30 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3 illa; presumably this entire portion <strong>of</strong> the maxilla is the homolog <strong>of</strong> the medial lamina <strong>of</strong> other ornithischians. In the rostral apex <strong>of</strong> the antorbital fossa is a moderately large foramen directed rostrally into the bone. It is unknown if the foramen expands into an accessory cavity comparable to the cavity described for Emausaurus ernsti. As in the latter taxon, the nasolacrimal duct passes dorsomedially around the antorbital cavity through the lacrimal bone. Among stegosaurs, the basal taxon Huayangosaurus taibaii exhibits a small but distinct external antorbital fenestra within the maxilla, lacrimal, and jugal (see Sereno and Dong, 1992). The internal antorbital fenestra is caudally situated within the antorbital cavity and is probably opposite the choana. A welldeveloped medial lamina <strong>of</strong> the maxilla walls the cavity internally, forming an antorbital fossa that undercuts the dorsal margin <strong>of</strong> the external antorbital fenestra. The cavity is partially walled-in laterally by a prominent supralveolar lamina and a lateral lamina <strong>of</strong> the lacrimal. The external antorbital fenestra is absent in Chungkingosaurus jiangbeiensis (Dong et al., 1983) and Tuojiangosaurus multispinus (Dong et al., 1983). It is present, however, in Stegosaurus stenops, in which the antorbital cavity is similar to Huayangosaurus taibaii but smaller and shallower (Sereno and Dong, 1992). The Ankylosauria are a highly apomorphic group: they lack an external antorbital fenestra, most <strong>of</strong> the skull sutures have been obliterated by overlying dermal ossifications, and their snouts enclose a complicated pattern <strong>of</strong> cavities (Coombs, 1971, 1978; Maryanska, 1977; Coombs and Maryanska, 1990). As a result, the specified osteological correlates are difficult to assess. There clearly is a cavity within the maxillae <strong>of</strong> probably all ankylosaurids that may be homologous with the antorbital cavity (e.g., Euoplocephalus tutus, AMNH 5843, Fig. 19A; Ankylosaurus magniventris, AMNH 5894; see also Maryanska, 1977; Coombs, 1971, 1978; Tumanova, 1987; Coombs and Maryanska, 1990). This cavity-the "maxillary sinus" <strong>of</strong> the aforementioned authors-communicates rostrally with the narial region via a foramen in the premaxilla ventral or lateral to the true naris (e.g., Pinacosaurus grangeri, AMNH 6523; Euoplocephalus tutus, AMNH 5843; Ankylosaurus magniventris, AMNH 5214, 5895; see Maryanska, 1977). Maryanska (1977) suggested that the maxillary sinus lodged the glandula nasalis, which is not unreasonable since the cavity opens into the caudal part <strong>of</strong> the naris; furthermore, such a communication between the antorbital cavity and naris would be unique. However, the cavity seems inordinately large for a gland, and it communicates with other paranasal cavities (e.g., in Ankylosaurus magniventris, AMNH 5895). Moreover, Maryanska (1977) described a vertical septum within this cavity in Pinacosaurus grangeri and noted that both parts <strong>of</strong> the cavity open into the nasal cavity proper. Perhaps strictly the caudal portion <strong>of</strong> the cavity is the homolog <strong>of</strong> the antorbital cavity. Unfortunately, the communications <strong>of</strong> the various sinuses with each other and with the nasal cavity proper are poorly known (Coombs and Maryanska, 1990). Although similar paranasal cavities have not been reported for nodosaurid ankylosaurs, it is not clear if sufficient material providing the necessary views is available to regard this as a positive absence. The frequently figured cross-section <strong>of</strong> Edmontonia longiceps (AMNH 3076; Coombs, 1971, 1978; Norman, 1985; Coombs and Maryanska, 1990) indeed shows a single, large cavity on each side within the snout (Fig. 19B). However, this cavity is partially subdivided by a dorsally projecting maxillary ridge that housed the tooth roots (Coombs, 1971). Since the cross-section (produced by a natural break) passes through that portion <strong>of</strong> the snout in which the antorbital cavity is lodged in other ornithischians, the possibility remains that the maxillary ridge is a septum between nasal cavity proper and antorbital cavity, the snout having broken transversely at the position <strong>of</strong> the ostium (which would be dorsal to the septum). The point is that nodosaurids may indeed have paranasal cavities similar to, although simpler than, those in ankylosaurids. Dinosauria: Ceratopsia-There are three major groups <strong>of</strong> ceratopsian ornithischians, all