Final answer:
The gracile australopithecines had a more pronounced face projection, smaller teeth, and no sagittal crest, indicating a varied diet. Robust australopithecines had features like a sagittal crest and larger molars for a diet of hard plant materials, and they lived approximately a million years longer than graciles. Paranthropus aethiopicus presents mixed traits of both groups.
Step-by-step explanation:
Differences Between Robust and Gracile Australopithecines: The anatomical differences between robust and gracile australopithecines are distinct. Gracile australopithecines, including species such as Australopithecus anamensis, A. afarensis, A. africanus, A. garhi, and A. sediba, are characterized by a more pronounced face projection or prognathism, less flared cheeks, absence of a sagittal crest, and smaller teeth and jaws. These features suggest they had a varied diet that did not rely heavily on tough plant materials. In contrast, the robust australopithecines, categorized under the genus Paranthropus which includes species such as Paranthropus robustus, P. boisei, and P. aethiopicus, had a sagittal crest, more pronounced zygomatic arches, and larger molars, indicating adaptation to chewing hard, fibrous plant materials. Their facial structure was flatter due to the reduction in the size of the incisors and canines, giving less projection of the jaw. Chronologically, gracile australopithecines appeared around 4 million years ago (MYA) and vanished by 2 MYA, while the robust species survived for another million years thereafter. Paranthropus aethiopicus, known as the 'black skull', shows a mix of both gracile and robust features, indicating it may fall somewhere between the two groups. The morphological differences and diet between these two groups show a fascinating divergence in the human evolutionary tree.