The Origin and Evolution of Mammals - Moodle

146 THE ORIGIN AND EVOLUTION OF MAMMALS
retained a number of more primitive features. The
first two vertebrae, the atlas and axis respectively,
are cynodont-like insofar as the elements of the
atlas have not fused to form a ring-shaped bone.
Nevertheless, they may well have functioned as in
the modern mammals, with extensive rotation of
the head plus atlas about the anterior odontoid
process of the axis vertebra behind (Kemp 1969a).
The ribcage is fully mammalian in appearance, with
the individual ribs connecting ventrally to a row
of sternebrae rather than with a flat, continuous
sternal plate. There is an abrupt transition to the
lumbar region of the vertebral column, indicated by
a change in orientation of the neural spines from
posteriorly directed to vertical, as well as a complete
absence of distinguishable lumbar ribs. This
arrangement indicates both the presence of a
diaphragm at the back of the thorax, and also the
development of dorso-ventral flexibility at, and
behind the point of transition between the thoracic
and the lumbar regions. Only three sacral vertebrae
are present, compared to the five of typical later
mammals.
The structure of the morganucodontan pectoral
girdle and forelimb indicates the evolution of a
greater degree of flexibility and, probably, amplitude
of movement compared to the primitive cynodont
condition. The coracoid plate is reduced and the
acromium process at the base of the scapula blade
is large, which together indicate the presence in
life of a large supraspinatus muscle originating
from the inner face of the scapula blade. However,
there is no development of a scapula spine or of a
supraspinatus fossa in front of the acromium,
which are so characteristic of modern mammalian
scapulae. An interclavicle is still present. The glenoid
fossa is more open than in cynodonts, which permits
a greater range of movements of the humerus at the
shoulder joint. The humerus itself is a more slender
version of the cynodont humerus. The pelvis and
hindlimb are very similar indeed to typical small,
modern mammals. The ilium is expanded forwards
and is divided by a longitudinal ridge into an upper
area for the gluteal musculature and a lower area
for the iliacus. It no longer extends posteriorly behind
the level of the acetabulum. The pubo-ischiadic plate
has rotated backwards and gained a large obturator
fenestra. The femur has developed the inturned
head, large trochanter major, and a trochanter minor
replacing the primitive internal trochanter.
The locomotion of morganucodontans must have
resembled a modern small, non-cursorial mammal
(Jenkins 1971a, 1974; Jenkins and Parrington 1976).
The knee and elbow would have been turned
inwards at least halfway towards the body and
both fore and hind feet placed underneath it. Both
humerus and femur were oriented approximately
horizontally and in both cases the principal movements
would have been in oblique vertical planes,
with large extension–flexion movements of the
lower limb at the elbow and knee, respectively. At
least half of the amplitude of the stride of the smaller
forelimb would have resulted from movement of the
shoulder girdle on the ribcage, controlled by the
clavicle. The larger hindlimb would have produced
virtually all of the locomotory force. As discussed
elsewhere (page 113), possible benefits of re-modelling
of the limb action in mammals, particularly
getting the feet close to the midline, include both
increased agility, and increased ventilation for sustained
aerobic activity.
There are several genera of morganucodontans
known, distinguished from one another mainly on
differences in molar tooth form. Most are still represented
only by isolated teeth from various localities
in Europe (Clemens 1986; Kielan-Jaworowska
et al. 2004), but two virtually complete skeletons
have been found in the Red Beds of the Stormberg
Series of South Africa (Crompton 1974; Crompton
and Jenkins 1978), which are dated as Late Triassic,
possibly Norian but otherwise Rhaetian. In
Megazostrodon (Fig. 5.3(d)), the three main cusps of
the upper molar teeth form an obtuse triangle, and
the surrounding cingulum is better developed and
bears far larger cingular cusps than in Morganucodon.
The other South African form is Erythrotherium, represented
by a juvenile specimen that is very similar
to Morganucodon.
North American morganucodontans have been
discovered in the Kayenta Formation of Arizona,
which is probably Lower Jurassic in age (Sues
1986b). There are specimens of Morganucodon itself,
but the genus Dinnetherium is more interesting
(Jenkins et al. 1983; Crompton and Luo 1993). It is
very similar to other morganucodontids in most
respects, but the central cusp on both the upper and