Among the issues that remain unresolved in regard to the origins of primates is the locomotor mode of the ancestral euprimate. In living primates, quadrupedal running and walking is the most common form of locomotion (Rose, 1973). Most analyses of fossil material also suggest that the ancestral primate locomotor type included some amount of quadrupedalism (e.g., Cartmill, 1972, 1974, 1992; Dagosto, 1988; Ford, 1988; Gebo, 1989a; Godinot and Beard, 1991; Godinot and Jouffroy, 1984; Martin, 1972; Szalay and Dagosto, 1980; Szalay and Delson, 1979; however, for a contrary view, see Napier, 1967; Napier and Walker, 1967). That most mammalian species are quadrupedal makes it tempting to view quadrupedalism as simply a primitive retention in primates. However, several lines of research indicate that the form of quadrupedal locomotion displayed by primates differs from that observed in other small to medium mammals (see Larson, 1998; Vilensky, 1987, 1989). These differences include use of a diagonal sequence/diagonal couplets walking gait pattern (Hildebrand, 1967; Howell, 1944; Prost, 1965,

Susan G. Larson • Department of Anatomical Sciences, Stony Brook University School of Medicine, Stony Brook, NY 11794-8081

1969; Rollinson and Martin, 1981; Vilensky, 1989; Vilensky and Larson,

1989); infrequent use of a running trot (Hildebrand, 1967); atypical patterns of shoulder muscle recruitment (Larson, 1998; Larson and Stern, 1989a,b; Whitehead and Larson, 1994), greater reliance on hindlimbs for both support and propulsion (Demes et al., 1992, 1994; Kimura, 1985; 1992; Kimura et al., 1979; Reynolds, 1985; however, see Schmitt and Lemelin, 2002); relatively low stride frequencies (Alexander and Maloiy, 1984; Demes et al.,

1990), relatively longer stride lengths (Alexander and Maloiy, 1984; Reynolds, 1987; Vilensky, 1980) brought about by relatively long limb bones (Alexander et al., 1979) and large limb angular excursions (Larson, 1998; Larson et al., 2000, 2001; Reynolds, 1987), and use of a compliant gait (Schmitt, 1994, 1998, 1999). The ubiquitous nature of these characteristics among primate quadrupeds suggests a shared origin early in the evolution of primates. However, exactly how these characteristics figure into the debate about the locomotor mode of the ancestral euprimate depends on identifying morphological correlates of these behaviors in fossil forms to determine when and where they might have arisen.

Unfortunately, other than relative limb bone length and functional differentiation between fore- and hindlimbs (by differences in bone cross-sectional properties, e.g., Burr et al., 1989; Demes and Jungers, 1989, 1993; Demes et al., 1991; Kimura, 1991, 1995; Ruff and Runestad, 1992; Schaffler et al., 1985), none of these locomotor characteristics have been related to any known morphological features. The goal of this chapter is to identify morphological correlates of one aspect of the unusual form of primate quadrupedal locomotion, namely, the uniquely protracted posture of the primate forelimb at the beginning of a walking step that helps generate their distinctively large forelimb excursion angles (Larson, 1998; Larson et al., 2000; Schmitt, 1994; Schmitt and Lemelin, 2002). Such morphological correlates can then be used to identify where and when this aspect of the unique form of primate quadrupedal locomotion arose in the course of primate evolution.

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