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2 SOCIETY OF VERTEBRATE PALEONTOLOGY, MEMOIR 3
torbital fenestra, determination of the function of the antorbital
cavity becomes problematic. Birds, however, retain an antorbital
cavity and external fenestra that are directly comparable
to those of fossil archosaurs (Witmer, 1987b, 1990, 1995b).
This situation is thus a striking example of the insidious nature
of paraphyletic taxa, because by excluding birds one ignores
the only extant taxon retaining a "typical" antorbital cavity. It
is tempting to wonder how the history of the debate would have
progressed had birds as well as crocodilians been more explicit
parts of the earlier comparisons.
Three hypotheses for the function (or soft-tissue relations) of
the antorbital cavity have been proposed in the literature: (1) it
housed a gland, (2) it housed a muscle, or (3) it housed an air
sac. In all three cases the cavity would serve to "house" a softtissue
structure. Thus, the function of the antorbital cavity is
correctly viewed as a "soft tissue problem." It should be noted
that the function and biological role of the soft tissues enclosed
within the cavity is an issue that can be addressed only after
the function of the antorbital cavity itself is established and is
taken up in the last section. I previously provided a brief review
of the three hypotheses and found the air-sac hypothesis to be
best supported by what meager evidence was available then
(Witmer, 1987b).
The problem is re-investigated here, making use of new studies
of both extant and extinct archosaurs. It also provides the
opportunity to implement (and indeed was the impetus to develop)
a methodology for reconstructing soft tissues in extinct
organisms. This methodology-the extant phylogenetic bracket
approach-is outlined in the next section and is more fully presented
elsewhere (Witmer, 1992a, 1995a). This approach is then
applied specifically to the problem of the antorbital cavity of
archosaurs. Since each of the major hypotheses proposes a different
soft-tissue component within the antorbital cavity, these
components are studied in detail in extant archosaurs to determine
(or at least to hypothesize about) the causal associations
of particular soft tissues and their osteological correlates. In
doing so, this paper draws heavily on a companion paper on
the homology of these and other facial structures among extant
amniotes (Witmer, 1995b). Given these homologous causal associations
and the distribution of the osteological correlates in
fossil archosaurs, it is possible to infer which soft-tissue elements
are associated with the antorbital cavity and which are
not.
Before proceeding, the anatomy will be introduced and some
terms clarified. As much as possible, the terminology will follow
that codified in Nomina Anatomica Avium (Baumel and
Witmer, 1993; see also Witmer, 1994). The most obvious facial
feature is the antorbital fenestra, bounded principally by the
maxilla and lacrimal with varying contributions from the jugal
and nasal. The lateral surfaces of the bones surrounding the
fenestra are often excavated into a depression usually termed
the antorbital fossa (Fig. I). When precision is required, the
name of the bone bearing the depression will be added: e.g.,
maxillary antorbital fossa. Previously (Witmer, 1987b, 1990) I
regarded the fenestra, depression, and adjacent cavity together
as the "antorbital fossa," but here restrict fossae to actual bony
structures. "Antorbital cavity" replaces my previous usage
(Witmer, 1994). The antorbital cavity may be defined simply as
the space rostral to the orbit, external to the cartilaginous nasal
capsule, and internal to the surface of the snout (Witmer,
1995b). It is like the other named craniofacial cavities (e.g.,
orbit, adductor chamber, nasal cavity) in having bony limits and
a variety of soft-tissue contents. It communicates with other
spaces, requiring slight modification of the traditional sense of
"antorbital fenestra." The lateral aperture of the cavity becomes
the external antorbital fenestra (Fig. 1). When there is
an antorbital fossa externally, the peripheral rim of the fossa
marks the external antorbital fenestra. The bony edge internal
to the antorbital fossa is the internal antorbital fenestra and
roughly corresponds to the "antorbital fenestra" as traditionally
conceived (Fig. 1). The internal and external antorbital fenestrae
are difficult to distinguish in archosauriforms having poorly
developed laminae walling the antorbital cavity. For example,
in some archosaurs (such as some derived parasuchians and
most pterosaurs) there really is just a single fenestra.
The rostral limits of the antorbital cavity typically are within
the maxilla, although in parasuchians and some birds it may
extend rostrally into the premaxilla. The bony floor of the cavity
is formed by the palatine bone to a variable extent (Fig. I). The
maxilla also contributes to the floor in those taxa with at least
moderate development of maxillary palatal processes. The size
and caudal extent of the choana constrain maxillary and palatine
involvement in the floor. Medially, the antorbital cavity
tends to open broadly to the nasal cavity. In life, the lateral wall
of the cartilaginous nasal capsule forms the medial wall of the
cavity although there is a gap (i.e., an ostium) joining the nasal
and antorbital cavities in extant archosaurs and, as argued below,
extinct archosaurs as well. The medial laminae of the maxilla
and/or lacrimal are so extensive in some archosaurs (many
ornithischians and some crocodylomorphs) that they almost
completely enclose the cavity medially such that the internal
antorbital fenestra essentially becomes a foramen between nasal
and antorbital cavities (Fig. 1). The antorbital cavity almost
always communicates with the orbit, although the nature of this
communication is seldom described in fossil taxa. This communication
in the bony skull is termed the postnasal fenestra
(Fig. 1; see also Witmer, 1994, 1995b). In extant archosaurs a
variety of structures (e.g., nerves, vessels, musculature, pneumatic
diverticula, ducts of glands) may pass through the postnasal
fenestra, and the fenestra itself is partially occluded by
these structures and portions of the cartilaginous nasal capsule.
SOFT-TISSUE INFERENCE AND THE EXTANT
PHYLOGENETIC BRACKET
As mentioned above, each of the major hypotheses for the
function of the antorbital cavity proposes that some soft-anatomical
component is lodged within the cavity. How do we
approach this problem when fossils hardly ever preserve such
soft tissues? Furthermore, why should we even care? These
questions are addressed at length elsewhere (Witmer, 1992a,
1995a) and discussed briefly in this section. The general importance
of soft-tissue considerations will be examined first,
followed by methodological issues.
The Significance of Soft Tissues
Accurate evolutionary and/or functional interpretations of extinct
vertebrates may not be possible with information from
bones alone. Soft tissues are important for two main reasons.
First, in general, soft tissues often have morphogenetic primacy
over skeletal tissues, such that the existence, position, maintenance,
and form of bones and bony structures-largely are controlled
by particular soft tissues. In effect, bony structures are
products of their epigenetic systems (Smith and Hall, 1990;
Herring, 1993). Thus, evolutionary changes in nonskeletal aspects
of anatomy are transmitted via the epigenetic machinery
to the skeleton. Questions about the direction and cause of evolutionary
change based on inferences from only the skeleton
thus may be posed at the wrong hierarchical level. Moss' (1968)
slogan-soft tissues evolve, bones respond-somewhat overstates
the issue, but it does emphasize the notion that bones
must be studied in concert with their associated soft tissues.
The second reason for the importance of soft tissues stems
directly from the perspective offered by the slogan. Soft tissue
considerations reside at the base of a whole host of paleobiological
inferences. This point is not always appreciated and