The character is generated from metric data and is defined by an index of mid-interorbital breadth divided by orbital height (see earlier). A narrow interorbital has often been argued to by a synapomorphy of the Ponginae and is associated with patterns of airorynchy (see Shea, 1985, 1988; Andrews, 1992; Cameron, 2002). As such, this character is of developmental and phylogenetic interest. The extant hominid mean is 0.45 (n = 123), with one standard deviation of 0.11. Thus, the intermediate range is 0.34-0.56. A narrow interorbital is an index less than 0.34, while a broad interorbital is an index beyond 0.56. It was noted that an additional character state was required because a few hominids were beyond the extant hominid maximum value for this feature; that is, an index greater than 0.77. Thus, a fourth character state is recognized as an "extremely broad" interorbital. All indices for the fossil hominins have been generated from the data provided in B.A. Wood (1991), unless stated otherwise.
A narrow interorbital (= 0) is observed in Pongo with a mean index of 0.32 (n = 22). The intermediate condition (= 1) is observed in Gorilla with a mean index of 0.50 (n = 35), and Pan with a mean index of 0.46 (n = 66). A broad interorbital (= 2) is observed in Dryopithecus specimen RUD 77 (data from Kordos & Begun, 1997) with an index of 0.58; Graecopithecus specimen XIR-1 (original) has an index of 0.60; Australopithecus with a mean of 0.63 (n = 2); P walkeri specimen KNM-WT 17000 (cast) with an index of 0.60; K. rudolfensis with a mean index of 0.73 (n = 2); H. habilis with a mean index of 0.71 (n = 2); H. ergaster with a mean index of 0.63 (n = 2); and H. sapiens with a mean index of 0.65 (n = 6). An extremely broad interorbital (= 3) is observed in P. boisei, with a mean index of 0.72 (n = 2), and P. robustus, with a mean index of 0.83 (n = 2); that is, they are beyond the maximum values observed in the extant hominids.
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