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NUMBER 89 307
FIGURE 4.—Wing of the European Starling (Sturnus vulgaris Linnaeus) during
upstroke in frontal view (after Dial et al., 1991) at the downstroke-upstroke
transition (A), midupstroke (B), and upstroke position (c) at
maximum humeral rotation and elevation. At the downstroke-upstroke position,
the humerus is depressed 10° below the horizontal and the hand is
pronated. During upstroke, the humerus rotates 80° on its longitudinal axis
and elevates 55° above the horizontal, and the hand is fully supinated. Scale
bar=l cm.
we examined (see Figure 3), is a bipinnate muscle with relatively
short but numerous fascicles. This architecture is favorable
for high forces and limited excursion. Additionally, the tendon
of insertion inserts circumferentialy on the long axis of the humems,
further contributing to the role of the supracoracoideus
as a humeral rotator. The moment arm of the tendon of insertion
in both species is short; we estimate its maximum to be 2
mm in the starling and 4 mm in the pigeon. Although the mechanical
advantage of the supracoracoideus is low, its high input
force, particularly at the downstroke-upstroke transition, is
favorable for the production of high-velocity movements at the
distal portion of the wing. We predict that during the upstroke,
the distal portion of the wing experiences extremely high rotation
velocity. Although still to be determined, these rotary forces
may act to augment supination at the wrist in addition to supination
provided by the trochlea carpalis-cuneiform complex.
THE FOSSIL EVIDENCE
In view of the evidence for the role of the supracoracoideus
during the wingbeat cycle in modem birds, the obvious question
before us is, when did the supination/humeral rotation action
of the supracoracoideus come into play? Rotation of the
humems by the supracoracoideus is enhanced by the leverage
provided by the tuberculum dorsale (external tuberosity), the
derived site of insertion of the supracoracoideus (Figure 3). Reorientation
of the supracoracoideus to this insertion on the humems
is accomplished by the passage of the tendon through
the triosseal canal and around the acrocoracoid. At what point
in the fossil record can we recognize any of these features?
None of these features have been noted in any of the specimens
of Archaeopteryx (Figure 5). As reported by Sereno and
External
Tuberosity
Head
ARCHAEOPTERYX
Deltopectoral Crest -————^——
Deltopectoral
Internal Tuberosity
Bicipital Crest
CATHARTES
Ectepicondyle
FIGURE 5.—Comparison of the humeri of Archaeopteryx and a modem flying
bird, the Turkey Vulture (Cathartes aura (Linnaeus)), in dorsal aspect. Humeri
are drawn to unit length for easy comparison; each scale bar equals 3 cm
(external tuberosity=tuberculum dorsale). The humerus of Archaeopteryx is
devoid of most of the tubercles and crests that are well developed in most modem
birds. Most of these features are the attachment sites of muscles that retract
and rotate the humerus. (After Ostrom, 1976a.)