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SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
FIGURE 30. Albertosaurus libratus. A, stereophotographs of left maxilla of RTMP 83.35.100 in lateral view. B, same in medial view. C,
Interpretive drawing of A. Arrow points to the promaxillary fenestra concealed behind the ascending ramus. D, interpretive drawing of B. Arrow
passes through the caudal fenestra in the maxillary antrum, between the postantral and interfenestral struts. E, drawing of 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 of the antorbital cavity. An isolated maxilla of
T. pusillus (BMNH 48260) has ample space for an internalized
antorbital cavity between the lateral wall of 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 of 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 of 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 of 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 of the major clades of archosaurs,
it is possible to infer the presence of an epithelial air sac
within the antorbital cavities of 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 of an
epithelial air sac. In other words, all of the osteological correlates
of the air sinus that were observed in extant birds and
crocodilians (the extant phylogenetic bracket) were observed in
virtually all clades of 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 of 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 of the external antorbital
fenestra is a derived condition. Although a large antorbital
cavity and a prominent external fenestra remain important features
of Archosauria as a whole, the external antorbital fenestra
independently closes in at least ten different clades: a number
of crocodylomorphs clades, a few birds (e.g., some owls), Stegosauridae,
Ankylosauria, Hadrosauridae, most Pachycephalosauria,
Psittacosauridae, some Ceratopsidae. In almost all of
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,