The single bony element forming the lower jaw of living mammals, the dentary, has been interpreted as representing the culmination of a long and gradual evolutionary trend. Numerous fossil nonmammalian synapsids (“mammal-like reptiles”) show varying degrees of enlargement of the dentary and concomitant reduction in the postdentary bones. To quantitatively reexamine patterns of morphological change in the evolution of the mammalian lower jaw, measurement and discrete character data were collected from 322 fossil synapsid mandibles spanning Late Carboniferous through Jurassic time. Measurements confirm that the relative contribution of the dentary increased in theriodont (advanced therapsid) evolution with regard to both stratigraphic and phylogenetic position. However, dentary enlargement and postdentary reduction failed to typify all therapsid subclades. Qualitative characters of the mandible were used to quantify morphological similarity with regard to the early mammal Morganucodon. Analyses contrasting stratigraphic and phylogenetic position with mammalian similarity indicate that mandibular evolution was primarily conservative, with only anomodont therapsids evolving substantial morphological novelty. Scaling analyses comparing the area of the dentary and postdentary regions to jaw length uniformly show isometry or slight positive allometry, although cynodont therapsids have a smaller postdentary region than any other therapsid subgroup. These results suggest that body size decreases cannot fully explain the reduction of the postdentary bones. Finally, step size bias was tested as a mechanism for explaining long-term trends. Qualitative data reveal no significant difference in the magnitude of character changes occurring in mammalian and nonmammalian directions.