On the cranial anatomy of some gorgonopsids and the synapsid middle ear.

Abstract

SUMMARY. Part of a collection of gorgonopsids from Upper Permian rocks of the Ruhuhu Valley in Tanganyika is described. Detailed accounts are given of the quadrate‐articular hinge mechanism and the reflected lamina of the angular from fragmentary remains, and skulls of Scylacops capensis Broom and Dixeya quadrata Haughton are described. An attempt is made to restore the jaw musculature of gorgonopsids. It is concluded that the posterior pterygoideus muscle was inserted on the ventro‐medial surface of the articular and that a masseter was present and was inserted on the reflected lamina of the angular. The evolution of the masseter muscle is discussed and it is concluded that it arose in captorhinomorph cotylosaurs, and that it was inserted on the deep Made of the angular in primitive pelycosaurs, and on the reflected lamina of the angular in all more advanced synapsids except the most advanced theriodonts. The mechanics of retroarticular processes are examined. It is concluded that the deflected articular process present among synapsid reptiles generally was a true retroarticular process, giving attachment to a depressor mandibuli muscle, and that its ventral deflection was a modification to permit the wide gape necessary in forms having large canine teeth. The nature of the typical synapsid middle ear is discussed. It is concluded that the available data are best accounted for by assuming that an external auditory meatus, a post‐quadrate tympanum and an extra‐stapes were present, and that the ear was sensitive to airborne sound.