra vomeronasalis externa posteriorly before tapering to a lateral point. A pronounced septomaxillary facet is present on the dorsal surface. The posterolateral surface forms the ventrolateral margin of a palatine groove. At its base, the septomaxillary process is pierced by the vomerine foramen within the lacrimal groove ( Fig. 12A ).

The flat posterior flange of the vomer establishes a tongue-in-groove contact with the palatine, its tip variably in contact with the pterygoid. A vomer- pterygoid contact is unusual among squamates, known only in some iguanians, some amphisbaenians ( Kearney, 2003) some teioids ( Estes, 1983; Estes et al., 1988; Gao & Norell, 2000), Anguis , and some specimens of Odaxosaurus ( Meszoely, 1970) .

The vomer of Shinisaurus is subequal in length to the palatine, a common condition among nonanguid and nonvaranoid squamates. In contrast, Carusia possesses a vomer that is shorter than the palatine ( Borsuk-Bialynicka, 1985; Gao & Norell, 1998, 2000).

Some significant differences exist between the vomers of Shinisaurus and Xenosaurus . The vomers contact for almost their entire length in Xenosaurus , and the ventral parasagittal ridge originates from a more medial position not corresponding with the premaxilla–maxilla contact and is much straighter than in Shinisaurus . The septomaxillary process is smaller and is supported by the upturned, lateral edge of the vomer rather than a special groove. The posterior flange does not taper, but rather is a sheet that is curled laterally forming the lacrimal groove, and forms a flat butt joint posteriorly, similar to the condition seen in anguids.

Palatine: The paired palatines ( Figs 1C , 2C , 12B ) do not contact at midline. Each palatine is irregularly shaped with a maxillary process, a thin vomerine process, and a plate-like pterygoid process. The palatine contacts the vomer, maxilla, prefrontal, lacrimal, jugal, and pterygoid. It forms the posterior and posteromedial margin of the internal naris, may contribute to the interpterygoid vacuity (dependent upon presence or absence of contact between the vomer and pterygoid), and is pierced by the suborbital foramen. The palatine is slightly longer than wide, though to a lesser degree than is seen in some anguids. Palatine teeth are absent.

C apl

Anteriorly, the vomerine and maxillary processes project ventrolaterally and anteroventrally, respectively, forming the posterior margin of the internal naris in a short choanal groove as it opens into the mouth. The medial vomerine process is narrow and overlies the vomer dorsolaterally. An elongate, but shallow, groove on the dorsal side of the vomer receives this process in a broad tongue-in-groove joint; a facet for this contact extends for about the anterior one-half of the vomerine process. The maxillary process is triradiate and overlaps the dental shelf of the maxilla in a gentle arc. The dental arm is ventrolaterally orientated and the two infraorbital arms are laterally orientated ( Fig. 12B ); the latter are constricted medially, but expanded laterally, and the distal tips contact and may fuse to completely encircle the infraorbital foramen. The posterior infraorbital arm is more ventral than the anterior arm. A common maxillary facet covers the ventral surface of the posterior infraorbital arm and the dental arm. Only the lateral surface of the dorsal infraorbital arm contacts the maxilla.

Between the maxillary and vomerine processes, the main body of the palatine curves dorsally into a scarf joint with the prefrontal. A palatine foramen pierces the bone at the level of the anterior infraorbital arm of the maxillary process ( Fig. 12B ). The posterior palatine canal runs through the palatine from the pterygoid facet to the area between the three arms of the maxillary process.

The horizontally orientated, posterolaterally tapering pterygoid process constitutes more than half the length of the palatine ( Fig. 12B ). An elongate triangular facet for a lap joint with the pterygoid begins at the level of the vomerine process on the ventral surface near the medial edge and expands posterolaterally. Consequently, the exposed ventral surface of the pterygoid process is a posteriorly orientated triangle ( Figs 1C , 2C ). A sharp process laterally overlaps the pterygoid at the extreme posterior of the palatine.

Xenosaurus platyceps differs from Shinisaurus in the morphology of the maxillary process and the pterygoid process. Whereas Shinisaurus has elongate infraorbital arms of the maxillary process that are only narrowly connected distally, X. platyceps has strongly fused distal tips of its relatively short infraorbital arms. The dental arm of the maxillary process is more elongate and subequal in length to the vomerine process. The pterygoid process overlaps the pterygoid much less with in X. platyceps , not approaching the vomerine process.

Pterygoid: The paired pterygoids ( Figs 1C , 2C , 12C ) are medially separated by the interpterygoid vacuity. Each pterygoid is triradiate, with palatine and transverse processes anteriorly and a quadrate process posteriorly. The pterygoid contacts the palatine, ectopterygoid, epipterygoid, basisphenoid, quadrate, and sometimes the vomer (see above). It forms the posterior border of the suborbital fenestra, most of the lateral border of the interpterygoid vacuity, and the ventral borders of the infratemporal vacuity and the basicranial fossae.

