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
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
WZTMER-ANTORBITAL CAVITY OF ARCHOSAURS 59<br />
yon et al., 1982; Lanyon, 1987; Thomason, 1995; and references<br />
therein). Once again, the transduction mechanism by<br />
which functional strains are converted into adaptive remodeling<br />
is enigmatic, but the phenomenon is real.<br />
The final corollary <strong>of</strong> the epithelial hypothesis is that there<br />
is a "struggle" between the conflicting tendencies <strong>of</strong> pneumatization<br />
and bone deposition. Five quite different examples will<br />
be presented here to support this claim. (1) The mere fact that<br />
pneumatic cavities are usually supported by bony struts and<br />
buttresses tends to support this notion <strong>of</strong> a compromise between<br />
pneumatization and biomechanical demands. Furthermore, the<br />
position <strong>of</strong> the struts are non-random and <strong>of</strong>ten correspond to<br />
the locations <strong>of</strong> high stress. For example, in Alligator mississippiensis,<br />
the caviconchal and postvestibular sinuses pneumatize<br />
most <strong>of</strong> the snout (see Witmer, 1995b), but the two sinuses<br />
usually remain separated by a bony strut (Fig. 15B) that is situated<br />
directly opposite the largest maxillary tooth, which is<br />
presumably a site <strong>of</strong> high bite loads. (2) An interesting example<br />
<strong>of</strong> these conflicting tendencies may be seen in cases where an<br />
epithelial air sac is closed <strong>of</strong>f from the main air-filled chamber.<br />
For example, chronic otitis media in humans leading to obstruction<br />
<strong>of</strong> the auditory tube results in the paratympanic air sacs<br />
losing communication with the pharynx; in the absence <strong>of</strong> a<br />
patent sinus ostium, new bone formation increases and fills the<br />
cavity (Tos et al., 1984). The same findings (i.e., new bone<br />
filling in a pneumatic cavity upon closure <strong>of</strong> its ostium) result<br />
from experimental studies <strong>of</strong> auditory tuba1 occlusion in rats<br />
(Kuijpers et al, 1979) and pneumatic foramen closure in chicken<br />
humeri (Ojala, 1957). Likewise, clinical studies <strong>of</strong> humans<br />
(e.g., Proctor and Naclerio, 1996; Batsakis and El-Naggar,<br />
1996) have repeatedly shown that a normally functioning mucosa<br />
and patent sinus ostium are necessary for normal sinus<br />
growth. Although these situations represent pathological cases,<br />
they show that nonpathological epithelial air sacs are competent<br />
to maintain the cavity and keep new bone formation at bay. (3)<br />
As discussed above, the skull ro<strong>of</strong>s <strong>of</strong> most birds are pneumatized<br />
by paratympanic diverticula and produced into a multistoried<br />
lattice. Chapin (1949:691) noted, however, that the<br />
skull ro<strong>of</strong> <strong>of</strong> woodpeckers (Picidae) "is composed <strong>of</strong> a single<br />
layer <strong>of</strong> bone, thicker and stiffer than that" <strong>of</strong> most other birds.<br />
He attributed this (p. 691) to "direct adaptation to their hammering<br />
and the use <strong>of</strong> the beak as a chisel." This reasoning<br />
seems sound. In a sense, the mechanical rigors <strong>of</strong> repeated axial<br />
loading <strong>of</strong> the skull have apomorphically tipped the balance<br />
toward bone deposition and away from pneumatization (nonpicid<br />
piciforms retain the pneumatic skull ro<strong>of</strong>s). (4) In another<br />
avian example, Strasser (1877), Miiller (1908), and subsequent<br />
authors noted that the position <strong>of</strong> the pneumatic foramina on<br />
avian postcranial long bones is non-random, and the pulmonary<br />
diverticula pneumatize the bones only at locations under relatively<br />
little biomechanical stress. (5) Finally, Sicher (1952) and<br />
DuBrul (1988) noted that in elderly humans that have lost their<br />
teeth, the paranasal sinuses greatly expand, crossing sutural<br />
boundaries into adjacent bones, and reducing the facial bones<br />
to thin shells. Having lost their teeth, the skulls <strong>of</strong> these individuals<br />
are no longer subjected to the repetitive masticatory<br />
stresses that require substantial cross-sectional area <strong>of</strong> bone. As<br />
a result, the sinuses expand in a relatively unconstrained manner.<br />
In summary, the epithelial hypothesis for pneumatic function<br />
is supported by a considerable amount <strong>of</strong> data, and can explain<br />
quite disparate findings. It is a testable hypothesis, as are all <strong>of</strong><br />
its corollary hypotheses. Questions remain about the details <strong>of</strong><br />
the mechanisms <strong>of</strong> pneumatization and bone strain transduction,<br />
but these do not compromise the applicability <strong>of</strong> the hypothesis<br />
or falsify it.<br />
