A large sample of fossils from Sterkfontein, South Africa, represent the species Australopithecus africanus. Most of these fossils come from Member 4 at the site, with an age estimated between 2.5 and 2 million years ago. Excavations from Sterkfontein have yielded more than 600 fossil hominin specimens, most of them teeth but with many partial skulls, mandibles, and postcranial bones.

This lab includes three notable Member 4 Sterkfontein skulls. Sts 5 is the most famous fossil from the site, uncovered in 1947. Stw 13 is a partial skull with similar facial and cranial morphology to Sts 5 where it is preserved, but the skull bears considerable distortion from the process of fossilization. Stw 505 is a large skull of Au. africanus, with a tall and broad face and large brain size compared to the smaller Sts 5. The lab also includes a human skull for comparison.

A number of features can be observed in these Sterkfontein crania that characterize Au. africanus.

• Bell-shaped cranial profile. When viewed from the back, Au. africanus skulls have their maximum width across the mastoid region of the temporal bone. The base of the skull flares slightly outward, while the sides of the skull curve upward and toward the midlline of the skull, giving it a bell-like shape. This is especially visible on Sts 5.
• Subnasal prognathism. The face of Au. africanus skulls projects forward, especially the part below the nose.
• Anterior pillars. A raised bar of bone extends upward from the canine roots to make up both the left and right sides of the nasal aperture. All three of these Au. africanus skulls show this feature.
• Brain size. The endocranial volumes of these skulls are around a third the size of recent humans. Sts 5 has a volume of around 485 ml, while the Stw 505 is larger, between 580 and 600 ml. The range of brain sizes in the known Au. africanus sample is between 400 and 600 ml.

In its overall cranial shape, Au. africanus is similar to known fossil skulls attributed to Au. afarensis and Homo habilis. Anthropologists recognize these similarities. The differences between the skulls of these species are in their teeth and mandibles, details of their faces related to the orientation of the jaws, and the size and orientation of the muscles of the neck.

Materials in this lab

• The original Sts 5 fossil is curated at the Ditsong Museum of Natural History in Pretoria, South Africa. The model in this virtual lab is based upon a 3D surface scan of a cast in the Biological Anthropology collection of UW-Madison.
• The original fossils of Stw 13 and Stw 505 are curated at the University of the Witwatersrand in Johannesburg, South Africa. The models in this virtual lab are based upon casts in the Biological Anthropology collection of UW-Madison.
• The model of the human calvaria is based on an anatomical model created by Hannah Newey. The model is available on Sketchfab with a Creative Commons Non-Commercial Share-alike (CC-BY-NC-SA) license. I reduced the polygon count of the model for this virtual lab.

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