Hyracotherium at the onset of the equid evolution had four metacarpals. The horse lineage exhibits the most extreme digit reduction, resulting in the monodactyl forelimb of Equus. Within the mani of perissodactyls other than the horse, rhinoceroses possess three fully formed digits and tapirs have four complete digits. For example, in cervids, splint metacarpals and external hooves are present, but the side toes lack phalanges, whereas in suids, the side toes are complete but small. The reduction of the side toes in artiodactyls is variable. Among the artiodactyls, digits III and IV are fused centrally, digits II and V are reduced into smaller side toes, and I is often entirely eliminated. In many carnivores, digit I is removed from the locomotory apparatus, but persists on the limb as the dewclaw. In contrast to the squamates, where entire limbs are lost completely, mammals exhibit a lesser degree of reduction where only individual digits are lost or simplified. We show shades of pentadactyly within the Equus manus.ĭigit reduction is common among mammals. We relate this revised interpretation of the Equus forelimb to Laetoli footprints, and suggest the Hipparion side impressions are created from the hooves of I and V, rather than from II and IV. Distally, the frog, a V-shaped structure on the ventral hoof represents digits II and IV, and the wings and hoof cartilages of the distal phalanx are digits I and V. The carpal articulations of the five metacarpals match those of pentadactyl taxa. These ridges are present in the tridactyl Mesohippus, but are absent in the tetradactyl Hyracotherium. Metacarpals II and IV each exhibit a ventral ridge, which we suggest represents the undifferentiated digits I and V. The current understanding is that the horse distal forelimb consists of one complete digit (III) and two reduced splint metacarpals (II and IV). Histological, osteological and palaeontological evidence suggest that the Equus distal forelimb is more complex than traditionally conceived. Osteological descriptions of selected tetradactyl, tridactyl and monodactyl equids demonstrate the evolution of the forelimb. We revisit digit reduction in the horse and propose that all five digits are partially present in the modern adult forelimb.
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