The Origin and Evolution of Mammals - Moodle

no significant novelties to what had evolved by the
tritylodontid–tritheledontid grade (Jenkins and
Parrington 1976), but a number of refinements were
added subsequently and are found as basic features
of modern mammals. These include the fusion of
the atlas intercentrum and neural arches to form the
ring-shaped atlas vertebra, which rotates on the
odontoid process of the axis, increasing the amplitude
of head movements (Kemp 1969a). Fusion of
the cervical ribs to the vertebrae may also be associated
with increased head movements.
In the shoulder girdle, a vertical spine appeared,
that is the homologue of the anterior edge of the
primitive scapula blade. It separates the supraspinatus
muscle in front of it from the infraspinatus muscle
behind that is attached to what was the original
lateral face of the scapula blade, and also gives origin
to the deltoideus muscle. This increase in size and
elaboration of the protractor musculature, which is
functionally the main weight-supporting musculature
of the forelimb, yet again points to the support
rather than thrust-generating function, even in mammals.
In the hindlimb, one of the most unexpected
adaptations was the evolution of superposition of the
astragalus on the calcaneum, a process initiated in
later cynodonts (Jenkins 1971a) but not coming to full
expression until the mammals. The intratarsal joint
between the astragalus and the calcaneum that was
associated with the therapsid-stage dual gait had
allowed rotation between these two bones about a
transverse axis. With adoption of an obligatory
parasagittal gait, the extra joint was no longer necessary,
but instead of losing it, it became modified to
allow pronation–supination movements of the foot
relative to the lower leg. The astragalus shifted on to
the top of the calcaneum, losing its contact with the
ground. The axis of rotation therefore shifted to longitudinal,
which permitted the foot to raise and lower
its inner and outer edges. Thus another element was
added to the increased manoeuvrability of mammals
compared to primitive, sprawling-limbed tetrapods.
Functional significance of mammalian locomotion
The functional significance of the profound evolutionary
changes in the anatomy of the postcranial
skeleton and mechanics of locomotion from pelycosaur
to mammal is open to debate. Modern
non-cursorial mammals have neither greater
EVOLUTION OF MAMMALIAN BIOLOGY 113
maximum speed, nor greater mechanical efficiency
than sprawling-gaited lizards of the same body
weight, as demonstrated long ago by Bakker (1974).
The mammal does have much greater locomotor
stamina, being able to maintain a given speed for far
longer before the oxygen debt accrues, but as discussed
later this is a function of the animal’s physiology,
not its morphology. There are two current
hypotheses to explain the anatomical changes. One is
that by getting the feet underneath the body and the
main musculature high up at the top of the limb,
agility of locomotion was enhanced (Kemp 1982).
This is a difficult property to measure, but it can be
argued that the repositioning of the feet closer
together decreased stability and therefore increased
manoeuvrability. Mammals certainly seem more
capable than reptiles of rapid changes in speed and
direction of movement, and of coping more effectively
with a highly irregular terrain including tree
climbing. The second hypothesis is that by abandoning
lateral undulation of the vertebral column, and
getting the belly permanently off the ground, the rate
of respiration could increase (Carrier 1987). Lateral
undulation supposedly prevents breathing and running
simultaneously, because as the lung on one side
of the body is expanded, that on the other is compressed.
Furthermore, a diaphragm at the back of the
ribcage could not work so effectively if the belly was
on the ground. This issue is discussed in a broader
context later, but for the moment it may be noted that
these two explanations are not mutually exclusive.
Sense organs and brain
Of the many features that distinguish mammals
from reptiles, the extraordinary hearing mechanism
utilising the old reptilian jaw hinge bones as ear ossicles,
the vast expansion in range and sensitivity of
olfactory ability, and the monumental increase in
brain volume by some ten times, mean the neurosensory
system was as dramatically modified as any
other. Most of the evolutionary changes affected soft
tissues and so little is known of them, but several
aspects can be inferred from the fossil record.
Hearing
The enlargement of the dentary bone, and concomitant
reduction in size of the postdentary bones and