The evolution of the unguligrade manus in artiodactyls

Artiodactyls show a step-wise morphological transition from groups with a primitive and pentadactyl limb configuration to groups with more derived and didactyl limb configurations. An analysis of proportions of the forelimb segments indicates that artiodactyls can be separated based on digit number along a morphometrical axis indicating relatively longer segments of manus (carpus, metacarpus, and phalanges). The three digit configurations in artiodactyls (pentadactyl, tetradactyl, didactyl) are identified and the relevant bony characteristics of each configuration are described. Among tetradactyl groups, there are two grades of anatomy in the manus. In the more derived tetradactyl groups (tayassuids, tragulids, and early camelids), the proximal volar metacarpal process becomes flatter and joints of the digit become more ginglymal in comparison with the less derived tetradactyl groups (ticholeptine and merychyine oreodonts, leptomerycid traguloids, primitive cainotheres, oromerycids, and protoceratids). The foot posture of members of the pentadactyl groups is interpreted as digitigrade. The evolution of unguligrady took place between the morphological condition of the primitive type of tetradactyl manus and the derived type of tetradactyl manus. This transition to unguligrady occurred at least three times and was facilitated each time by the formation of ginglymal joints of the digit and the transition to interosseus ligaments from muscular precursors.