Trends in the Evolution <strong>of</strong> the Facial Skeleton<br />
What does this "new perspective" on the function <strong>of</strong> pneumaticity<br />
tell us about archosaurs? Can it provide any insight<br />
into the observed patterns <strong>of</strong> morphological evolution? What<br />
role, if any, does this antorbital air sac play in the evolution <strong>of</strong><br />
the facial skeleton in various clades <strong>of</strong> archosaurs? This section<br />
will briefly examine trends in three groups <strong>of</strong> archosaurs+rocodylomorphs,<br />
ornithopod ornithischian dinosaurs, and theropod<br />
saurischian dinosaurs. These three clades were selected because<br />
they each show fairly clear evolutionary trends in the<br />
anatomy <strong>of</strong> the antorbital cavity: namely, a reduction in the size<br />
<strong>of</strong> the cavity and a tendency for closure <strong>of</strong> the external antorbital<br />
fenestra in crocodylomorphs and ornithopods; and expansion<br />
<strong>of</strong> the cavity with the development <strong>of</strong> pneumatic accessory<br />
cavities in theropods (Witmer, 1992b). In some cases, there are<br />
concurrent trends in other anatomical systems that impact on<br />
the interpretation <strong>of</strong> the antorbital cavity. The intent is to document<br />
the broad phylogenetic changes in the antorbital cavity<br />
in each clade, compare these changes with modifications in other<br />
anatomical systems, and evaluate these data in light <strong>of</strong> the<br />
epithelial hypothesis for pneumatic function. Thus, examination<br />
<strong>of</strong> these trends is a sort <strong>of</strong> test <strong>of</strong> the epithelial hypothesis, albeit<br />
one <strong>of</strong> plausibility and consistency.<br />
It is understood that "evolutionary trend" is a term and concept<br />
loaded with considerable (and formidable) intellectual baggage<br />
(Nitecki, 1988; Gould, 1988, 1990). Actually, the causal<br />
basis <strong>of</strong> the trends is not the focus here (although it is occasionally<br />
too difficult to resist the temptation <strong>of</strong> causal explanation).<br />
Rather, the patterns themselves are <strong>of</strong> interest for these<br />
allow an evaluation <strong>of</strong> the functional question with fewer assumptions<br />
about the action (or non-action) <strong>of</strong> natural selection.<br />
Strictly speaking, the trends examined here are not <strong>of</strong> the conventional<br />
"change-through-time" variety that dominate the literature<br />
(e.g., papers in McNamara, 1990). They are phylogenetic<br />
trends. The goal simply is have a sense <strong>of</strong> the historical<br />
pattern <strong>of</strong> acquisition <strong>of</strong> the features <strong>of</strong> interest. Detailed resolution<br />
is not sought here, and only a handful <strong>of</strong> taxa will serve<br />
to illustrate each trend. The broad outlines <strong>of</strong> the trends will be<br />
sufficient to examine the role <strong>of</strong> the antorbital cavity and air<br />
sinus in facial evolution.<br />
Crocodylomorpha-The primitive archosaurian condition<br />
for the antorbital cavity, as described earlier, is to have a relatively<br />
large cavity excavating an antorbital fossa on the maxilla<br />
and lacrimal, no pneumatic accessory cavities, and large internal<br />
and external antorbital fenestrae. Extant crocodilians, <strong>of</strong><br />
course, have closed their external antorbital fenestrae, and thus<br />
have markedly diverged from the primitive condition. At the<br />
risk <strong>of</strong> constructing an arbitrary "anagenetic highway" (Gould,<br />
1990:7), the basic outline <strong>of</strong> the trend is as follows (Fig. 37).<br />
Basal crocodylomorphs such as the basal sphenosuchians Terrestrisuchus<br />
gracilis, Saltoposuchus connectens, and Pseudhesperosuchus<br />
jachaleri can be scored as having essentially the<br />
primitive condition. In derived sphenosuchians such as Dibothrosuchus<br />
elaphros, Sphenosuchus acutus, and the Kayenta<br />
sphenosuchian, the antorbital cavity is smaller, the fossa occupies<br />
less <strong>of</strong> the maxilla, and the internal fenestra is more caudally<br />
placed. Protosuchians (basal crocodyliforms) show a similar<br />
reduction in the antorbital cavity and the external antorbital<br />
fenestra. Ignoring thalattosuchians for the moment, basal metasuchians<br />
such as Uruguaysuchus aznarezi and Araripesuchus<br />
gomesii continue the reduction and enclosure <strong>of</strong> the cavity, but<br />
to a relatively minor extent. At the level <strong>of</strong> Neosuchia (e.g.,<br />
Theriosuchus pusillus and Alligator mississippiensis), the external<br />
antorbital fenestra is closed or extremely small and the<br />
antorbital cavity is completely internalized. Thus, the trend is<br />
for the antorbital cavity to become reduced and restricted to the<br />
caudal portion <strong>of</strong> the snout and the external antorbital fenestra