The Origin and Evolution of Mammals - Moodle

92 THE ORIGIN AND EVOLUTION OF MAMMALS
and the skull table as a whole narrowed. Reduction
in cranial width reduced the medially directed
component of the adductor mandibuli, and so
increased the net bite force available.
Sphenacodontine grade
Several important mammal-like innovations are
evident in Dimetrodon and the other sphenacodontines.
To start with, the dentition had become differentiated
to some extent. The anterior, incisor teeth
are large and round in cross-section for grasping and
possibly ripping up prey. They are followed by an
enlarged canine in both upper and lower jaws for an
initial disabling bite. Behind these are the smaller
postcanines, laterally compressed, sharp and slightly
recurved, which would have aided in retaining the
still struggling prey of these top-carnivore animals.
The morphology and functioning of the jaw musculature
of eupelycosaurs (Fig. 4.2(c)) has been
analysed in most detail by Barghusen (1973). By analogy
with modern primitive amniotes, he assumed
that the main jaw-closing muscle, the adductor
mandibuli, was divided into separate slips. An
external adductor mandibuli originated from the
edges of the temporal fenestra and from an aponeurotic
sheet of connective tissue covering the fenestra.
It inserted into both faces of a vertical aponeurotic
sheet attached to the dorsal edge of the jaw. A posterior
adductor mandibuli originated from the
posterior part of the temporal fenestra and inserted
into the hind region of the vertical aponeurosis and
adjacent areas of the jaw. Third, an anterior pterygoideus
muscle originated from the dorsal surface
and posterior edge of the palate, especially from
the large lateral pterygoid processes and massive
quadrate rami of the pterygoid further back. Insertion
of the anterior pterygoideus muscle was low down
on the internal surface of the lower jaw. While this
basic arrangement no doubt also occurred in more
primitive pelycosaurs, important innovations in the
form of processes on the lower jaw had evolved in
the sphenacodontines, which increased the effectiveness
of the muscle action (Fig. 4.3(d)). The dentary
and surangular bones had extended dorsally to
form a prominent coronoid eminence, the forerunner
of the mammalian coronoid process; the angular
had expanded ventrally as a large keel-like structure
below the level of the jaw articulation. These
processes have a dual effect. One is simply to
increase the area available for the insertion of muscle.
The other is to create lever arms so that the
torque generated by the muscles on the jaw
increased. In the case of the coronoid eminence,
the adductor mandibuli muscle ran from it in a
postero-dorsal direction and so had a longer lever
arm with respect to the jaw articulation than if it
had been attached to the dorsal edge of a jaw without
a coronoid eminence. The effect is further
increased by the ventral reflection of the articular
bone, depressing the position of the jaw hinge. In
the case of the enlarged angular keel, here a ventral
extension of the lower jaw creates a larger lever arm
for a muscle that runs antero-dorsally, which is the
orientation of the anterior pterygoideus muscle. A
curious morphological effect follows from the combination
of the development of the large angular
keel with the ventral deflection of the articular
region of the jaw. A space exists between the keel
laterally and the articular region medially. It is
unencumbered below, and part of the pterygoideus
muscle may have wrapped around the ventral edge
of the main part of the jaw to insert within it.
Although not very prominent at this stage, the
space is the forerunner of the therapsid condition
discussed shortly, where a much larger recess is
bounded by the reflected lamina of the angular.
Given this arrangement of the major jaw-closing
muscles, a further consequence arises, although not
of great importance at this stage of evolution. The
adductor mandibuli has a posteriorly directed component
to its line of action, while the anterior pterygoideus
has an anteriorly directed component. These
will tend to cancel out, and therefore the tendency
for the lower jaw to be pulled either backwards or
forwards during the bite is reduced. Not only would
this have relieved some of the stress acting at the jaw
hinge, but it also endows the potential to control precisely
any antero-posterior shifts of the jaw. As it
happens, the jaw articulation of Dimetrodon indicates
that there were still large stresses, and no possibility
of such propalinal movement of the jaw, but this
potential property was to be realised in the subsequent
evolution of the mammalian jaw mechanism.
The sphenacodontian jaw articulation is complex.
There are two quadrate condyles in the form
of elongated cylinders. The outer one is at a higher