Comparative functional morphological study of the tarsal joint mobility in artiodactyls and perissodactyls in light of astragalar morphological differences.

In most mammals, the astragalus features a single-pulley surface for articulation with the tibia; in contrast, artiodactyls possess pulley structures at both the proximal and distal ends of the astragalus, a condition known as the double-pulley astragalus. This structure has been hypothesized to increase the force exerted during plantarflexion by elongating the lever arm formed by the calcaneus. However, this hypothesis, based on skeletal simulations, has lacked empirical confirmation under ex vivo conditions. To address this, the present study used CT scanning to examine the hindlimbs obtained from necropsies of 15 mammalian species-including artiodactyls, perissodactyls, and a cheetah-and generated three-dimensional (3D) reconstructions of the tarsal joint to measure mobility using the obtained 3D models. Distances between the tuber calcanei and astragalar trochleae, interosseous angles, and ranges of motion (ROMs) were quantified in maximally dorsiflexed and plantarflexed postures. In artiodactyls, the distances between the tuber calcanei and the proximal and distal trochleae of the astragalus varied with joint posture and changed strictly in antiphase, reflecting a shift in the functional locus of rotation within the astragalus. Despite these postural changes, the effective lever length relative to the instantaneous axis of rotation remained nearly constant. In species with a single-trochlea astragalus, including perissodactyls and the cheetah, almost the entire ROM of the tarsal joint was attributable to motion at the crurotarsal joint, and the calcaneus and astragalus were virtually immobile relative to each other. By contrast, artiodactyls exhibited substantial calcaneo-astragalar mobility, with changes in relative orientation reaching up to 90°. Tarsal joint mobility in artiodactyls was distributed between the proximal and distal astragalar joints, with ruminants showing particularly large ROMs at the distal astragalar joint. Clear differences were observed between plesiomorphic and apomorphic taxa: equids and ruminants displayed greater overall tarsal joint mobility than Tapiridae and Suina, respectively. Although Suina did not differ from ruminants in ROM at the proximal astragalar joint, they were markedly restricted in mobility at the distal astragalar joint and in calcaneo-astragalar motion. Llamas, and likely camelids in general, exhibited tarsal joint characteristics closely resembling those of ruminants, a pattern most plausibly interpreted as convergent acquisition of similar joint configurations. These results demonstrate that the double-pulley astragalus is associated with a distinctive pattern of joint partitioning and interosseous mobility within the tarsus, rather than with simple elongation of the calcaneal lever arm, highlighting its anatomical significance in shaping artiodactyl tarsal joint morphology.