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WITMERANTORBITAL CAVITY OF ARCHOSAURS 45
antrum and fenestra are innovations of Tetanurae or Neotetanurae,
but new discoveries could easily change this conclusion.
Clearly there has been a fair amount of homoplasy, especially
among the earlier lineages; this variation becomes, in effect,
sorted out in Coelurosauria in which the maxillary recesses are
fairly consistent. Despite the phylogenetic ambiguities, the
point emerging from this discussion is that the complicated and
variable nature of the maxillary recesses of theropods are indeed
compelling morphological evidence for the presence of
paranasal air sinuses.
Paul's (1988a) compromise solution to the function of the
antorbital cavity is pertinent at this point. He suggested that the
dorsal pterygoideus muscle attached to the margin of the internal
antorbital fenestra and that the surrounding antorbital fossa
lodged paranasal air sinuses; the epithelial diverticulum would
have traversed the maxillary fenestra to pass from the nasal
cavity to the antorbital cavity. Notwithstanding the problems
detailed earlier regarding the muscular hypothesis, Paul's hypothesis
fails in that it cannot apply to all archosaurs or even
to all theropods because maxillary fenestrae are limited perhaps
to just a single clade, thus leaving the fossae of other archosaurs
unkxplained. Furthermore, with regard to theropods, it reverses
the direction of ~neumatization. Although - the maxillarv sinuses
of some theropods (e.g., Allosaurus fragilis) have medial apertures,
the following taxa can be shown to have no internal
fenestrae, demonstrating that the subsidiary diverticula must
have entered the accessory cavities (passing through the promaxillary
and/or maxillary fenestrae) from the external side of
the maxilla, not the internal side: PiatnitzkysaurusJoresi (Bonaparte,
1986), Megalosaurus hesperis (BMNH R332), Afrovenator
abakensis (UC OBA I), Ornitholestes hermanni
(AMNH 619), Marshosaurus bicentesimus (UUVP 4695,
1846), and perhaps Deinonychus antirrhopus (YPM 5232).
Thus, Paul's (1988a) compromise is not applicable even for
these theropods.
The lacrimal recess is another accessory cavity associated
with the antorbital cavity. In many theropods (especially large
forms), the lacrimal has a large recess expanding within the
caudodorsal portion (body) of the bone that opens rostrolaterally
into the antorbital cavity (Fig. 29A). In most cases, the
opening of the cavity is a single, rather broad aperture located
in the rostroventrolateral surface of the body of the bone, and
is continuous with the smooth surfaces of the lacrimal antorbital
fossa. Examples include Ceratosaurus nasicornis (USNM
4733, Afrovenator abakensis (UC OBA 1; Sereno et al., 1994),
Allosaurus fragilis (UUVP 2133, YPM-PU 14554, Fig. 29A),
and Yangchuanosaurus shangyuensis (Dong et al., 1983),
among others. In Tyrannosaurus rex (AMNH 5027, CM 9401,
others), this single aperture is restricted to a relatively small,
more discrete foramen. In a number of taxa, additional openings
may be present rostral to the main aperture: e.g., Acrocanthosaurus
atokensis (Stovall and Langston, 1950), Giganotosaurus
carolinii (Coria and Salgado, 1995), some individuals of Allosaurus
fragilis (e.g., UUVP 5814, BYU 5125), Sinraptor dongi
(Cume and Zhao, 1994a), and Daspletosaurus torosus (CMN
8506). A number of taxa lack any lateral apertures into the body
of the lacrimal bone, although they may retain moderately deep
lacrimal antorbital fossae; examples include Coelophysis bauri
(CM 31374; Fig. 14), Dilophosaurus wetherilli (UCMP 77270),
abelisaurids (Novas, 1992), Torvosaurus tanneri (Britt, 1991),
Monolophosaurus jiangi (Zhao and Cume, 1994), Troodon formosus
(RTMP 82.19.23; Cume, 1985; Witmer, 1990), Dromiceiomimus
brevitertius (CMN 12228), Utahraptor ostrommaysi
(Kirkland et al., 1993), Archaeopteryx lithographica (BMNH
37001; Fig. 16B), and Erlikosaurus andrewsi (PST 100/111;
see also Clark et al., 1994). Given the absence of such openings
in most small maniraptorans it is probably significant that Deinonychus
antirrhopus (YPM 5232; MOR 747; see also Witmer
and Maxwell, 1996) shows the more-or-less typical condition
of more basal tetanurans, a moderately large aperture; furthermore,
as in some of the other theropods noted above, there is
an additional, smaller foramen just rostral to the main opening.
