Rodent
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.
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, chinchillas and degus. 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.

# 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.  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
Many rodents are small; the tiny African pygmy mouse is only 6 cm in length and 7 grams in weight. On the other hand, the capybara can weigh up to 65  (Expression error: Missing operand for *. ), and the largest known rodent, the extinct Josephoartigasia monesi, is estimated to weigh about 1,000  (Expression error: Missing operand for *. ), and possibly up to 1,534  (Expression error: Missing operand for *. ) or 2,586  (Expression error: Missing operand for *. ).
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.
Rodents 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.
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
The 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, suggests 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. 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.

# Classification

## 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
- 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
- Superfamily Castoroidea
Family Castoridae: beavers
- Family Castoridae: beavers
- Superfamily Geomyoidea
Family Geomyidae: pocket gophers (true gophers)
Family Heteromyidae: kangaroo rats and kangaroo mice
- 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
- Family incertae sedis Diatomyidae: Laotian rock rat
- Infraorder Ctenodactylomorphi
Family Ctenodactylidae: gundis
- 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
- 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
- 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
- Superfamily Dipodoidea
Family Dipodidae: jerboas and jumping mice
- 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
- 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
- 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
- Ctenodactylidae (gundis)
- Hystricognathi containing:
Hystricidae
An unnamed clade containing:
Phiomorpha
Caviomorpha
- Hystricidae
- An unnamed clade containing:
Phiomorpha
Caviomorpha
- Phiomorpha
- Caviomorpha
- An unnamed clade contains:
Gliridae
Sciuroidea containing:
Aplodontiidae
Sciuridae
- Gliridae
- Sciuroidea containing:
Aplodontiidae
Sciuridae
- Aplodontiidae
- Sciuridae
- Myodonta includes:
Dipodoidea
Muroidea
- 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. 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. A minority scientific opinion briefly emerged arguing that guinea pigs, degus, and other caviomorphs are not rodents, while several papers were put forward in support of rodent monophyly. Subsequent studies published since 2002, using wider taxon and gene samples, have restored consensus among mammalian biologists that the order Rodentia is monophyletic.

# 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.  
- 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". 
- 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 : 
- Rodent Photos