The orderRodentia has been the most spectacularly successful 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 (they are the only placental order other than the bats to reach Australia without human introduction), most islands, and in all habitats bar the oceans.
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 Latinrodere, to gnaw. These teeth are used for cutting wood, biting through the skin of fruit, or for defence. Nearly all rodents feed on plants, seeds in particular, but there are a few exceptions which eat insects or even fish.
Recent work has suggested that the rodents may actually be biologically polyphyletic (i.e. have evolved more than once), specifically the Myomorpha may constitute a distinct and separate lineage, in which case the order would have to be revised.
