The "Black Skull",field number KNM WT 17000, is pictured to the right. It gets its name from the black coloration of the fossil due to the high manganese content of the matrix it was embedded in. This find is a nearly complete fossil skull of a "robust" early human, and has been assigned to the species Australopithecus aethiopicus. Note the widely flaring zygomatic arches (the bones arching around the side of the skull to join below the eyes, forming the cheeks). These arches are typical of the robust face. The prominent sagittal crest running along the midline at the top of the skull is the largest ever discovered in the human lineage. Both of these features were adaptations for heavy chewing, and the cheek teeth are correspondingly large. However, there were problems with this classification as the skull was a curious mixture of primitive and derived traits. It's cranial capacity was the smallest ever recorded in an adult early human, and at 410 cc it was not much larger than that of a modern chimpanzee. A prominent sagittal crest is typical of later P. boisei. But, note the position of the sagittal crest in the "Black Skull" and compare it with the position of the sagittal crest in P. boisei. Compare the shape of the back of the skull with A. afarensis. Note the degree of prognathism (the degree to which the face projects in front of the brain case) and compare it with the above two species. It is more prognathic than P. boisei, and the position of the sagittal crest is far to the rear of the head, not near the top of the skull.