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The evolution of the feeding apparatus is described from Dimetrodon (a primitive mammal-like reptile representing a pre-therapsid stage of evolution) through theriodont therapsids (moderately and fully advanced carnivorous mammal-like reptiles from which mammals were ultimately derived). Osteological changes are analyzed in terms of modifications in the adductor jaw musculature. In Dimetrodon, very primitive theriodonts, and therocephalians, adductor jaw musculature did not descend beneath the zygomatic arch to insert over the lateral surface of the lower jaw. With the exception of M. pterygoideus, the jaw musculature in these animals was confined within the temporal fossa where part of this muscle mass approached the arrangement of M. adductor mandibulae externus (filling the temporal fossa lateral to the mandibular branch of the trigeminus nerve) in living reptiles. The insertion of this external adductor was probably characterized by a concentrated tendinous attachment to the coronoid eminence or abbreviated coronoid process and a broad fleshy attachment to the dorsal and dorsomedial surfaces of the jaw. In addition there was no origin of adductor jaw musculature from the medial surface of most of the zygomatic arch. Two groups of theriodont therapsids present evidence that each independently departed from the more primitive arrangement of jaw musculature. In gorgonopsians adductor musculature may have descended beneath the zygomatic arch to insert on the lateral surface of the angular. In cynodonts, on the other hand, two major changes occurred. The first change was the appearance of a masseter muscle whose development was intimately related to the posteroventral expansion of the dentary including the formation of an angular process. The descent of the insertion of this muscle onto the exposed lateral surface of the dentary was accompanied by the establishment of the entire length of the zygomatic arch as an area of muscular origin. The second change was the development of a pattern of insertion characteristic of the mammalian temporalis muscle, accomplished through the posterodorsal expansion of the coronoid process of the dentary. The result of these modifications was that cynodonts established an arrangement of adductor jaw musculature closely approaching that in living mammals. For this and other reasons cynodonts appear to be excellent candidates for the ancestors of all mammals.