Memoir cover 0.tif - Ohio University College of Osteopathic Medicine
Memoir cover 0.tif - Ohio University College of Osteopathic Medicine
Memoir cover 0.tif - Ohio University College of Osteopathic Medicine
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
24 SOCIETY OF VERTEBRATE PALEONTOLOGY. MEMOIR 3<br />
air-filled epithelial diverticulum <strong>of</strong> the nasal cavity. Although a<br />
number <strong>of</strong> earlier workers regarded some <strong>of</strong> the various cavities<br />
surrounding the antorbital fenestra, principally in theropods, as<br />
being pneumatic (e.g., Gilmore, 1920; Stovall and Langston,<br />
1950; Osm6lska, 1976; Barsbold, 1983; Molnar, 1973, 1985),<br />
Osm6lska (1985) was the first to suggest that the antorbital<br />
cavity<br />
.<br />
<strong>of</strong> probably all archosaurs was associated with the nasal<br />
-<br />
cavity in general-and perhaps a paranasal air sac in particular.<br />
While investigating the early evolution <strong>of</strong> avian crani<strong>of</strong>acial<br />
pneurnaticity (Witrner, 1990), 1 independently came to similar<br />
conclusions (Witrner. 1987b). Osm6lskn's (1985) formulation<br />
was intimately invoked with the biological'role bf the air sac<br />
itself and the ecology and physiology <strong>of</strong> fossil archosaurs; as<br />
indicated earlier, these matters are secondary to elucidating the<br />
function (i.e., s<strong>of</strong>t-tissue relations) <strong>of</strong> the cavity, but, <strong>of</strong> course,<br />
are necessary for interpreting its paleobiological significance.<br />
In some respects, the concept <strong>of</strong> a pneumatic antorbital cavity<br />
is frustrating in that the function <strong>of</strong> crani<strong>of</strong>acial air sacs in general<br />
are obscure (e.g., Negus, 1958; Blanton and Biggs, 1969;<br />
Blaney, 1990; see below). However, in other respects, this is an<br />
advantage in that determination <strong>of</strong> the function <strong>of</strong> the bony<br />
cavity does not become muddled with the perceived "utility"<br />
<strong>of</strong> the enclosed structure, which has been a failing <strong>of</strong> both the<br />
glandular and muscular hypotheses. Issues relating to the function<br />
<strong>of</strong> an antorbital air sac are addressed in a later section.<br />
The Extant Phylogenetic Bracket<br />
Because air sacs are unusual organ systems, their basic properties<br />
will be briefly outlined here (see also Witmer, 1990,<br />
1995b, and references therein). Paranasal air sinuses in general<br />
are air-filled epithelial outgrowths or diverticula <strong>of</strong> the nasal<br />
cavity that evaginate beyond the cartilaginous nasal capsule,<br />
always maintaining communication with the external environment<br />
via the naris. They should not be confused with the various<br />
intracapsular recesses or cavities, as only the extracapsular<br />
air sacs are competent to pneumatize bone (Witmer, 1995b).<br />
Not all air sacs pneumatize bone; some insinuate themselves<br />
among other s<strong>of</strong>t tissues or even reach a subcutaneous position<br />
(Bignon, 1889; Witmer, 1990).<br />
Birds and crocodilians have highly pneumatic skulls. Birds<br />
have only one major diverticulum <strong>of</strong> the nasal cavity proper,<br />
the antorbital sinus (Witmer, 1990), whereas crocodilians have<br />
five types <strong>of</strong> diverticula <strong>of</strong> the nasal cavity proper and several<br />
more diverticula <strong>of</strong> the nasopharyngeal duct (Wegner, 1958;<br />
Witmer, 1987b, 1995b). Based on a variety <strong>of</strong> criteria derived<br />
from study <strong>of</strong> extant amniotes, there exists strong evidence that<br />
one <strong>of</strong> the crocodilian paranasal sinuses, the caviconchal sinus,<br />
is homologous to the antorbital sinus <strong>of</strong> birds. Detailed comparison<br />
<strong>of</strong> all the diverticula and the justification for the homology<br />
is presented elsewhere (Witmer, 1995b).<br />
Extant Birds-The avian antorbital sinus evaginates the nasal<br />
cavity toward the caudoventral margin <strong>of</strong> the cartilaginous<br />
nasal capsule, directly opposite the lacrimal bone and near the<br />
caudal margin <strong>of</strong> the choana. The proximal portion <strong>of</strong> the sinus<br />
diverticulum is closely associated with the major branch <strong>of</strong> the<br />
maxillary nerve in this region (i.e., the nasopalatine nerve), and<br />
the air sac tends to partially surround the nerve. The sinus expands<br />
into the area lateral to the nasal capsule, filling the available<br />
space within the antorbital cavity (Fig. 6B). Within the<br />
antorbital cavity, the sinus lies directly adjacent to the maxilla,<br />
lacrimal, palatine, and usually also the jugal and nasal. In most<br />
cases, diverticula <strong>of</strong> the antorbital sinus pneumatize many <strong>of</strong><br />
these bones, producing characteristic pneumatic foramina or<br />
