SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3 FIGURE 30. Albertosaurus libratus. A, stereophotographs <strong>of</strong> left maxilla <strong>of</strong> RTMP 83.35.100 in lateral view. B, same in medial view. C, Interpretive drawing <strong>of</strong> A. Arrow points to the promaxillary fenestra concealed behind the ascending ramus. D, interpretive drawing <strong>of</strong> B. Arrow passes through the caudal fenestra in the maxillary antrum, between the postantral and interfenestral struts. E, drawing <strong>of</strong> the same specimen in oblique rostrodorsomedial view. Arrow at right passes from the antorbital cavity through the promaxillary fenestra into the promaxillary recess. Arrow at left passes through the caudal fenestra in the maxillary antrum. Curved arrows point into the interalveolar pneumatic recesses. internalization <strong>of</strong> the antorbital cavity. An isolated maxilla <strong>of</strong> T. pusillus (BMNH 48260) has ample space for an internalized antorbital cavity between the lateral wall <strong>of</strong> the nasal cavity and the tooth-bearing portion. A maxillary groove was not observed in any more derived neosuchians, and its absence may correlate with complete closure <strong>of</strong> the external antorbital fenestra. Goniopholidids have an unusual feature aptly termed the "maxillary depression" by Buffetaut (1982, 1986). In Goniopholis sirnus (BMNH 41098), this depression is completely within the maxilla, just dorsal to the tooth row, and, although somewhat subdivided by two low septae, does not open medially. Buffetaut (1982) identified a small foramen between the lacrimal and maxilla in this specimen and in Eutretauranosuchus delfsi that he regarded as the antorbital fenestra. Although he suggested that the foramen was associated with the maxillary depression, I could not confirm the presence <strong>of</strong> this foramen in BMNH 41098, and it is not clear how such a foramen would relate to the depression. In any case, the maxillary depression is an unusual feature <strong>of</strong> problematic function. All higher neosuchians (i.e., Dyrosauridae, Bernissartia fagesii, Eusuchia) have completely closed the external antorbital fenestra (Norell and Clark, 1990). Given their similarity to extant eusuchians, the extinct higher neosuchians presumably internalized the antorbital cavity in a similar manner, although appropriate material was not available to confirm this for all taxa. Conclusions Having examined in detail all <strong>of</strong> the major clades <strong>of</strong> archosaurs, it is possible to infer the presence <strong>of</strong> an epithelial air sac within the antorbital cavities <strong>of</strong> all Archosauria with very little speculation (a level I inference). Whereas previous hypotheses failed to implicate the antorbital fenestrae and fossa in housing a gland or jaw muscle, these bony structures are, in fact, inti- mately (almost certainly causally) related to the presence <strong>of</strong> an epithelial air sac. In other words, all <strong>of</strong> the osteological correlates <strong>of</strong> the air sinus that were observed in extant birds and crocodilians (the extant phylogenetic bracket) were observed in virtually all clades <strong>of</strong> extinct archosaurs. For example, the internal antorbital fenestra is the bony aperture through which the epithelial diverticulum passes. The antorbital fossae are pneumatic fossae excavated by the air sac. The external antorbital fenestra develops as a fontanelle around the paranasal air sac. Thus the original hypothesis survives the congruence test, and the avian antorbital sinus is indeed homologous to the caviconchal sinus <strong>of</strong> crocodilians. Therefore, given that the osteological correlates are indeed causally associated with the air sac, all extinct archosaurs possessed a homologous paranasal air sinus, unless it was apomorphically lost. The term developed for birds for the epithelial diverticulum-the antorbital sinus (Witmer, 1987b, 1990)-may be applied to all archosaurs, reserving the designation "antorbital cavity" for the bony space (Witmer, 1994). Given the above conclusions, closure <strong>of</strong> the external antorbital fenestra is a derived condition. Although a large antorbital cavity and a prominent external fenestra remain important features <strong>of</strong> Archosauria as a whole, the external antorbital fenestra independently closes in at least ten different clades: a number <strong>of</strong> crocodylomorphs clades, a few birds (e.g., some owls), Stegosauridae, Ankylosauria, Hadrosauridae, most Pachycephalosauria, Psittacosauridae, some Ceratopsidae. In almost all <strong>of</strong> these taxa, there is evidence that the antorbital cavity is still present, just internalized. ACCESSORY CAVITIES AS COMPELLING MORPHOLOGICAL EVIDENCE FOR PARANASAL PNEUMATICITY Accessory cavities are recesses located in the bones surrounding the antorbital cavity (e.g., lacrimal, maxilla, nasal,
WITMER-ANTORBITAL CAVITY OF ARCHOSAURS FIGURE 30. (Connnued)