Originally labelled "Zinjanthropus", A. boisei has been identified based on the size of the attachment areas for the masticatory muscles ( note the well developed sagittal crest) and the massive back teeth (molars). These features characterize this hominid group.

You can see the large mandible and ramus of A. boisei. The face predominates the entire cranium. A. boisei had a brain capacity of about 500 cc. This is relatively small compared to the robustness of this hominid. A. boisei and A. robustus were larger than early australopithecines.

Note the relatively flat face.

This recently discovered fossil find has changed the way paleoanthropologists have traditionally looked at the development of the specialized robust australopithecine group. WT17000 has a very small brain capacity of only 410 cc as compared to A. robustus with an average of about 500cc and A. boisei of about 530 cc. It has a massive sagittal crest. WT17000 is clearly in the A. boisei lineage but it is older than even the A. robustus finds from southern Africa. The dating of WT17000 is now set at about 2.6 to 2.65 million years ago. This specimen is also known as the "Black Skull".

The Wt17000 specimen has the most prominent sagittal crest of any hominid. The crest anchored huge jaw muscles, probably needed for chewing tough plant fibers. In contrast to A. africanus, note the position of the eyes relative to the top of the cranium.

The angle of reflexion (the base of the skull and the long axis of the face) is different from A. africanus. However, the face exhibits marked prognathism in contrast to the later A. boisei individuals who had a relatively flat face. This is a characteristics more similar to A. africanus.

The cranial capacity for WT17000 is only 410 cc. This is even smaller than some specimens of A. afarensis.

For further reference read A Skull to Chew On by Donald C. Johanson (Natural History, 5/93, pages 52-53) and this reading from Rick Potts' Humanity's Descent may help you put things together a bit more.

This is an A. boisei from Koobi Fora
(KNM-ER 406). This fossil shows the well developed sagittal crest and robust (wide) face of A. boisei. The cranial capacity for this individual was about 510 cc. Taken in relative terms to body size, this is virtually the same size as for A. africanus.

This is the side view for the previous specimen. A. boisei had massive zygomatics and mastoid process. This is associated with the well developed chewing apparatus that marks this hominid specialized diet of hard foods. From a look at the cranium from the top we can see the flaring zygomatic arches that would have been filled with muscules reaching downward from the sagittal crest to the large mandible (jaw).

This top view also shows the relationship of the small brain to the face. The robust forms of australopithecine had small brains relative to their body size and particularly to the remainder of the cranium - the face region. They specialized in a diet based on dry vegetable materials requiring a massive face and massicating apparatus to grind food. Afterall, it does require a lot of brain power to "hunt down" plants of the savanna.

This is an example of A. robustus from south African. This specimen illustrate the similar features that mark A. boisei and A. robustus - sagittal crest, massive face, and large posterior (rear) teeth. The mandible of the robust australopithecines is very robust and the ramus is wide in comparison to other hominids. Note that A. robustus was not as large as its east African counterpart.

SEXUAL DIMORPHISM Sexual dimorphism among the early hominids was pronounced. You have already observed the difference between males and females in A. afarensis. This illustrates the difference between a male on the left and a female on the right for A. boisei.

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