This description focuses on two excellently preserved specimens of Gobihadros mongoliensis. One is a complete and uncrushed skull (MPC-D100/763), while the second—the holotype (MPC-D100/746)–is a virtually complete skull and postcranial skeleton. Except for the skull, this latter specimen is fully articulated. The skull was found closely associated, but not articulated except for the braincase, the right and left mandibles, and the right and left prefrontals and lacrimals. Only minimal distortion is present in both cranial and postcranial elements. The available specimens represent a range of sizes and presumed ontogenetic stages from subadult to adult. The osteology described here is consistent across the known size variation and is hypothesized to characterize the adult morphology. A comparative table of measurements for select Mongolian hadrosauroids is provided in Supporting Information S1 File.

Skull.

In lateral view, the skull of MPC-D100/763 is 294 mm long, rising gradually from its rostral tip to the orbits, and the quadrate height is 154 mm (Fig 2). Equivalent measurements of MPC-D100/746 are 226 mm and 115 mm. The transverse width of the postorbital region of the skull in dorsal view is broad; width is maintained from the orbit to the head of the quadrate. The external naris is relatively short (20% basal skull length) and the dorsal and caudoventral premaxillary processes do not meet caudal to external nares. Instead, its v-shaped caudal margin is formed by the nasal. The caudalmost apex of the external naris is formed equally by the nasal (dorsally) and the premaxilla (ventrally). In its proportions and composition, it differs from Tenontosaurus tilleti Ostrom, Jinzhousaurus yangi Wang & Xu, Iguanodon bernissartensis Mantell, and Altirhinus kurzanovi Norman, but is similar to Telmatosaurus and Probactrosaurus [7, 14, 44–46]. The antorbital foramina and fossa appear to be absent (similar to Eolambia, but different from many other non-hadrosaurid hadrosauroids; [10], although a small opening along the jugal-lacrimal suture may well be the last vestige of this system opening onto the lateral surface of the skull. The nearly circular orbits are 18% basal skull length. The supratemporal fenestra is nearly subequal in transverse (38 mm) and rostrocaudal (33 mm) dimensions. The infratemporal fenestra is somewhat trapezoidal, 80 mm in maximal length and wider ventrally than dorsally. The foramen magnum is subcircular, slightly prolonged and pointed ventrally. A modest fontanel with irregular edges, found between the paired nasals and frontals, testifies to the immature ontogenetic status of this individual. The vomer, ectopterygoids, and stapes appear to be missing.

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larger image TIFF original image Download: Fig 2. Skull and mandible of Gobihadros mongoliensis. Skull and mandible (MPC-D100/763) in left lateral (A), dorsal (B), ventral (C), and posterior (D) views. Abbreviations: an, angular; ar, articular; at, atlas; atr, atlantal rib; ax, axis; boc, basioccipital; bsp, basisphenoid; cop, coronoid process; c3, 3rd cervical vertebra; d, dentary; exo, exoccipital; f, frontal; gl, glenoid for the lateral quadrate condyle; hy, hyoid; j, jugal; l, lacrimal; mx, maxilla; n, nasal; p, parietal; pa, palpebral; pat, proatlas; pd, predentary; pf, prefrontal; pl, palatine; pm, premaxilla; po, postorbital; poc, paroccipital process; pt, pterygoid; q, quadrate; qj, quadratojugal; rap, retroarticular process; s, surangular; scl, sclerotic ring; soc, supraoccipital; sq, squamosal; v, vomer. https://doi.org/10.1371/journal.pone.0208480.g002

