The Origin and Evolution of Mammals - Moodle

108 THE ORIGIN AND EVOLUTION OF MAMMALS
stride, the front part of the articulating surface of
the humerus contacted the front of the glenoid,
making contact at two points, one on the upper and
one on the lower part of the glenoid. In this position,
the shaft of the humerus extended laterally
and horizontally and its leading edge was elevated
so that the radius and ulna ran antero-ventrally to
the forefoot, which was placed on the ground
further forwards. The humerus was then retracted
while remaining in the horizontal plane, and simultaneously
rotated about its long axis. The movement
was controlled by the rolling of the cylindrical
humerus surface across the convex glenoid surface,
there always being two points of contact between
the two. By the final position, the humerus was
still horizontal but retracted by about 45º, and the
rotation about its long axis had placed the forefoot
further back relative to the end of the humerus. The
structure of the joint prevented adduction of the
humerus below the horizontal, but did permit elevation
to occur at any stage. This, along with a certain
amount of freedom over the extent of the long
axis rotation gave the humerus a much wider variety
of movement than the highly constrained movement
found in the pelycosaurs. The reason for this
unusual joint mechanism may be related in a very
particular way to the musculature that activated it
(Kemp 1982). With the freeing of the shoulder girdle,
there was a need to separate the muscles
responsible for movement of the humerus relative
to the girdle from those responsible for movement
of the girdle relative to the ribcage. The roller mechanism
causes the front and the hind points of the
humerus head to move medially and laterally during
the stride cycle. Therefore muscles attached to
these points can run medially and so attach exclusively
to the girdle, rather than to the body wall.
Under these circumstances, the shoulder girdle was
left unhampered to undergo its own independent
movements relative to the body, controlled by muscles
running between the shoulder girdle and the
ribcage, body fascia and back of the skull. This
unexpected design of the shoulder joint is manifestly
weak. First, with so little area of direct contact
between the articulating faces of the glenoid and
humerus, even after allowing for a generous layer
of cartilage the joint could not have withstood particularly
large imposed forces. Second, mobility of
the pectoral girdle on the body wall using only soft
tissues for attachment would also limit the size of
locomotor forces that could be imposed without
disarticulation. All this points again to the fundamental
principle of tetrapod locomotion mentioned,
that the primary function of the forelimb
was to support the weight of the front part of the
animal, and not to generate locomotor forces.
The actual musculature of the shoulder girdle did
not differ greatly from the sphenacodontine grade.
The main retractor was the subcoraco-scapularis,
originating on the internal face of the scapulocoracoid,
emerging immediately above and behind
the glenoid, and inserting on the posterior part
of the proximal head of the humerus. The main
retractor musculature consisted of muscles from the
procoracoid and lower part of the scapula inserting
on the anterior part of the humerus head. The probable
origin of a supracoracoideus muscle is indicated
by a shallow concave area in front of the glenoid,
and of a scapulo-humeralis anterior by a large area
in the antero-ventral region of the scapular blade.
Adduction remained the function of a large
pectoralis muscle running ventrally from the still
massive delto-pectoral crest of the humerus, and
elevation was the consequence of the action of deltoideus
and latissimus dorsi muscles.
The functioning of the lower forelimb was not radically
different from the sphenacodontine stage. As
there, here also the radius and ulna were as stout as
one another, and shorter than the humerus. The
design of the joints was similar, with the sigmoid
notch of the ulna making a strong hinging joint at the
elbow and also resisting the powerful torque arising
from long-axis rotation of the humerus. The radius,
by contrast, accommodated the rotation at the elbow.
At the wrist, the roles were reversed, with the ulna
forming a rotatory joint and the radius a hinge joint
with the manus. The manus itself is short and robust,
with approximately equal lengths of digits although
this was achieved by reduction in size of some of the
phalanges: the phalangeal formula of biarmosuchians
and gorgonopsians is still 2-3-4-5-3. Although never
studied in detail, the relatively short metacarpals
indicate that manus was probably plantigrade.
Hindlimb. The pelvic girdle and hindlimb of the
primitive therapsid grade of evolution was also
radically modified from the sphenacodontine grade,
and Kemp (1978) proposed that the anatomy of