Rodent

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Rodents
Fossil Range: Paleocene - Recent
Image:Capybara.png
Capybara, the largest living rodent
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Order:Rodentia
Bowdich, 1821
Families

See Classification Section

The order Rodentia is the most numerous of all the branches on the mammal family tree. Currently there are, depending on the authority consulted, between 2000 and 3000 species of rodent—roughly half of all mammal species. Rodents are found in vast numbers on all continents except Antarctica (they are the only placental order other than bats (Chiroptera) to reach Australia without human introduction), most islands, and in all habitats bar the oceans.

Groups commonly confused with rodents, or erroneously thought to be rodents, include the aforementioned Chiroptera (bats), Scandentia (tree shrews), Insectivora (moles, shrews and hedgehogs), and Lagomorpha (hares, rabbits and pikas) and carnivore mink.

Most 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.

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. These teeth are used for cutting wood, biting through the skin of fruit, or for defence. These 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 even fish.

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 a model organism in animal testing, and even in detecting landmines[1].

Contents

Natural History

The fossil record of rodents began after the extinction of the dinosaurs some 65 million years ago. The earliest rodents resembled squirrels and from these stem rodents, they diversified. By the end of the Eocene epoch, beavers and squirrels appeared in the fossil record. Their origins were from Laurasia, the joined continents of North America, Europe, and Asia. Some species colonized Africa, giving rise to the earliest hystricognaths. From there they rafted to South America, an isolated continent during the Oligocene and Miocene epochs. By the Miocene, Africa collided with Asia, allowing rodents such as porcupines to spread intoEurasia. During the Pliocene, rodent fossils appeared in Australia. Even though marsupials are prominent residents in Australia, it is the rodents that dominate the mammalian fauna: making up almost 25% of the mammals on the continent. Meanwhile, the Americas collided. Rodents expanded into unknown territory: mice headed south and porcupines headed north.

Some Prehistoric Rodents
Castoroides, a giant beaver
Epigaulus, a horned burrowing rodent
Flores Giant Rat, a rat that grew to large size alongside small elephants and humans 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 110 pounds
Phoberomys pattersoni, the largest known rodent
Telicomys, a giant South American rodent

Classification

The rodents are part of the clades: Glires (along with lagomorphs), Euarchontoglires (along with lagomorphs, primates, tree shrews, and colugos), and Boreoeutheria (along with most other placental mammals). The order Rodentia may be divided into suborders, infraorders, superfamilies and families. This is a common classification scheme:

  • ORDER RODENTIA

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:



The positions of the Castoridae, Geomyoidea, Anomaluridae, and Pedetidae are still being debated.

References

  • 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.
  • 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.


Mammals
Monotremata

Placentalia: Xenarthra | Dermoptera | Desmostylia | Scandentia | Primates | Rodentia | Lagomorpha | Insectivora | Chiroptera | Pholidota | Carnivora | Perissodactyla | Artiodactyla | Cetacea | Afrosoricida | Macroscelidea | Tubulidentata | Hyracoidea | Proboscidea | Sirenia

Marsupialia: Didelphimorphia | Paucituberculata | Microbiotheria | Dasyuromorphia | Peramelemorphia | Notoryctemorphia | Diprotodontia

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