Anteriorly, the palatine process of the pterygoid is narrow and thin in dorsal view ( Fig. 12C ). Ventrally, a row of small curved teeth runs along its long axis from the level of the transverse process to near the point where it begins to taper anteriorly. Diverging anterolaterally from the palatine process, the transverse process extends for about one-half the length of the palatine process and joins the ectopterygoid ( Figs 1 , 2 ). The transverse process is flat mediolaterally and about twice the width of the palatine process. It is distally expanded dorsoventrally and, to a lesser degree, posteriorly. It bears a pair of facets for articulation with the ectopterygoid: a ventral facet covers the ventral three-fourths of the lateral surface and extends for about one-half the length of the transverse process; a dorsal facet is very narrow and elongate, reaching to the point of contact between the transverse and palatine processes. A longitudinal swelling narrowly separates these facets and is contiguous with a thin suborbital lamina joining the transverse and palatine processes for about one-third the length of the transverse process ( Fig. 12C ).

From the common base of the palatine and transverse processes, the pterygoid angles posterolaterally as an elongate quadrate process. This process is subequal in length to the palatine process. It is mediolaterally thin and is laterally convex in cross section. The quadrate process tapers posteriorly and the posterolateral tip bears a narrow facet for reception of the quadrate. A deep columellar fossa for reception of the epipterygoid is located on the quadrate process, just posterior to the point where the three processes unite ( Fig. 12C ). A postcolumellar process medially supports this columellar fossa. The postcolumellar process and a similar but sharper process on the ventral surface of the pterygoid together define a laterally orientated pterygoid notch. This is an oval depression for reception of the basipterygoid processes on the braincase.

In contrast to Shinisaurus , the suborbital lamina is more extensive in most other anguimorphs. Xenosaurus and observed anguids possess a suborbital lamina joining the distal tip of the palatine process to the posterior two-thirds of the transverse process ( Fig. 4B, C ). Carusia seems to possess a very similar state ( Gao & Norell, 1998, 2000; Fig. 4A ).

Pterygoid teeth are present in many anguimorphs, but are absent in Varanus , Estesia ( Norell et al., 1992) , Cherminotus longifrons Borsuk-Bialynicka ( Borsuk-Bialynicka, 1984) , some Heloderma , Anniella , some Anguis ( Gao & Norell, 1998) , some diploglossines, some gerrhonotines, and Xenosaurus . When present, palatal teeth (both pterygoid and palatine) are usually arranged in multiple rows or ovoid patches in many anguids ( Criley, 1968; Meszoely, 1970; Sullivan, 1979, 1989) and platynotans ( Borsuk-Bialynicka, 1984; Gao & Fox, 1996). This in contrast to the single row seen in other anguids, Shinisaurus , Heloderma , Lanthanotus , and mosasauroids ( Russell, 1967; Bell, 1997).

Ectopterygoid: The short, robust ectopterygoid ( Figs 1 , 2 , 12D ) contacts the maxilla, jugal, and pterygoid. Contrary to Hu et al. (1984), the ectopterygoid does not contact the palatine, but defines the posterolateral margin of the suborbital fenestra and the anteroventral border of the infratemporal vacuity. The general shape is a posteromedially orientated cone, bifurcate laterally, with a thin ventral lamina.

The arms of the lateral process of the ectopterygoid grasp the posterior tip of the maxilla. The posterior arm wraps around the outside of the maxilla and contacts the ventromedial portion of the jugal on the lateral skull surface.

More medially, the ectopterygoid is constricted anteroposteriorly and dorsoventrally before greatly expanding dorsoventrally into a medial process. The medial process is bifurcate with a cone-shaped dorsal process ending in a sharp point and a thin ventral lamina ( Fig. 12D ). The medial process is directed ventromedially and serves as a facet for the larger of the pterygoid contacts. The ventromedial tip is rounded and slightly expanded anteriorly complementing a similar swelling on the pterygoid. The pointed, dorsal process overlaps the transverse process of the pterygoid.

Many anguimorphs possess the condition in which the ectopterygoid communicates with the lateral surface of the skull. This is the case in Xenosaurus , some anguids (at least Peltosaurus , gerrhonotines, and diploglossines), extant varanoids, and perhaps also in Proplatynotia longirostrata ( Borsuk-Bialynicka, 1984) and Estesia ( Norell et al., 1992) . It is possible that this condition also occurs in other fossil platynotans, but poor preservation or disarticulation of the palate makes this difficult to determine.

