Final answer:
The robust australopithecines had a ridge of bone on top of the skull called a sagittal crest, which anchored large temporalis jaw muscles for chewing tough plant materials.
Step-by-step explanation:
The robust australopithecines, which include species like Paranthropus robustus, P. boisei, and P. aethiopicus, had a distinctive feature on their skulls known as a sagittal crest. This pronounced ridge of bone was located at the top of the skull and provided an anchoring point for the large temporalis jaw muscles, which were necessary for chewing tough plant materials. In comparison, gracile australopithecines, such as Australopithecus afarensis and A. africanus, had smaller teeth and jaws and did not possess this sagittal crest.