Rodents range in size from pygmy mice weighing 5 gms to capybaras, the largest of which weigh over 70 kg. They are found around the world except in Antarctica, New Zealand, and on some oceanic islands. Ecologically, they are incredibly diverse. Some species are highly aquatic, while others are equally specialized for life in deserts. No rodent has more than one incisor in each quadrant, and no rodent has canines.
Rodent incisors are rootless , growing continuously.
Their anterior and lateral surfaces are covered with enamel , but their posterior surface is not. During gnawing, as the incisors grind against each other, they wear away the softer dentine , leaving the enamel edge as the blade of a chisel. Multituberculates, a very large and successful but now extinct group of early mammals, had a similar pattern. So do wombats , hyraxes , aye-ayes , and lagomorphs , to give a few examples chosen from modern mammals. Rodents have specialized in gnawing to an extreme, however, seen in few or no other groups of vertebrates.