example, rodents can usually be divided into granivores and herbivores. Granivores typically feed on seeds and cache food (Valone & Brown 880). In contrast, herbivores consume vegetation and do not cache their food (Eisenberg 82-111).
During the past 55 million years of mammalian evolution, rodents have had considerable adaptive success. Between 330 and 400 genera, and some 1800 to 2300 rodent species have been recorded in the Recent fauna (Hartenberger 1-33). Moreover, Anderson (1967b) has recognized 9 extinct rodent families (Nowak & Paradiso 494). The success and abundance of the order has attracted the attention of numerous comparative and evolutionary biologists (Luckett & Hartenberger vii). Given the "bewildering variety of adaptations" which occur among the animals, the subordinal classification of rodents has "met with extreme difficulties (Eisenberg 82-111)." Such problems include, for example, the animals' high frequency of convergent evolution, parallel evolution, or both.
Typically, the order Rodentia is subdivided into the following extant suborders: the Myomorpha (the rat-like rodents), the Hystricomorpha (the porcupine-like rodents), and the Sciuromorpha (the squirrel-like rodents) (Graur et al. 649). Simpson (1945) distinguished these suborders on the basis of jaw and jaw musculature differences. For instance, in the typical myomorph, the masseter is attached anteriorly
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