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FIGURE 6.—Lateral surface of jaw adductors in lateral view: A, Rhea, Rhea
americana; B, White-naped Crane, Grus vipio. Aex=M. adductor mandibulae
extemus; Ap=M. adductor mandibulae posterior; Dm=depressor mandibulae
muscle; Pss=pseudotem-poralis superficialis muscle; Pvl=ventrolateral portion
of the pterygoid muscle. (B, after Kuular and Dzerzhinsky, 1994.)
pression stresses mn from the bill tip through the premaxillary
and maxillary bones to the palatine and then almost directly to
the apex of the basipterygoid process. In the roof of the mouth,
the ostrich possesses a broad gap that is closed only by skin.
This patch of skin gains a gliding support from the parasphenoidal
rostmm via a long, thin, anterior extension of the vomer.
Source of the Medial Mandibular Process
The processus mandibulae medialis is highly specific for
Aves. For example, in Gobipteryx, a fossil Mongolian bird, Elzanowski
(1974) regarded this process as a distinctly avian
character. The functional properties of the muscular portion
(ventromedial portion of the pterygoid muscle) inserting on its
tip are influenced significantly by the particular position of the
tip. It is placed extremely high in the sagittal plane, so corresponding
muscular forces pass almost through the pivot of the
quadrato-mandibular joint (Figure 9A) and therefore apply a
negligible adductory component to the mandible as compared
to the retractory one. In the frontal plane, the tip of the process
is extremely close to the midline, and therefore those muscular
forces tend to rotate the caudal part of mandibular branch and
so expand the lower jaw as a whole (Figure 9B; Yudin, 1961).
The functional conditions discussed above, however, do not
seem to account for the first steps in the evolution of the medial
mandibular process. There is a peculiarity in the paleognath
jaw musculature that is more useful in this respect: the abovementioned
M. retractor palatini. I suggest that this muscle may
have arisen by a joining of two muscular units—the ventromedial
portion of the pterygoid muscle and the depressor mandibulae
muscle. Thus, their primitive interconnection via the caudal
portion of the mandible formed a two-link chain that
foreshadowed the recent M. retractor palatini (Figure 10). The
cmcial event in their further evolution has been an optimization
of their ability to exert a single force, which has been ensured
by alignment of both muscular links, due to the displacement
Ls Fr
FIGURE 7.—Lateral view of tinamou skull (Tinamiformes), showing the loose articulation of the frontal with
adjacent bones. Fr=frontal; Ls=laterosphenoid; Pa=parietal.