The mammalian lower jaw comprises a single bone, the dentary, which is a unique feature among vertebrates. The lower jaws of extinct non-mammalian synapsids were composed of the dentary and several postdentary bones. Synapsid fossils exhibit variation in the dentary size relative to the overall lower jaw. An evolutionary trend toward dentary enlargement and postdentary reduction in non-mammalian synapsids has long been documented but has not been established using modern phylogenetic comparative methods. In this study, we examine the evolutionary pattern of dentary size relative to the lower jaw through phylogenetic analyses of measurements in a broad range of non-mammalian synapsid taxa. Our analyses revealed an evolutionary trend toward dentary area enlargement relative to the overall lower jaw in the lateral view across all non-mammalian synapsids. This trend is likely due to vertical expansion of the dentary given that the same trend is not evident when looking at anterior to posterior measurements of the dentary relative to the lower jaw as a whole in lateral view. Ancestral character reconstructions revealed that the evolution of the measurements was not unidirectional in non-mammalian synapsids. Our results provide no evidence of an evolutionary trend toward the dentary enlargement at the expense of postdentary bones across non-mammalian synapsids. This implies that the evolutionary origin of the mammalian lower jaw is not adequately explained by the evolutionary trend of dentary enlargement throughout non-mammalian synapsids. Instead, selection that occurred during the transition from non-mammalian cynodonts to early mammals may have produced the mammalian lower jaw.