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Society of Vertebrate Paleontology Memoir 3
Journal of Vertebrate Paleontology
Volume 17, Supplement to Number 1
O 1997 by the Society of Vertebrate Paleontology
THE EVOLUTION OF THE ANTORBITAL CAVITY OF ARCHOSAURS: A
STUDY IN SOFT-TISSUE RECONSTRUCTION IN THE FOSSIL RECORD
WITH AN ANALYSIS OF THE FUNCTION OF PNEUMATICITY
LAWRENCE M. WITMER
Department of Biological Sciences and College of Osteopathic Medicine, Ohio University, Athens, Ohio 45701
ABSTRACT-The most commonly cited apomorphy of Archosauriformes is an opening in the snout known as the
antorbital cavity. Despite the ubiquity and prominence of the antorbital cavity, its function and importance in craniofacial
evolution have been problematic. Discovering the significance of the antorbital cavity is a two step process:
first, establishing the function of the bony cavity (that is, its soft-tissue relations), and second, determining the biological
role of the enclosed structure. The first step is the most fundamental, and hence is examined at length. Three hypotheses
for the function of the antorbital cavity have been advanced, suggesting that it housed (1) a gland, (2) a muscle, or (3)
a paranasal air sinus. Thus, resolution is correctly viewed as a "soft-tissue problem," and is addressed within the
context of the extant phylogenetic bracket (EPB) approach for reconstructing the unpreserved features of fossil organisms.
The soft-anatomical relations of the antorbital cavity (or any bony structure) are important because (1) soft tissues
generally have morphogenetic primacy over bony tissues and (2) inferences about soft tissues are the foundation for a
cascading suite of paleobiological inferences. The EPB approach uses the shared causal associations between soft
tissues and their osteological correlates (i.e., the signatures imparted to the bones by the soft tissues) that are observed
in the extant outgroups of the fossil taxon of interest to infer the soft-anatomical attributes of the fossil; based on the
assessment at the outgroup node, a hierarchy characterizing the strength of the inference can be constructed. This
general approach is applied to the problem of the function of the antorbital cavity, taking each hypothesized soft-tissue
candidate-gland, muscle, and air sac-in turn, (1) establishing the osteological correlates of each soft-tissue system
in the EPB of any fossil archosaur (i.e., extant birds and crocodilians), (2) formulating a hypothesis of homology based
on similarities in these causal associations between birds and crocodilians, (3) testing this hypothesis by surveying
fossil archosaurs for the specified osteological correlates, and (4) accepting or rejecting the hypothesis based on its
phylogenetic congruence. Using this approach, fossil archosaurs can be reliably reconstructed with a Glandula nasalis,
M. pterygoideus, pars dorsalis, and Sinus antorbitalis that are homologous with those of extant archosaurs; however,
the osteological correlates of only the antorbital paranasal air sinus involve the several structures associated with the
antorbital cavity. Additional evidence for the pneumatic nature of the antorbital cavity comes from the presence of
numerous accessory cavities (especially in theropod dinosaurs) surrounding the main antorbital cavity. To address the
origin of the antorbital cavity, the EPB approach was applied to basal archosauriforms; the data are not as robust, but
nevertheless suggest that the cavity appeared as a housing for a paranasal air sinus. The second step in discovering the
evolutionary significance of the antorbital cavity is to assess the function of the enclosed paranasal air sac. In fact, the
function of all pneumaticity is investigated here. Rather than the enclosed volume of air (i.e., the empty space) being
functionally important, better explanations result by focusing on the pneumatic epithelial diverticulum itself. It is
proposed here that the function of the epithelial air sac is simply to pneumatize bone in an opportunistic manner within
the constraints of a particular biomechanical loading regime. Trends in facial evolution in three clades of archosaurs
(crocodylomorphs, ornithopod dinosaurs, and theropod dinosaurs) were examined in light of this new perspective.
Crocodylomorphs and ornithopods both show trends for reduction and enclosure of the antorbital cavity (but for
different reasons), whereas theropods show a trend for relatively poorly constrained expansion. These findings are
consistent with the view of air sacs as opportunistic pneumatizing machines, with weight reduction and design optimality
as secondary effects.
INTRODUCTION
With more than 10,000 species, archosaurs are the most diverse
group of terrestrial vertebrates living today and have been
so since almost the beginning of the Mesozoic Era when they
radiated into virtually all habitats. Correlated with this high taxic
diversity is a remarkable morphological diversity in skull
form. A prominent aspect of skull anatomy in archosaurs is a
usually large opening and space in the side of the snout called
the antorbital fenestra and cavity, respectively. The antorbital
cavity is such a characteristic feature of archosaurs that for
many decades it has been the major diagnostic feature of the
group. In fact, this feature stands as a synapomorphy of a slightly
more inclusive group, Archosauriformes (Gauthier et al.,
1988). Any attempt to understand the evolution of the skulls of
archosaurs therefore must take the antorbital cavity into account.
It is thus somewhat surprising, given both the ubiquity
and prominence of the antorbital cavity, that the cavity has re-
mained a functional enigma. This paper asks the simple question,
what is the antorbital cavity for? That is, what is the function
of the bony antorbital cavity? "Function" is a term with
many biological meanings and has been used at many different
levels of organization. In the present context, "function" relates
to the fundamental mechanical and physical relationships of
soft tissue to bone, and, as such, its usage conforms fairly closely
to the definitions advanced by Bock and von Wahlert (1965)
and Lauder (1995). Thus, to phrase it another way, this paper
asks, what are the soft-tissue relations of the bony antorbital
cavity? The logical next question is, what is the function (or
even biological role) of the enclosed soft-tissue structure, and
what does this determination tell us about facial evolution in
the group?
Previous efforts to answer these questions have been hampered
by treatment of Archosauria as a paraphyletic assemblage
excluding birds and leaving crocodilians as the only extant archosaurs.
Thus, since extant crocodilians lack an external an-