Rodentia

(vertebrate zoology) An order of mammals characterized by a single pair of ever-growing upper and lower incisors, a maximum of five upper and four lower cheek teeth on each side, and free movement of the lower jaw in an anteroposterior direction.

The mammalian order consisting of the rodents, often known as the gnawing mammals. This is the most diverse group of mammals in the world, consisting of over 2000 species, more than 40% of the known species of mammals on Earth today. Rodents range in size from mice, weighing only a few grams, to the Central American capybara, which is up to 130 cm (4 ft) in length and weighs up to 79 kg (170 lb). Rodents have been found in virtually every habitat and on every continent except Antarctica. Rodents have adapted to nearly every mode of life, including semiaquatic swimming (beavers and muskrats), gliding (“flying” squirrels), burrowing (gophers and African mole rats), arboreal (dormice and tree squirrels), and hopping (kangaroo rats and jerboas). Nearly all rodents are herbivorous, with a few exceptions that are partially insectivorous to totally omnivorous, such as the domestic rat. The great adaptability and rapid evolution and diversity of rodents are mainly due to their short gestation periods (only 3 weeks in some mice) and rapid turnover of generations.

The most diagnostic feature of the Rodentia is the presence of two pair of ever-growing incisors (one pair above and one below) at the front of the jaws. These teeth have enamel only on the front surface, which allows them to wear into a chisellike shape, giving rodents the ability to gnaw. Associated with these unique teeth are a number of other anatomical features that enhance this ability. Behind the incisors is a gap in the jaws where no teeth grow, called a diastema. The diastema of the upper jaw is longer than that of the lower jaw, which allows rodents to engage their gnawing incisors while their chewing teeth (molars and premolars) are not being used. The reverse is also true; rodents can use their chewing teeth (also called cheek teeth) while their incisors are disengaged.

The entire skull structure of rodents is designed to accommodate this task of separating the use of the different types of teeth. Rodent skulls have long snouts; the articulation of the lower jaw with the skull is oriented front to back rather than sideways as in other mammals; the jaw muscles (masseter complex) are extended well forward into the snout; and the number of cheek teeth is less than in most other mammals—all features unique to rodents.

The classification of rodents has always been difficult because of the great diversity of both Recent and fossil species. Traditionally, there are two ways that rodents have been divided: into three major groups based on the structure of the attachment of the jaw muscle on the skull (Sciuromorpha, Hystricomorpha, Myomorpha); or into two groups based on the structure of the lower jaw (Sciurognathi, Hystricognathi). The difficulty in using these groups (usually considered suborders or infraorders) is that the distinctive adaptations of one group of rodents are also present in others, derived in completely separate ways.