The Origin and Evolution of Mammals - Moodle

98 THE ORIGIN AND EVOLUTION OF MAMMALS
muscle was even more impressive. The zygomatic
arch has expanded to form a broad, dorsally arched
sheet of bone providing origin for masseter muscle
fibres all along its length. The more posterior part
of the masseter muscle still inserted onto the external
surface of the angular bone. By this stage, however,
the dentary had expanded ventrally to form a large
angular region. The greater part of the masseter
muscle inserted on this new region of the dentary,
as indicated by the smooth, shallow masseteric
fossa occupying most of its surface. The functional
effect of the continued evolution of the adductor
musculature was threefold. First, the magnitude of
the laterally directed component of the force generated
by the masseter muscle more closely matched
the medially directed component of the temporalis,
and therefore a closer approach to an actual balance
between these two was achieved. Second, the
increasingly horizontal line of action of the temporalis
muscle from its area of origin high up in the
coronoid process, and the increasingly vertical orientation
of the masseter muscle as it expanded forwards
along the zygomatic arch, together created a
tendency to concentrate the net adductor muscle
force on the teeth, and away from the jaw articulation.
This is better explained in the context of
the next evolutionary stage, the eucynodont grade,
where the effect had become fully expressed. The
third consequence arose from the capture by the
enlarged dentary of an increased proportion of
the total jaw muscle fibres, leaving very few still
inserting on the reduced postdentary bones. Stresses
across the sutures between the dentary on the one
hand and the attached postdentary bones on the
other were reduced, permitting the latter to be less
firmly attached.
Apart from these important quantitative modifications
to the jaw musculature, little else of great
significance occurred at the Thrinaxodon stage. The
one that should be mentioned is a minor technicality
at this stage. The surangular bone, which lies
immediately lateral to and above the articular bone
of the jaw hinge, has developed a small boss at the
back which faces towards, but does not quite contact
a process on the squamosal bone adjacent to the
quadratojugal bone. At this stage, a ligamentous
connection occurred, helping to stabilise the jaw
hinge and relieve some of the stress on the quadrate
bone (Crompton 1972b; Crompton and Hylander
1986). The significance of this new feature is that it
is actually the incipient forerunner of what was
eventually to become the new, mammalian jaw
hinge. Apart from this detail, the jaw articulation of
Thrinaxodon differs little from Procynosuchus except
for a relative reduction in size of the participating
bones (Luo and Crompton 1994).
Compared to Procynosuchus, there is a reduced
number of teeth in Thrinaxodon, which has only four
upper and three lower incisors, and around eight
postcanines. The anteriormost and posteriormost
cuspules of the cingulum of the molariform teeth are
enlarged to the extent that they can be described as
accessory cusps. This elaboration is no doubt correlated
with the greater force and accuracy of the
available bite. Although there is a range of molar
tooth-form in basal epicynodonts, that of Thrinaxodon
in particular resembles several eucynodont and early
mammal groups, so it is probably a good model for
this hypothetical ancestral stage.
Eucynodont grade
The eucynodont grade is characterised by completion
of the trend towards balancing the adductor muscle
forces so that the net force is concentrated at the
point of bite and the stress at the jaw hinge reduced,
theoretically to zero. A form such as Chiniquodon
(�Probelesodon) illustrates this (Fig. 4.4(c)). The dentary
has enlarged to the extent that it now completely
dominates the lower jaw. The coronoid
process reaches the level of the top of the temporal
fenestra, and the angular region below has also
expanded, enlarging yet further the available area
for insertion of the masseter muscle. The dentary
has also for the first time expanded posteriorly to
form an articular process that overlies and supports
the relatively minute postdentary bones. While
impossible to be sure, it is likely that by this stage
the magnitude of the laterally directed component
of the masseter muscle force generated during the
bite equalled the magnitude of the medially directed
component of the temporalis so that the two virtually
cancel out. Therefore, any tendency for the
lower jaw to be forced inwards or outwards was
abolished, and there was zero transversely directed
stress generated between the articular and the
quadrate at the jaw articulation.