Rodentia: Extraordinary Diversification of a Morphologically Distinctive and Stereotyped Order

Abstract

The defining characteristic of the mammalian order Rodentia is a pair of continually growing incisors (Figure 1). There are rodents that scamper, hop, burrow, swim, and glide; that eat seeds, grass, nuts, insects, and worms; that live in the tropics, arctic, and temperate regions, in, on, or above the surface of the earth; in desert, grassland, savannah, and forest, but all are easily recognized by the incisors and correlated features. The lower incisors grow as helices approximating arcs. The upper incisor represents a relatively larger arc of a smaller circle than the lower incisor, which is a smaller arc of a larger circle. The root of each incisor is far back in the jaw. Enamel is essentially restricted to the labial surface of the incisors and comprises two layers. The wear of lower incisors against uppers, the distribution of enamel, the relative hardness of enamel versus dentine, and continual growth results in the maintenance of a sharp chisel edge. Thus, the gnawing incisors are primary food-gathering organs for rodents, but can also function in a number of other ways, such as in digging or as forceps.