CHAPTER XI 

 THE RODENTS 



A STUDY IN THE EVOLUTION OF HERBIVOROUS TYPES 



Rodent suborders — History of the Simplicidentata — Primitive 

 modern squirrel-like forms — The adaptation of the Rat 

 to an omnivorous diet — Increasing specialization in the 

 Porcupines — Extreme herbivorous adaptation in the Pata- 

 gonian Cavy — Dentition of the Rabbit — History of the 

 Duplicidentata — Convergent evolution in Rodents, Mar- 

 supials and Primates. 



The Rodents, the skulls of which are easily distinguished by 

 chisel-like incisors and the hiatus between these and the cheek 

 teeth, are of world-wide distribution, some even having reached 

 Australia. So numerous and varied are they that it is impos- 

 sible to describe them adequately w r ithin the restricted compass 

 of this volume. The best we can do is to indicate the general 

 trend of evolution within the order. 



As a group the Rodents are separated into two suborders. 

 The Rabbits, Hares and Picas which have two pairs of upper 

 incisors are termed Duplicidentata whereas the Simplicidentata 

 possess only a single pair of upper incisors. The latter sub- 

 order includes the following sections: Sciuromorpha, squirrel- 

 like; Myomorpha, mouse-like; and Hystricomorpha, porcupine- 

 like Mammals. Of these the Sciuromorpha are the most primi- 

 tive and the Hystricomorpha the most highly specialized. 



Even in the Oligocene the Duplicidentata were almost as 

 greatly differentiated from contemporary Sciuromorpha as at 

 the present date. Hence the two suborders must have had 



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