Sereno et al. (1994, 1996) listed "lacrimal pneumatic excavation"
as a synapomorphy of their Tetanurae, and this seems
reasonable although the lacrimal recess of Ceratosaurus nasicornis
is well within tetanuran variation (Witmer, 1995~).
Holtz's (1994) analysis definitely showed a lot of homoplasy in
this feature, and, in fact, he probably significantly underestimated
the amount of homoplasy (e.g., the discovery of lacrimal
pneumatic recesses in D. antirrhopus noted above; see also Witmer,
1995~).
These lacrimal pneumatic apertures lead into cavities of variable
size, such that the body and cornual process (if present, as
in Ceratosaurus nasicornis and Allosaurus fragilis) is hollow.
The internal architecture of the lacrimal recess can be assessed
for only a few taxa. For example, in one specimen of A. fragilis
(UUVP 2133), the lacrimal recess has three cavities partly subdivided
by internal ridges; the recess does not extend far into
either the rostral or ventral rami. Molnar (1991) also identified
three cavities within the lacrimal of Tyrannosaurus rex, and the
recess extends far into the rostral ramus in one specimen (CM
9401). In Albertosaurus sarcophagus (CMN 5601, RTMP
86.114.1; see also Carr, 1996), the cavity within the rostral ramus
opens laterally within the antorbital cavity via a foramen.
The ventral ramus of the lacrimal usually has no evident cavities,
although it appears to be hollow in one tyrannosaurid specimen
(RTMP 83.30. I).
Jugal recesses are found in a number of theropods, and, in
most cases, take the form of a slit-like or occasionally round
foramen at the caudoventral apex of the jugal antorbital fossa.
Such a foramen is found in all tyrannosaurids and has been
regarded as a synapomorphy of that group (Bakker et al., 1988;
Molnar et al., 1990; Molnar, 1991). The distribution of this
feature, however, is more complicated. It is fairly certain that
ceratosaurians and at least some basal tetanurans (Bakker et al.,
1992) lack jugal recesses. Sereno et al. (1994, 1996) regarded
a "jugal pneumatic excavation" as a synapomorphy of Tetanurae,
and deep jugal antorbital fossae with pneumatic foramina
similar to tyrannosaurids are found in Monolophosaurus jiangi
(Zhao and Currie, 1994), Afrovenator abakensis (UC OBA 1;
Sereno et al., 1994), Sinraptor dongi (Curie and Zhao, 1994a),
some individuals of Allosaurus fragilis (Currie and Zhao,
1994a), Acrocanthosaurus atokensis (Stovall and Langston,
1950), and Carcharodontosaurus saharicus (Sereno et al.,
1996). Most coelurosaurs (other than tyrannosaurids), however,
appear to lack jugal recesses; this seems to be the case for
Omitholestes hermanni (AMNH 619), oviraptorosaurs, troodontids,
and ornithomimosaurs, although additional preparation
(and the proper search image) could change this assessment.
For example, further preparation of specimens of Deinonychus
antirrhopus (YPM 5210, 5232; MOR 747; Maxwell and Witmer,
1996) reveals a well developed jugal pneumatic recess.
The internal structure of the jugal recess is known for few taxa,
but, in Sinraptor dongi (Currie and Zhao, 1994a) and Tyrannosaurus
rex (Molnar, 1973, 1991), the pneumatic foramen
leads into a series of cavities within the m&illary and postorbital
rarni of the jugal.
Nasal recesses, foramina, and cavities within the nasal bones
associated with the nasal antorbital fossa, are not common.
They are absent in ceratosaurians, although Gilmore (1920:82)
identified "a number of pneumatic cavities" in the nasal of
Ceratosaurus nasicornis. Allosaurus fragilis (UUVP 3839,
BYU 5124, USNM 4734, YPM-PU 14554; Fig. 29A), Sinraptor
dongi (Cume and Zhao, 1994a), Yangchuanosaurus shangyouensis
(Dong et al., 1983), and Carcharodontosaurus saharicus
(Sereno et al., 1996) exhibit one to three foramina lead-