Premaxilla (Fig 3): The paired premaxillae were found in articulation and no attempts have been made to disarticulate them. In dorsal view, the oral margin is straight and bears a row of large denticles (three or four denticles per premaxilla), unlike in Mantellisaurus, Ouranosaurus, Protohadros, Eolambia, and Dakotadon, which have only two [10, 12, 47–50]. There is a second, more caudal (more oral) row of smaller denticulations separated from the aforementioned row by a deep sulcus bearing vascular foramina, as in hadrosaurids. The external surface of the oral region of the premaxilla is slightly rugose, indicating the presence of a keratinous rhamphotheca. More caudally, the premaxilla is expanded laterally less than twice the width of the premaxilla at its narrowest point. The lateral profile of the rostral margin of the premaxilla is slightly less than 70°. Although the dorsal process is slightly dorsoventrally crushed, it was very long, terminating 35 mm prior to the caudalmost extent of the caudolateral premaxillary process. There is no reflected rim around the oral margin, nor is there a circumnarial depression, which occurs in hadrosaurids. The narial fossa extends in front of and beneath the rostral and ventral margins of the external naris onto the expanded narial platform. A canal traverses the premaxilla from the rostral surface of the narial fossa to the front of the premaxillary palatal surface, 7 mm from the midline. There is no accessory narial foramen. The flat caudolateral process shrouds the upper articular surface with the maxilla, ending as it covers the front of the lacrimal. There is no outer (accessory) narial fossa. In ventral and caudal view, just beneath the ventral margin of the external naris is a small, shallow, but well-defined excavation that accommodates the styloid rostromedial process of the maxilla. Immediately caudolateral on the ventral surface is a larger, more rounded fossa into which the blunt rostrolateral process of the maxilla fits.

Nasal (Fig 4): The nasal is a long and thin element (no more than a millimeter thick), which forms the dorsal wall of the nasal cavity. Rostrally and medially, it forms a very long articulation with the dorsal process of the premaxilla; here it also forms a modest crescentic contribution to the caudal margin of the external naris, as described above. Making up the ventral middle third of the nasal is a grooved surface for reception of the caudolateral process of the premaxilla. The outer surface is generally smooth, marked only by a few neurovascular foramina. There is no indication of a crest or other visual display structure, and there is no circumnarial depression, which is present in Eotrachodon and hadrosaurids [29]. Caudally, the nasal twists slightly to articulate with the frontal along a slightly fluted scarf joint, as seen in all hadrosauroids.

Maxilla (Fig 5): As in Camptosaurus, Probactrosaurus, Eotrachodon, and saurolophine hadrosaurids [7, 26, 46, 51], the maxilla is isosceles-triangular in shape and nearly half as high as long, in lateral view. Rostrally, two processes articulate with the premaxilla: the rostromedial process, which fits into a small, well-defined pit, and the flattened, lobate rostrolateral process, which articulates with a fossa on the undersurface of the premaxillary body. These processes are also seen in numerous species of hadrosauroids, including Bactrosaurus, Altirhinus, Shuangmiaosaurus, Mantellisaurus, and saurolophine hadrosaurids [7, 15, 45, 47, 51, 52]. The body of the maxilla diverges markedly from the midline of the skull. The oblique dorsal margin of the bone is marked by a shallow facet for articulation with the caudolateral process of the premaxilla. The apex (dorsal maxillary process), set slightly rostral to the midpoint, is tall and sharply peaked. Contact between the maxilla and lacrimal is along a linear groove directly rostral to the pointed end of the jugal. The articular facet for the jugal dominates the dorsal process. Immediately medial to the front of the jugal facet is a rostrocaudal channel that opens on the premaxillary articular surface and terminates as a curved groove in the middle of the dorsal maxillary process. Beneath the sigmoid ventral margin of the jugal facet, the lateral surface of the maxilla overhangs the tooth row and forms the dorsal extent of the buccal cavity. Four or five neurovascular foramina ranging in diameter from 1 to 2 mm, run rostrocaudally ventral to this buccal shelf; these are also seen in Protohadros, Jinzhousaurus, Probactrosaurus, Bactrosaurus, and numerous hadrosauroids [14, 46, 49, 51–53]. The contact with the vomer appears to be along the shoulder where the rostromedial process of the medial maxillary edge lies immediately caudal to the dorsal process. The ectopterygoidal shelf slightly overhangs the lateral wall of the maxilla; continuation of the suture with the ectopterygoid is on the blunt, angular, caudal end of the maxilla. Contact with the pterygoid occurs just caudal to that of the palatine and onto the small process at the caudal extremity of the maxilla. Medially, the maxilla is flat, marked by alveolar foramina and a distinct, slightly arched neurovascular canal that links them. Ventrally, the tooth row is very slightly concave laterally and contains 19 tooth families, or 1.5 families/1.0 cm. The low number of tooth families is consistent with most non-hadrosaurid hadrosauroids, but similar tooth family density is found in Bactrosaurus, Telmatosaurus, Probactrosaurus.

