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apparent. It is now clear that this semilunate carpal is characteristic
of the clade, but the adaptive meaning of these clade-common
unique wrist movements that seem to have been typical of
maniraptoran theropods remains unknown.
THE WRIST OF Archaeopteryx
Revelation of the remarkable details of the anatomy preserved
in the Eichstatt specimen (the fifth) of Archaeopteryx
(Wellnhofer, 1974) caused renewed interest in the question of
the origin of birds, culminating in the 1984 International Archaeopteryx
Conference (Dodson, 1985) in Eichstatt, Germany
(Hecht et al., 1985). Although not unanimous, that conference
reached a consensus that the ancestral stock from which birds
arose was probably a primitive archosaurian, but the details of
this origin soon dissolved into three distinctly different hypotheses
that persist to this day: the primitive thecodontian theory
(Hecht and Tarsitano being the principal advocates), the crocodylomorph
theory (championed effectively by Walker, Martin,
and Whetstone), and the theropod ancestral theory (argued
by Ostrom, Padian, and Wellnhofer, sometimes by Gauthier,
and occasionally by others).
The most important evidence provided by the Eichstatt specimen
is the well-preserved semilunate carpal almost exactly as
it is preserved in Velociraptor and Sinomithoides from Mongolia
and China, respectively (Figure 2). The semilunate carpal
appears to have functioned in the same way in these forms, just
as originally visualized in Deinonychus (Ostrom, 1969b); flexion
at the wrist forced a pronounced supination (circumduction)
of the metacarpus-manus. If this interpretation also is correct
for Archaeopteryx, as we believe, that carpal manipulation
has profound implications regarding the flight capability of the
Urvogel and has even stronger implications concerning the early
stages of flapping flight in birds.
Because of this apparent wrist action in Archaeopteryx, supination
was initially equated with the modem avian wrist (Ostrom,
1976a), where the shape of the modem carpometacarpus
is so similar to that formed by the semilunate carpus-metacarpus
complex of the Eichstatt specimen. In fact, the trochlea carpalis
of the modem carpometacarpus forms the key articulation
essential for modem flapping bird flight (Vazquez, 1992). It
was proposed that the maniraptoran-like semilunate carpal,
through time, fused with metacarpals I and II to form the modem
carpometacarpus with its distinctive trochlea carpalis and
its unique action so characteristic of all flying birds (Ostrom,
1976a).
As Vazquez (1992) described, flexion of the wrist of the
modem avian wing forces the more distal wing segments to supinate,
streamlining those wing components for the ensuing upstroke.
Flexion at the wrist displaces the cuneiform distally,
causing it to slide along the trochlea carpalis, which results in
supination, although there are no muscles that directly supinate
the hand (Vazquez, 1995, and references therein). Thus, supination
is dependent on the trochlea carpalis-cuneiform com­
plex and air resistance on the dorsal surface of the wing during
upstroke. In addition to the wrist's osteology, we propose herein
that a derived supracoracoideus contributes to supination by
rapidly rotating the humems on its longitudinal axis. Below we
report the experimental evidence to support this position.
The Role of the M. Supracoracoideus in Flapping Flight
Numerous derived features characterize the pectoral girdle
and associated musculature of the Neornithes. The most striking
of these, and the one that represents an extreme departure
from a primitive tetrapod organization, is that of the M. supracoracoideus.
The supracoracoideus in all birds possessing
powered flapping flight lies deep to the pectoralis, arises from
the carina, sternum, and coracoclavicular membrane, and possesses
a bipinnate architectural organization of its fascicles.
The most distinctive feature of the supracoracoideus, however,
is the course of its tendon of insertion (Figure 3). The tendon
passes dorsally through the triosseal canal (formed by the coracoid,
scapula, and furcula) and attaches on the dorsal aspect of
the humems above the glenohumeral joint. The seemingly obvious
function of this dorsally inserting tendon is that the supracoracoideus
is for wing elevation. The presence or absence
of this anatomical arrangement has been a central question in
debates concerning the evolution of flapping flight and has
been given considerable attention in interpreting the flight capabilities
of the Late Jurassic bird Archaeopteryx (Ostrom,
1976a, 1976b; Olson and Feduccia, 1979).
We studied the in situ contractile properties of the supracoracoideus
to clarify its role during flapping flight in two species
of extant birds, the European Starling {Sturnus vulgaris Linnaeus)
and a pigeon {Columba livia Gmelin). Starlings and pigeons
contrast in their wing loading (wing area/body weight)
and flight styles. In both species, we measured the absolute
force generated by the supracoracoideus, the humeral excursion
(elevation and rotation), and the forces of humeral elevation
and humeral axial rotation.
Electrical activity of the supracoracoideus of a starling flying
in a wind tunnel (Dial et al., 1991) and pigeons in free flight
(Dial et al., 1988) begins in late downstroke and ends prior to
the upstroke-downstroke transition. The electrically active period
is not coincident in time with force. The electromechanical
delay reported in the pectoralis during flight in starlings (Biewener
et al., 1992) and pigeons (Dial and Biewener, 1993) suggests
electrical activity anticipates force at burst onset by several
milliseconds (ms). After electrical activity ceases,
however, force continues for 20-25 ms, leading us to conclude
the force produced by the supracoracoideus in both species is
sustained through most of the upstroke. We used as a reference
for our physiological measurements the wing kinematics for
European Starlings reported in the cineradiographic study by
Dial et al. (1991). Kinematic data of comparable precision are
not available for the pigeon; we made estimates from Brown
(1951) and Simpson (1983). The downstroke-upstroke transi-