The Origin and Evolution of Mammals - Moodle

106 THE ORIGIN AND EVOLUTION OF MAMMALS
limbs, and was doubtless slow and clumsy by modern
standards. Nevertheless it was the starting
point for the evolution of what became the agile,
long-limbed parasagittal gait seen in typical mammals.
Indeed, one hint of the mammalian design
has already been seen in pelycosaurs, namely the
reduction of the lateral undulation of the vertebral
column. However, even in the most basal of therapsids,
numerous significant new developments in
the direction of the mammals are to be found.
Ideally at this point the postcranial skeleton of
the biarmosuchian grade should be described and
interpreted, but unfortunately it is very poorly
known. Sigogneau and Chudinov (1972) and
Sigogneau-Russell (1989b) have described several
isolated postcranial bones of Biarmosuchus, as has
Boonstra (1965) for the South African Hipposaurus.
As far as it is known, it shares a general similarity
to the far better understood gorgonopsian skeleton.
Several complete and many partial specimens of
the latter taxon have been described (Sigogneau-
Russell 1989) and aspects of the functional anatomy
considered (Colbert 1948; Kemp 1982; Sues 1986a).
Furthermore, they are not too dissimilar from a basal,
brithopian dinocephalian such as Titanophoneus.
Taken together, these groups can be combined to
create a general picture of the structure and functioning
of the primitive therapsid-grade postcranial
skeleton.
Axial skeleton. Considering first the axial skeleton,
the very attachment of the vertebral column to the
skull was modified to increase the mobility of the
head relative to the body (Kemp 1969a). The occipital
condyle broadened to a kidney shape, which permitted
the head to rotate about the longitudinal axis
(‘shaking’) through many more degrees without the
first vertebra, the atlas, damaging the spinal cord. It
also increased the extent of dorso-ventral rotation of
the skull about a transverse axis (‘nodding’) without
undue stretching of the spinal cord because each of
the paired neural arches of the atlas articulated to
the side rather than directly below the foramen
magnum. Behind the atlas and axis, there are five
cervical vertebrae. These have broad, horizontal
zygapophyses, and intercentra are retained between
adjacent vertebrae, both features indicating that large
lateral movements of the head were accommodated
by the intervertebral joints of the neck. The dorsal
region of the vertebral column between the pectoral
and pelvic girdles is relatively undifferentiated at
this stage (Fig. 3.16(a)). There is no significant
anatomical distinction between thoracic and lumbar
vertebrae, and moveably attached, ventrally directed
ribs extend for the full length. The zygapophyses
are close to vertically oriented, and intercentra are
absent, both indicating that lateral undulation was
virtually eliminated, but as yet dorso-ventral bending
of the vertebral column had not evolved. Three
sacral vertebrae are present, behind which the tail
was probably very much reduced in length compared
to pelycosaurs, although no completely preserved
specimen is yet known.
Forelimb. The basal therapsid shoulder girdle and
forelimb (Fig. 4.7(a)) have undergone a profound
change. The dermal shoulder girdle arch, formed
from the interclavicle and clavicles, is far less massive
and the contact between the clavicle and the
scapulo-coracoid looser. The scapula blade is much
narrower and the coracoid plate shorter. Significant
movements of the scapulo-coracoid relative to
the ribcage had evolved by this stage, adding to
the total stride length of the forelimb. Although to
a limited degree at this stage, the change anticipates
the fuller expression of scapulo-coracoid
mobility in later cynodonts and most extremely in
mammals.
The most remarkable evolutionary change in the
forelimb concerns the shoulder joint (Fig. 4.7(a)).
The glenoid no longer has the elongated screw
shape found in the pelycosaurs, but is a short notch
formed equally between the scapula above and the
coracoid below. It faces postero-laterally, and the
surface is concave from top to bottom, but convex
from front to back. The corresponding articulating
surface of the humerus is curiously very different,
for it is in the form of a hemicylinder on the proximal
surface of the head that is much longer than
the glenoid. The extreme incongruity between the
respective articulating surfaces of the shoulder joint
can only be explained by a very subtle mechanism
(Kemp 1980c, 1982). The shoulder joint must have
functioned by a roller action between the two
opposing articulating surfaces, analogous to a
wheel passing over the ground, rather than the sliding
action between the opposing surfaces found in
most joints (Fig. 4.7(b)). Beginning at the start of the