The Origin and Evolution of Mammals - Moodle

Locomotion
Ancestral amniote grade
Amniote locomotion is compounded from several
separate movements of the limbs and body. Lateral
undulation of the vertebral column contributes a significant
fraction of the overall stride length: as waves
of contraction pass down alternate sides of the body,
the limbs are passively protracted and retracted.
Added to this, the limbs actively protract and retract
relative to the vertebral column, which increases the
length of the stride. The transversely oriented
humerus and femur also undergo rotation about
their long axes, which has the effect of shifting the
foot forwards and backwards relative to the body.
Finally, the limbs as a whole are extensible struts so
the stride can be increased yet further by extension at
the joints once the foot is behind the level of the limb
girdle. The change from this design to that of mammals
involved altering the relative contributions of
these four elements of the stride. Lateral undulation
and long-axis rotation of the propodials were lost,
while active retraction–protraction, and extension of
the limbs were retained and developed. Additionally,
two new elements were added: movement of the
shoulder girdle on the ribcage, and dorso-ventral
bending of the lumbar region of the vertebral column.
The reconstructions of the hypothetical ancestral
stages of mammal-like reptiles illustrate much of
how and by inference why this radical remodelling of
the locomotor system occurred.
No understanding of the functioning of locomotor
systems, or the transition from primitive amniote to
mammalian is possible without appreciating that in
all non-specialised tetrapods the function of the forelimb
differs in important respects from the hindlimb,
which accounts for several differences in their design.
The forelimb is primarily to maintain the front of the
animal off the ground during locomotion, and produces
virtually no net locomotor force, analogous to
the wheel of a wheelbarrow. The humerus has a simple,
predetermined stride pattern and there is about
the same volume of protractor as retractor musculature.
It is the hindlimb that generates the necessary
thrust. Therefore, the hindlimb is the larger, movement
of the femur is much less constrained, and the
retractor musculature is far larger than the protractor
musculature.
EVOLUTION OF MAMMALIAN BIOLOGY 101
Sphenacodontine grade
As reviewed by Kemp (1982), Romer’s (1922) classic
reconstruction of the musculature and locomotor
mechanism of the pelycosaur Dimetrodon is still
the basis for the sphenacodontine-grade ancestor,
although subsequent studies, especially of the
joints, have added further detail.
Vertebral column. A significant modification of the
vertebral column towards the eventual mammalian
condition occurred, even at this early stage in which
otherwise the locomotor apparatus is little modified
from the ancestral amniote. The articulating surfaces
of the zygapophyses of adjacent vertebrae are
no longer horizontal, but oblique in orientation.
Even in more primitive pelycosaurs, such as ophiacodontids,
the prezygapophyses face inwards at an
angle of about 30º from the horizontal, while in
Dimetrodon the angle is increased to about 45º. This
indicates that the lateral undulation component of
locomotion was reduced in pelycosaurs, and probably
virtually abolished in the latter genus. The small
size of the intercentra, small bones between the ventral
margins of adjacent vertebrae, also relates to the
reduction of mobility between adjacent vertebrae,
and presumably the loss of lateral undulation permitted
certain pelycosaurs to evolve hugely long neural
spines. Otherwise, the axial skeleton (Fig. 4.5(a)) has
changed little from a primitive amniote condition.
Moveably attached and ventrally directed ribs occur
all the way back to the pelvic region, with no abrupt
distinction between dorsal and lumbar regions, indicating
that prevention of the body from sagging
while walking still depended on the intercostal muscles
and tendons between the ribs, rather than on the
specialised lumbar musculature that evolved later.
Forelimb. The action of the forelimb of pelycosaurs
was heavily constrained by the design of the shoulder
and elbow joints. The pectoral girdle (Fig. 4.5(b))
is a massive structure. The clavicles and interclavicle
together form a U-shaped arch around the thorax
that would have prevented any extensive
movements of the shoulder girdle relative to the
ribcage. The scapula is very broad, and the coracoid
plus procoracoid bones together form a broad ventral
plate. The glenoid fossa is elongated from front
to back and its articulatory surface is described as