Jugal (Fig 6): As in hadrosauroids in general, the jugal of Gobihadros is roughly triradiate, consisting of a rostral process, where it articulates with the maxilla, a postorbital process, where it contacts the postorbital, and an expanded caudal process or blade, which articulates with the quadratojugal and the quadrate. The rostral process is slightly dorsoventrally flared and pointed where it contacts the maxilla. The dorsal margin of the rostral process has pushed the lacrimal dorsally to lie completely above the level of the maxilla. Here the articular facet for the jugal process of the lacrimal is divided into two by a gap or foramen between the pillar-like caudal process of the lacrimal that fits into a slot on the jugal and the thinner, linear jugal-lacrimal contact. This foramen may be the opening for the antorbital sinus. Medially, a very small facet directly caudoventral to the rim of the maxillary articulation marks the contact with the ectopterygoid, as also seen in Iguanodon, Mantellisaurus, Altirhinus, Eolambia, Protohadros, Probactrosaurus, and Bactrosaurus, but not in hadrosaurids [7, 10, 44–47, 49, 51, 52]. Contact with the palatine is along the dorsalmost portion of the rim of the maxillary suture.

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larger image TIFF original image Download: Fig 6. Jugal of Gobihadros mongoliensis. Jugal (MPC-D100/746, right) in lateral (A), medial (B), and dorsal (C) views. Abbreviations: cdp, caudal process; fvf, free ventral flange; itf, infratemporal fenestra; lap, lacrimal process; ls, lacrimal suture; mxf, maxillary facet; mxp, maxillary process; orm, orbital margin; plf, palatine facet; pof, postorbital facet; pop, postorbital process; qjf, facet for the rostral process of the quadratojugal; rop, rostral process. https://doi.org/10.1371/journal.pone.0208480.g006

The orbital margin of the jugal is smoothly curved. The postorbital process extends vertically 68 mm above the lowest point of the infratemporal fenestra. Most of this length forms the contact surface for the jugal process of the postorbital; at its terminus, it reaches to the body of the postorbital, making this a relative long postorbital process compared to most hadrosauroids if not all ornithopods. The jugal makes up the entire ventral infratemporal margin. It is also dorsoventrally constricted beneath this fenestra to set off the caudal jugal process, thereby giving this edge of the jugal its sigmoidal silhouette. The depth-to-length ratio of the caudal process is relatively small (0.70–0.90), compared to that of Jinzhousaurus and Altirhinus [14, 45], where the ratio ranges upward of 1.3. Striations on the lateral aspect of the ventral jugal margin may indicate the presence of soft tissue covering the buccal region.

Lacrimal (Fig 7): The lacrimal is plate-like and triangular in lateral view. Along its base, it articulates via a ridge and groove suture with the maxilla. At the caudal extreme of this edge, the lacrimal forms the concave margin of a small fenestra, which is caudally limited by the jugal process of the lacrimal. The rostral edge is marked by a shallow scarf suture for the caudolateral premaxillary process. The base of the lacrimal makes a linear contact with the lateral maxillary wall directly in front of the dorsal process. The dorsal aspect of the lacrimal slips beneath the ventral edge of the prefrontal. The caudal edge of the lacrimal forms the rostral orbital margin. Here the element is thickest where it forms the entrance of the nasolacrimal canal, which exits on the medial aspect of the lacrimal, immediately rostral to the jugal process.

Prefrontal (Fig 7): The majority of the crescentic prefrontal is smoothly convex externally, but flares dorsolaterally to form the thin, everted, and wing-like rostrodorsal margin of the orbit. Ventrally it overlaps the dorsal margin of the lacrimal. There is a small facet on the external surface at the base of the prefrontal that marks the articulation of the terminus of the caudolateral process of the premaxilla. Immediately above this facet are one or two neurovascular foramina. The internal surface of the prefrontal is markedly concave except where it contacts the frontal. This articulation is via a complex, tongue-and-groove joint on the caudomedial surface of the prefrontal.

Frontal (Fig 8): In dorsal view, the flat, subtriangular frontal is two-thirds as wide as long. Rostrally, it bears a broad, grooved, and shallow articular facet for the nasal and immediately lateral a complex excavated pit for the prefrontal. The orbital margin is short and grooved on its ventral surface. An upward doming over a braincase is absent. The interfrontal joint is interdigitated along its length. Caudally, the paired frontals part to accommodate a rostral process of the parietal. The caudolateral corner of the frontal is complexly excavated for the postorbital as well as for the head of the laterosphenoid. The caudal surface of the frontal receives the rostral face of the parietal and the laterosphenoid.

