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Humeral Rotation and Wrist Supination:
Important Functional Complex for the Evolution
of Powered Flight in Birds?
John H. Ostrom, Samuel O. Poore, and G.E. Goslow, Jr.
ABSTRACT
To achieve a better understanding of the function of the M.
supracoracoideus in extant birds, we measured the mechanical
properties and actions of the supracoracoideus in the European
Starling (Sturnus vulgaris Linnaeus) and a pigeon (Columba livia
Gmelin). We performed three sets of acute in situ experiments by
direct nerve stimulation. We measured length-active and
length-passive tension, forces of humeral elevation and rotation
(torque), and humeral excursion (elevation and rotation). The
supracoracoideus is capable of generating a tetanic force 7-10
times the bird's body weight, imparts a torque about the longitudinal
axis that is greater than its force of humeral elevation, and,
when tetanically stimulated, elevates the humems a limited
50°-60° above the horizontal but rotates it through 80°. We conclude
that the primary role of the supracoracoideus is high-velocity
rotation of the humerus, a movement critical to achieving the
upstroke portion of the wingbeat cycle. In addition, we propose
that high-velocity humeral rotation may also serve to augment
supination of the wrist during upstroke.
A morphologically derived supracoracoideus to produce rapid
humeral rotation and the skeletal features associated with it, an
acrocoracoid, triosseal canal, and tuberculum dorsale, are not evident
in Archaeopteryx or Sinornis. These features also appear
undeveloped in Iberomesornis and Concornis, dirt unknown in
Cathayornis, and apparently are not preserved in the most recent
find, Confuciusomis.
Introduction
In order for a flapping wing stroke to be effective, the wing
surface must be converted from an aerofoil to a "nonaerofoil"
surface as the wing changes from the powered downstroke to
the recovery upstroke, either powered or unpowered. The wing
of birds is pronated and extended during the downstroke to provide
maximum surface area for both lift and thrust, but it is supinated
during the recovery upstroke so as to minimize the surface
area and thereby reduce drag. The focus of this
contribution centers around the M. supracoracoideus and its role
in modem birds for augmenting supination at the wrist. These
data provide a new perspective on the fossil record of birds.
The statement written twenty years ago that "any consideration
of the evolution of flight must start with Archaeopteryx"
(Ostrom, 1976a:3) is more important today than when it was
written. This is because more is now known about the anatomical
details of Archaeopteryx, especially after publication of the
last three finds (Wellnhofer, 1974, 1988, 1992, 1993), and because
of advances in our understanding of bird flight mechanics
(Gauthier and Padian, 1985; Ostrom, 1986, 1994, 1995; Jenkins
et al., 1988; Rayner, 1988a; Dial et al., 1991; Vazquez, 1992;
Pennycuick, 1993). This fact is brought home in a most compelling
manner when the now seven nearly complete, articulated
specimens of Archaeopteryx are compared with the often incomplete,
solitary Mesozoic bird specimens {Iberomesornis,
Sinornis, Cathayornis, Concornis, Otogornis, Confuciusomis,
and others) that have been reported since the Eichstatt specimen
was recognized. Recognition in the Eichstatt specimen of the
maniraptoran-like semilunate carpal (Ostrom, 1976a, 1976b)
resulted in a careful reevaluation of the hypothesis of the theropod
origin of birds (Hecht et al., 1985; Schultze and Tmeb,
1991) and the origins of flight in birds (Padian, 1986; Gauthier
and Padian, 1989; Bock and Buhler, 1995). These arguments
aside, however, from a functional standpoint this same semilunate
carpal was central to the maintenance of pronation during
downstroke and to the execution of supination during upstroke.
JohnH. Ostrom, Division of Vertebrate Paleontology, 170 Whitney Av­
After meticulous investigations of the morphology of the
enue, P.O. Box 6666, New Haven, Connecticut 06511, United States. avian carpal-metacarpal complex and functional morphology
Samuel O. Poore and G.E. Goslow, Jr., Brown University, Department of the pigeon carpometacarpus, Vazquez (1992, 1995) demon­
of Ecology and Evolutionary Biology, Box G-BMC 204, Providence, strated that the articular surface of the trochlea carpalis in mod­
Rhode Island 02912, United States.
em birds acts to automatically supinate the hand upon wrist
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