The planum nuchale: tilt and height

The tilt of the planum nuchale

The planum nuchale of the occipital squama is tilted at only a slight angle to the plane of the foramen magnum which, in turn, is tilted upwards at an angle of 7° to the F.H. (see section on ‘The Plane of the Foramen Magnum’). Thus the planum nuchale of Zinjanthropus is much more horizontal than is that of the Pongidae, in which the nuchal surface rises steeply upwards to the nuchal crest. This low, nearly horizontal planum nuchale is characteristic, too, of Australopithecus (e.g. MLD 37/38 and Sts 5), though in the very unusual and apparently deformed cranium VII from Sterkfontein, the planum is much steeper and more pongid in appearance. Broom and Robinson (1950, p. 25) suggested that this unique feature in Sts VII, which they likened to the effects of artificial deformation on Amerindian crania, was due to ‘slow postmortem crushing without very manifest breaking of the bones’. In Sts VIII, as well, the planum nuchale ‘makes an angle with the main part of the base of the basioccipital of about 49°’ (Broom and Robinson, 1950, p. 28). The latter plane is thus not nearly as horizontal as in Zinjanthropus. Of the Paranthropus crania published, the only ones in which this area is satisfactorily preserved are those of two juveniles from Swartkrans, in which the plane of the nuchal surface is very similar to that in Zinjanthropus (Broom and Robinson, 1952, pp. 26–30).