Can distal fibular morphology be used to infer Au. afarensis locomotor behavior?

The fibula has rarely been considered in comparative morphological studies. The bones normally used to investigate early hominin locomotor behavior are the largest bones of the hindlimb, femur and tibia. The reason for this choice is probably due to the relatively minor role of the fibula in carrying mechanical loads. However differences in morphology (and inferred function) of the fibula between human and non-human great apes, and within non-human great apes, have been noted in the past and related to differences in positional behavior. Recent research (Marchi, 2007; Marchi and Shaw, 2011) have pointed out the correlation present between diaphyseal structural properties of the fibula and locomotor behavior in living hominids (i.e. Pongo, Gorilla, Pan and Homo), and its possible application to inferring early hominin locomotor behavior. The problem with the method proposed in these studies is the extreme rarity of complete fibular diaphyses in the early hominin fossil record. However, distal fibular fragments are present in the fossil record. In particular, five distal fibulae of Australopithecus afaresis (i.e. AL 288-1at, AL 333-1a, AL 333-1b, AL 333-85, AL 333w-37) from the Hadar region of Ethiopia are available. Several morphological traits in the distal part of the fibula have been used in the past to infer locomotory adaptations and in particular the degree of arboreality in fossil hominins (Stern and Susman, 1983). However, the descriptions of these traits are often qualitatite. In this preliminary research articular measurements of the distal fibula of living hominids (Pongo, Gorilla, Pan and Homo, N = 107) are quantified. Besides articular area and breadth of the fibulotalar articular surfaces (i.e. proximal and distal), quantitative analysis is provided here of: 1. the angles formed by the fibulotalar articular surfaces with the longitudinal axis of the fibula; 2. the angle between the proximal portion of the fibulotalar articular surface and the subcutaneous triangular area of the fibula, which was hypothesized to be related to the degree of arboreality in great apes (Stern and Susman, 1983). Results show that the morphological characteristics of the distal fibula bear signals correlated with different locomotor behaviors particularly useful to distinguish between bipedal and non-bipedal hominids. The application of this method to the five distal Au. afarensis fibulae available shows how these morphological traits can be added to the ones normally investigated by palaeoanthropologists to better understand early hominin locomotory behavior.