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42 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
palatine). In extant forms, they are produced by pneumatization
and house subsidiary epithelial diverticula of the antorbital paranasal
air sinus (Witmer, 1994, 1995b). Therefore, accessory
cavities are the osteological correlates of the subsidiary diverticula.
In most cases, the cavities are smooth-walled, strutted
recesses and may be multi-chambered. Sometimes they communicate
with the antorbital cavity via a relatively small pneumatic
foramen, other times through a very broad aperture (Wegner,
1958; Witmer, 1990, 1995b). Although both clades of extant
archosaurs (birds and crocodilians) have such subsidiary
diverticula and accessory cavities, none could be hypothesized
to be homologous (i.e., they failed the both the similarity and
congruence tests). Thus, the presence of accessory cavities was
not posited as an ancestral feature of Archosauria, and was not
listed among the osteological correlates of paranasal pneumaticity
in the previous section. Indeed, few accessory cavities
characterize more than a small clade. Nevertheless, such spaces
are present in many fossil archosaurs, and, making use of an
argument for "compelling morphological evidence" (see discussion
of the EPB approach above and in Witmer, 1995a),
strongly corroborate the pneumatic nature of the antorbital cavity
although requiring a level I1 or level I11 inference. These
accessory cavities are discussed in detail below, following
roughly the same systematic order as above.
The Diversity of Accessory Cavities
Theropoda-The greatest diversity of accessory cavities occurs
within Theropoda. As a result, these are given a fairly
thorough (but not exhaustive) treatment. The complicated morphology
of these bony sinuses requires the creation of a number
of new anatomical terms (Witmer, 1994 and in preparation).
The following discussion is intended as an initial characterization
of the anatomical diversity. Further study will likely expand
the known variation, and the interpretation of the accessory
cavities may require revision, especially as theropod phylogeny
becomes better resolved.
The maxillary recesses (or sinuses) are among the most common
accessory cavities in theropods below the level of ornithurine
birds. They (Fig. 29) tend to be separable into three
named cavities: (1) a promaxillary recess rostrally, (2) a maxillary
antrum caudally, and (3) an excavatio pneumatica within
the ascending ramus of the maxilla. All three systems communicate
laterally with the space associated with the maxillary
antorbital fossa: the promaxillary recess via the promaxillary
fenestra, the maxillary antrum via the maxillary fenestra, and
the excavatio pneumatica via a usually broad fossa. The promaxillary
and maxillary fenestrae are the "accessory" or "subsidiary
antorbital fenestrae" of previous authors (e.g., Osborn,
1912; Ostrom, 1969). The plan of this section is first to discuss
the three named cavities in taxa (in general, neotetanurans) for
which their identification is less problematic. More basal theropods
(i.e., Ceratosauria, including abelisauroids) will be taken
up after this basic pattern is established, because they present
a problem in that they have just a single fenestra or no fenestra
at all. The problem rests on the identification of the single fenestra
in ceratosaurians: Is it a maxillary fenestra or a promaxillary
fenestra?
The promaxillary fenestra (Carpenter, 1992) is located just
rostra1 to the maxillary fenestra, the two being separated by the
pila promaxillaris. In lateral view, the promaxillary fenestra
usually forms a slit-like aperture just caudal to the rim of the
external antorbital fenestra within the antorbital fossa. Examples
of this pattern are common, including Ornitholestes hermanni
(AMNH 619), Marshosaurus bicentesimus (Fig. 29C;
UUVP 4695, 1846; referral of these specimens to this taxon is
uncertain but will be employed for purposes of discussion),
Deinonychus antirrhopus (YPM 5232, MCZ 8791), Velocirap-
tor mongoliensis (AMNH 6515, cast of PIN 3143/8), Archaeopteryx
lithographica (cast of Eichstatt specimen; Fig. 16B),
Sinornithoides youngi (Russell and Dong, 1994), and Saurornithoides
junior (Barsbold, 1974). Ornithomimosaurs also probably
fall in this class, but here the promaxillary fenestra is situated
somewhat more ventrally (e.g., Struthiomimus altus,
ROM 1790; Dromiceiomimus samueli, ROM 841; Parks, 1928;
Barsbold and Osm6lska, 1990). Although the promaxillary fenestra
is usually smaller than the maxillary fenestra, the reverse
seems to be the case in the sinraptorids Yangchuanosaurus spp.
