Final answer:
Australopithecines had brain sizes and skull shapes most similar to early hominids and great apes, but displayed bipedal locomotion with humanlike feet indicative of their evolutionary trajectory towards modern humans.
Step-by-step explanation:
In terms of brain size and skull shape, australopithecines were most similar to other early hominids and great apes. Australopithecines had relatively small brains compared to modern humans, with cranial capacities ranging from about 350 to 500 cubic centimeters (cc), which is less than the approximately 1,400 cc of modern human brains. Their skull shape featured a more sloped forehead, a pronounced jaw, and larger teeth, akin to that of great apes. However, australopithecines were bipedal with humanlike feet, indicating a significant evolution towards modern human morphology.
The skeletal structure also displays differences in arm bones and pelvis shape that indicate a locomotion partly similar to and partly distinct from modern humans. Some australopithecines, like Australopithecus afarensis, had long arms and curved finger and toe bones, suggesting a mix of arboreal and bipedal habits. Additionally, gracile australopithecines exhibited less pronounced facial features and smaller teeth compared to robust australopithecines, which had larger jaw muscles and a sagittal crest to support them.