Memoir cover 0.tif - Ohio University College of Osteopathic Medicine

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