Murids include most of the familiar rats and mice, but the family also encompasses an enormously diverse array of other rodents. Here, we follow recent authorities in treating murids as members of a single, very large family with a number of subfamilies. The systematic relationships of these groups among themselves and with other rodents has proved to be an extremely difficult problem, one that is by no means resolved.
A number of characters link most murids. Not surprisingly in such an old and diverse group, even the most basic characters have been subject to continuing evolutionary change; most of the characters listed as diagnostic in the next paragraph do in fact show some variation within the group. All, however, are believed to have characterized primitive murids.
Members of this family range from a few grams to a kilogram or more in weight. Most have relatively large eyes. External ears vary from almost absent in, for example, diggers like mole-rats (Spalax), to moderately large (e.g., deer mice, long-eared mice). Some species, especially fossorial murids, have relatively short and powerful legs; others are longer but no murids are really cursorial. The feet of some are relatively small; those of swimmers like muskrats are large and almost fully webbed; others (for example, many climbers) are broad and strongly grasping. Four clawed digits are found on each forefoot (the pollex or "thumb" is small and bears a nail); the hind foot in most has five clawed digits (but sometimes the hallux or first toe has a nail). Tails range from nearly absent in Siberian hamsters to much longer than the head and body in some tropical climbing rats. In many species the tail are nearly naked as in Norway rats; in others they are well furred; and in some, for example Philippine cloud rats, they are bushy. The pelage is also highly variable, in consistency (from densely wooly to spiny), pattern, and color.
In the skull of murids, the infraorbital foramen, which primitively transmits nerves to the rostral region of the skull, lies mostly above the zygomatic plate. It is enlarged above for the passage of a slip of muscle, the medial masseter (masseter medialis) that originates on the side of the rostrum and inserts on the lower jaw. Below, the foramen is narrowed, but it nevertheless houses nerves and blood vessels en route to the rostrum. The foramen thus has a distinctive, "keyhole" shape in most forms (but the narrow ventral portion is lost in a few species). The zygomatic plate, formed by the anterior base of the zygomatic arch, is broad and a conspicuous feature of the cranium. It serves as attachment for another branch of the masseter, the lateral (masseter lateralis). This arrangement of the masseter, with one branch passing through the infraorbital foramen and and another originating on the zygomatic plate, is termed myomorphous. The jugal, one of the bones that participates in the zygomatic arch, is small and does not contact the lacrimal. The frontals are constricted above the orbits and there is no postorbital process or bar. Posteriorly, an interparietal bone is present and usually conspicuous. The lower jaw is sciurognathus.
As in all rodents, one upper and one lower incisor are always found on each side of the jaw, and canines are always absent. Follow the incisor is a diastema. Premolars are never present, and no more than three cheekteeth occur on each side (but this number is sometimes reduced to two or even one). The nature of the molars (shape, size, surface structure, number of roots) varies greatly.
Members of the family can be found on all continents except Antarctica and on many oceanic islands. They occupy ecosystems ranging from dry desert to wet tropical forest, from tundra to savanna to temperate woodland. Some species are semiaquatic; others live underground; yet others spend their entire lives in the canopy of tropical forest. Their food habits range from true omnivory to specialization in earthworms, subterranean fungi, even aquatic invertebrates. Their importance to mankind cannot be overstated. Some species cause millions of damage to agricultural lands and stored foods. Others are the vectors or reservoirs of a number of diseases that have periodically devasted human populations (and continue to do so). On the other hand, many species are beneficial to man. Some are important biological controls of pestiferous insects. Others may be essential ("keystone") species in maintaining the health of our forests, through their role in dispersing seeds or spreading mycorrhizal fungi. Some even provide a source of meat. And a few species play an essential role as "domestic animals" used in medical research that has been enormously beneficial to mankind.
Around 1325 living species of murid rodents have been described, making the Muridae by far the largest family of mammals -- but surely many more remain to be discovered. Murid species are currently placed in 281 genera, which are distributed among 17 subfamilies. The fossil record of the family extends to the late Eocene.
Families of Order Rodentia Suborder Sciurognathi Family Aplodontidae (mountain beaver, sewellel) Family Sciuridae (squirrels) Family Castoridae (beavers) Family Geomyidae (pocket gophers) Family Heteromyidae (kangaroo rats, pocket mice, and allies) Family Dipodidae (birch mice, jumping mice, jerboas) Family Muridae (familiar rates and other rodents) Family Anomaluridae (scaly-tailed squirrels) Family Pedetidae (spring hare, springhaas) Family Ctenodactylidae (gundis) Family Myoxidae (dormice and hazel mice) Suborder Hystricognathi Family Bathyergidae (mole rats, blesmols, and rats) Family Hystricidae (Old World porcupines) Family Petromuridae (rock rat or dassie rat) Family Thryonomyidae (cane rats or grasscutters) Family Erethizontidae (New World porcupines) Family Chinchillidae (Chinchillas and viscachas) Family Dinomyidae (pacarana, branick rats, false paca) Family Caviidae (cavies and guinea pigs) Family Hydrochaeridae (capybara) Family Dasyproctidae (agoutis, acouchis) Family Agoutidae (pacas) Family Ctenomyidae (tuco-tucos) Family Octodontidae (degus, coruros, rock rats) Family Abrocomidae (chinchilla rats, chinchillones) Family Echimyidae (spiny rats) Family Capromyidae (hutias, zagouties, cavies, Indian coneys) Family Heptaxodontidae (Quemi, giant hutias) Family Myocastoridae (nutria, coypu)
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