presenting different conformations <strong>of</strong> the snout. In the basal ceratopsian Psittacosaurus spp. (Sereno, 1986), the external antorbital fenestra is probably absent. Earlier reports <strong>of</strong> the presence <strong>of</strong> a homologue <strong>of</strong> the maxillary antorbital fossa (Sereno, 1987; Sereno and Chao, 1988; Sereno et a]., 1988) were later corrected (Sereno, 1990). Most specimens <strong>of</strong> Psittacosaurus spp. have an unossified gap between the lacrimal and premaxilla opening into a cavity in which the nasolacrimal canal opens (Sereno, 1987, 1990; Sereno et al., 1988). It is possible that this gap represents the remnants <strong>of</strong> an external antorbital fenestra, with the expansive caudodorsal lamina <strong>of</strong> the premaxilla restricting the size <strong>of</strong> the opening as it overlaps the lacrimal. The course <strong>of</strong> the nasolacrimal duct (Sereno, 1987; Sereno et al., 1988) tends to support this assessment. In juveniles <strong>of</strong> Psittacosaurus mongoliensis (AMNH 6535, 6536; see also Coombs, 1982; Sereno, 1987), this gap (perhaps best viewed as a fontanelle) is somewhat larger, although preservational artifact may increase its apparent size. Arguing against this notion is the observation <strong>of</strong> a specimen <strong>of</strong> this species in which the gap is fully within the lacrimal bone (Sereno, 1987), which would be unlike an external antorbital fenestra. Thus, it is probably judicious to follow Sereno (1990) in regarding the external antorbital fenestra as absent. The snouts <strong>of</strong> basal neoceratopsians ("protoceratopsians") are well known from many specimens (Brown and Schlaikjer, 1940; Maryanska and Osm6lska, 1975; Osmolska, 1986). In Protoceratops andrewsi (many AMNH specimens), Bagaceratops rozhdestvenskyi (Maryanska and Osmolska, 1975), and Leptoceratops gracilis (Sternberg, 195 l), there is a small internal antorbital fenestra between the maxilla, lacrimal, and usually the jugal (Fig. 20A, B). The internal fenestra is relatively larger in young individuals (e.g., Protoceratops andrewsi, AMNH 6421). This aperture clearly communicates solely with the nasal cavity directly opposite the caudal portion <strong>of</strong> the choana (Osmolska, 1986). In most specimens, there is a wellmarked antorbital fossa excavated into the maxilla, lacrimal, jugal, and, in Bagaceratops rozhdestvenskyi, the nasal. In most specimens <strong>of</strong> Protoceratops andrewsi, the antorbital cavity undercuts the margins <strong>of</strong> the external antorbital fenestra so that there are shallow lacrimal and jugal recesses and much deeper maxillary recesses; in many larger skulls (AMNH 6433, 6429, 6414, 6466) the rostral portion <strong>of</strong> the fossa is not deeply excavated into the surface (Fig. 20A, B). In B. rozhdestvenskyi and P. andrewsi, an accessory cavity is present within the body <strong>of</strong> the maxilla (Maryanska and Osmblska, 1975; Osmolska, 1986; see below). The course <strong>of</strong> the nasolacrimal duct is unknown, and in fact, despite excellent material <strong>of</strong> Protoceratops andrewsi (AMNH 6429), no evidence <strong>of</strong> the nasolacrimal canal could be dis<strong>cover</strong>ed. Sternberg (1951) also could not locate a canal in Leptoceratops gracilis. It should be noted that Bagaceratops rozhdestvenskyi has an additional opening in the snout rostral to the antorbital cavity, between maxilla and premaxilla (Maryanska and Osmblska, 1975); this structure remains poorly understood, but it appears to have little to do with the antorbital cavity. In Ceratopsidae, the specified osteological correlates are easily interpreted, although the antorbital cavity is apparently very small. The internal antorbital fenestra is usually little more than an oval foramen situated between maxilla and lacrimal with varying contribution from the nasal and jugal (Dodson and Currie, 1990). It opens medially into the nasal cavity opposite the caudal portion <strong>of</strong> the choana; the palatine sends a lateral process to form the caudomedial border <strong>of</strong> the internal fenestra (Hatcher et al., 1907). Although there is sometimes a small maxillary
WITMERANTORBITAL CAVITY OF ARCHOSAURS 3 1 cav nas S1 c cav nas FIGURE 19. Transverse sections <strong>of</strong> snouts <strong>of</strong> Ankylosauria. A, Euoplocephalus tutus (AMNH 5403). rostral view <strong>of</strong> caudal part <strong>of</strong> skull. B, Edmontonia longiceps (AMNH 3076), caudal view <strong>of</strong> rostral part <strong>of</strong> skull. (Drawings after Coornbs, 1978.)
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