The calvaria of Zinjanthropus is a curious blend of extraordinary robusticity in parts and unusual thinness in others. Basically a thin-walled calvaria, the brain-case is modified in parts by (a) massive ectocranial superstructures, and (b) excessive pneumatisation.

Robusticity owing to pneumatisation

One could multiply measurements apace to demonstrate the unusual thickness and robusticity of different parts of the cranium of Zinjanthropus. A good area in which to demonstrate this is the region of the parietomastoid and occipitomastoid sutures, since comparative data are available, once again provided mainly by Weidenreich (1943, p. 66).

The first thickness is that of the parietomastoid suture just in front of asterion. In Zinjanthropus, this thickness measures 22·2 mm. on the left and 24·1 mm. on the right. For comparison the values in three crania (five sides) of Homo erectus pekinensis range from 15 to 18 mm.; and in modern man from 3·5 to 7·0 mm.

The second thickness is that of the occipitomastoid suture ‘medial to the mastoid process’. As in anthropoid apes, it is extremely difficult to take this measurement in Zinjanthropus because the whole pars mastoidea (not just the mastoid process) is pneumatised. For instance, in Zinjanthropus, the parietomastoid suture just in front of asterion is not the thickest part (which it is in H. e. pekinensis); instead the thickest part is to be found just behind the right mastoid process proper, where readings of 26–29 mm. can be taken.