Six measurements were taken on the foot of each individual using Mitutoyo digital calipers: maximum talar articular length, talar trochlea length, cuboid length, first metatarsal (MT1) length, fifth metatarsal length (MT5), and fourth proximal phalanx (PP4) length. Talar neck length was derived by subtracting the talar trochlea length from maximum talar articular length. Maximum talar articular length is defined as the maximum proximodistal distance between the most proximal margin of the talar trochlea and the most distal point on the talar head. Talar trochlea length is defined as the maximum proximodistal distance between the most proximal margin of the talar trochlea and the most distal point of the talar trochlea. Cuboid length is defined as the proximodistal distance between the dorsal margin of the calcaneal facet and the most distal point of the tarsometatarsal joint, taken in dorsal view in approximate anatomical orientation. The purpose of measuring cuboid length in this manner is to explicitly avoid the cuboid beak or calcaneal process since it varies extensively among great apes (Lewis, 1983) and because it confounds the cuboid length measurement as a representation of midfoot length since it is articular and housed within a corresponding concavity on the cuboid facet of the calcaneus. MT1 length is defined as the maximum proximodistal distance between the most proximal points on the metatarsal base (with calipers held flush) and the most distal point of the metatarsal head. MT5 length is defined as the proximodistal distance between the most proximal point of the cuboid-MT4 articular margin and the most distal point on the metatarsal head. PP4 length is defined as maximum proximodistal distance between the most proximal point of the phalangeal base and the most distal point of the trochlea. The MT5 was chosen to represent non-hallucal metatarsal length because it is preserved in the ARA-VP-6/500 foot. The fossil is missing most of its metatarsal head and its length was estimated by Lovejoy et al. (2009a) using a combination of anatomical and statistical estimation. There is a nearly complete third metatarsal of Ar. ramidus, but it derives from a different locality and is therefore associated with a different individual. There is a partially preserved second metatarsal of Ar. ramidus also from a different individual (Lovejoy et al., 2009a). Therefore, for this study, the metrics are based on the preserved elements of the ARA-VP-6/500 foot of Ar. ramidus.

The individual elements of the bony foot skeleton contribute to the production of three movements used in various forms of primate locomotor behavior that are hypothesized to be reflected in intrinsic foot proportions: hallucal adduction and flexion, non-hallucal digital flexion, and plantarflexion at the talocrural joint. Increasing the length of the first metatarsal should increase the moment arm of the intrinsic adductor musculature such as the m. adductor hallucis across a range of hallucal abduction angles, and therefore should increase the hallucal adduction force during grasping in non-human primates (Cartmill, 1979). Increasing hallucal metatarsal and non-hallucal phalangeal lengths also contributes to increasing the span of the pedal grasp in taxa with a mobile hallux, which also helps to maintain a friction grip in pedal grasping (Cartmill, 1979). Previous studies have modeled the foot skeleton as a second-class lever, where the fulcrum is at the metatarsophalangeal joints, the load passes through the talocrural joint at the rearfoot, and the force is produced by the plantarflexor muscles. The load arm is the distance between the fulcrum and the load, whereas the effort arm (or ‘power arm’) is the distance between the insertion of the ankle plantarflexors on the calcaneal tuberosity and the talocrural joint (Schultz, 1963a; Schultz, 1963b; Strasser, 1992). Increasing the effort arm of the foot relative to the load arm increases the mechanical advantage of the foot skeleton as a lever (Schultz, 1963a; Schultz, 1963b; Strasser, 1992). However, increasing the load arm increases the range of motion for a given amount of plantarflexor contraction (Schultz, 1963a; Schultz, 1963b). The length of the effort arm of the Ar. ramidus foot is unknown because its calcaneus is not well preserved (Lovejoy et al., 2009a). There are multiple anatomical strategies for increasing the length of the foot skeleton’s load arm. The length of the load arm can be increased by lengthening the metatarsals, tarsals (e.g., the talus and/or cuboid), or any combination of these elements. Increasing the length of the metatarsals subjects them to greater bending moments during stance phase, and therefore increases the possibility of injury, so humans achieve a longer load arm by instead increasing the length of the tarsals. As such, in a bipedal heel-strike plantigrade foot, the load arm is increased by lengthening the cuboid and other midtarsal elements. In contrast, in quadrupedal semiplantigrade or digitigrade primates, and indeed other terrestrial cursorial mammals (Taylor, 1976), the load arm is lengthened by increasing the length of the metatarsals. One implication of these differing anatomical arrangements (e.g., increasing metatarsal versus tarsal lengths), is that foot proportions may also be a correlate of foot postures (plantigrady and digitigrady or semiplantigrady), which is one of the hypotheses tested by this study using evolutionary modeling.