Abstract The evolutionary increase in body size of terrestrial quadrupeds is closely associated with skeletal adaptations to support their mass. Following Cope's Rule, proboscideans evolved from relatively small early forms to increasingly large ones. Their limb long bones exhibit corresponding adaptations to these shifts in body mass through two main weight‐bearing strategies. The ankle joint is essential for weight bearing and locomotion and is therefore expected to show corresponding adaptations. This study investigates the morphological variation of the astragalus and calcaneus in extant and extinct proboscideans using three‐dimensional geometric morphometrics in a phylogenetic context to explore morphological adaptations related to body mass, locomotion and posture. The results show that the acquisition of a more columnar posture during the Miocene is accompanied by a reorganization of the foot anatomy reducing axial compression constraints. Astragalus and calcaneus morphologies of graviportal proboscideans are influenced by body mass, especially at the level of articular facets and ligament and muscle attachment sites. The results highlight two distinct morphotypes among graviportal proboscideans: Elephantidae, Mammutidae and Stegodontidae have an ankle morphology optimized for stability and rapid movement, but Gomphotheriidae, Amebelodontidae and Deinotheriidae show an ankle morphology adapted to powerful strokes, particularly in plantar–dorsiflexion. Both morphotypes are observed in taxa of similar mass, and, once acquired, remain fixed within the lineage even in cases of insular dwarfism. Finally, the overall hindlimb morphology highlights different combinations of robustness between the long‐bone and tarsal morphotypes depending on the taxon, reflecting different locomotor and weight‐bearing strategies within Proboscidea.
Tetaert et al. (Sun,) studied this question.