20 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3 culature is very clear (BMNH R11956, R8501, RUB 17), passing through a canal formed by maxilla, lacrimal, jugal, and perhaps palatine. The maxilla and lacrimal (BMNH R11956) are both strongly grooved for the neurovascular bundle. In fact, the dorsal groove on the maxilla extends rostrally within the floor <strong>of</strong> the antorbital cavity, ending at a slit-like foramen directed ventrally into the bone and opposite the internal antorbital fenestra (BMNH R11956). This slit almost certainly conducted the maxillary neurovasculature because all <strong>of</strong> the external neurovascular foramina are directed toward it (Fig. 7A). Among Ornithopoda, as in Lesothosaurus diagnosticus, Heterodontosaurus tucki (BMNH R8 179 [cast <strong>of</strong> SAM 3371) probably had a neurovascular canal between maxilla, lacrimal, and jugal. However, more similar to Hypsilophodon foxii (discussed below), the neurovascular branches exited more or less separately from the antorbital cavity rather than from a single foramen. Better data are available for Hypsilophodon foxii (BMNH R197, R192, R5862, R2477). The dorsal surface <strong>of</strong> the palatine (BMNH R2477) has a clear fossa extending rostrally up to the choana (Fig. 8A). Unlike Lesothosaurus diagnosticus, there is no nasal fossa rostral to it, but the postchoanal strut is grooved dorsally for the nasal capsule. As in L. diagnosticus, however, there is a very clear neurovascular canal formed by maxilla, jugal, lacrimal, and a dorsal, laterally arching process <strong>of</strong> the palatine (Fig. 8A). The portion <strong>of</strong> the maxilla forming the ventral rim <strong>of</strong> the external antorbital fenestra (supralveolar lamina) is drawn up dorsally such that the nerves and vessels would be lodged in a deep groove. As alluded to above, the neurovasculature exited the antorbital cavity ventrolaterally via several large foramina (Fig. 8B). Whereas these more basal ornithopods retain the basic crani<strong>of</strong>acial architecture <strong>of</strong> other archosaurs, the facial and palatal skeleton in iguanodontian ornithopods becomes highly-transformed. with reduction <strong>of</strong> the antorbital cavity and development <strong>of</strong> larger dentitions (Weishampel, 1984; se; section on trends below). Probably above the level <strong>of</strong> Iguanodon spp. within Iguanodontia, the palatine assumes a much more vertical orientation (Fig. 11A; Lambe, 1920; Heaton, 1972; Norman, 1980; Norman and Weishampel, 1990; Weishampel and Horner, 1990), largely occluding the postnasal fenestra and restricting the rostral advance <strong>of</strong> any pterygoideus musculature. The more basal iguanodontians retaining an antorbital cavity, such as Camptosaurus dispar (UUVP 5946), C. prestwichii (Galton and Powell, 1980), and Iguanodon ather-eldensis (BMNH R5764, R11521), nevertheless resemble more basal ornithischians with regard to the relationship <strong>of</strong> the neurovasculature. There is a deep groove in the dorsal surface <strong>of</strong> the maxilla which is formed into a canal by the addition <strong>of</strong> the jugal and palatine (the jugal excludes the lacrimal from the canal in at least Iguanodon atherfieldensis; Fig. 11B). This groove extends in the floor <strong>of</strong> the small antorbital cavity before entering the neurovascular foramen located opposite the antorbital fenestra (as in Lesothosaurus diagnosticus; see Fig. 1lB for Iguanodon ather-eldensis). Hadrosaurs have a completely closed external antorbital fenestra (Weishampel and Horner, 1990), yet retain a shallow fossa on the palatine and the same basic structure <strong>of</strong> the neurovascular canal (e.g., Edmontosaurus regalis, CMN 2289; Corythosaurus casuarius, AMNH 5338; Hypacrosaurus sp., MOR-609-88-9 I) The evidence provided by the basal thyreophoran Scelidosaurus harrisonii (BMNH R1111) is particularly important because many higher thyreophorans are so derived that direct correspondences are difficult to identify. S. harrisonii resembles Hypsilophodon foxii in that the caudodorsal surface <strong>of</strong> the palatine is excavated into a fossa extending up to the choana and the postchoanal strut is grooved for the nasal cartilages. Likewise, very similar to the other ornithischians discussed here, there is a neurovascular canal formed by maxilla, lacrimal, ju- gal, and palatine that is carried rostrally as a groove in the floor <strong>of</strong> the antorbital cavity, entering the body <strong>of</strong> the maxilla at a foramen opposite the