REPTILElS, BIRDS, AND MAMMALS 



467 



Fig. 28.32. Diagram of the evolution of the horse. Lowest panel, Phenacodus, a generalized 

 "archaic" ungulate of the Paleocene. The remaining figures are horses. From bottom to top 

 those shown are Eohippus, Mesohippus, Merychippus, Pliohippus, and Equus. Within 

 each panel, from left to right, appear a reconstruction of the horse, the fore and hind foot 

 showing the progressive reduction in lateral digits, one of the permanent molar teeth, and 

 the skull. Only the teeth are drawn to scale. 



