Memoir cover 0.tif - Ohio University College of Osteopathic Medicine
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IVITMER-ANTORBITAL<br />
Pseudhesperosuchus jachaleri) diverted the primary choana<br />
caudally. Because <strong>of</strong> the fundamental morphogenetic relationship<br />
between the primary choana and antorbital sinus (discussed<br />
above; see also Witmer, 1995b), the entire system shifted<br />
caudally where, encroaching on the orbit and its contents, there<br />
simply was less space available for the antorbital cavity; in<br />
other words, it was constrained by "packing" phenomena. It<br />
turns out, however, that formation <strong>of</strong> a nasopharyngeal duct in<br />
mesoeucrocodilians seems to have had virtually no effect on<br />
the subsequent reduction or enclosure <strong>of</strong> the antorbital cavity,<br />
because forms such as Notosuchus terrestris, Uruguaysuchus<br />
aznarezi, and Araripesuchus gomesii retain an antorbital cavity<br />
similar to that <strong>of</strong> protosuchians. This situation probably results<br />
because the development <strong>of</strong> a nasopharyngeal duct involves the<br />
presence and position <strong>of</strong> the secondary choana, not the primary<br />
choana (Witmer, 1995b); the primary choana (i.e., the rostra1<br />
end <strong>of</strong> the duct) had not moved relative to the antorbital cavity.<br />
In fact, all this is further evidence affirming the causal relationship<br />
<strong>of</strong> primary choana and antorbital sinus.<br />
So, if the initial reduction <strong>of</strong> the antorbital cavity perhaps has<br />
its causal basis in the evolution <strong>of</strong> palatal processes <strong>of</strong> the maxilla,<br />
what factors are involved in the ultimate reduction <strong>of</strong> the<br />
cavity? As discussed earlier, extreme reduction and closure <strong>of</strong><br />
the external antorbital fenestra occurs multiple times in Crocodylomorpha:<br />
once or twice in Thalattosuchia, at least once or<br />
twice among basal metasuchians, and at least once or twice in<br />
Neosuchia. Concomitant internalization <strong>of</strong> the antorbital cavity<br />
occurred clearly at least twice: at least once in Thalattosuchia<br />
and at least once in Neosuchia. In most <strong>of</strong> these cases, closure<br />
<strong>of</strong> the external fenestra can be shown to be associated with<br />
apomorphic flattening <strong>of</strong> the snout. Skull flattening, the other<br />
imuortant concurrent trend mentioned above, has been well<br />
documented by Langston (1973) and especially Busbey (1995).<br />
The biomechanical consequences <strong>of</strong> dorsoventral flattening<br />
are considerable. Flattening the snout (Fig. 38B) moves it away<br />
from the design optimum <strong>of</strong> a cylinder (Fig. 38A), potentially<br />
making it less competent to resist sagittal bending and torsional<br />
loads (Witmer, 1992b). Busbey's (1995) elegant functional analysis<br />
<strong>of</strong> the trend from oreinirostral (tall-snouted) sphenosuchians<br />
and protosuchians to platyrostral (flat-snouted) neosuchians<br />
confirmed these mechanical sequelae and suggested mechanisms<br />
to resist these stresses. In particular, Busbey (1995) noted<br />
that platyrostral taxa show increased cross-sectional area <strong>of</strong><br />
bone through (1) thickening <strong>of</strong> the bones and (2) the development<br />
<strong>of</strong> a secondary palate (interestingly, even an incomplete<br />
secondary palate has biomechanical benefits). Furthermore,<br />
Busbey (1995) was correct in noting that platyrostral skulls<br />
loaded in sagittal bending exhibit stress concentrations at the<br />
caudal end <strong>of</strong> the snout, just in front <strong>of</strong> the orbits. These stress<br />
concentrations are in precisely the position <strong>of</strong> the external antorbital<br />
fenestrae. Therefore, an external fenestra severely decreases<br />
the ability <strong>of</strong> the snout to resist these torsional and<br />
especially sagittal loads, because such a gap or discontinuity<br />
would produce a so-called "open section" (Fig. 38B). In fact,<br />
it would result in the coincidence <strong>of</strong> an open section and a stress<br />
concentration-a potentially catastrophic design. Therefore, I<br />
would suggest (see also Witmer, 1992b) that the ultimate closure<br />
<strong>of</strong> the external fenestra is causally linked to platyrostry as<br />
another mechanism to increase cross-sectional area <strong>of</strong> bone.<br />
Thus, in light <strong>of</strong> the epithelial hypothesis for the function <strong>of</strong><br />
pneumaticity, the evolutionary trends in the snouts <strong>of</strong> crocodylomorphs<br />
support the claim <strong>of</strong> a "struggle" between the conflicting<br />
tendencies <strong>of</strong> pneumatization and maintenance <strong>of</strong> adequate<br />
strength. The caudal shift <strong>of</strong> the primary choana resulting<br />
from the development <strong>of</strong> maxillary palatal processes pushed the<br />
whole pneumatic system caudally and constricted it as it competed<br />
for space with the orbital contents. Nevertheless, the cavity<br />
remained tolerably large in the oreinirostral metasuchians<br />
CAVITY OF ARCHOSAURS<br />
A primitive condition:<br />
cylindrical snout<br />
fen<br />
antorb<br />
ext<br />
"dangerous" derived condition:<br />
dorsoventrally flattened snout<br />
retaining external fenestra<br />
agitta<br />
bending<br />
open cross section<br />
produced by fenestra<br />
fen antorb ext<br />
FIGURE 38. Biomechanical implications <strong>of</strong> platyrostry (flattening <strong>of</strong><br />
the snout). A, the primitive condition (manifested by sphenosuchians,<br />
protosuchians, many basal metasuchians) is to have a much taller, more<br />
cylindrical snout. B, the dorsoventral flattening observed in some clades<br />
(most notably neosuchians) makes the snout particularly susceptible to<br />
failure under sagittal bending andlor torsion. Furthermore, the stress<br />
concentrations resulting from platyrostry are in precisely the locations<br />
<strong>of</strong> the external antorbital fenestra, which would produce an open cross<br />
section further weakening the snout. Ultimate closure <strong>of</strong> the external<br />
antorbital fenestra and internalization <strong>of</strong> the antorbital cavity was probably<br />
largely a biomechanical consequence <strong>of</strong> platyrostry.<br />
such as Araripesuchus gomesii. However, the flattening <strong>of</strong> the<br />
snout produced a biomechanical weak spot at exactly the location<br />
<strong>of</strong> the external antorbital fenestra. Thus closure <strong>of</strong> the<br />
external fenestra and internalization <strong>of</strong> the paranasal air sac was<br />
a biomechanical solution to the problem (and nasal rotation was<br />
probably the morphogenetic mechanism accomplishing this closure;<br />
Witmer, 1995b). It would seem that in crocodylomorphs,<br />
the paranasal air sinus "loses" in the metaphorical struggle, and<br />
this is probably a fair conclusion. But, interestingly, in the<br />
broad- but flat-snouted alligatorines, pneumaticity has rebounded,<br />
and the snout is a multi-chambered maze with stout bony<br />
struts at biomechanically predictable locations.<br />
Ornithopoda-Turning to ornithischian dinosaurs, the focus<br />
will be Omithopoda, but most major clades <strong>of</strong> ornithischians