Three character states are recognized: large (0), intermediate to small (1), and absent (2). The frontal sinus in Dryopithecus is developed because it extends below nasion (Begun, 2002). A well-developed frontal sinus is observed in P. boisei (Tobias, 1967), P. robustus (Tobias, 1991), K. rudolfensis as defined by KNM-ER 1470 (B.A. Wood, 1991), and H. ergaster specimen KNM-WT15000 (Walker & Leakey, 1993b). Gorilla, Pan, and H. sapiens share a similar development (Spoor & Zonneveld, 1999), which Begun (1992) and Rae (1999) consider similar to Dryopithecus. Thus, all of the foregoing genera are allocated to the primitive hominoid condition of a developed frontal sinus system. Graecopithecus also appears to have a moderately developed frontal sinus (see Begun, 2002). In the "garhi group", the frontal sinus is restricted to the mid-frontal just above glabella; thus, it is intermediate to small (= 1) (Asfaw et al., 1999). This also applies to Australopithecus and H. habilis, in which it is described as small (Tobias, 1991). A frontal sinus is absent in Pongo (= 3), and while some pneumatization of the frontal may occur, it is regarded as an extension of the maxillary sinus (Cave, 1940; Koppe & Ohkawa, 1999).
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