Rodents Fossil range: Late Paleocene - Recent |
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Ammospermophilus leucurus |
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Scientific classification |
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Suborders
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Sciuromorpha |
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, with over 40 percent
of mammalian species belonging to the order.[1] 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)
There are between 2000 and
3000 species of rodents, which are found in vast numbers on all continents
except Antarctica, most islands, and in all habitats except for oceans. They
are the only placental order other than bats (Chiroptera) to reach Australia
without human introduction.
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, 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 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 model organisms in animal testing,
for food, and even in detecting landmines[2].
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 for
rodents. Rodents have a carrier-immunity to rabies virus, making them immune to
the potentially infectious and lethal disease. They are carriers for most other
animal-to-human illnesses, however, and still should not be agitated.
The fossil record of
rodents begins long before the extinction of the dinosaurs 65 million years
ago, as early as the Jurassic Period. By the end of the Eocene epoch, beavers
and squirrels appeared in the fossil record. They originated in 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 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
Flores Giant Rat, 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
Phoberomys pattersoni, the largest known rodent
Telicomys, a giant South American rodent
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)
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.
1.
^ Myers, Phil (2000). Rodentia. Animal
Diversity Web. University of Michigan Museum of Zoology. Retrieved on
2006-05-25.
2.
^ Wines, Michael. "Gambian
rodents risk death for bananas", The Age, The Age Company Ltd.,
2004-05-19. 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.
Wikipedia
http://en.wikipedia.org/w/index.php?title=Rodent&action=history
http://www.gnu.org/copyleft/fdl.html