238 GEOLOGY. 



little is known of the cats until the Miocene, when they were abun- 

 dant and wide-spread. 



The emergence of the edentates. — The ancestral edentates, the Gano- 

 donta, were very similar in general appearance to the Condylarthra and 

 Creodonta, but their dentition and certain peculiarities of structure 

 brought to knowledge by the researches of Wortman and Osborn have 

 led to the recognition of their edentate relations. The slight degree of 

 differentiation in the earliest Eocene seems to imply that the three orders 

 had but recently diverged from their common ancestors. Wortman 

 holds that the South American edentates were derived from these north- 

 ern forms and that there must hence have been a land connection about 

 the time of the early Eocene, which permitted their migration. It is 

 not .'mprobable that such a connection was formed during the transi- 

 tion epoch from the Cretaceous to the Eocene, which might have con- 

 tinued long enough to serve this function without permitting a migra- 

 tion of all forms. 



The ancestral rodents. — In the early Eocene there were very primi- 

 tive rodents whose incisors had just begun to assume their specific gnaw- 

 ing functions. By the middle of the period they became a notable factor 



Fig. 432. — The skull and jaw of a large Eocene rodent, Tillotherium fodiens Marsh, 

 from the Bridger formation, Wyoming, about \ natural size. 



of the fauna in the form of tillodonts, the Tillotherium of the Bridger 

 formation having finely specialized incisors (Fig. 432). For a rodent, 

 this was a large animal, half the size of a tapir. The primitive squirrel 

 type appeared in Europe in the latter part of the period. Even to-day, 

 the rodents retain many primitive characters, and since the Miocene 

 they have undergone few radical changes. This slow evolution implies 



