Rodents

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, and guinea pigs. 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 45 kg (100 pounds) and the extinct Phoberomys pattersoni is believed to have weighed 700 kg. The extinct Josephoartigasia monesi weighed about one ton, and the biggest could have been more than 2.5 tons.

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.

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 unnnamed 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. 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.