59 8 



HISTORICAL GEOLOGY 



Ancestors of the Hoofed Mammals (Ungulates). — Interest in 

 the Eocene centers not so much upon such groups of animals as 

 the Amblypoda, which, though the largest and most conspicuous 

 of their time, left no descendants, as upon those animals that are 

 either actually the ancestors of recent mammals or so closely related 

 to them that they help us to understand the evolution and past 

 history of the mammals living to-day. 



Some of the earliest Eocene herbivorous mammals (Condylarthra) 

 are so generalized that many groups seem to converge in them, even 

 the carnivores and herbivores not being easily distinguishable. These 

 ancestral herbivores were small or of moderate size and walked flat 

 on the foot (plantigrade) and not on the toes (digitigrade), as do 

 the horse and cow. The ends of the toes were not quite in the form 



either of hoofs or of 

 claws. One of the 

 best known forms 

 (Phenacodus) (Fig. 

 539), although not the 

 direct ancestor of any 

 of the modern mam- 

 mals, is of great in- 

 terest since it prob- 

 ably differed but 

 slightly from those in 

 the direct line of descent. It resembles the carnivores in having 

 an arched back, strong legs, and five toes on its feet. It walked 

 somewhat on its toes and the toes ended in a flat " nail " which 

 may be considered as the beginnings of a hoof. The teeth were 

 short-crowned (that portion of the tooth above the jaw being short) 

 and comparatively simple, showing that their possessor was om- 

 nivorous in habit. The head is remarkably small and the nearly 

 smooth brain is small, even for a head of this size. It apparently 

 had no means of defense and sought safety in flight. Some species 

 of the genus attained the size of a sheep. 



It was from some such animal, so simple in structure that it might 

 almost equally well be ancestral to the carnivores (the dog and lion) 

 and to the hoofed mammals (ungulates, — horse, ox, camel), that the 

 modern hoofed mammals, such as the horse, ox, rhinoceros, and 

 elephant are descended. It is interesting in this connection to note 

 that Huxley and Cope had independently pictured, what the an- 



Fig. 539. — Phenacodus, an Eocene mammal which in 

 many particulars is like the ancestor of the hoofed 

 mammals or ungulates. (After Scott.) 



