Functional, morphogenetic and phylogenetic significance of conjunction between cardioid foramen magnum and enlarged occipital and marginal venous sinuses

A quarter of a century ago the study of Australopithecus boisei , a hyper‐robust ape‐man first found at Olduvai Gorge in Tanzania (Tobias, 1967), revealed that in A. boisei and A. robustus the drainage of the intracranial venous sinuses had occurred predominantly by means of enlarged occipital and marginal venous sinuses, instead of, or as a supplement to, the customary hominoid pattern of drainage by way of the transverse and sigmoid sinuses. Later the Hadar hominids (usually assigned to A. afarensis ) were found to show similar selective drainage through enlarged occipital and marginal sinuses. The same 1967 study led to the discovery of a hitherto unrecorded pattern of the foramen magnum which was broad, truncated, almost heart‐shaped or cardioid and which occurred in some of the early hominids ( A. boisei , probably A. robustus ). In a new study on Hadar casts, the senior author has concluded that, in these Ethiopian hominids as well, the shape of the foramen magnum was most probably cardioid. Thus, in A. boisei and probably in A. robustus and Hadar, we have the remarkable association of enlarged occipital and marginal sinuses and of cardioid foramina magna. This conjunction we propose is not fortuitous, but results from a functional and morphogenetic correlation between the two sets of traits. A careful study of the contents of the foramen magnum leads us to infer that the occurrence of enlarged occipital and marginal sinuses drastically modified the pattern of drainage of the blood around the foramen magnum, with consequential changes in the shape of the foramen, especially of its anterior and posterolateral margins. The occurrence of this uniquely different, shared‐derived (synapomorphic) character complex in these three groups of early hominids provides further evidence that they had seemingly become specialised away from the lineage leading to later hominids of the genus Homo .