Temporal Line Orientation

The development and orientation of the temporal lines is clearly associated with the development and orientation of the temporalis muscle, which itself will be influenced by sexual dimorphism (Kimbel, 1988; Rak, 1983). As such, only male specimens (where available) are considered. While some might argue against the inclusion of such "obviously functional" features within a hominin phylogenetic analysis (Skelton & McHenry, 1992), the concept of "phylogenetic niche conservatism" as discussed in the main text suggests that they are of phylogenetic interest. Other features, such as postorbital constriction, degrees of cranial base angulation, frontal lobe development, and neuro-orbital disjunction, will also influence the development and orientation of the temporal lines. As such, the development of this feature is probably quite complex, due to the numerous mosaic processes impacting the development of upper facial and frontal form.

Temporal lines converging anteriorly to help define an anterior frontal trigon (= 0) occur in Dryopithecus specimen RUD 44 (Begun, 1994, 2002) and species within Paranthropus (Rak, 1983), Pongo, and Gorilla. While this condition is not preserved entirely in either Graecopithecus, the surviving temporal lines, supraorbital tori, and frontal parts suggest that it was characterized by a marked frontal trigon (de Bonis & Koufos, 1993). Moderate anteromedial incursion of the temporal lines (= 1), but not resulting in a frontal trigon, is observed in Sahelanthropus (Brunet et al., 2002), Praeanthropus (Kimbel, 1988; Kimbel et al., 1994), Australopithecus (Rak, 1983; Kimbel et al., 1994), the "garhi group" (Asfaw et al., 1999), and most male specimens of Pan. The condition in H. habilis is undeveloped (= 2) (Strait et al., 1997; Strait & Grine, 2001). Kenyanthropus platyops is described as having a similar pattern to that observed in the "1470 group" (K. rudolfensis), which is similar to H. habilis in this regard (M.G. Leakey et al., 2001 — see Table 2). Thus, Kenyanthropus is coded the same condition as H. habilis, as is H. ergaster (B.A. Wood, 1991).

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