标题：rodent
Summary
Rodentia is an order of mammals also known as rodents, characterised by two continuously-growing incisors in the upper and lower jaws which must be kept short by gnawing.[1][2]
Forty percent of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, rats, squirrels, chipmunks, gophers, porcupines, beavers, hamsters, gerbils, guinea pigs, degus, chinchillas, and groundhogs.[1] Rodents have sharp incisors that they use to gnaw wood, break into food, and bite predators. Most eat seeds or plants, though some have more varied diets. Some species have historically been pests, eating human seed stores and spreading disease.
Contents
1 Size and range of order 
2 Characteristics 
3 Evolution 
4 Classification 
4.1 Standard classification 
4.2 Alternate classifications 
4.3 Monophyly or polyphyly? 
5 Notes 
6 Links 
7 References
Size and range of order
In terms of number of species — although not necessarily in terms of number of organisms (population) or biomass — rodents make up the largest order of mammals. There are about 2,277 species of rodents (Wilson and Reeder, 2005), with over 40 percent of mammalian species belonging to the order.[3] Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods. (Lambert, 2000)
Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only placental order, other than bats (Chiroptera) and Pinnipeds, to reach Australia without human introduction.
Characteristics
The capybara, the largest living rodent, can weigh up to 65 kg (140 lb).Many rodents are small; the tiny African pygmy mouse can be only 6 cm (2.4 in) in length and 7 g (0.25 oz) in weight at maturity, and the Baluchistan Pygmy Jerboa is of roughly similar or slightly smaller dimensions. On the other hand, the capybara can weigh up to 80 kg (180 lb)[4], and the largest known rodent, the extinct Josephoartigasia monesi, is estimated to weigh about 1,000 kg (2,200 lb), and possibly up to 1,534 kg (3,380 lb)[5] or 2,586 kg (5,700 lb)[6].
Rodents have two incisors in the upper as well as in the lower jaw which grow continuously and must be kept worn down by gnawing; this is the origin of the name, from the Latin rodere, to gnaw, and dens, dentis, tooth. These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside and exposed dentine on the inside, so they self-sharpen during gnawing. Rodents lack canines, and have a space between their incisors and premolars. Nearly all rodents feed on plants, seeds in particular, but there are a few exceptions which eat insects or fish. Some squirrels are known to eat passerine birds like cardinals and blue jays.
Typical rodent tooth systemRodents are important in many ecosystems because they reproduce rapidly, and can function as food sources for predators, mechanisms for seed dispersal, and as disease vectors. Humans use rodents as a source of fur, as pets, as model organisms in animal testing, for food, and even in detecting landmines.[7]
Members of non-rodent orders such as Chiroptera (bats), Scandentia (treeshrews), Insectivora (moles, shrews and hedgehogs), Lagomorpha (hares, rabbits and pikas) and mustelid carnivores such as weasels and mink are sometimes confused with rodents.
Evolution
Wood MouseThe fossil record of rodent-like mammals begins shortly after the extinction of the non-avian dinosaurs 65 million years ago, as early as the Paleocene. Some molecular clock data, however, suggest that modern rodents (members of the order Rodentia) already appeared in the late Cretaceous, although other molecular divergence estimations are in agreement with the fossil record.[8][9] By the end of the Eocene epoch, relatives of beavers, dormouse, squirrels, and other groups appeared in the fossil record. They originated in Laurasia, the formerly joined continents of North America, Europe, and Asia. Some species colonized Africa, giving rise to the earliest hystricognaths. There is, however, a minority belief in the scientific community that evidence from mitochondrial DNA indicates that the Hystricognathi may belong to a different evolutionary offshoot and therefore a different order. From there hystricognaths rafted to South America, an isolated continent during the Oligocene and Miocene epochs. By the Miocene, Africa collided with Asia, allowing rodents such as the porcupine to spread into Eurasia. During the Pliocene, rodent fossils appeared in Australia. Even though marsupials are the prominent mammals in Australia, rodents make up almost 25% of the mammals on the continent. Meanwhile, the Americas became joined and some rodents expanded into new territory; mice headed south and porcupines headed north.
Some Prehistoric Rodents 
Castoroides, a giant beaver 
Ceratogaulus, a horned burrowing rodent 
Spelaeomys, a rat that grew to a large size on the island of Flores 
Giant hutias, a group of rodents once found in the West Indies 
Ischyromys, a primitive squirrel-like rodent 
Leithia, a giant dormouse 
Neochoerus pinckneyi, a giant North American Capybara that weighed 50 kg (110 lb) 
Josephoartigasia monesi, the largest known rodent 
Phoberomys pattersoni, the second largest known rodent 
Telicomys, a giant South American rodent
Classification
2/3 of rodent species are in the superfamily Muroidea. The families Muridae (blue) and Cricetidae (red) make up the bulk of the Muroidea.
