The Origin and Evolution of Mammals - Moodle

100 THE ORIGIN AND EVOLUTION OF MAMMALS
the mammalian sound transmitting ear ossicles, as
explained later in the chapter.
Given the general similarity between the teeth of
Thrinaxodon, the eucynodont Probainognathus, the
tritheledontan Brasilitherium, and the basal mammal
Morganucon, it is safe to assume that the postcanine
teeth of the eucynodont grade hypothetical ancestor
would have been triconodont, with a main cusp
flanked by smaller anterior and posterior accessory
cusps, and an internal cingulum of cuspules.
Tritylodontid grade
The taxonomic uncertainty about the interrelationships
of tritylodontids, tritheledontans, and mammals
has been discussed, and this spills over into
ambiguity about the details of the next stages
towards the fully mammalian condition that can be
inferred from the fossils. The only useful contribution
to the sequence of evolutionary events illustrated
by the tritylodontids is the inference that loss
of the postorbital bar preceded the development of
the new jaw articulation between the dentary and the
squamosal. Otherwise, the extreme specialisation of
the jaw and dentition of tritylodontids obscures any
other possible innovations that appeared at this evolutionary
stage.
Tritheledontan grade
The tritheledontids (Fig. 3.24) represent the stage at
which the new jaw articulation occurred. It had
evolved as a consequence of an increase in length of
the articular process of the dentary to the point where
it invaded and replaced the pre-existing contact
between the surangular bone and the squamosal,
and lies immediately alongside the primary hinge
between articular and quadrate. At this stage the
contact between dentary and squamosal is simple,
there being no development of a prominent dentary
condyle or squamosal glenoid fossa. The function was
mainly to free yet further the primary hinge bones for
their evolving hearing function. But the new arrangement
may also have increased the degree of fine control
of horizontal position of the jaw. Wear facets
on the postcanine teeth of Pachygenelus indicate that
a significant change in the occlusal mechanism
had occurred; the outer sides of the lower teeth wear
against the inner sides of the uppers. The only way
this could happen is by a slight initial lateral shift of
the lower tooth row, placing it below the upper tooth
row. As the jaw closed and the teeth met, the lower
teeth moved upwards but also shifted slightly medially
during the tooth-to-tooth contact. Given this
geometry, occlusal biting could only occur on one side
at a time, right or left, but not both simultaneously.
At this stage the postcanine teeth lack precise patterns
of tooth wear, indicating that the occlusal was a
general contact between opposing tooth rows, rather
than the precise action of a specific lower tooth with
a specific upper tooth that characterises mammals.
Mammalian grade
Morganucodon represents the ancestral mammalian
stage best (Fig. 5.3(b)). The principal innovation is
the evolution of a definite ball-like condyle on the
dentary that articulates with a well-defined glenoid
cavity in the squamosal, alongside but functionally
replacing the articular-quadrate hinge. This is at first
sight paradoxical. The eucynodont jaw articulation
was exceedingly weak, but did not have to bear large
reaction forces. Why now replace it with a new articulation
that promptly evolved into a robust structure
capable once again of withstanding large forces?
Crompton and Hylander (1986) answered the question
by pointing out that in primitive mammals only
one side of the dentition is used at a time during
mastication, as in tritheledontans. The difference in
the mammals is that the adductor jaw musculature
on both sides of the head contributes simultaneously
to the bite force between the teeth on one side.
However, while the ipsilateral musculature can in
principle behave as described in eucynodont-grade
skulls and avoid generating a jaw hinge reaction, the
contralateral muscles cannot do so. They can only
contribute their muscle force to the bite force of the
teeth of the opposite side by transmitting it via a firm
symphysis between the front ends of the respective
jaws, plus a jaw articulation strong enough to withstand
a significant component of the force. The
benefit of the arrangement is a yet greater bite force,
and therefore this final evolutionary innovation of a
strengthened jaw articulation was the last step in
achieving not only a combination of extremely large,
but also extremely accurately applied, bite forces.
The final fate of the primary jaw hinge bones and
other postdentary bones is discussed in the context
of the evolution of mammalian hearing.