Long-term morphological trends in the evolution of the Equidae

"Heads and teeth are drawn to scale, showing the increase in size in the lineages. Front feet and not to scale. Other trends include: a shift from cusps to complex ridges on the grinding surface of the premolars and molars; elongation of the face and of the space between incisors and cheek teeth; an anterior shift of the cheek teeth in the Merychippus-Equus lineage so that they lie forward of the eye; a deeper lower jaw bone; reduction and loss of the side toes; enlargement of the terminal phalanx (hoof); elongation and enlargement of the central metapodial (the longest bone in the foot)." [text from Futuyma 1997; drawing from Stanley 1993, after Simpson 1951]