The shape of the hominin talus: Evolutionary timing and relationships of the talar facets

The human talus occupies a pivotal position between the leg and foot and is adapted to bipedal locomotion. Great debate surrounds interpretation of hominin talar functional morphology and influences of locomotor diversity. Here we analyze hominin whole talar morphology, using individual and combined talar facets to assess timing of the emergence of a human-like talus. A template of 251 (semi)landmarks was used to analyze 200 hominoid tali: 94 H. sapiens, 86 great apes, and 20 fossil hominins (A. afarensis, A. sediba, P. robustus, H. habilis, early Homo, H. erectus sensu lato, H. floresiensis, H. neanderthalensis). Digital reconstructions were used to estimate missing (semi)landmarks on partially damaged fossils with cracks and/or gaps. Incomplete tali were only analyzed for complete facets. Shape variation was evaluated through Generalized Procrustes superimposition and PCA. Talar shape separates H. sapiens from other apes, with australopiths plotting closer to great apes and all Homo fossils closer to modern humans. Analyses of individual facets showed the navicular facet to have the most discriminatory power, with other facets showing overlap in PCA scores within the extant sample. When combining articular surfaces, the combined head, trochlea and posterior calcaneal facet perform the best, clearly separating australopiths from Homo. Our results suggest that the relationship of talar facets is notable in recognizing a human-like talus, suggesting that to discriminate between groups the relative shape (size, orientation and position) of facets should be considered. This approach can contribute to the assignment (taxonomic and functional) of fragmented fossil tali.