Question: what is the main morphological difference between the gracile and the robust australopiths?
Answer: The "gracile" australopiths are ancestral to only the the "robust" australopiths. ... The "robust" australopiths are ancestral the "gracile" australopiths. e. The "robust" australopiths are ancestral to both the "gracile" australopiths and the genus Homo.