te. If modern views of
selection are applied to the problem of explaining the appearance of
temporal fenestrae, the possibility cannot be ignored that:

1. Selective pressures causing the inception of temporal
fenestrae differed from those causing the continued
expansion of the fenestrae.

2. The selective pressures both for the inception and
continued expansion of the fenestrae differed from group to
group.

3. Selection perhaps involved multiple pressures operating
concurrently.

4. Because of different genotypes the potential of the
temporal region to respond to selective demands varied from
group to group.

[Illustration: FIG. 9. _Captorhinus._ Diagram, showing some
hypothetical lines of stress. Approx. x 1.]

[Illustration: FIG. 10. _Captorhinus._ Diagram, showing areas of
internal thickening. Approx. x 1.]

[Illustration: FIG. 11. _Captorhinus._ Diagram, showing orientation of
sculpture. Approx. x 1.]

Secondly, the vectors of mechanical force associated with the temporal
region are complex (Fig. 9). Presumably it was toward a more efficient
mechanism to withstand these that selection on the cheek region was
operating. The simpler and more readily analyzed of these forces are:

1. The force exerted by the weight of the skull anterior to
the cheek and the distribution of that weight depending
upon, for example, the length of the snout in relation to
its width, and the density of the bone.

2. The weight of the jaw pulling down on the suspensorium
when the jaw is at rest and the compression against the
suspensorium when the jaw is adducted; the distribution of
these stresses depending upon the length and breadth of the
snout, the rigidity of the anterior symphysis, and the
extent of the quadrate-articular joint.

3. The magnitude and extent of the vectors of force
transmitted through the occiput from the articulation with
the vertebral column and from the pull of the axial
musculature.

4. The downward pull on the skull-roof by the adductor
muscles of the mandible.

5. The lateral push exerted against the cheek by the
expansion of the mandibular adductors during contraction.

6. The necessity to compensate for the weakness in the skull
caused by the orbits, particularly in those kinds of
primitive tetrapods in which the orbits are large.