Standard classification
The rodents are part of the clades: Glires (along with lagomorphs), Euarchontoglires (along with lagomorphs, primates, treeshrews, and colugos), and Boreoeutheria (along with most other placental mammals). The order Rodentia may be divided into suborders, infraorders, superfamilies and families.
Classification scheme
ORDER RODENTIA (from Latin, rodere, to gnaw)
Suborder Anomaluromorpha 
Family Anomaluridae: scaly-tailed squirrels 
Family Pedetidae: springhares 
Suborder Castorimorpha 
Superfamily Castoroidea 
Family Castoridae: beavers 
Superfamily Geomyoidea 
Family Geomyidae: pocket gophers (true gophers) 
Family Heteromyidae: kangaroo rats and kangaroo mice 
Suborder Hystricomorpha 
Family incertae sedis Diatomyidae: Laotian rock rat 
Infraorder Ctenodactylomorphi 
Family Ctenodactylidae: gundis 
Infraorder Hystricognathi 
Family Bathyergidae: African mole rats 
Family Hystricidae: Old World porcupines 
Family Petromuridae: dassie rat 
Family Thryonomyidae: cane rats 
Parvorder Caviomorpha 
Family †Heptaxodontidae: giant hutias 
Family Abrocomidae: chinchilla rats 
Family Capromyidae: hutias 
Family Caviidae: cavies, including guinea pigs and the capybara 
Family Chinchillidae: chinchillas and viscachas 
Family Ctenomyidae: tuco-tucos 
Family Dasyproctidae: agoutis 
Family Dinomyidae: pacaranas 
Family Echimyidae: spiny rats 
Family Erethizontidae: New World porcupines 
Family Myocastoridae: nutria 
Family Octodontidae: octodonts 
Suborder Myomorpha 
Superfamily Dipodoidea 
Family Dipodidae: jerboas and jumping mice 
Superfamily Muroidea 
Family Calomyscidae: mouse-like hamsters 
Family Cricetidae: hamsters, New World rats and mice, voles 
Family Muridae: true mice and rats, gerbils, spiny mice, crested rat 
Family Nesomyidae: climbing mice, rock mice, white-tailed rat, Malagasy rats and mice 
Family Platacanthomyidae: spiny dormice 
Family Spalacidae: mole rats, bamboo rats, and zokors 
Suborder Sciuromorpha 
Family Aplodontiidae: mountain beaver 
Family Gliridae (also Myoxidae, Muscardinidae): dormice 
Family Sciuridae: squirrels, including chipmunks, prairie dogs, & marmots
Alternate classifications
The above taxonomy uses the shape of the lower jaw (sciurognath or hystricognath) as the primary character. This is the most commonly used approach for dividing the order into suborders. Many older references emphasize the zygomasseteric system (suborders Protrogomorpha, Sciuromorpha, Hystricomorpha, and Myomorpha).
Several molecular phylogenetic studies have used gene sequences to determine the relationships among rodents, but these studies are yet to produce a single consistent and well-supported taxonomy. Some clades have been consistently produced such as:
Ctenohystrica contains: 
Ctenodactylidae (gundis) 
Hystricognathi containing: 
Hystricidae 
An unnamed clade containing: 
Phiomorpha 
Caviomorpha 
An unnamed clade contains: 
Gliridae 
Sciuroidea containing: 
Aplodontiidae 
Sciuridae 
Myodonta includes: 
Dipodoidea 
Muroidea 
The positions of the Castoridae, Geomyoidea, Anomaluridae, and Pedetidae are still being debated.
Monophyly or polyphyly
In 1991, a paper submitted to Nature proposed that caviomorphs should be reclassified as a separate order (similar to Lagomorpha), based on an analysis of the amino acid sequences of guinea pigs.[10] This hypothesis was refined in a 1992 paper, which asserted the possibility that caviomorphs may have diverged from myomorphs prior to later divergences of Myomorpha; this would mean caviomorphs, or possibly hystricomorphs, would be moved out of the rodent classification into a separate order.[11] A minority scientific opinion briefly emerged arguing that guinea pigs, degus, and other caviomorphs are not rodents,[12][13] while several papers were put forward in support of rodent monophyly.[14][15][16] Subsequent studies published since 2002, using wider taxon and gene samples, have restored consensus among mammalian biologists that the order Rodentia is monophyletic.[17][18]
Notes
Adkins, R. M. E. L. Gelke, D. Rowe, and R. L. Honeycutt. 2001. Molecular phylogeny and divergence time estimates for major rodent groups: Evidence from multiple genes. Molecular Biology and Evolution, 18:777-791. 
Carleton, M. D. and G. G. Musser. 2005. Order Rodentia. Pp 745-752 in Mammal Species of the World A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore. 
