CHAPTER XIII 



THE PERISSODACTYL UNGULATES 



Great divergence within the order — History of the Tapirs — 

 The Rhinoceroses — Paleontological record of the Horse — 

 Features of the equine dentition — Ancestral relationships 

 of the order. 



Perissodactyla the zoological term for the odd-toed Ungulates 

 must not be taken to mean literally that representatives inva- 

 riably possess an odd number of toes, the Tapirs for example 

 having four toes on the forefeet. Rather, as previously pointed 

 out, the term must be construed as indicating that the axis of 

 each foot is formed by the third digit. Included within this 

 order are the modern Tapirs, Rhinoceroses and Horses but in 

 ancient times it comprised also many other families, some of 

 them quite grotesque in appearance. Different as the three 

 modern representatives are from each other today their lower 

 Eocene fore-runners are clearly of common stock. Starting from 

 unknown but common Cretaceous ancestors, the Perissodactyls 

 developed into a great multitude of varied forms of which the 

 existing representatives are a mere handful. 



Of modern forms the Tapirs have changed least ; they re- 

 main almost as they were in Miocene times. Indeed Ave must 

 compare the modern with the Oligocene Tapirs if we desire to 

 see any marked differences in skeletal structure. From refer- 

 ence to these early forms we note that the main dental change 

 which has occurred in developmental progression with increase 

 m size of the animal is approximation to the molar pattern 

 of at first only the hindmost premolar but ultimately and at 

 the present day of all except the first, together with reduction 

 in size of the upper canine and consequent enlargement of the 



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