枕骨圆枕的变异

VARIATION IN THE OCCIPITAL TORUS

对印第安人、阿留申人、华北人和云南人总共５０７具头骨的枕骨圆枕发育程度进行观察，结果表明枕骨圆枕任一发育级别的出现率在相同人种且性别相同的人群之间并无显著差别．枕骨圆枕的发育程度可以被认为是“遗传标记”．作者认为在化石人类中枕骨圆枕的发育程度会有较大的变异；枕骨圆杭的发育程度不是一个有效的鉴别性状．

The occipital torns refers to a thickened band of bone usually betWeen the superior and supreme nuchal lines. Since Weidenreich published his study in 1940, the size of the oCCipital tornshas been thought to decrease over the course of human evolution. A strong torns can be aden incrania of Homo e.rectus, whereas a relatively weak one is presented in HOrn o sa￣s. A well--developed occipital torns therefore may be useful in the diagnosis of Homo erectus.Recent finds and dating resultS in China indicate that extensive variahon occurred in thedevelopment of the occipital torns during the period 190--230 by B.P. Thus, some authors havesuggested the co--existence of Hobo erectal and Homo swim in China during that time. However. it is also possible that the occipital torns is not a genehc marker, but rather may be anadaptive skeletal response to veriable nuchal muscle action.To help establish whether middle Pleistocene oCCipital variations in China reflecttaxonomy, adaptation, dimorphism, or mechanical consequences of differing cramal form, observations and measurementS were taken on 507 modern human skulls. These skulls representpopulations of Mongoloid affinity across a wide range of adaptive conditions, including samples of American indians, Aleuts. North Chinese. and from Yunnan.In eaCh skull the degree of oeeipital torns development was recorded and 14 measurementswere taken rcflCCting cramal and occipital forms. The degree of occipital torns development wasdivided intO four categorics:0: No torns. Supreme nuchal line (SMNL) cannot be idenhficd. Superior nuchal line afL)may or may not be discernible. The surface between SMNL and SNL is smooth and levelwith the occipital and nuchal planes.l: Slight elevation betWeen SMNL and SNL, and SMNL shows a trace only. Alternatively,two small rounded elevations, one on right and left sides, appear between SMNL andSNL, and the SMNL presents itS median portion only.2: Moderately developed elevation between SMNL and SNL. The SMNL and SNL arc didtinct. The SNL extends laterally toward asterian.3: Strong elevation with distinct demarcations (SMNL and SNL). The SMNL and SNL are'pronounced and elevated. The SNL extends laterally toward asterian.ResultS of these observations are given in Table 1. Testing the occurrence of each degree ofdripital torns development, xZ testS indicate that there are no significant differences betweendifferent populations in either the male or the female samples (P>0.05). There is. however, aaignificant difference between male and female samples within populations (P < 0.01).To examine the correlation between occipital torns development and cramal or occipitalform, statistics for the 14 measurements are given by sex and population. Analysis suggestS thatonly occipital angle and nuchal plane length may be correlated with torns development withinmale or female samples of some (but not all) populations (Tables 2 alld 3). This correlation isnot widespread, and the development of the occipital torns is not necessarily affected by overallcramal and oedpital form.To summarize, the incidence of each category of oCCipital torns development appears tO beidentical across populations, controlling for sex. Moreover, the incidence is not sttongly affected by geographic (environmental) differences in male or female samples of populations havingthe same raceal affinity. Genetic factors thus may be operating in the development of theOCCipital torns. In this sense, oedpital torns size may be considered a genetic marker.Wide variahon in the development of the oCCipital torns can be talced back to Homo erectus. Comparedespecially with torns shape, the development (size) of the torns does not appear to be an cffcobye taxonomic character, although genetically controlled.This work was funded by SFPPAS of Acadcmia Sinica, the Office of Fellowships andGrants of the Smithsonian institution, and a grant from the American Museum of Natllral History. The authors wish to express their grateful thanks for helpful suggestions to Prof. M.H.Wolpoff, Prof. E. Delson, and Dr. I. Tattersall, and fo