Epipterygoid: The epipterygoid ( Figs 1 , 2 ) is a thin rod bowing laterally at midlength and contacting the pterygoid and the prootic. The ventral surface is flattened and fits into the columellar fossa on the pterygoid. From about mid-length to the ventral margin, the epipterygoid is somewhat flattened anteroposteriorly; the dorsal one-third is similarly flattened mediolaterally. Dorsally, it contacts the anterolateral surface of the crista alaris of the prootic.

Quadrate: The quadrates ( Figs 1 , 2 , 13 ) form the connection between the dermal skull roof, braincase, palate, and lower jaw. Each quadrate contacts the squamosal, parietal, pterygoid, prootic, otooccipital, and articular. The quadrate forms the posterior border of the infratemporal vacuity and is pierced by a small quadrate foramen. It is broadest near the cephalic condyle in lateral view and of relatively uniform width mediolaterally.

Shinisaurus is streptostylic. A single broad articular facet receives the otooccipital and the supratemporal. Lateral to this is a weakly separated facet for reception of the squamosal ( Fig. 13 ). Anterior to the cephalic condyle, the dorsal outline of the quadrate is L-shaped or subtriangular, depending upon the variable development of the dorsal flange connecting the cephalic condyle and tympanic crest. This surface of the quadrate is dorsally convex, but does not directly articulate with the squamosal. It is joined to the supratemporal arch via the superficial aponeurosis ( Haas, 1960).

The articular condyle is saddle-shaped, the medial portion of the condyle being narrower than the lateral ( Fig. 13B ). Laterally, the condyle is broadly joined with the tympanic crest and the anterodorsal margin is mostly flat. The posterodorsal margin is chevronshaped and is contiguous with the posterior crest.

Both the posterior and tympanic crests are well developed and are of subequal lengths dorsally. The posterior crest arches anteroventrally and terminates on the main body of the quadrate just dorsal to the articular condyle ( Fig. 13 ). The tympanic crest is prominent, although not as extensive as in Xenosaurus . It is relatively straight dorsoventrally.

A very weakly developed medial crest of the quadrate contacts the braincase dorsally and the palate ventrally. The dorsomedial portion possesses a surface for a loose contact with the prootic. Ventral to this contact, the crest recedes somewhat before expanding

A ast/ex B ast/ex again for contact with the quadrate process of the pterygoid on the posteromedial surface of the crest ( Fig. 13A ).

The quadrate foramen ( Figs 1E , 2F , 13 ) runs obliquely through the quadrate. The posterior opening lies just medial to the posterior crest and the lateral opening lies just laterally, near the dorsal margin of the articular condyle.

Whereas Xenosaurus possesses a broader tympanic crest than Shinisaurus , the posterior crest is weaker and merges with the main body of the quadrate about a quarter of the way down the shaft and the medial crest is absent altogether. Consequently, the quadrate foramen runs from the medial surface anteriorly to about the midpoint of the tympanic crest. There is no direct contact between the quadrate and braincase in Xenosaurus .

Braincase

Stapes: The stapes ( Figs 1A , 2A ) is a simple rod, bowed somewhat posteriorly. The overall form is similar to that of other anguimorphs. There is no stapedial foramen and the distal tip the stapes contacts the posterior crest of the quadrate.

Supraoccipital: The supraoccipital ( Figs 1 , 2 , 14 , 15 ) lies at the midline and contacts the parietal, prootics, and otooccipitals. It forms the dorsal surface of the braincase, the dorsal part of the inner ear capsule, and the dorsal margin of the foramen magnum. It is subrectangular in dorsal view ( Fig. 14B ) and inverted an U-shape in anterior view.

The dorsal surface of the supraoccipital expands posterolaterally to contact the otooccipital and then becomes narrower toward the foramen magnum ( Fig. 14B ). The posterolateral portion of the supraoccipital roofs the inner ear capsule. Ventrolateral processes descend from the ventral surface and contact the prootic. The prootic contact is chevron-shaped in lateral view and that with the otooccipital is V-shaped in dorsal view. The dorsolateral contacts with the prootic and otooccipital are a continuous butt joint that extends to the paroccipital processes. The ventrolateral contact with the prootic forms the anterior and ventral border of the membranous labyrinth. The contact with the otooccipital becomes a scarf joint posteriorly.

Located at the anteromedial apex of the supraoccipital, the elongate cartilaginous processus ascendens fits into the parietal fossa of the parietal. The base of this process abuts a dorsoventrally narrow fossa on the supraoccipital ( Fig. 14A, B ). Posterior to this contact the supraoccipital crest is expressed as a low swelling on the supraoccipital. The posterior border of the supraoccipital is sinuous, with anterior concavities bo located at the foramen magnum and at the junction of the dorsal surface with the prootic processes.

The supraoccipital contribution to the osseous labyrinth is a dorsal fossa on its ventral surface. The tympanic bullae approach one another near the midline, constricting the brain cavity into an inverted keyhole shape in posterior view ( Fig. 1E, F ). The medial opening of the endolymphatic foramen faces anteromedially. Posteriorly the vagus foramen lies on the suture between the supraoccipital and otooccipital.