Ventrally, the most prominent feature is the endocranial surface for the olfactory bulbs and tracts on the rostral half of the frontal. The ventral surface of the orbital margin is perforated by a few small neurovascular foramina. On its caudal half, just behind and forming the lateral wall of the olfactory tract, the ventral surface of the frontal articulates with the orbitosphenoid and, more laterally, with the body of the laterosphenoid.

Parietal (Fig 8): The parietal is stout and hour-glass in shape in dorsal view, possessing only a modest, straight to slightly down-warped sagittal crest that is more than half the length of the supratemporal fenestrae. Rostrally an interfrontal process slips between the paired frontals. Two ridges diverge from the sagittal crest to set off the rear of the interfrontal process. Contact with the frontal is extensive, covering nearly its entire caudal surface. A small portion of the parietal lateral to this frontal suture contacts the postorbital. The squamosal rides over the rear margin of the parietal. Ventrally, the parietal articulates in series with the laterosphenoid rostrally, with the prootic intermediately, and with the opisthotic/exoccipital complex caudally. Midway along this suture, the parietal forms the dorsal margin of the foramen for the median cerebral vein.

Postorbital (Fig 9): The postorbital is a triradiate element with medial, caudal, and ventral processes. It closely resembles the postorbital of Bactrosaurus johnsoni [52]. Centrally it forms the thickened caudodorsal border on orbital rim. Medially, its contact with the frontal is made up a complex tongue-and-groove articulation. Ventral to the postorbital-frontal suture is a pit that accommodates the head of the laterosphenoid. The caudal process overlaps the squamosal for most of the supratemporal bar; here it is a simple non-digitate suture. The caudal end of the process is not bifurcated, as is seen frequently in hadrosaurids. Finally, contact with the jugal is by way of a long, thin scarf joint on the caudomedial surface of the ventral (postorbital) process.

Squamosal (Fig 8): The squamosal is roughly triradiate in form. Medially, it forms the caudal margin of the supratemporal fenestra, continuing on toward the midline and its counterpart, separated here by a narrow band of the parietal, as in Jinzhousaurus, Jintasaurus, Probactrosaurus, Bactrosaurus, Levnesovia, and Tanius [14, 18, 19, 46, 54]. The cranial process forms much of the upper temporal arcade; here it is marked externally by the single angular articulation for the caudal process of the postorbital. The ventral process descends behind the quadrate cotylus to form a deep, nearly vertical, and rugose postquadratic process. The caudal surface of this process forms a butt joint with the base of the paroccipital process and also contacts the lateral margin of the supraoccipital in caudal view. Externally, the deep cotylus for the head of the quadrate dominates the squamosal in external view. Immediately rostral to the cotylus is the prequadratic process; sandwiched between it and the articular surface for the postorbital is the small, triangular scar for m. adductor mandibulae externus superficialis [55].

Quadrate (Fig 10): The quadrate in lateral view is moderately robust. The dorsal head is nearly triangular in dorsal view, and it is very shallow, as in Mantellisaurus, Jeyawati, Bactrosaurus, and Telmatosaurus. Immediately beneath the head, the rostral surface is beveled and strongly striated for sutural ligaments that bind this region with the prequadratic process of the squamosal. The caudal margin is buttressed where it contacts the postquadratic process of the squamosal. This squamosal articulation positions the quadrate in a canted position, giving the skull a triangular outline in occipital view (Fig 2D). In lateral view, the caudal aspect of the quadrate shaft is modestly concave. Beneath midshaft, the lateral aspect of the quadrate shaft is dominated by the articular surface for the quadratojugal. There is a modest gap here, indicating that the paraquadratic opening was reasonably well defined. The lower margin of the quadrate here is slightly buttressed. The ventral end of the quadrate is only slightly expanded and obviously bicondylar. The lower and larger lateral condyle articulates with the surangular glenoid, while the slightly higher, smaller medial condyle articulates with the articular glenoid. The lateral condyle is not large and globular as in hadrosaurids [51], but is similar in relative size and morphology to Bactrosaurus, Gilmoreosaurus, and Jeyawati [56–58]. Together, the ventral quadrate gives a slight medial elevation to the axis of mandibular rotation. Medially, the thin, platelike pterygoid wing extends rostromedially at approximately 45° from the lateral quadrate wall. The lower margin of the ala is thickened; the central region is markedly convex laterally, forming the reciprocal surface for the quadrate ala of the pterygoid.

