Mammalia : Classification 



779 



Paleocene and Eocene but extinct before the end of the Eocene. The 

 condylarthrans had five digits on each foot and the third digit was 

 strongest, suggesting a tendency toward the mesaxonic type of foot. 

 Some of these condylarthrans had claws; some had hoofs. The teeth 

 were definitely of ungulate pattern. The origin of the group is quite 

 obscure, but it seems likely that the Perissodactyla may have been 

 derived from Condylarthra. The history of horses (Equidae) is well 

 known back to the early Eocene (possibly 50,000,000 years ago), 

 beginning with Eohippus (Fig. 597), a little "odd-toed" North 

 American ungulate, horselike in many ways, but only about 20 inches 

 long and having four well-developed digits on the manus and three on 

 the pes. Along the later line of "horses," there was progressive increase 

 in size of the animal and decrease in number of digits in the mesaxonic 

 foot to the present one-toed limit (Fig. 598). 



The Artiodactyla were already well developed in the early Eocene. 

 The teeth of the oldest known artiodactyls were not definitely of the 

 herbivorous type, even resembling somewhat those of creodonts. It is 

 possible that the group may have arisen in close relation to the early 

 creodont line rather than from the Condylarthra — therefore quite 

 independently of Perissodactyla. 



The Proboscidea, in retaining all five digits, show their independ- 

 ence of both perissodactyl and artiodactyl lines of descent. A line of 

 more or less elephant-like mammals has been traced back into the 

 late Eocene, where it becomes lost in the general obscurity of ungulate 

 origins. The Hyracoidea must have emerged from that same obscurity 

 and, in retaining most of their digits and in some skeletal features, they 

 show a possible remote affinity to elephants. But the conies remained 

 relatively primitive while the elephants became the most highly 



Fig. 596. Phenacodus, a primitive ungulate (Condylartha), about 53>2 feet long. 

 (After Osborn. Courtesy, Romer: "Vertebrate Paleontology," University of 



Chicago Press.) 