antorbital fenestra. Emausaurus ernsti (Haubold, 1990) is an even more basal thyreophoran, retaining a much larger antorbital cavity. Although the existence <strong>of</strong> a muscular fossa on the palate is unknown, the course <strong>of</strong> the maxillary neurovasculature in E. ernsti closely resembles that in Scelidosaurus harrisonii and other ornithischians. The basal ceratopsian Psittacosaurus mongoliensis (AMNH 6535; see Sereno, 1987) holds similar importance as a more basal member <strong>of</strong> a very specialized group. It also has a shallow fossa on the dorsolateral surfaces <strong>of</strong> the palatine and pterygoid extending rostrally to the choanal margin and a neurovascular canal surrounded by maxilla, palatine, and jugal. The neurovascular canal is large and leads to a series <strong>of</strong> ventral foramina opening into the buccal cavity. Saurischians differ in that the maxillary neurovasculature is not typically enclosed in a canal formed by several bones; a canal is presumably an ornithischian apomorphy associated with the tendency to wall in the antorbital cavity (see section on trends below). Otherwise, some saurischians, such as Plateosaurus engelhardti, resemble such ornithischians as Lesothosaurus diagnosticus and Hypsilophodon foxii in the osteological correlates <strong>of</strong> the dorsal pterygoideus and maxillary neurovasculature. Most basal prosauropods (e.g., Thecodontosaurus antiquus, Kermack, 1984; Anchisaurus polyzelus, Galton, 1976) are poorly known with regard to the relevant features and the following discussion focuses mostly on Plateosaurus engelhardti (principally AMNH 6810; see also Galton, 1984, 1985c, 1990). The dorsolateral surface <strong>of</strong> the palatine <strong>of</strong> P. engelhardti has a postchoanal strut that is strong laterally and weakens rostrodorsally, delimiting a caudal (muscular) fossa from a flatter, rostral, nasal area (Fig. 12D). Although Galton (1985~) did not list the palatine as an attachment site for the dorsal pterygoideus in P. engelhardti, the fossa compares well with the presumptive muscular fossa <strong>of</strong> other archosaurs. As in ornithischians, there is, within the antorbital cavity, a dorsal groove on the maxilla that passes rostrally until it enters a ventral, slit-like foramen opposite the rostral margin <strong>of</strong> the internal antorbital fenestra (Fig. 12D); as in Lesothosaurus diagnosticus, the external neurovascular foramina are all directed toward this slit (Fig. 12A). The dorsal groove and foramen apparently are present in Sellosaurus gracilis (Galton, 1985b), and almost identical relationships <strong>of</strong> the external neurovascular foramina also are seen in that taxon and Massospondylus carinatus (Gow et al., 1990). Caudally in Plateosaurus engelhardti, processes <strong>of</strong> the lacrimal and palatine approach each other in the vicinity <strong>of</strong> their junction with the maxilla and jugal (Fig. 12D), but do not form the complete bony canal observed in the ornithischians (although it was probably completed with s<strong>of</strong>t tissue). As a whole, sauropods are poorly known with regard to these features. The palatines and pterygoids <strong>of</strong> Camarasaurus lentus (CM 11338; see also Madsen et al., 1995) and Diplodocus longus (CM 11161) are more or less vertically oriented in the vicinity <strong>of</strong> the antorbital cavity, with perhaps a slight concavity to their caudodorsolateral surfaces (Fig. 13A). The palatine <strong>of</strong> Euhelopus zdanskyi has a stronger dorsolateral fossa, undercutting the postchoanal portion <strong>of</strong> the bone (Mateer and McIntosh, 1985). Zhang's (1988) reconstruction <strong>of</strong> the adductor musculature attaching to this portion <strong>of</strong> the palate in Shunosaurus lii seems reasonable, although he carried the muscle rostrally onto the vomer which is unlikely. Janensch (1935-36) reconstructed the pterygoideus musculature <strong>of</strong> Brachiosaurus brancai as extending over the entire dorsal surface <strong>of</strong> the palatine to attach to the antorbital fenestra; however, the palatine <strong>of</strong> this form resembles that <strong>of</strong> Camarasaurus lentus (McIntosh, 1990; Mad-
WITMER-ANTORBITAL CAVITY OF ARCHOSAURS 21 FIGURE 13. Camarasaurus lentus. A, stereophotographs <strong>of</strong> skull <strong>of</strong> CM 11338 in left lateral view. B, same in ventral view. C, interpretive drawing <strong>of</strong> A. D, interpretive drawing <strong>of</strong> B.
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