Final answer:
Dart thought that the killer ape was Australopithecus africanus, which was discovered in South Africa and represented an extinct race of apes intermediate between living anthropoids and man.
Step-by-step explanation:
The killer ape that Dart thought was responsible for the Taung Child skull is Australopithecus africanus. This species of australopithecine was discovered by Dart in 1924 in South Africa. The Taung Child skull provided evidence of bipedalism and an upright posture, characteristics that distinguish it from modern apes and suggest a closer relationship to humans.