Enamel Structure of Early Mammals and Its Role in Evaluating Relationships among Rodents

Abstract

The evolution of mammalian enamels is a matter of multidisciplinary interest, and in recent years considerable attention has been paid to the subject (Grine, 1978; Fosse et al., 1973; Sahni, 1979). Work has concentrated on studying Recent reptilian and mammalian enamels (Cooper and Poole, 1973; Poole, 1957; Boyde, 1966), while other studies have centered on the micro- and ultrastructure of various “ancestral” groups such as the therapsid (mammal-like) reptiles in the hope of documenting evidence of the change from the thin, nonprismatic structure of reptilian enamels to the thicker, prismatic enamels of mammals (Poole, 1956; Moss, 1969; Osborn and Hillman, 1979). The enamels of various fossil and Recent rodents have invited special attention since the classic work of Tomes (1850). Rodent enamels are the most complex and highly organized within Mammalia. There are distinct subordinal variations among various groups; material from both fossil and Recent rodents is readily available and, furthermore, the process of amelogenesis (enamel secretion) can be observed throughout the life history of an individual because of the phenomenon of evergrowing incisors (Korvenkontio, 1934; Kiel, 1966).