(Dong et al., 1983) and Sinraptor dongi (Cume and Zhao,
1994a). My interpretation of the situation in the latter conflicts
somewhat with that of Currie and Zhao (1994a: fig. 4). According
to my scheme, their "maxillary fenestra" is actually
the large promaxillary fenestra and their "pneumatic opening
8" is the small maxillary fenestra. Dromaeosaurus albertensis
(AMNH 5356; see also Currie, 1995) also appears to have a
relatively large, round promaxillary fenestra, but its size relative
to the maxillary fenestra is unknown.
In some taxa, the promaxillary fenestra is not visible in lateral
view, but rather is tucked up rostromedially, concealed by
the lamina lateralis of the ascending ramus of the maxilla; in
these cases, the presence of the promaxillary fenestra is easily
overlooked (e.g., Paul [1988b] regarded it as absent in Proceratosaurus
bradleyi). Examples of this concealed promaxillary
fenestra are P. bradleyi (BMNH R4860), Allosaurus fragilis
(UUVP 5427; Fig. 29A, B), Tyrannosaurus rex (UCMP
118742), Albertosaurus libratus (RTMP 83.35.100, AMNH
5664; Cam, 1996; Fig. 30), Troodon formosus (CMN 12392;
see also Currie, 1985), and perhaps Afrovenator abakensis (UC
OBA 1; see also Sereno et al., 1994). A concealed promaxillary
fenestra may well characterize other taxa but remains unrecognized
or even unprepared.
Owing to their delicate construction, the internal structure of
any of the accessory cavities is known for very few species
(see Madsen [1976b] for Allosaurus fragilis). Where known,
the promaxillary fenestra leads into a single or, more often, a
series of interconnected cavities within the ascending ramus of
the maxilla (Figs. 29, 30; see Molnar [I9911 for tyrannosaurids).
In some cases, it can be shown that the rostralmost cavity
of the promaxillary fenestra inflates that portion of the maxilla
flooring the naris, forming a sort of thin-walled bulla within the
nasal vestibule. A bulla vestibularis can be observed in, for
example, Proceratosaurus bradleyi (BMNH R4860), A. fragilis
(UUVP 5427, BYU 5126, YPM-PU 14554, Fig. 29), probably
Sinraptor dongi (Cume and Zhao, 1994a), Albertosaurus libratus
(RTMP 83.35.100, Fig. 30), Marshosaurus bicentesimus
(UUVP 1846, Fig. 29), Deinonychus antirrhopus (YPM 5232),
cf. Velociraptor mongoliensis (IGM 100/972), and Troodon formosus
(CMN 12392). The vestibular bulla does not open into
the naris (with perhaps one or two equivocal exceptions). The
promaxillary recesses and vestibular bullae of Afrovenator
abakensis (UC OBA I), M. bicentesimus (UUVP 4695, 1846;
Fig. 29C, D), as well as probably D. antirrhopus (YPM 5232)
and Ornitholestes hermanni (AMNH 619) lack dorsal or medial
openings, but such openings are present in A. fragilis (Fig.
29B), apparently Sinraptor dongi (Currie and Zhao, 1994a), and
at least some tyrannosaurids. It should be reiterated, however,
that the presence or absence of such openings is often problematical
given that the delicate bone of the maxillary recesses is
often damaged.
The maxillary fenestra (Gauthier, 1986) is much more consistent
in morphology than the promaxillary fenestra, usually
being a large, more-or-less round opening in the lamina medialis
of the maxillary ascending ramus. Again, it appears to be
relatively small in sinraptorids and also Afrovenator abakensis
(UC OBA 1; Sereno et al., 1994). It is separated from the promaxillary
fenestra by the promaxillary strut (pila promaxillaris)