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WITMER-ANTORBITAL CAVITY OF ARCHOSAURS 5 3
nlax
antorb
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ten
suborb
FIGURE 36. Chasmatosaurus vanhoepeni. A, drawing of snout in left
lateral view. B, same in ventral view. (Modified from Cruickshank,
1972.)
torbital fossa in proterosuchids and basal erythrosuchids, but a
well-developed fossa excavates a cavity within usually both the
maxilla and lacrimal in more derived erythrosuchids (Parrish,
1992), Euparkeria capensis (Ewer, 1965), and the proterochampsids
Chanaresuchus bonapartei (MCZ 4309), Gualosuchus
reigi (Romer, 1971), and Cerritosaurus binsfeldi (Price,
1946) (Sereno and Arcucci, 1990; Sereno, 1991b). In fact, the
general structure of the antorbital cavity differs very little between
these basal archosauriforms and many archosaurs. Thus,
the morphological evidence is probably sufficiently "compelling"
to infer the presence of an antorbital air sinus in basal
archosauriforms, although again this inference (a level I1 inference)
requires more speculation than that for Archosauria (a
level I inference).
The Origin of the Antorbital
Fenestra and Cavity
Given the assumption of an antorbital air sac in basal archosauriforms,
what is the impact of the pneumatic hypothesis on
the origin of the antorbital fenestra and cavity? Based on the
discussion above, it is almost certain that the function of the
bony antorbital cavity at its inception was to house an epithelial
paranasal air sinus. If the ontogeny of birds is a reliable guide
(Witmer, 1995b), the external antorbital fenestra of the ancestral
archosauriform species formed as an embryonic fontanelle that
did not close, not a hole that opened within contiguous bones
during ontogeny. The air sac may have been (and remains today)
an important functional matrix controlling structure in this
region of the face, physically intervening between the growing
facial bones and preventing their apposition (see Moss and
Young [I9601 for air sinuses as functional matrices). The bones
comprising the antorbital cavity and external fenestra thus
formed around this pneumatic functional matrix (Witmer,
1995b). In extant akhosaurs, accessory pneumatic cavities
within the facial bones develop subsequent to hatching (i.e.,
after the air sac becomes aerated) via pneumatically induced
processes of resorption (Witmer, 1995b). Thus, if air sinuses
are indeed functional matrices, we have a causal basis for explaining
the origin of the antorbital fenestra, fossa, and cavity
of archosauriforms. In other words, the focus shifts from skeletal
to soft-tissue concerns.
The origin of the epithelial diverticulum itself (i.e., the antorbital
sinus) remains problematic and is perhaps intractable.
Relative to their outgroups (e.g., Prolacertiformes, Trilophosaurus,
Choristodera, Rhynchosauria; Gauthier et al., 1988; Evans,
1988; Benton and Clark, 1988), archosauriforms are not drastically
divergent in facial structure, and thus their evolution of
a paranasal sinus is not easily comprehensible. At least two
possibilities exist for the origin of the antorbital air sac. First,
perhaps a novel genetic program for evaginating air sacs
evolved. This scenario may suggest an initial adaptive value to
the air sac, but not necessarily so. Second, perhaps archosauriforms
evolved some unique conformation of the cartilaginous
nasal capsule, choana, and neurovasculature that permitted
evagination of an epithelial diverticulum. Such an evolutionary
change could have originated simply as a heterochronic modification
of the pattern of tissue interactions. For example, Brylski
and Hall (1988) showed that the novel evolution of external,
fur-lined cheek pouches in geomyoid rodents results from a
simple change in developmental timing of the interaction between
oral and dermal tissues. Thus, according to the second
scenario the air sac may be selectively neutral, being a by-
-product of other (perhaps adaptive, perhaps not) aspects of
facial conformation (i.e., it is a non-aptation sensu Gould and
Vrba [1982]). The mysterious function of paranasal pneumaticity
perhaps offers some support to this notion, such that the
antorbital sinus may have originated for "no particular reason"
and was subsequently co-opted for specific roles (e.g., its pneumatizing
capabilities, a flotation device; see the next section).
THE FUNCTION OF PNEUMATIC SINUSES: A NEW
PERSPECTIVE AND ITS IMPACT ON ANALYSIS OF
SOME TRENDS IN FACIAL EVOLUTION
It has been emphasized throughout this study that elucidation
of the function of the antorbital cavity must precede discussion
of the function of the structure enclosed within the cavity. The
previous sections have focused on the former issue, concluding
that the antorbital cavity housed a paranasal air sinus. We now
are in a position to ask the question, what is the function of
this antorbital air sac? As will be seen, the functions and biological
roles of any of the pneumatic systems of amniotes-not
just the paranasal system but also the paratympanic, and pulmonary
systems-remain controversial. This section explores
some of the general issues surrounding the functions of pneumatic
diverticula, examining previously published ideas, and
then presenting a novel alternative hypothesis. Predictions of
this new hypothesis are tested by analyzing the transformation
of the antorbital cavity in the course of morphological trends
in facial structure in selected clades.
The Enigmatic Biological Role of Pneumaticity
Of all of the pneumatic systems, the paranasal system has
received the most attention, probably because the high incidence
of sinusitis and "sinus headache" in humans has made
the system clinically very important. Despite this attention and
interest, the basic function of paranasal pneumaticity continues
to be obscure. The topic has been reviewed on various occa-