Sagittal Cresting in the South African Australopithecines

The evidence for sagittal cresting, and more generally the position of the temporal lines is reviewed in the South African australopithecine sample. The position of the lines is dependent on both the allometric relation of the masticatory apparatus to cranial size and on individual variation. In the Swartkrans specimens, with generally bigger body size, the influence of allometry predominates, actually overshadowing the influence of individual variation. At Sterkfontein and Makapansgat with generally smaller body size and a resulting smaller allometric ratio, individual variation has a greater influence. Of the eleven adult South African specimens, the four largest are crested. The one smaller crested specimen comes from Sterkfontein. The crested Makapan specimen is intermediate in size. The pattern of australopithecine cresting is somewhat different from other hominoids, and is part of a total morphological pattern suggesting adaptation to a diet requiring powerful crushing during mastication. Sagittal crests in hominoids are compound crests caused by the pull of both temporalis muscles when they meet at the midline of the cranium. Functionally, the crest provides additional area for the attachment of m. temporalis when the area provided by the external cranial surface is insufficient. Formation of a sagittal crest is under the direct control of the superior fascial attachment of m. temporalis, and does not necessarily depend on the meeting of two simple temporal crests. If the two superior fascial attachments of m. temporalis meet at the midline, the fasciae run together to the underlying bone, and a two layered fibrous septum between the muscles is formed (Scott, '54). The resulting osteogenic activity leads to ossification extending into this septum, producing a sagittal crest. Ossification proceeds along the union of the fascia and the outer layer of the periosteum, and the resulting crest consists of two laminae which later fuse together. The cresting process is the same for all hominoids. However, the position of the crest when it first forms, and its anterior and posterior extension, depend on the relative development of the anterior and posterior fibers of m . temporalis. Zuckerman ('66) has shown that the fiAM. J. PHYS. ANTHROP., 40. 3 9 7 4 0 8 bres of m . temporalis first come together posteriorly in many primates, and that the sagittal crest first forms in the posterior region, often in conjunction with a compound temporahuchal crest. This does not seem to be true in australopithecines where the temporal lines first meet at bregma or anterior to it, and the compound or intermediate temporal/nuchal crest, when i t occurs, is only along the lateral third of the nuchal crest (Robinson, '58; Tobias, '67). This indicates a relatively greater development of the anterior fibers of m. temporalis. This anterior development also occurs in H . sapiens where the closest approach of the temporal lines in crania with high lines occurs in the region of bregma (Riesenfeld, '55). There are two variables which systematically influence the relation of temporalis size, and area available for attachment The first is the allometric relation between the size of the masticatory apparatus and the size of the cranium. As Huxley ('32) noted for baboons, increase of body size in the same, or even very similar, species leads to far greater differences in the masticatory apparatus than in the size of the cranium, resulting in a disproportionate 1 This research was supported by NSF grant (25.33035.