Dental, occlusal, and associated jaw morphology of the woolly spider monkey Brachyteles arachnoides are described in a functional and comparative context. The masticatory and digestive systems of this rare geographic isolate show a high order of concordance with homologous structures in the Alouattinae and differ in significant ways from the other Atelinae. Brachyteles is viewed as a facultative leaf-eater. These shared traits, many of a primitive nature, reflect common descent from a leaf-eating, brachiating proto-atelinine-alouattine ancestor. Lagothrix and Ateles are highly concordant in their masticatory characteristics, the form-functional attributes of which represent an evolutionary trend away from the primitive tooth form and transverse jaw functioning found in Brachyteles and Alouatta.