Final answer:
Robust australopithecines had small front teeth and large back teeth along with a sagittal crest indicative of strong jaw muscles, adaptations for a tough plant-based diet. Gracile australopithecines, on the other hand, had smaller teeth and less pronounced facial features.
Step-by-step explanation:
The distinctive traits of robust australopithecines, classified under the genus Paranthropus, include both small front teeth (canines and incisors) and a big face with features like a sagittal crest. Specifically, Paranthropus species such as Paranthropus robustus and Paranthropus boisei possessed adaptations for a diet composed mainly of hard nuts and seeds. These adaptations manifested as very large back teeth (molars) for grinding down tough plant material and a sagittal crest to anchor large jaw muscles necessary for such a diet. Conversely, gracile australopithecines had more pronounced jaw projection and smaller teeth, indicating different dietary adaptations.