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WITMERANTORBITAL CAVITY OF ARCHOSAURS
Sinraptor
Dilophosaurus
Albertosaurus
Deinonychus
Coelurosauria
Tetanurae
I
Theropoda
/
FIGURE 40. Facial trends in Theropoda. In the course of theropod evolution, the antorbital cavity expands with the development of diverse
bony accessory cavities (shaded areas) to house the subsidiary diverticula. The bewildering array of pneumatic accessory cavities in neotetanurans
is good evidence for the expansive nature of pneumatic diverticula. Skull drawings modified from Russell (1970), Barsbold (1983), Welles (1984),
Paul (1988a,b), Witmer (1990), and Currie and Zhao (1994a).
morphs, ornithopods present another example in which the biomechanical
requirements for adequate cross-sectional area of
bone (in this instance coupled to the rigors of repetitive masticatory
bite loadings) apparently prevail over the tendency for
pneumatic expansion.
Theropoda-Whereas crocodylomorphs and ornithopods
both manifest trends for reduction of the antorbital cavity, theropod
dinosaurs show the opposite trend: expansion of the antorbital
paranasal air sinus and formation of bony accessory
cavities to house these subsidiary diverticula. At its earliest appearance,
the antorbital cavity of theropods was very extensive
and almost always is the most conspicuous aspect of facial
structure. The accessory cavities of theropods received fairly
extensive treatment in a previous section, so detailed discussion
is not required here. The trend is quite simple (Fig. 40): in basal
theropods such as Eoraptor lunensis, Herrerasaurus ischigualastensis,
Dilophosaurus wetherilli, or Coelophysis bauri,
there are very few or no pneumatic accessory cavities, whereas
there is both a much greater diversity and frequency of pneumatic
recesses in more derived theropods (certainly at and
above the level of Neotheropoda). The trend is carried to its
extreme in Oviraptor philoceratops in which virtually all of the
facial elements are highly pneumatic. It is difficult to identify
specific concurrent trends in other anatomical systems in the-
ropods. In fact, it is even difficult to present an orderly pattern
of acquisition of the accessory cavities (Witmer, 199%). Consider,
for example, the pneumatic recesses in the nasal bone:
They are present in Sinraptor dongi and Allosaurusfragilis, but
not in any tyrannosaurid. Oviraptor philoceratops has them, but
ornithomimosaurs lack them; they are present in Deinonychus
antirrhopus, but Velociraptor mongoliensis lacks them, etc. Although
the example seems whimsical, other similar patternless
instances could be cited.
In fact, this almost haphazard pattern of highly homoplastic
pneumatic characters is compelling evidence for the epithelial
hypothesis of pneumatic function in that these subsidiary diverticula
of the antorbital sinus appear to be expanding in a
very invasive and opportunistic manner. Numerous examples of
"swollen" or "inflated" pneumatic bones can be cited, such as
the palatines of large tyrannosaurids and the lacrimals and vestibular
bullae of many theropods. Given the phylogenetic distribution
of these recesses, pneumaticity seems to be a fairly
poorly constrained system in theropods. Nevertheless, the main
structural members (e.g., the ventral ramus of the lacrimal, the
ascending ramus of the maxilla, etc.) never appear to be compromised.
In fact, in Tyrannosaurus rex, an apomorphically
massive form capable of generating enormous bite forces, many
of the pneumatic apertures have become apomorphically re-