fossae collectively termed "accessory cavities" (Witmer, 1994).<br />
Extant birds are variable with respect to these features, but what<br />
remains constant is the presence <strong>of</strong> a large air sac within the<br />
antorbital cavity lodged principally in the caudal portion <strong>of</strong> the<br />
maxilla and rostral to the lacrimal. In addition to the diverticula<br />
into the facial bones, the antorbital sinus has another diverticulum,<br />
the suborbital diverticulum (Witmer, 1990; Fig. 6B). This<br />
diverticulum extends <strong>of</strong>ten far caudally beyond the postnasal<br />
fenestra, interleaving between the pterygoideus and external adductor<br />
muscles, curving ventrally around the eyeball, and <strong>of</strong>ten<br />
reaching the region <strong>of</strong> the trigeminal foramen (see Bignon,<br />
1889).<br />
Laterally, the antorbital sinus extends to the margins <strong>of</strong> the<br />
external antorbital fenestra (except dorsally where the nasolacrimal<br />
duct passes) and, in many cases, excavates an antorbital<br />
fossa, especially on the maxilla (Fig. 15A). The presence <strong>of</strong><br />
antorbital fossae appears to be a size- and age-related attribute,<br />
with larger and older individuals tending to have more deeply<br />
excavated fossae. The external antorbital fenestra and antorbital<br />
sinus are <strong>cover</strong>ed with skin laterally. The nasolacrimal duct<br />
passes lateral to (or, in most ratites, partially through) the lacrimal<br />
bone and then through the dorsal portion <strong>of</strong> the internal<br />
antorbital fenestra, after which it turns sharply ventromedially<br />
to open into the nasal cavity proper toward the rostral end <strong>of</strong><br />
the choana (Figs. 6B, 15A). Thus, the nasolacrimal duct passes<br />
dorsomedial to the antorbital sinus and is among the contents<br />
<strong>of</strong> the antorbital cavity. As mentioned in the previous section,<br />
the dorsal pterygoideus commonly reaches into the caudoventral<br />
portion <strong>of</strong> the antorbital cavity where it comes into direct<br />
contact with the antorbital sinus.<br />
Extant Crocodilians-The crocodilian caviconchal sinus,<br />
the homolog <strong>of</strong> the avian antorbital sinus, evaginates the nasal<br />
capsule directly opposite the primary choana (the primary choana<br />
is the rostral [nasal] end <strong>of</strong> the nasopharyngeal duct and is<br />
homologous to the choana <strong>of</strong> birds and other archosaurs; Witmer,<br />
1995b), passing dorsally over the palatine to enter the antorbital<br />
cavity. The sinus expands early in ontogeny into the<br />
area <strong>of</strong> the fontanelle between maxilla, lacrimal, nasal, and prefrontal<br />
(fonticulus antorbitalis; Witmer, 1995b), but later is<br />
housed almost completely within the bony caviconchal recess<br />
<strong>of</strong> the maxilla (Fig. 15B). The maxilla usually is extensively<br />
pneumatized by the sinus.<br />
The maxillary nerve is closely associated with the diverticulum<br />
and travels within the bony recess (most alligatorids) or<br />
within a bony canal adjacent to the recess (most crocodylids).<br />
In adult crocodilians. the nasolacrimal duct makes only a glancing<br />
contact with the caviconchal sinus, passing dorsomedially<br />
over the diverticulum before the latter enters the aperture <strong>of</strong> the<br />
caviconchal recess (Figs. 6A, 15B), although earlier in ontogeny<br />
the duct and sinus contact each other more broadly. As<br />
indicated earlier, the caudal portion <strong>of</strong> the antorbital cavity in<br />
crocodilians is dominated by the large dorsal pterygoideus muscle,<br />
restricting the sinus to the rostral, largely intramaxillary<br />
portion <strong>of</strong> the cavity; in other words, the suborbital diverticulum<br />
observed in birds is absent in crocodilians.<br />
Osteological Correlates-The osteological correlates <strong>of</strong> paranasal<br />
pneumaticity in each clade <strong>of</strong> extant archosaurs are very<br />
clear. In both clades, the paranasal air sac leaves the nasal capsule<br />
and passes through a bony aperture (corresponding to the<br />
internal antorbital fenestra) situated directly lateral to the primary<br />
choana, passing laterally dorsal to the palatine bone and<br />
expanding to occupy a large, recessed cavity within the caudal<br />
portion <strong>of</strong> the maxilla (Fig. 15). In both clades, the paranasal<br />
sinus pneumatizes the maxilla via additional diverticula, resulting<br />
in pneumatic foramina and bony accessory cavities opening<br />
into the main sinus chamber. The nasolacrimal duct passes dorsomedially<br />
around the sinus, producing canals and/or grooves<br />
in the adjacent bones; in adult crocodilians, the duct is partly<br />
enclosed in bone such that it makes only slight contact with the<br />
diverticulum, whereas, in birds, the duct generally broadly contacts<br />
the sinus.<br />
In birds, the sinus fills the antorbital cavity and extends to