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 P redmost , Czechoslovakia, also shows a very high incidence of occipital-marginal sinus patterns (approximately 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).