Final answer:
Robust australopithecines have larger teeth and jaws, a sagittal crest, and flared zygomatic arches compared to gracile australopithecines, to chew tough plant material.
Step-by-step explanation:
The robust australopithecines are so-called because they have larger teeth and jaws than gracile australopithecines. Unlike their gracile counterparts, robust australopithecines had adaptations suited for heavy chewing, such as the presence of a sagittal crest and flared zygomatic arches indicating large temporalis jaw muscles. These characteristics allowed them to process tough plant materials in their diet. In contrast, gracile australopithecines had smaller dental features and lacked these robust cranial adaptations, suggesting a diet that included softer food items.