Reconstruction of the locomotor repertoire of early primates in the light of astragalar and calcaneal shape.

The locomotor behavior of the earliest euprimates is key to our understanding of the origin and early diversification of the group. Postcranial traits suggest that major locomotor shifts occurred during the early evolution of this clade. Two tarsal bones, the astragalus and the calcaneus, have been extensively studied because of their functional importance. To provide further insights into early primate evolution, we use a three-dimensional high-density sliding semilandmark geometric morphometric approach to quantify tarsal shape on an extensive (936) sample of astragali and calcanei from extant and extinct primates as well as other euarchontans. We reconstruct the locomotor repertoire for a total of 37 extinct taxa, representing all major Paleogene primate groups, using a partial least squares regression between astragalar/calcaneal shape and locomotor percentages compiled from the literature. Our results concur with previous studies and confirm that the astragalar/calcaneal shape exhibits a strong functional signal, allowing to accurately estimate the locomotor repertoire of extinct species. Locomotor estimates based on fossils indicate that early euprimates displayed a diverse array of locomotor repertoires comparable to extant species, highlighting cases of convergent evolution among distantly related groups. Locomotor differences between plesiadapiforms and early euprimates include a greater use of leaping by the latter, suggesting that the origin and early diversification of euprimates involved an important locomotor shift. Based on tarsal shape, this study improves our understanding of early primate locomotion and evolution, providing the most extensive taxonomic scope to date.