The functional morphology of the manus in the tridactyl equidsMerychippusandMesohippus: paleontological inferences from neontological models

Abstract

An investigation of the kinematics and mechanics of the manus of extant Equus and Tapirus is used as the basis for interpreting the functional morphology of the manus in the extinct tridactyl equids Mesohippus and Merychippus. The shape of the articular surfaces of each joint in the manus of the extant forms is related to the kinematics of the limb, and the probable soft-tissue anatomy and locomotory kinematics of the two fossils are reconstructed by comparing joint shapes among the four genera. The results support the conclusion of Camp and Smith (1942) that Mesohippus had a padded subunguligrade manus, similar to that of Tapirus, while Merychippus was fully unguligrade like Equus. However, all three equids have similar cursorial adaptations, not shared with Tapirus, which include the structure of the carpus and elongation of the distal segments of the limb. The differences in foot structure between Mesohippus and the unguligrade equids may be related to size. Only two, relatively minor, anatomical changes were necessary for the descendents of Mesohippus to become unguligrade: (1) loss of the digital pad, and (2) increase in the curvature of the metacarpophalangeal joints to allow more rotation of the digits. The eventual reduction in digital number, and the specialization in soft-tissue elements seen in Equus, were apparently unnecessary for the transition in function, and may also have been size dependent.