The Origin and Evolution of Mammals - Moodle

the therocephalian grade was not only retained but
the face of the sagittal crest became deeper, increasing
the area for the origin of the temporalis muscle.
The posterior part of the zygomatic arch is also
deeper and its internal face indicates that it gave
origin to a small, but definite masseter muscle. The
principal innovation of the musculature at this
stage was the invasion of the lateral surface of the
lower jaw by adductor musculature. At the therocephalian
grade, at best only a very small fraction
of the total muscle mass had gained an external
insertion on the jaw. In Procynosuchus, the temporalis
had extended its insertion to a shallow fossa,
the adductor fossa, which occupies part of the
external surface of the broadened coronoid process.
There is a similarly broad, shallow concavity occupying
most of the external face of the angular bone onto
which the enlarged masseter inserted. The increased
overall size of the adductor musculature increased
the magnitude of the potential bite force available.
Moreover, the particular way in which it was
arranged introduced yet another highly significant
property. The temporalis had a medially directed
component as in therocephalians, but now there
was also a masseter muscle, running between the
external face of the jaw and the more laterally
placed zygomatic arch, that had a laterally directed
component of its force. To some degree at least,
these respective medial and lateral components acting
on the jaw would have cancelled each other out,
reducing the net tendency of the back of the jaw to
be forced medially, and therefore reducing the
net stress between the articular and quadrate bones
of the jaw hinge. Furthermore, this arrangement
increased the potential ability to control movements
of the lower jaw in the transverse direction.
Insofar as even primitively multicusped teeth like
those of Procynosuchus required accurate placing of
the lower teeth relative to the upper teeth during
closure, an increase in the control of the precise
positioning of the lower jaw during mastication
was necessary. By evolving the masseter muscle
laterally as well as increasing the temporalis muscle
medially, the procynosuchian grade saw the initiation
of an essential requirement of musculature for
operating large, complex, occluding teeth: a combination
of a large bite force with fine precision of
movement of the lower jaw in the horizontal plane.
EVOLUTION OF MAMMALIAN BIOLOGY 97
Two other points about the jaw musculature
should be noted. The first is that the reflected lamina
of the angular is greatly reduced in size, and
could no longer have supported significant jaw
musculature. Any tongue, hyoid, or jaw-opening
muscles that may have inserted on the reflected
lamina in the therocephalian grade had presumably
shifted to the main body of the angular and
adjacent bones. There is a longitudinal groove
low down on the inner face of the angular of
Procynosuchus, which probably marks part of the
attachment area for such musculature. The second
point is that the pterygoid processes of the palate
were further reduced and by now the pterygoideus
musculature would have been involved in fine
control of jaw movements, rather than contributing
significantly to the production of bite forces.
The structure of the jaw articulation of the procynosuchian
grade corresponds to the new arrangement of
the adductor muscles. As in the therocephalian grade,
the axis of the joint is still oblique rather than transverse,
indicating that the net adductor muscle force
still had a medially directed component. However,
the angle of the jaw hinge to the transverse is less,
indicating that the net medially directed component
of the jaw-closing force had been reduced.
While the quadrate retained its ability to rotate in
order to maintain the patency of the jaw hinge, the
movement was less extensive (Kemp 1979; Luo and
Crompton 1994).
Basal epicynodont grade
The next cynodont grade is represented by
Thrinaxodon (Fig. 4.4(b)), in which there are differences
in degree rather than innovations compared
to the procynosuchian grade. The temporal fenestra
is larger, having expanded posteriorly and laterally,
and the sagittal crest deeper. Therefore, there is
attachment for a much larger temporalis muscle
from the medial and posterior walls of the fenestra as
well as from the connective tissue aponeurosis covering
it. The corresponding increase in area for the
muscle’s insertion on the lower jaw comes from a
large expansion of the coronoid process, which now
extends up into the temporal fenestra, almost to the
height of the skull itself; the medial and lateral
surfaces of the process were both available for the
insertion. The increase in size of the masseter