Variation in the pattern of cranial venous sinuses and hominid phylogeny

In 1967 Tobias noted that Australopithecus boisei cranium O.H.5 exhibited a cranial venous sinus pattern in which the occipital sinus and the marginal sinuses of the foramen magnum appeared to have replaced the transverse‐sigmoid sinuses as the major venous outflow track. Specimens of A. robustus and several more recently recovered A. boisei crania also show evidence of enlarged occipital‐marginal sinuses. In contrast, A. africanus and H. habilis retain a dominant transverse‐sigmoid system that characterizes the great majority of extant apes and modern human cadaver samples. Pliocene A. afarensis exhibits a high frequency of occipital‐marginal drainage systems. An examination of several series of precontact North American Indian crania shows that the frequency distribution of the occipital‐marginal sinus pattern is spatiotemporally disjunct, ranging from 7.5% to 28%. The Late Pleistocene sample from Předmost, Czechoslovakia, also shows a very high incidence of occipital‐marginal sinus patterns ( ∼ 45%). These observations suggest that occipital‐marginal and transverse‐sigmoid sinus patterns are adaptively equivalent character states. This conclusion is supported by the fact that enlarged occipital‐marginal and transverse‐sigmoid sinus systems often coexist on the same and/or contralateral sides of the head. It is well known that the frequencies of such adaptively neutral traits are often heavily influenced by population‐specific epistatic interactions. The utilization of such traits in phylogenetic reconstruction entails a substantial risk of mistaking parallelism for synapomorphy. It is concluded that using functional‐adaptive criteria in the definition of morphologic characters is a more reliable method to guide phylogeny reconstruction. In light of this, the distribution of venous sinus variants in Plio‐Pleistocene hominids gives little or no basis for revising the phylogenetic scheme of Johanson and White (1979), or the functional‐adaptive interpretation offered by White et al. (1981).