Calcaneal proportions in primates and locomotor inferences in Anchomomys and other Palaeogene Euprimates

Foot proportions, and in particular the lengthening of the tarsal elements, play a fundamental role in the discussion on the locomotor adaptations of Palaeogene primates. The elongation of the distal portion of the tarsus, particularly the anterior part of the calcaneus, is frequently interpreted as an adaptation to leaping and has played a fundamental role in the reconstruction of the locomotor adaptations of the earliest primates. Here, we report an allometric analysis of calcaneal proportions in primates and other mammals, in order to determine the actual differences in calcaneal proportions. This analysis reveals that primates as a group display a relatively longer distal calcaneus, relative to both total calcaneal length and body mass, when compared with other mammals. Contrary to current expectations, morphofunctional analysis indicates that a moderate degree of calcaneal elongation is not an adaptation to leaping, but it is merely a compensatory mechanism to recover the lost load arm (metatarsal length) when the foot adopts a grasping function, in order to maintain the same locomotor efficiency. Leaping can be inferred only when anterior calcaneal length departs from the scaling of non-specialized primate groups. The role of leaping on the inferred locomotor repertoire of earliest primates needs to be revised considering the results of this work.