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270 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
steeply, i.e., at a great angle to the jugal bar, and so produces a
small retractory effect. It may, however, include (e.g., in Rhea)
a very inclined portion, the so-called intramandibularis muscle
(Figure 5B).
In some birds another inclined portion of this muscle has
evolved. The so-called caput absconditum (Hofer, 1950) apparently
is a derivative of the main part of M. pseudotemporalis
superficialis that is situated in the posterior temporal fossa of a
typically diapsid ancestor (Figure 5A; Dzerzhinsky and Yudin,
1979). It is found in Sphenisciformes, Procellariiformes (Figure
5B), and Pelecaniformes but never in recent paleognathous
birds.
3. M. adductor mandibulae extemus (Figure 6) acting on the
upper jaw via the pterygoid muscle is a very important retractor
in most birds (e.g., cranes). In paleognaths, however, it shows a
rather modest development and, except Apteryx, does not
spread up over the temporal wall of the braincase. Thus, its origin
is limited to the zygomatic process of the squamosal. This
restriction might have resulted from a reduction of the main ancestral
origin of the muscle, the upper jugal arch. Otherwise, it
might be a result of the change of functional requirements in
the muscle during the course of development of the long bill
and the cranial kinesis.
I presume that the immediate ancestors of birds had an akinetic
skull that possessed some prerequisites of cranial kinesis,
such as a loose basipterygoid articulation (Yudin, 1970). It
seems likely that kinetic mobility appeared first in the most
Pss
loaded zone, i.e., within the slender upper jaw (Figure 1), and
thus resulted in an archaic rhynchokinesis (Yudin, 1970, 1978).
One of the questions about the functional morphology of the
avian skull is the influence of sharp strokes, such as are associated
with pecking or with accidental strokes against hard substrates
while gathering grain or catching small, agile prey. In
tinamous, the loose articulation of the frontal bone with the adjacent
parietal and laterosphenoid (Figure 7) is equivalent to
the so-called "articulating frontoparietal joint" described by
Houde (1981) in early Tertiary North American carinates. It
does not allow significant rotary movements of any cranial
part, so in my opinion it is not associated with ancient mesokinetic
mobility. Rather it is for damping shocks received along
the dorsal bill stalk while pecking.
The ventral stalk of the upper jaw was initially compliant,
and it had to be supported by some solid framework able to
transfer to the braincase large, but not dangerous, forces. This
framework is formed by the bony palate, and among recent paleognaths
it is strongest in Apteryx (Figure 8A), doubtless due
to its specialization for probing.
In tinamous, Rhea (Figure 8B), and, apparently, recent Casuariiformes,
the main trajectory of compression stresses mns
from the palatine process of the premaxillary bone to the
vomer, then to the pterygoid, the quadrate, and finally via the
quadrate's otic process to the braincase (Dzerzhinsky, 1983).
In ostriches (Figure 8C), where the palatal processes of the premaxillary
are missing and the vomer is partly reduced, com-
Psp aim aps
FIGURE 5.—Comparison of the pseudotemporalis muscle in lateral view: A, lizard, Cyclura nubilis Gray; B, procellariiform
bird, Northern Fulmar, Fulmarus glacialis (Linnaeus) (mandible and side wall of braincase partly
destroyed and removed). aca=aponeurotic insertion lobe of the caput absconditum of the pseudotemporalis
superficialis muscle; Aex=M. adductor mandibulae extemus; aim= aponeurotical lobe of origin of the M. intramandibularis;
apm=aponeurosis of insertion of the pseudotemporalis profundus muscle; aps=aponeurosis of
insertion of the pseudotemporalis superficialis muscle; Ca=caput absconditum of the pseudotemporalis superficialis
muscle; Im=intramandibularis muscle, part of the pseudotemporalis superficialis muscle;
Psp=pseudotemporalis profundus muscle; Pss=pseudotemporalis superficialis muscle; *=especially inclined
part of the pseudotemporalis muscle. (A, after Iordansky, 1990; B, after Dzerzhinsky and Yudin, 1979.)
aca