The Origin and Evolution of Mammals - Moodle

screw-shaped: the front part faces backwards and
downwards, the middle part faces directly outwards,
and the back part faces forwards and upwards. This
shape matches that of the articulating area on the
head of the humerus (Fig. 4.5(e)), which occupies
the end of the expanded proximal region of the
bone. It too is elongated from front to back and is in
the form of a spiral ribbon. At the front it faces
antero-dorsally, in the centre medially, and at the
hind end postero-ventrally. The movement of the
humerus in the glenoid fossa (Fig. 4.5(g)) was
wholly constrained to a single track (Jenkins 1971a).
At the start of an active stride, the spiral of the
humerus head fitted completely into the screwshaped
glenoid. The humerus extended laterally
and its distal end was elevated and faced anteroventrally.
This caused the lower leg to extend forwards
and forefoot to be placed on the ground a
little in front of the rest of the limb. As the humerus
was retracted from this position, the anterior part
of the head passed forwards beyond the confines of
the glenoid. As retraction continued, the shape of
the middle and posterior parts of the articulating
surfaces forced the distal end of the humerus to
lower and the humerus as a whole to rotate about its
long axis so that the lower limb comes to project
postero-ventrally.
The movements and stresses that had to be
accommodated by the rest of the limb during the
locomotory cycle were complex, and explain several
features of the design of the sphenacodontine
grade forelimb that had implications for its subsequent
evolution. The elbow joint (Fig. 4.5(f)) must
act as a hinge joint so that the lower leg can extend
and flex on the end of the humerus. Also, if the foot
is to remain stationery on the ground while the
humerus is retracted in a horizontal plane, then a
relative rotation between the foot and the distal end
of the humerus about a vertical axis must be accommodated
via the lower leg. Third, with rotation of
the humerus about its long axis playing a role in the
stride, torsional stress has to be resisted by the
elbow joint. It is a principle of vertebrate skeletal
design that no single joint can have too many
degrees of freedom, and so for complex, multiple
movements, several associated joints, each designed
to control one or at most two specific movements,
tend to evolve. Nowhere is this principle better
EVOLUTION OF MAMMALIAN BIOLOGY 103
illustrated than in the relationship between the
distal end of the humerus and the foot in sprawlinggaited
tetrapods. The two epipodial bones, radius
and ulna, participate in four joints altogether, two
at the elbow and two at the wrist, and these joints
have differing functions. At the elbow, the radius has
a concave surface that articulates with the ventrally
facing, hemispherical capitulum of the humerus.
The function of this joint is to control the rotation of
the humerus relative to the forefoot. It is capable
of passively accommodating an applied hinging
movement but is not designed to control it, and it
provided little resistance to the torsion between
humerus and lower leg. The ulna, on the other
hand, is designed to control the extension–flexion
movements at the elbow, and also to transmit the
torsion stress from the humerus to the lower leg,
when the humerus was rotating about its long axis.
To achieve these two functions, the articulating surface
of the ulna is in the form of a deep sigmoid
notch into which fits the articulating facet of the
humerus. This joint is, however, incapable of accommodating
the rotation of the ulna about its long axis
on the humerus. Instead, the rotation of the ulna
occurs at the joint it makes with the forefoot. Thus,
as far as the rotation is concerned, the radius and
ulna behave independently of one another. The
radius rotates at the top on the humerus; the ulna
rotates at the bottom on the forefoot. This is a strong
arrangement, resistant to disarticulation during
powerful locomotory activity. The forefoot has a
large number of separate ossifications indicating
a general flexibility rather than precise functions at
specific joints, and the digital formula is still the
primitive amniote 2-3-4-5-3.
The musculature of the forelimb (Fig. 4.5(c) and
(d)) has been reconstructed on the basis of comparison
with living primitive amniotes along with the
morphology of the bones (Romer 1922). The main
depressor, or adductor muscle complex was the
pectoralis, originating on the massive ventral parts
of the shoulder girdle, interclavicle, sternum, and
clavicle, and inserting on the huge delto-pectoral
crest of the humerus. Retraction of the limb was
brought about by the subcoraco-scapularis, a muscle
complex originating on the inner face of the
scapulo-coracoid and emerging behind to insert on
the hind part of the humerus head. Its action pulled