Most of the glenohumeral joint is composed of a large hemispheroidal
humeral head articulating with an ovoid, concave glenoid
(Fig. 6a). Typically, this kind of joint geometry permits three
degrees of freedom: rotation (supination/pronation), flexion/
extension (elevation/depression) and protraction/retraction. In the
case of Tiktaalik, however, some restriction of humeral mobility is
engendered by cranial extensions of the articular surfaces of the
glenoid and humeral head (Figs 3d and 6a; see also Supplementary
Information). These accessory facets—a shallowly concave, elongate
humeral facet and a convex glenoid facet—are brought into contact
as the humerus is pronated, flexed and protracted. Pronation, flexion
and protraction are movements that could have been effected by
the large musculotendinous apparatus passing posterolaterally
through the coracoid foramen and inserting on the ventral surface
of the humerus. The simultaneous apposition of the reversed
concavoconvex geometries of the anterior and posterior parts of
the articulation represents a close-packed, or most stable, joint
position. Additional stability would be contributed through the
action of the trans-coracoid musculature. With the anterior facets
of the glenohumeral joint in full contact, however, protraction and
supination are inhibited, and flexor forces simply compress and
stabilize the humerus against the anterior portion of the glenohumeral
joint.