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24 SOCIETY OF VERTEBRATE PALEONTOLOGY. MEMOIR 3
air-filled epithelial diverticulum of the nasal cavity. Although a
number of earlier workers regarded some of the various cavities
surrounding the antorbital fenestra, principally in theropods, as
being pneumatic (e.g., Gilmore, 1920; Stovall and Langston,
1950; Osm6lska, 1976; Barsbold, 1983; Molnar, 1973, 1985),
Osm6lska (1985) was the first to suggest that the antorbital
cavity
.
of probably all archosaurs was associated with the nasal
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cavity in general-and perhaps a paranasal air sac in particular.
While investigating the early evolution of avian craniofacial
pneurnaticity (Witrner, 1990), 1 independently came to similar
conclusions (Witrner. 1987b). Osm6lskn's (1985) formulation
was intimately invoked with the biological'role bf the air sac
itself and the ecology and physiology of fossil archosaurs; as
indicated earlier, these matters are secondary to elucidating the
function (i.e., soft-tissue relations) of the cavity, but, of course,
are necessary for interpreting its paleobiological significance.
In some respects, the concept of a pneumatic antorbital cavity
is frustrating in that the function of craniofacial air sacs in general
are obscure (e.g., Negus, 1958; Blanton and Biggs, 1969;
Blaney, 1990; see below). However, in other respects, this is an
advantage in that determination of the function of the bony
cavity does not become muddled with the perceived "utility"
of the enclosed structure, which has been a failing of both the
glandular and muscular hypotheses. Issues relating to the function
of an antorbital air sac are addressed in a later section.
The Extant Phylogenetic Bracket
Because air sacs are unusual organ systems, their basic properties
will be briefly outlined here (see also Witmer, 1990,
1995b, and references therein). Paranasal air sinuses in general
are air-filled epithelial outgrowths or diverticula of the nasal
cavity that evaginate beyond the cartilaginous nasal capsule,
always maintaining communication with the external environment
via the naris. They should not be confused with the various
intracapsular recesses or cavities, as only the extracapsular
air sacs are competent to pneumatize bone (Witmer, 1995b).
Not all air sacs pneumatize bone; some insinuate themselves
among other soft tissues or even reach a subcutaneous position
(Bignon, 1889; Witmer, 1990).
Birds and crocodilians have highly pneumatic skulls. Birds
have only one major diverticulum of the nasal cavity proper,
the antorbital sinus (Witmer, 1990), whereas crocodilians have
five types of diverticula of the nasal cavity proper and several
more diverticula of the nasopharyngeal duct (Wegner, 1958;
Witmer, 1987b, 1995b). Based on a variety of criteria derived
from study of extant amniotes, there exists strong evidence that
one of the crocodilian paranasal sinuses, the caviconchal sinus,
is homologous to the antorbital sinus of birds. Detailed comparison
of all the diverticula and the justification for the homology
is presented elsewhere (Witmer, 1995b).
Extant Birds-The avian antorbital sinus evaginates the nasal
cavity toward the caudoventral margin of the cartilaginous
nasal capsule, directly opposite the lacrimal bone and near the
caudal margin of the choana. The proximal portion of the sinus
diverticulum is closely associated with the major branch of the
maxillary nerve in this region (i.e., the nasopalatine nerve), and
the air sac tends to partially surround the nerve. The sinus expands
into the area lateral to the nasal capsule, filling the available
space within the antorbital cavity (Fig. 6B). Within the
antorbital cavity, the sinus lies directly adjacent to the maxilla,
lacrimal, palatine, and usually also the jugal and nasal. In most
cases, diverticula of the antorbital sinus pneumatize many of
these bones, producing characteristic pneumatic foramina or
fossae collectively termed "accessory cavities" (Witmer, 1994).
Extant birds are variable with respect to these features, but what
remains constant is the presence of a large air sac within the
antorbital cavity lodged principally in the caudal portion of the
maxilla and rostral to the lacrimal. In addition to the diverticula
into the facial bones, the antorbital sinus has another diverticulum,
the suborbital diverticulum (Witmer, 1990; Fig. 6B). This
diverticulum extends often far caudally beyond the postnasal
fenestra, interleaving between the pterygoideus and external adductor
muscles, curving ventrally around the eyeball, and often
reaching the region of the trigeminal foramen (see Bignon,
1889).
Laterally, the antorbital sinus extends to the margins of the
external antorbital fenestra (except dorsally where the nasolacrimal
duct passes) and, in many cases, excavates an antorbital
fossa, especially on the maxilla (Fig. 15A). The presence of
antorbital fossae appears to be a size- and age-related attribute,
with larger and older individuals tending to have more deeply
excavated fossae. The external antorbital fenestra and antorbital
sinus are covered with skin laterally. The nasolacrimal duct
passes lateral to (or, in most ratites, partially through) the lacrimal
bone and then through the dorsal portion of the internal
antorbital fenestra, after which it turns sharply ventromedially
to open into the nasal cavity proper toward the rostral end of
the choana (Figs. 6B, 15A). Thus, the nasolacrimal duct passes
dorsomedial to the antorbital sinus and is among the contents
of the antorbital cavity. As mentioned in the previous section,
the dorsal pterygoideus commonly reaches into the caudoventral
portion of the antorbital cavity where it comes into direct
contact with the antorbital sinus.
Extant Crocodilians-The crocodilian caviconchal sinus,
the homolog of the avian antorbital sinus, evaginates the nasal
capsule directly opposite the primary choana (the primary choana
is the rostral [nasal] end of the nasopharyngeal duct and is
homologous to the choana of birds and other archosaurs; Witmer,
1995b), passing dorsally over the palatine to enter the antorbital
cavity. The sinus expands early in ontogeny into the
area of the fontanelle between maxilla, lacrimal, nasal, and prefrontal
(fonticulus antorbitalis; Witmer, 1995b), but later is
housed almost completely within the bony caviconchal recess
of the maxilla (Fig. 15B). The maxilla usually is extensively
pneumatized by the sinus.
The maxillary nerve is closely associated with the diverticulum
and travels within the bony recess (most alligatorids) or
within a bony canal adjacent to the recess (most crocodylids).
In adult crocodilians. the nasolacrimal duct makes only a glancing
contact with the caviconchal sinus, passing dorsomedially
over the diverticulum before the latter enters the aperture of the
caviconchal recess (Figs. 6A, 15B), although earlier in ontogeny
the duct and sinus contact each other more broadly. As
indicated earlier, the caudal portion of the antorbital cavity in
crocodilians is dominated by the large dorsal pterygoideus muscle,
restricting the sinus to the rostral, largely intramaxillary
portion of the cavity; in other words, the suborbital diverticulum
observed in birds is absent in crocodilians.
Osteological Correlates-The osteological correlates of paranasal
pneumaticity in each clade of extant archosaurs are very
clear. In both clades, the paranasal air sac leaves the nasal capsule
and passes through a bony aperture (corresponding to the
internal antorbital fenestra) situated directly lateral to the primary
choana, passing laterally dorsal to the palatine bone and
expanding to occupy a large, recessed cavity within the caudal
portion of the maxilla (Fig. 15). In both clades, the paranasal
sinus pneumatizes the maxilla via additional diverticula, resulting
in pneumatic foramina and bony accessory cavities opening
into the main sinus chamber. The nasolacrimal duct passes dorsomedially
around the sinus, producing canals and/or grooves
in the adjacent bones; in adult crocodilians, the duct is partly
enclosed in bone such that it makes only slight contact with the
diverticulum, whereas, in birds, the duct generally broadly contacts
the sinus.
In birds, the sinus fills the antorbital cavity and extends to