The Adductor Muscles of the Jaw In Some Primitive Reptiles

by Richard C. Fox

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.

Dimetrodon

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.

Dimetrodon and Captorhinus differ in the bones associated with the adductor mechanism; the area behind the orbit in Dimetrodon is relatively shorter, reducing the comparative longitudinal extent of the adductor chamber. Furthermore, the dermal roof above the adductor chamber slopes gently downward from behind the orbit to its contact with the occipital plate in Dimetrodon. Temporal fenestrae are, of course, present in Dimetrodon.

Musculature

The adductor musculature of the lower jaw in Dimetrodon was divided into lateral and medial groups (Figs. [5], [6]). The lateral division consisted of temporal and masseter masses. The temporal arose from the upper rim of the temporal opening, from the lateral wall of the skull behind the postorbital strut, and from the dorsal roof of the skull. The bones of origin included jugal, postorbital, postfrontal, parietal and squamosal. This division may also have arisen from the fascia covering the temporal opening (Romer and Price, 1940:53). The muscle passed into the Meckelian fossa of the mandible and inserted on the angular, surangular, prearticular, coronoid and dentary bones. Insertion on the lips of the fossa also probably occurred.

The lateral division arose from the lower rim of the temporal opening and from the bones beneath. Insertion was in the Meckelian fossa and on the dorsal surface of the adjoining coronoid process.

Fig. 5. Dimetrodon. Internal aspect of skull, showing masseter and temporal muscles. Skull modified from Romer and Price (1940). Approx. × 1/4.