Quadratojugal (Fig 11): The quadratojugal is a thin, oblong element located between the jugal and quadrate. Both external and internal surfaces are beveled rostrally and caudally respectively from a median thickened body. Externally, this beveled region is covered by the jugal, while internally it marks the regions that contact the quadrate.

Supraoccipital (Fig 8): The supraoccipital is a relatively large, subtriangular element that makes up the dorsal region of the occiput. Its caudal surface is inclined steeply forward at approximately 45°, unlike in Probactrosaurus, Ouranosaurus, Dakotadon, Lurdusaurus, Eolambia, Iguanodon, and Mantellisaurus, where the caudal supraoccipital surface is nearly vertical [10, 44, 46–48, 50, 59]. The ascending process contacts the parietal dorsally and laterally. There is a prominent median nuchal crest. On each side of this crest is a low boss separated by a furrow; this boss forms the area of insertion of m. rectus capitis caudalis [60]. In addition to contacting the caudolateral margin of parietal rostrally, the lateral supraoccipital process forms a prominent horizontal ridge with the opisthotic/exoccipital complex immediately above where the latter forms the dorsal margin of the foramen magnum. The ventrolateral corners of the supraoccipital are inset under the squamosals such that the supraoccipital is “locked” between them. The post-temporal foramen is found at the junction of the squamosal, opisthotic/exoccipital complex, and supraoccipital. The ventral margin of the supraoccipital forms a prominent horizontal ridge where it contacts the exoccipital/opisthotic complex.

Exoccipital/Opisthotic Complex (Fig 8): The ventral margin of the fused exoccipital-opisthotic complex contacts the basioccipital and basisphenoid. The exoccipital contributes a small occipital condylid and directly above forms the lateral and dorsal margins of the oval foramen magnum. The dorsal border of the exoccipital-opisthotic complex meets the supraoccipital along a prominent horizontal ridge. Immediately above this ridge, the ventromedial margins of the post-temporal foramen are formed by the exoccipital/opisthotic complex. Laterally, the caudal aspect of the complex is excavated to form the attachment site of m. obliquus capitis magnus [60]. Beneath the ridge at the supraoccipital suture is the m. rectus capitis caudalis scar. The outer margin of the exoccipital-opisthotic complex arches dorsolaterally to form the large caudolaterally-projecting paroccipital process. At their extremities, the paroccipital processes are curved rostrally.

In lateral view, the ventral edge of the paroccipital process forms a sharp rostroventral crest (crista tuberalis) that splits rostroventrally into two ridges. One, the more caudal and rounder of the two, merges with the dorsal margin of the exoccipital condylid, while the other, sharper ridge, continuing toward the suture between the basioccipital and basisphenoid, marks the suture between the exoccipital/opisthotic complex and the prootic. The concave ventrolateral aspect of the exoccipital-opisthotic complex is pierced by three foramina. Two, just above the suture for the basioccipital transmitted the hypogossal nerve (c.n. XII). The exit for the spinal accessory and vagus nerves (c.nn. X and XI) is found immediately behind crista tuberalis. The auditory foramen opens in front of crista tuberalis directly above the suture with the basisphenoid. At its base, contact with the basioccipital is somewhat difficult to discern given its nearly fused state, but it appears to correspond to a line extending from the base of the occipital condylids to the base of the basal tubera.

Prootic (Fig 8): With the laterosphenoid and exoccipital/opisthotic, the prootic forms the lateral wall of the braincase. It forms a butt joint with the laterosphenoid immediately above the foramen for the trigeminal nerve (c.n. V). From this foramen, the largest of all of the cranial nerve foramina, a groove accommodating the ophthalmic branch of the trigeminal nerve (V 1 ) follows the base of the laterosphenoid. A canal extending ventrally from the trigeminal foramen marks the pathway of the combined maxillary and mandibular divisions (V 2 , V 3 ). The surface of the prootic between the first and second (with the third) divisions marks the scar for m. constrictor ventralis. Directly caudal to the groove for the second and third divisions is a caudolateral flange that shrouds the pathway of the hyomandibular branch of the facial nerve (c.n. VII). At the top of this groove is the foramen of the facial nerve. Above and continuing caudodorsally, a roughened eminence traverses the lateral surface of the exoccipital-opisthotic complex known as crista prootica (the scar for m. constrictor dorsalis). This eminence marks an elongate scarf articulation between the prootic and opisthotic and also a continuation of crista prootica. Below the contact between the prootic and opisthotic is a small, deep depression marking the caudalmost extent of the otic vestibule. Dorsal to the prootic articulation and crista prootica, the rostrodorsal surface of the base of the paroccipital process is excavated for contact with the supraoccipital.