The dorsal surface of the supraoccipital is shorter anteroventrally and dorsoventrally in Xenosaurus , probably because of its flattened skull. The supraoc-

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Figures

Figure 1. Skull and mandible of Shinisaurus, palpebrals removed in this and in Fig. 2. A, skull and mandible, right lateral view. B, skull and mandible, dorsal view. C, skull, ventral view. D, right mandible, medial view. E, skull, posterior view. Skull lengths: A, B = 27 mm (UF 62316); C = 33 mm (UF 62497). Scale of specimens varying, see Fig. 2.

Figure 2. Skull and mandible of Shinisaurus. A, skull, right lateral view. B, skull, dorsal view. C, skull, ventral view. D, right mandible, lateral view. E, right mandible, medial view. F, skull, posterior view. Scale bars = 5 mm. A, B, D (UF 62316); C, E, F (UF 62497); Scales same in A, B and in C, F.

Figure 3. Line drawings of selected anguimorph crania in dorsal view. Palpebrals removed for clarity. A, Carusia intermedia. B, Xenosaurus grandis. C, Pseudopus apodus. D, Heloderma suspectum. E, Varanus salvator. A, composite redrawn after Borsuk-Bialynicka (1985), Gao & Hou (1996), and Gao & Norell (1998, 2000); others redrawn after Rieppel (1980). All scaled to same approximate cranial depth (see Fig. 5).

Figure 4. Anguimorph crania in ventral view. A, Carusia intermedia. B, Xenosaurus grandis. C, Pseudopus apodus. D, Heloderma suspectum. E, Varanus salvator. A, composite redrawn after Borsuk-Bialynicka (1985) and Gao & Norell (1998, 2000); others redrawn after Rieppel (1980). All scaled to same approximate cranial depth (see Fig. 5).

Figure 5. Anguimorph skulls in lateral view. A, Carusia intermedia. B, Xenosaurus grandis. C, Pseudopus apodus. D, Heloderma suspectum. E, Varanus salvator. A, composite redrawn after Borsuk-Bialynicka (1985), Alifanov (2000), Gao & Hou (1996), and Gao & Norell (1998, 2000); others redrawn after Rieppel (1980). Scale bars = 5 mm. All scaled to same approximate cranial depth.

Figure 7. Antorbital skull roofing bones, right side of the skull. A, external and B, internal views of the premaxilla. C, lateral and D, medial views of the maxilla. E, dorsal and F, ventral views of the nasal. G, lateral and H, medial views of the prefrontal. I, lateral and J, medial views of the lacrimal. Scale bar = 5 mm.

Figure 8. Skull in posteroventral view detailing palpebral contact with the prefrontal (UF 62578).

Figure 9. Paired postorbital skull roofing bones excluding the supratemporal, right side of skull. A, lateral and B, medial views of the jugal. C, lateral and D, and medial views of the postorbitofrontal. E, lateral and F, medial views of the squamosal. G, detail of the contacts of the jugal, postorbitofrontal, and squamosal in lateral view, illustrating the absence of a jugal–squamosal contact in juvenile (left) and adult (right) specimens. A-F, UF 57112; G, UF 62316 (left) and UF 61139 (right). Scale bars = 5 mm.

Figure 10. Posterior view of the right orbital wall in selected anguimorphs. A-C illustrate taxa possessing a jugal– prefrontal contact. D-F illustrate taxa lacking this contact. A, D, the two morphologies variably exhibited by Shinisaurus. B, Gerrhonotus liocephalus. C, Heloderma suspectum. E, Estesia mongoliensis. F, Varanus komodoensis. C, E, F, modified and redrawn after Norell & Gao (1998).

Figure 11. Frontal and parietal. A, dorsal and B, ventral views of the frontal. C, dorsal and D, ventral views of the parietal. E, F, relative lengths of the supratemporal processes of the parietal in E, a subadult and F, an adult specimen. A-D, UF 57112; E, 62316; F, UF 61139. Scale bars = 5 mm.

Figure 12. Dorsal view of left palatal elements. A, vomer. B, palatine. C, pterygoid. D, ectopterygoid. UF 57112. Scale bar = 5 mm.

Figure 13. Right quadrate. A, anteromedial view. B, and posterolateral view. UF 68203. Scale bar = 5 mm.

Figure 14. Braincase of Shinisaurus. A, right lateral view. B, dorsal view. UF 57112. Scale bar = 5 mm.

Figure 15. Detail of braincase foramina. A, posterolateral view of the braincase. B, posterodorsal view of braincase. C, anterior view of the basisphenoid. D, posteroventral view of braincase. UF 57112. Scale bars = 5 mm.