STAND UP TALL: THE POSITION OF THE FORAMEN MAGNUM IN BIPEDAL AND QUADRUPEDAL PRIMATES

Abstract

In human paleontology, the position of the foramen magnum has been widely used as an indicator of locomotion pattern in bipeds and quadrupeds. Regardless, previous studies have failed to comprehensively show a functional connection between an anteroposterior foramen magnum position, a shortening and broadening of the cranial base and bipedalism. Many have argued that early Pliocene primates, for example Sahelanthropus, Ardipithecus ramadis and Australopithecus are in fact the earliest known hominids, as they exhibit a relatively anterior foramen magnum and a short and wide basicranium. Metrical and morphological comparative analyses were conducted, using 3D computerised tomography scans, of the cranial base of 46 extant Homo fossils (one species: Homo sapiens) and 28 extant non-habitually bipedal primates fossils (four genera: Papio, Pongo, Macaca, Gorilla). Measurements of the two auricular points, the two carotid foramina, basion to the opisthion and the lateral tympanic length were taken on all samples. The data were then used to test the hypothesis that an anteroposterior position of the foramen magnum, deduced by a shortening and widening of the cranial base, is correlated with facultative bipedal locomotion. The results show that in bipeds, as represented by the Homo sapiens sample, the cranial base is shorter and broader than that of quadrupeds. Additionally, the carotid foramina has shifted laterally and the mediolateral tympanic length has shortened, indicating a relatively anterior position of the foramen magnum in Homo sapiens compared to the non-habitually bipedal primates. These changes to the cranial base are associated with hominid status and bipedalism but conclusions must be made with caution.