The Origin and Evolution of Mammals - Moodle

110 THE ORIGIN AND EVOLUTION OF MAMMALS
To summarise the primitive therapsid grade of
locomotion as inferred mainly from biarmosuchians
and gorgonopsians, the head was more mobile
on the body, and lateral undulation of the vertebral
column was completely lost. The forelimb operated
in sprawling mode, but its amplitude of movement
was increased by some mobility of the shoulder
girdle and by a rolling type of shoulder joint. It was
adapted only for support of the front of the animal
and not for production of any significant locomotor
thrust. The hindlimb was more versatile and
capable of at least two different gaits. For slow,
low-energy movement it operated in a primitive,
sprawling gait. For more active, faster locomotion
the knee was turned forwards, bringing the foot
below the body, and the limb was operated in a
mammal-like parasagittal mode. The ilio-femoralis
muscle, primitively an elevator of the femur had
started to expand forwards over the ilium to act as
a retractor muscle. The feet were plantigrade.
All the progressive features of these primitive
therapsids are also found in therocephalian-grade
therapsids and the functional anatomy of the limbs
was probably fundamentally the same. In fact, the
principal difference concerns the vertebral column.
In the therocephalians, it has developed a true lumbar
region in which the ribs are short, immobile, and
horizontally disposed. The complete skeleton of
Regisaurus (Fig. 3.17(d)) indicates a virtually mammalian
degree of vertebral differentiation, and also
mammalian proportions of limb length to body size.
Basal cynodont grade
Relatively little change in the postcranial skeleton
and locomotion had occurred within the basal
cynodont grades. Procynosuchus (Fig. 3.19(d)) was
adapted for an amphibious mode of life (Kemp
1980c) and therefore not very representative. The relatively
flattened limb and forefoot bones, wide-open
glenoid and acetabulum, and very long tail bearing
elongated haemal are evidently specialisations for
swimming. Thrinaxodon (Fig. 3.20(b)) is a better
model for this stage. The most striking innovation
was the development of overlapping costal plates on
the inner parts of the ribs. These are accompanied by
accessory zygapophyses, the anapophyses, immediately
below the normal pre- and postzygapophyses
of adjacent vertebrae. The effect of both structures
was to strengthen and stiffen the vertebral column,
although it is far from clear what the functional significance
of this was. Kemp (1980a) supposed that it
conveyed resistance to bending of the column in the
face of increased locomotory thrust from the
hindlimb. Others (Brink 1956; Jenkins and Bramble
1989) thought that it might relate in some way to the
evolution of a diaphragm, though the nature of the
functional connection between the two structures is
difficult to see. At any event, the presence of the
costal plates in the less-derived eucynodonts such as
Diademodon and Cynognathus indicates that they were
indeed a stage in the evolution of the mammalian
axial skeleton, subsequently reduced and finally lost
in more progressive eucynodonts and the mammals.
By these later stages, the complex of intervertebral
muscles and tendons, had presumably taken over
whatever function the osteological system of interlocking
ribs originally had (Jenkins 1971a).
The structure of the limbs and girdles of Thrinaxodon
were sufficiently similar to those of primitive
therapsids to assume that locomotory function was
not greatly modified. The scapula blade was concave
in lateral view, indicating larger muscles originated
from the medial surface to pass out laterally
and insert on the humerus. The shoulder joint had
remained basal-therapsid in structure, and the forelimb
continued to operate in a strictly sprawling
mode. The hindlimb was still capable of adopting
sprawling and parasagittal gaits (Blob 2001).
Eucynodont grade
There was a tendency to reduce the costal plates
within the eucynodonts. They were minute processes
in the traversodontid Massetognathus (Fig. 3.22(e))
and the chiniquodontids (Romer and Lewis 1973)
have lost all sign of them, leaving an axial skeleton
exactly as in mammals. Kemp (1980a) analysed the
shoulder joint of the traversodontid Luangwa,
showing that the humerus must still have acted in
a strictly sprawling mode because any attempt to
lower the distal end of the bone below the level of
the glenoid leads to disarticulation (Fig. 4.8(c)).
Indeed, the mechanical action of this joint was very
similar to that of primitive therapsids, although
there were significant changes in the associated
musculature (Fig. 4.8(a)). The anterior edge of the
scapula blade is strongly everted, at the base of
which is the acromion process to which the outer
end of the clavicle is attached. The gap between the