Sciuridae—Squirrels // Castoridae—Beavers // Heteromyidae—Kangaroo Rats and Pocket Mice // Geomyidae—Pocket Gophers // Cricetidae—Cricetid Mice and Rats // Erethizontidae—American Porcupines // Caviidae—Capybaras and Relatives
The modern rodents are the largest and most diverse of the mammalian orders, with 33 families, 481 genera, and 2,277 species recognized (Wilson and Reeder 2005). Not counting introductions, there are seven regional families, six of which are represented as fossils (taxa of the seventh, the Dipodidae, are limited in their geographic range in the region and have small populations; in addition, another family is recognized in the Pleistocene fossil record). They occur on all of the continents except Antarctica. All depend largely on plants for food, though many include arthropods as part of their diet and many will partake of other animal products, including meat, if the opportunity presents. Major adaptations include those for climbing, digging, jumping, gliding, and swimming.
Although morphologically diverse, universal characters include one upper and one lower, ever-growing incisor on each side, always separated by a diastema from the cheek teeth; there are no canines (Fig. 1). The enamel of the incisors is limited to the anterior surface. The bulk of the incisor is of the softer dentine, which wears more rapidly than the enamel. The unsupported portion of the enamel breaks off, leaving a constantly renewed sharp surface for gnawing. The maximum tooth formula for members of our region (and virtually all rodents) is 1/1 0/0 2/1 3/3 = 22; in many, the tooth number (tooth formula or dental formula) is reduced to 16 (1/1 0/0 0/0 3/3). The lower jaw is articulated such that it can move anteriorly and posteriorly to either bring the incisors into play for gnawing or the cheekteeth for mastication.
Fig. 1. Rodent skull (Thomomys bottae) showing a typical arrangement of incisors, diastema, and cheekteeth. There are one premolar and three molars in this taxon (the premolar looks like two teeth to the unwary).
The cheekteeth are highly varied within the order, and most regional rodents can be identified to the generic level by even isolated teeth. Identification to species, even on well preserved material, often is difficult to impossible. For some taxa, specific teeth (e.g., m1) may be necessary for identification to species. Especially problematical are identifications to the species level of members of the genus Peromyscus, and difficulties in the identification of the genus Microtus are especially vexing because members are of considerable value in reconstructions of past environments.
Fig. 2. A lower incisor (top), a medial view of an upper incisor of Cratogeomys (middle), and an external view of the upper incisor of Cratogeomys (lower). The chisel-like wear at the tips and the enamel limited to the anterior surfaces are visible. Fossil teeth from Dark Canyon Cave.
Literature. Wilson and Reeder 2005.
Last Update: 21 Nov 2008