The Origin and Evolution of Mammals - Moodle

112 THE ORIGIN AND EVOLUTION OF MAMMALS
acromion process above and the coracoid bone
below leads to the forward-facing internal face of
the scapula. The area of origin of the supracoracoideus
muscle of primitive therapsids had
expanded upwards and forwards through the gap,
to spread over this part of the internal face of the
scapula, which is the equivalent of the supraspinatus
fossa of mammals. A similar eversion of the
hind edge of the scapula blade has also occurred,
and so the posterior part of the inner scapula blade
faces largely posteriorly. This is the area of origin of
the retractor muscle, the subcoraco-scapularis.
Even though the shoulder joint, and also the
humerus, are still primitive in structure, the musculature
controlling the limb had increased in size.
This is explicable by recalling that the function of
the forelimb is to support the front of the animal off
the ground; enlargement of both the retractor and
protractor muscles would have increased the
strength of the support, and also the manoeuvrability
of the forelimbs in an animal increasingly able to
accelerate and rapidly change direction.
In contrast to the primitive nature of the forelimb,
the hindlimb of eucynodonts demonstrates the transition
to the more or less fully mammalian mode of
action, involving loss of the ability to undergo the
sprawling gait. The bulbous, inturned head of the
femur fits comfortably into the deep, hemispherical
acetabulum only when the femur is held in a virtually
parasagittal orientation (Fig. 4.8(d)). In this position,
the very prominent, mammal-like trochanter
major lies on the postero-lateral part of the femoral
head, in exactly the right position to accept the insertion
of a large ilio-femoralis muscle. To a varying
extent in different eucynodont taxa, the ilium has
extended further forwards, the pubis been reduced
and turned backwards, and the ischium turned more
horizontally (Fig. 4.8(e)). These pelvic girdle features
are correlated with an increasing development of the
gluteal (ilio-femoralis) and psoas–iliacus (puboischio-femoralis
internus) musculature as the prime
motivators of the locomotory cycle, in the mammalian
fashion.
Tritylodontid and tritheledontid grade
The postcranial skeleton of tritheledontids is as yet
insufficiently known to base a reconstructed stage
on that group. Although, as far as it goes, the
partial postcranial skeleton of Pachygenelus described
by Gow (2001) is similar to those of both tritylodontids
and early mammals. The postcranial
skeleton of tritylodontids is known in practically
every detail from the description of Oligokyphus
(Fig. 3.23(c)) by Kühne (1956), and the as yet unpublished
material of the North American form
Kayentotherium. It closely resembles the basal mammalian
stage represented by the morganucodontans
(Jenkins and Parrington 1976). At last the forelimb
had evolved a mammalian mode of action. The coracoid
plate is very small and the glenoid fossa is
widely open. The humerus is relatively slender with
a bulbous head that can move quite freely in the glenoid
as a ball-and-socket joint instead of the rolling
joint characterising the earlier stages. Adduction–
abduction as well as protraction–retraction movements
were freely possible. The elbow was turned
backwards and the forelimb is placed more or less
below the body, as in modern small mammals
(Jenkins 1971a). The anterior edge of the scapula and
acromion process are even more extremely everted,
and the supraspinatus muscle was very well developed,
as indicated by a large depression on the now
fully anterior-facing part of the internal scapula
surface.
In the hindlimb, the trend towards losing the posterior
process of the ilium and elongating the anterior
process was completed, with the latter bearing
the characteristically mammalian longitudinal ridge
separating gluteal musculature above from iliacus
musculature below (Fig. 4.8(f)). The pubis is fully
turned back to a level entirely behind the acetabulum,
and the obturator fenestra in the pubo-ischiadic
plate is full sized. In short, the pelvic girdle is completely
mammalian in form, as too is the femur, with
the head separated by a short neck from the distinct
trochanter major and trochanter minor. Actually, the
two trochanters and femoral head are in line with
each other, rather than set off at an angle as in typical
mammals, a feature that Gow (2001) attributed to
fossorial adaptations specific to tritylodontids.
Mammalian grade
At this rather generalised level of discussion, there is
little to add to the story of the origin of mammalian
locomotor mechanics, as directly inferred from the
preserved skeletons. The morganucodontans added