SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3 FIGURE 7. Lesothosaurus diagnosh'cus, facial skeleton. A, rostra1 part <strong>of</strong> skull in left lateral view. B, same in ventral view. C, stereophotographs <strong>of</strong> BMNH RUB 17 in right caudodorsolateral view, showing the palatal elements. D, interpretive drawing <strong>of</strong> C. (A,B modified after Sereno. 1991a.)
WITMER-ANTORBITAL CAVITY OF ARCHOSAURS 15 B - & fen antorb int prf , fen antorb lac .iua lam Jpra pmx max for neurovas FIGURE 8. Hypsilophodon foxii, facial skeleton. A, right maxilla, lacrimal, palatine, and jugal (BMNH R2477) in dorsal view. Thin arrow shows the course <strong>of</strong> the nasolacrimal duct. Thick arrow show the course <strong>of</strong> the maxillary neurovascular bundle. B, facial skeleton in left lateral view (modified after Galton, 1974.) the antorbital cavity. Here the muscle is just caudoventral to the ridge on the palatine supporting the nasal capsule and is in direct contact with the antorbital sinus (the avian homolog <strong>of</strong> the caviconchal sinus <strong>of</strong> crocodilians; see below and Witmer, 1995b). For much <strong>of</strong> its length, the muscle is just ventral to the suborbital diverticulum <strong>of</strong> the antorbital sinus (Witmer, 1990), and this air sac separates the pterygoideus from the other jaw muscles. As in crocodilians, the dorsal pterygoideus is ventral to the maxillary nerve (Fig. 6). In some cases (e.g., Gallus gallus), the neurovasculature closely adheres to the muscle, whereas in other cases (e.g., Anser anser) the nerve is dorsal to the muscle, suspended within the suborbital diverticulum by a double-walled, epithelial fold, a pneumatic "mesentery." Dollo (1884) reported that in birds the pterygoideus muscles attach to the rostral border <strong>of</strong> the antorbital cavity, but this is not the case in the birds I have personally dissected and appears not to have been reported otherwise in the literature. In a very few birds, a tendinous slip may contact the caudoventral tip <strong>of</strong> the maxilla (Lakjer, 1926) or the palatal process <strong>of</strong> the maxilla (H<strong>of</strong>er, 1950; Burton, 1984), but this is a rare condition. Thus the dorsal pterygoideus muscle <strong>of</strong> birds is indeed <strong>of</strong>ten one <strong>of</strong> the contents <strong>of</strong> the antorbital cavity, but never originates from any <strong>of</strong> the margins <strong>of</strong> the antorbital fenestrae. Osteological Correlates-The osteological correlates <strong>of</strong> M. pterygoideus, pars dorsalis are more consistent in crocodilians than in birds. In crocodilians, the medial surfaces <strong>of</strong> the jugal and maxilla tend to have a patchy striated or punctate pattern that increases somewhat in relief rostrally. The palatine usually has a raised dorsal ridge extending near the lateral edge rostral to the prefrontal articulation; this ridge marks the boundary between postconchal cartilage medially and the dorsal pterygoideus laterally. The bones surrounding the suborbital fenestra may display a slight excavation where the muscle attaches, but this fossa is usually weak and is absent in many individuals <strong>of</strong> Crocodylus spp. Thus, the direct evidence in crocodilians for the presence <strong>of</strong> a dorsal pterygoideus is strongest rostrally where the muscle attaches to the maxilla, jugal, and palatine. In birds, probably because <strong>of</strong> their small size, the muscle <strong>of</strong>ten leaves little evidence on the bone. Unlike crocodilians, the avian dorsal pterygoideus originates only from the palatine and pterygoids, never from the jugal and almost never from the maxilla. The rostral attachment sometimes excavates a fossa on the dorsolateral surface <strong>of</strong> the palatine near the choana. To determine if bony surfaces could be examined directly to ascertain if muscles were attached to them, bone samples from Alligator mississippiensis and Anser anser were examined by means <strong>of</strong> scanning electron microscopy. Samples came from surfaces known to be adjacent to gland, muscle, or air sac. The results indicated that bony surfaces are highly variable in these animals, and surface textures are not reliable indicators <strong>of</strong> the adjacent s<strong>of</strong>t tissues (although surface features usually are). Although samples occasionally had the predicted surface-texture pattern (e.g., Sharpey-fiber bone for muscle [Jones and Boyde, 19741 or uniformly smooth bone for gland and air sac), very <strong>of</strong>ten a sample showed the reverse pattern. Muscles are generally potent and well-understood functional matrices (sensu Moss, 1968, 1971). However, the ambiguity <strong>of</strong> the direct osteological correlates in both birds and crocodilians may reflect their archosaurian heritage in that their muscles are less likely to produce reliable bony evidence (i.e., scars) than those <strong>of</strong>, for example, mammals (Bryant and Seymour, 1990, and references therein). It also reflects the small sizes <strong>of</strong> virtually all the birds and many <strong>of</strong> the crocodilians in the sample. It was found that, in the more or less complete ontogenetic series available for this study (e.g., Alligator mississippiensis, Crocodylus porosus, Struthio camelus, Anser anser, Gallus gallus), older (and hence larger) individuals had more deeply etched muscle scars. Nevertheless, despite these ambiguities, extant birds and crocodilians do have similarities that can be hypothesized to have been present in their common ancestor. For example, in both groups <strong>of</strong> extant archosaurs, the dorsal pterygoideus is a relatively large muscle originating from the dorsal or dorsolatera1 surfaces <strong>of</strong> the pterygoid and palatine. The attachment on the palatine bone extends into the caudal portion <strong>of</strong> the antorbital cavity and usually excavates a fossa. This muscular fossa on the palatine is sometimes separated by a bony ridge from a fossa for the cartilaginous nasal capsule, the muscular fossa being generally caudoventrolateral to the nasal fossa. In neither group does the muscle fill the entire antorbital cavity, but rather is restricted to the caudal portion <strong>of</strong> the cavity, behind a homologous air sac (the crocodilian caviconchal sinus and avian antorbital sinus; see below and Witmer, 199513). Direct information regarding the extent and position <strong>of</strong> the muscle can be obtained by making use <strong>of</strong> the fact that in both birds and crocodilians (indeed in all sauropsids) the maxillary nerve always travels dorsal to the pterygoideus musculature (Fig. 6A, B). Thus, the position <strong>of</strong> the foramen or groove for
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