Final answer:
Robust australopithecines, including Paranthropus species, were unique due to their large jaw muscles supported by sagittal crests and flat faces, adaptations for a diet of hard plant materials.
Step-by-step explanation:
The unique aspect of the robust australopithecines of the late Pliocene was their adaptation to a specific dietary niche. Species such as Paranthropus robustus, Paranthropus boisei, and Paranthropus aethiopicus had large temporalis jaw muscles supported by sagittal crests, and their molars were adapted for grinding tough, fibrous plant materials. Unlike their gracile counterparts that had smaller teeth and projections of the jaw, robust australopithecines had flatter faces with reduced projection due to their larger teeth. This physical adaptation suggests a diet that relied heavily on hard nuts and seeds, contrasting with the partially carnivorous diet of other hominids at the time. Moreover, the robust species are not considered ancestors to modern humans, instead representing a side branch on the hominin family tree that eventually went extinct.