Laterosphenoid (Fig 8): The head of the laterosphenoid forms what appears to be a synovial joint with the underside of the postorbital and frontal. The frontal articulation continues transversely across the orbital surface to meet its suture with the orbitosphenoid. A butt joint is found between the laterosphenoid and prootic, and between the laterosphenoid and parietal.

Orbitosphenoid (Fig 8): The paired orbitosphenoids are found in articulation with the ventral surface of the frontals, as well as the laterosphenoids, basisphenoid, and each other. C.n. I emerges rostrally where the right orbitosphenoid articulates with the left. Contact with each frontal and laterosphenoid is via a laterally convex, digitate suture. Foramina for cranial nerves II, III, IV, and VI are found in the central and caudal aspects of the orbitosphenoid.

Basisphenoid (Fig 8): In ventral view, the basisphenoid is triangular and appears like a thin arrowhead. The handle of the blade, the basal tubera, is twice as wide as long. These tubera are rugose probably for attachment of m. rectus capitis ventralis. A sinuous groove across the tubera marks the suture with the basioccipital. More rostrally, the basipterygoid process forms lateral to the midline furrow and rostral to the entrance of the carotid canal. The basipterygoid process is relatively long (23 mm from the midline), and slightly ventrolaterally directed approximately 80° from the sagittal plane (the basipterygoid process appears to be deformed by dorsoventral compression). The basipterygoid articular facets are directed rostroventrally. A midline prong, immediately between the basipterygoid processes, is found in Gobihadros, Camptosaurus, Ouranosaurus, Bactrosaurus, and Levnesovia [18, 48, 52, 61]. Entrance to the carotid canal is located between the flange formed by the base of the basipterygoid process and basal tubera. This canal travels rostrally and probably slightly medially through the basisphenoid to exit through the caudoventral wall of the hypophyseal fossa. A vertical groove directly above the entrance of the canal corresponds to path of palatine ramus of facial nerve; the latter also enters the carotid canal. A style-like rostral projection from the body of the basisphenoid may be the separate center of ossification known as the parasphenoid. A foramen for the median palatine artery is found where the basisphenoid meets the orbitosphenoid.

Basioccipital (Fig 8). The basioccipital is formed principally by the occipital condyle, the intermediate collum, and the caudal half of the basal tubera. The occipital condyle is reniform as it wraps around, and forms 9 mm of, the ventral margin of the foramen magnum. Laterally it butts the condylids of the exoccipitals. The collum is short (10 mm) and equal in width of the occipital condyle. Farther forward, the basioccipital portion of the basal tubera projects rostrally, slightly laterally, and slightly ventrally to contact the basisphenoid portion of the basal tubera. The former is divided by a shallow midline sulcus and separated by a prominent transverse groove from the latter. There is no axial ridge between basal tubera, which is seen in Camptosaurus, Uteodon, Cumnoria, Dakotadon, and Jintasaurus [12, 19, 50, 61–64].

Palatine (Fig 12): The base of the palatine makes an inclined suture with the dorsomedial edge of the caudal process of the maxilla. A stout process located toward the front of this palatine–maxilla contact extends laterally to contact the jugal. The palatine also articulates with the pterygoid along its oblique dorsolateral margin.

Pterygoid (Fig 13): As in other ornithopods, the pterygoid of Go. mongoliensis is a tetraradiate element. The palatine ramus is incomplete, yet appears to extend dorsally and laterally where it contacts the palatine. Ventrally, the pterygoid sends a short process to contact the ectopterygoid and the rear end of the maxilla; this process is somewhat caudally deformed. Two alar projections on the caudal end of the pterygoid articulate with the quadrate. The larger upper ala fits against the pterygoid ala of the quadrate, while the thickened lower ala reinforces this contact with the quadrate. Additional buttressing is found between the ectopterygoid and upper quadrate rami. Halfway along this buttress, on the medial aspect of the pterygoid, is a well-defined saddle for the synovial articulation with the basipterygoid process of the basisphenoid.