David Lambert and the Diagram Group. The Field Guide to Prehistoric Life. New York: Facts on File Publications, 1985. ISBN 0-8160-1125-7 
Jahn, G. C. 1998. “When Birds Sing at Midnight” War Against Rats Newsletter 6:10-11. [1] 
Leung LKP, Peter G. Cox, Gary C. Jahn and Robert Nugent. 2002. Evaluating rodent management with Cambodian rice farmers. Cambodian Journal of Agriculture Vol. 5, pp. 21-26. 
McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8 
Nowak, R. M. 1999. Walker's Mammals of the World, Vol. 2. Johns Hopkins University Press, London. 
Steppan, S. J., R. A. Adkins, and J. Anderson. 2004. Phylogeny and divergence date estimates of rapid radiations in muroid rodents based on multiple nuclear genes. Systematic Biology, 53:533-553. 
University of California Museum of Paleontology (UCMP). 2007 "Rodentia". [2] 
Wilson, D. E. and D. M. Reeder, eds. 2005. Mammal Species of the World A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore.
Links
Website on African rodentia : http://projects.biodiversity.be/africanrodentia 
Rodent Photos
References
^ a b "rodent - Encyclopedia.com". Retrieved on 2007-11-03. 
^ "Rodents: Gnawing Animals". Retrieved on 2007-11-03. 
^ Myers, Phil (2000). "Rodentia". Animal Diversity Web. University of Michigan Museum of Zoology. Retrieved on 2006-05-25. 
^ Capybaras (Hydrochaeridae): Information and Much More from Answers.com 
^ Millien, Virginie (05 2008). "The largest among the smallest: the body mass of the giant rodent Josephoartigasia monesi". Proceedings of the Royal Society B 1: -1. doi:10.1098/rspb.2008.0087. Retrieved on 2008-05-27. 
^ Rinderknecht, Andrés; Blanco, R. Ernesto (01 2008). "The largest fossil rodent" (pdf). Proceedings of the Royal Society B 275: 923–928. doi:10.1098/rspb.2007.1645. Retrieved on 2008-05-27. 
^ Wines, Michael (2004-05-19). "Gambian rodents risk death for bananas", The Age, The Age Company Ltd.. Retrieved on 2006-05-25. "A rat with a nose for landmines is doing its bit for humanity" Cited as coming from the New York Times in the article. 
^ Douzery, E.J.P., F. Delsuc, M.J. Stanhope, and D. Huchon (2003). "Local molecular clocks in three nuclear genes: divergence times for rodents and other mammals and incompatibility among fossil calibrations". Journal of Molecular Evolution 57: S201. doi:10.1007/s00239-003-0028-x. 
^ Horner, D.S., K. Lefkimmiatis, A. Reyes, C. Gissi, C. Saccone, and G. Pesole (2007). "Phylogenetic analyses of complete mitochondrial genome sequences suggest a basal divergence of the enigmatic rodent Anomalurus". BMC Evolutionary Biology 7: 16. doi:10.1186/1471-2148-7-16. 
^ Graur, D., Hide, W. and Li, W. (1991) 'Is the guinea-pig a rodent?' Nature, 351: 649-652. 
^ Li, W., Hide, W., Zharkikh, A., Ma, D. and Graur, D. (1992) 'The molecular taxonomy and evolution of the guinea pig.' Journal of Heredity, 83 (3): 174-81. 
^ D'Erchia, A., Gissi, C., Pesole, G., Saccone, C. and Arnason, U. (1996) 'The guinea-pig is not a rodent.' Nature, 381 (6583): 597-600. 
^ Reyes, A., Pesole, G. and Saccone, C. (2000) 'Long-branch attraction phenomenon and the impact of among-site rate variation on rodent phylogeny.' Gene, 259 (1-2): 177-87. 
^ Cao, Y., Adachi, J., Yano, T. and Hasegawa, M. (1994) 'Phylogenetic place of guinea pigs: No support of the rodent-polyphyly hypothesis from maximum-likelihood analyses of multiple protein sequences.' Molecular Biology and Evolution, 11: 593-604. 
^ Kuma, K. and Miyata, T. (1994) 'Mammalian phylogeny inferred from multiple protein data.' Japanese Journal of Genetics, 69 (5): 555-66. 
^ Robinson-Rechavi, M., Ponger, L. and Mouchiroud, D. (2000) 'Nuclear gene LCAT supports rodent monophyly.' Molecular Biology and Evolution, 17: 1410-1412. 
^ Lin, Y-H, et al. "Four new mitochondrial genomes and the increased stability of evolutionary trees of mammals from improved taxon sampling." Molecular Biology and Evolution 19 (2002): 2060-2070. 
^ Carleton, Michael D., and Musser, Guy G. "Order Rodentia". Mammal Species of the World, 3rd edition, 2005, vol. 2, p. 745. (Concise overview of the literature)