Locomotor variability in Sterkfontein Member 4: Analysis of the external shape and internal bone structure of the StW 562 and StW 595 first metatarsals

Recent fossil discoveries have highlighted the diversity of foot morphology among fossil hominins. However, determining whether such morphological differences can be interpreted as a signal of substantively different gaits and/or contributions of arboreal locomotion to a species’ behavioural repertoire is unclear. Here, we address this question of behavioural diversity in two fossil first metatarsals from Member 4 of Sterkfontein that differ in their external morphology: StW 562 and StW 595.

We analyse external shape using landmark based geometric morphometrics and the internal structure using cross sectional geometry and whole-bone analysis of cortical and trabecular bone structure in StW 562, StW 595, and a comparative sample of humans (N=7), chimpanzees (N=14), bonobos (N=9), gorillas (N=7) and orangutans (N=11). Results of the shape analysis support previous descriptions with both fossil metatarsals being intermediate in shape between humans and extant great apes, with StW 562 more similar to the human sample and StW 595 more similar to extant apes. Cross-sectional geometric analysis demonstrates that StW 562 shares a robust shaft with humans and African apes, whereas StW 595 has a low bending rigidity and cross-sectional area, similar to the less robust orangutans. The pattern of distribution of cortical and trabecular bone differs between these fossils, adding further evidence of morphological and perhaps functional diversity.

These results suggest two foot morphotypes may be present in specimens from Sterkfontein Member 4. Future work will further explore the potential biomechanical implications of these differing morphologies and the taxonomic affiliation of these two specimens.