The posterior pterygoid muscle probably arose from the anterior portion of the pterygoid wing of the quadrate, from a ridge on the ventromedial surface. From the relationship of the muscle to the articulation of the jaw with the skull, it may be deduced that the muscle was limited in function to the stabilization of the quadrate-articular joint by keeping the articular surfaces in close contact with each other and by preventing lateral slipping.
Finally there is evidence for an adductor between the temporal and masseter masses. The anterior dorsal lip of the Meckelian fossa supports a small knob to which this muscle attached, much as in _Sphenodon_ (Romer, 1956:18, Fig. 12). Presumably the muscle was sheetlike and attached to the skull roof, medial to the attachment of the masseter.
A pseudotemporal may have been present, but evidence to indicate its extent and position is lacking. The muscle usually arises from the epipterygoid and nearby areas of the braincase and skull roof and inserts in the anterior parts of the fossa of the jaw. In _Captorhinus_ the lateral wing of the pterygoid cuts across the fossa, effectively blocking it from the upper and medial parts of the skull, the areas of origin for the pseudotemporal.
The morphology of the skull of _Dimetrodon_ closely resembles that of the primitive _Haptodus_ (Haptodontinae, Sphenacodontidae), and "hence may be rather confidently described as that of the family as a whole" (Romer and Price, 1940:285). The major differences between the two genera are in the increased specialization of the dentition, the shortening of the lacrimal, and the development of long vertebral spines in _Dimetrodon_. The absence of gross differences in the areas of the skull associated with the groups of muscles with which this study is concerned, implies a similarity in the patterns of musculature between the two groups. Romer and Price suggest that _Haptodus_, although too late in time to be an actual ancestor, shows "all the common features of the _Dimetrodon_ group on the one hand and the therapsids on the other." The adductors of the jaw of _Dimetrodon_ were probably little changed from those of the Haptodontinae and represent a primitive condition within the suborder.
