CENOZOIC ERA: AGE OF MAMMALS 



621 



to a burrowing animal it is difficult to imagine. They may have served as accessories 

 to the strong claws in digging, or they may prove to be sexual characters. 



In the Eocene there also appeared a race (Tillotherium) (Figs. 554 A, B) similar in 

 habits to the rodents, although not of this tribe, some members of which grew to be of 

 considerable size, one species being half as large as the tapir. In South America during 



Fig. 554. — A, skull ; and B, restoration of the head of Tillotherium, a peculiar 

 rodent-like creature (Eocene). (After H. F. Osborn.) 



the Miocene rodents were abundant, but all belonged to the great porcupine group 

 such as live on that continent to-day; the forms common in North America at that 

 time, the rats, mice, squirrels, beavers, marmots, hares, and rabbits, being absent. 



It is an interesting fact that, notwithstanding their failure to 

 develop a highly complex brain, rodents are, at present, the most 

 abundant of mammals. This has been possible because of their 

 fecundity and adaptability to varying conditions. 



REFERENCES FOR RODENTS 



Gidley, J. W., — A New Horned Rodent from the Miocene of Kansas: Proc. U. S. Nat. 



Mus., Vol. 32, No. 1554, 1913. 

 Woodward, A. S., — Vertebrate Paleontology, pp. 373-374. 



Edentates (Latin, edentatus, toothless). — The earliest Eocene edentates (Ganodonta) 

 were so similar to the ancestral herbivores (Condylarthra) and carnivores (Creodonta) 

 that it seems probable that the three orders were derived from a common ancestor 

 only a short time before. The most familiar living edentates are the armadillos, the 

 anteaters, and the sloths. The name, edentate, is misleading, since most of the order 

 have teeth which are much alike and are without enamel. Teeth, however, are lack- 

 ing in the front part of the mouth, and it was from this character that the name was 

 given. One modification — among a number — should be mentioned. In the earliest 

 forms the teeth were covered with enamel, as is commonly true of mammalian teeth, 

 and had definite roots. In the later forms the teeth become rootless, and the enamel 

 disappears except in narrow bands. It is rather surprising to find armadillos (Meta- 

 cheiromys) as far back as the Eocene, similar in many respects to the smaller armadillos 

 of to-day, but apparently possessing a leathery instead of a bony shield and with dif- 

 ferent teeth. 



