A partial cranium and mandible of Paranthropus robustus was discovered in 1938 by a schoolboy , 70 km south west of Pretoria in South Africa . A partial cranium and mandible of Paranthropus robustus was discovered in 1938 by a schoolboy , Gert Terblanche , at Kromdraai B ( 70 km south west of Pretoria ) in South Africa .