874 



TEXTBOOK OF ZOOLOGY 



times; (g) the developmental clianges of the chordate groups are 

 more completely read in the fossil record, with the history of the 

 mammals in fullest outline, partly because they are relatively recent 

 and partly because the mammalian skeleton is readily fossilized. The 

 most complete pedigree in fossils has probably been worked out for 

 the horse, and a great deal of its material has been located in the 

 southwestern part of the United States. 



The Rise of the Horse. — Primitive horselike animals are thought 

 probably to have arisen from an extinct group called Condylarthra, 

 which had five toes on each foot and a large part of the sole resting 



/J 



Wrist-- 



Fig. 442. — Positions of the human hand to show the comparative stages of 

 elevation of tlie horse's foot to the tip of the middle toe. (Courtesy of American 

 Museum of Natural History.) 



on the ground. The first unquestionable horselike form found in 

 America is the small Eohippus which was about one foot tall and 

 the fossils of which came from the rocks of Eocene times. It had 

 the outer four digits complete on the forefoot, but no trace of the 

 thumb, while the hind foot had three complete digits with vestiges 

 or splints of the first and fifth. Following the foxlike Eohippus, 

 later in the Eocene period came the Orohippus with an enlarged 

 central digit in the forefoot and the loss of the splints in the hind 

 foot. Mesohippus, about the size of a large dog with a three-toed 



