Geology . Condylarthra andCreodonta, but their dentition and certain peculiarities of structurebrought to knowledge by the researches of Wort man and Osborn haveled to the recognition of their edentate relations. The slight degree ofdifferentiation in the earliest Eocene seems to imply that the three ordershad but recently diverged from their common ancestors. Wortmanholds that the South American edentates were derived from these north-ern forms and that there must hence have been a land connection aboutthe time of the early Eocene, which permitted their migration. It isnot mprobable that such
Geology . Condylarthra andCreodonta, but their dentition and certain peculiarities of structurebrought to knowledge by the researches of Wort man and Osborn haveled to the recognition of their edentate relations. The slight degree ofdifferentiation in the earliest Eocene seems to imply that the three ordershad but recently diverged from their common ancestors. Wortmanholds that the South American edentates were derived from these north-ern forms and that there must hence have been a land connection aboutthe time of the early Eocene, which permitted their migration. It isnot 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 Bridgerformation having finely specialized incisors (Fig. 432). For a rodent,this wTas a large animal, half the size of a tapir. The primitive squirreltype appeared in Europe in the latter part of the period. Even to-day,the rodents retain many primitive characters, and since the Miocenethey have undergone few radical changes. This slow evolution implies THE EOCENE PERIOD. 239 that they may have extended farther back than the record derivation is not yet determined. The primitive insectivores.—Most of the present families of insec-tivores can be traced back to the Eocene. They retain even to thisclay many of their primitive characters, agreeing with the creodonts intheir low type of brain and in some skeletal features. They are theleast altered of the gre
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