Rodent Evolution
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The order Rodentia consists of small, terrestrial mammals with a specialized gnawing apparatus and a high rate of reproduction. Throughout the course of their evolution, these omnivores have experienced remarkable adaptive success. On the basis of jaw and jaw muscle morphology, the order is typically subdivided into the Myomorpha, the Hystricomorpha, and the Sciuromorpha suborders, among others. The evolutionary history of the rodents can be traced back to the emergence of the mammals. During the Triassic, the therapsid reptiles evolved the mammalian anatomical structure. It wasn't until the Eocene, however, that the rodents arose. The origin of Rodentia is primarily based on the acquisition of the following masticatory apparatus specializations: (1) specializations of the incisors; (2) cheek teeth; and (3) the associated musculoskeletal features needed for gnawing. Two hypotheses have been advanced for Rodentia origins. One, the primate hypothesis, suggests that the animals are derived from a primitive Paleocene primate. In contrast, the palaeoryctoid or eurymyloid hypothesis attempts to describe evolutionary relationships by comparing various physical attributes. Once rodents emerged, they rapidly radiated. Unfortunately though, instances of parallel and convergent evolution make Rodentia subordinal classification very difficult. For instance, it has even been suggested that the different rodent types actually represent grade le
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) and the family Heteromyidae (the pocket mice and kangaroo rates). Similarly, Muroidea includes the family Rhizomyidae (the bamboo rats), Spalacidae (the mole rats), Muridae (the Old World rats and mice), and Cricetidae (the voles and hamsters). Whereas Rhizomyidae and Spalacidae may be artificially "lumped" in the superfamily Muroidea (Eisenberg 82-111), Muridae and Cricetidae are very closely related.
Second, the suborder Hystricomorpha can be characterized by the production of precocial young. Newborns are generally born with a full coat of fur and their eyes open. Old World hystricomorph families include the Hystricidae (the Old World porcupines), the Petromyidae (the African rock rats), and the Thryonomyidae (the cane rats). New World hystricomorphs include Erethizontidae (the New World porcupines), Caviidae (the cavies), Hydrochoeridae (the capybara), Dinomyidae (Branick's giant rat), Dasyproctidae (the agoutis), and Chinchillidae (the chinchillas), among others.
Third, the suborder Sciuromorpha contains the following families: Aplodontidae (the mountain beaver) and Sciuridae (the squirrels). In addition, various other Rodentia suborders include Castorimorpha (the beavers) and Theridomyomorpha (the scaly-tailed f
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Some common words found in the essay are:
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Approximate Word count = 1590
Approximate Pages = 6 (250 words per page)
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