This feature measures the general robusticity of the mandibular symph-ysis. That is, a high index means the symphysis is robust, while a lower index means it is more gracile. As suggested by Hylander (1984, 1988), Hylander and Johnson (1994), and Hylander et al. (2000), the thickness of the symphysis has important implications associated with wishboning and its impact on the symphyseal region. All data for the fossil hominins have been taken from B.A. Wood (1991), and data for the extant hominids are either unpublished data collected by DWC, unless stated otherwise. The mean index for the extant hominids (n = 99) is 0.38, with one standard deviation of 0.05. A gracile symphysis is defined by an index of less than 0.33, an intermediate breadth is between 0.33 and 0.43, while a robust symphysis is defined by an index greater than 0.43. In addition to this, the minimum value for the extant hominids is 0.26, while the maximum value is 0.51.
A relatively gracile symphysis (= 0) is observed in Pongo with a mean index of 0.32 (n = 22). The intermediate condition (= 1) is observed in Kenyapithecus specimen KNM-FT 45 (reconstruction) with an index of 0.40 (data from Andrews, 1971), Gorilla with a mean index of 0.42 (n = 23), Pan with a mean index of 0.38 (n = 54), and H. sapiens with a mean index of 0.43 (n = 128 [data from Thorne, 1976]). A robust mandible (= 2) is observed in the "anamensis group" with a mean index of 0.50 (n = 3 [data from C.V. Ward et al., 2001]), Praeanthropus with a mean index of 0.50 (n = 4), and Australopithecus specimen Sts 52 with an index of 0.51. Finally, an extremely robust symphysis (= 3) is seen in P. boisei with a mean index of 0.56 (n = 7), K. rudolfensis with a mean value of 0.63 (n = 4), H. habilis with a mean index of 0.69 (n = 2), and H. ergaster with a mean index of 0.57 (n = 4).
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