The Adductor Muscles of the Jaw In Some Primitive Reptiles

by Richard C. Fox

In Didelphis the sagittal crest is high and dorsally convex in lateral aspect, arising posterior to and medial to the orbits, reaching its greatest height near the midpoint, and sloping down to its termination at the supraoccipital crest. Two low ridges extend posteriorly from the postorbital process to the anterior end of the sagittal crest and correspond to ridges in similar position in Thrinaxodon.

The supraoccipital crest flares upward to a considerable extent in Thrinaxodon and slopes posteriorly from the skull-roof proper. The crest extends on either side downward to its confluence with the zygomatic bar. The area of the crest that is associated with the temporal musculature is similarly shaped in Didelphis.

The zygomatic bar in each genus is stout, laterally compressed, and dorsally convex on both upper and lower margins. At the back of the orbit of Thrinaxodon, the postorbital process of the jugal extends posterodorsally. At this position in Didelphis, there is but a minor upward curvature of the margin of the bar.

In Thrinaxodon the dorsal and ventral postorbital processes, arising from the postorbital and jugal bones respectively, nearly meet but remain separate. The orbit is not completely walled off from the adductor chamber. The corresponding processes in Didelphis are rudimentary so that the confluence of the orbit and the adductor chamber is complete.

The adductor chamber dorsally occupies slightly less than half of the total length of the skull of Thrinaxodon; in Didelphis the dorsal length of the chamber is approximately half of the total length of the skull.

Fig. 7. Thrinaxodon. Showing masseter and temporal muscles. Skull after Romer (1956). Approx. × 7/10.

The coronoid process in Thrinaxodon sweeps upward posterodorsally at an angle oblique to the long axis of the ramus. Angular, surangular and articular bones extend backward beneath and medial to the process. The process extends above the most dorsal point of the zygomatic bar, as in Didelphis. The mandibular ramus is ventrally convex in both genera.

The relationships described above suggest that Thrinaxodon and the therapsids having similar morphology in the posterior region of the skull possessed a temporal adductor mass that was split into major medial and lateral components ([Fig. 7]). The more lateral of these, the masseter, arose from the inner surface and lower margin of the zygomatic bar and inserted on the lateral surface of the coronoid process.