Zinj and the mastication

This is a nice and rare illustration by Bonnie Miljou of Zinjanthropus boisei (‘Zinj’), the species defined by Louis Leakey for the iconic fossil OH 5 – later assigned to Paranthropus boisei. Due to the hyper-robust morphology of its dentition, OH 5 was nicknamed ‘Nutcracker Man’, which gives the name to this blog.

The morphology of this cranium is clearly associated with the masticatory function. In particular, the illustration highlights 5 features:

Zinjanthropus boisei

Zinjanthropus boisei. Illustration by Bonnie Miljou

  • Visor-like cheekbones: the infraorbital region has a shape of a visor extending far anteriorly and laterally from an imaginary vertical line drawn from the lateral orbital margins. The frontal process of the zygomatic bone widens considerably, in a similar way as in Australopithecus africanus. The transition between the front-facing infraorbital surface and the laterally-facing surface is very gradual, unlike P. robustus in which both regions are distinctly divided.
  • Infraorbital foramen: an opening in the maxilla located below the infraorbital margin. In Zinj this is very inferiorly positioned on the face – as well as in P. robustus, in contrast to the more superior position in Australopithecus.
  • Sagittal crest: an anchor for the strong temporal muscles, larger than in P. robustus although the largest crest belongs to P. aethiopicus (KNM WT 17000, ‘Black skull’). Several australopitecine specimens have a sagittal crest, though not so well-developed.
  • Frontal trigon: the temporal lines converging anteriorly form a concave smooth triangular area on the frontal bone just behind the orbits. This feature is present in Paranthropus but not in most of the Australopithecus.
  • Temporomandibular joint: this hinge connected Zinj’s massive lower mandible to its temporal bone, with a morphology oriented for strong transverse movements.

The following images depict more features of the OH 5 cranium – these are extracted from the post ‘Dear Boy: todo sobre OH 5, Nutcracker Man’ [access]


  • Zinjanthropus boisei. Illustration by Bonnie Miljour, senior scientific illustrator of the Museum of Paleontology at the University of Michigan.
  • The Australopithecine Face. Yoel Rak. Academic Press, 1983
  • An Introduction to Human Evolutionary Anatomy. Leslie Aiello. Elsevier, 2002


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