Age variation of the third upper molar in Eothenomys smithii

Abstract

A study was made of age variation in the size and enamel patterns of the third upper molar of 99 Eothenomys smithii specimens from Japan. No significant age variation was found in either the frequency of the patterns, or the width of the dentine confluent space between the second and the third triangles. Deep lingual reentrant folds, on the posterior loop, appear in specimens where the condylobasal length (CBL) is of 22-24 mm, then the pattern with a shallow reentrant fold increases in frequency in larger CBL classes. The depth of the inner fold showed the same tendency as the changes in the patterns. A significant association, however, between five enamel patterns and age classes, depends on classification according to CBL or body weight. This proved insignificant in five CBL classes, but significant in three CBL or body weight classes. A gradual transition in the age variation of the posterior loop patterns was found among Eothenomys species which have rootless molars throughout life. The simple enamel pattern form significantly increased in frequency with advancing age in E. andersoni and E. shanseius, resembling Clethrionomys glareolus and C. rufocanus ; on the other hand, in E. regulus, E. inez, E. eva, E. chinensis, E. wardi, E. custos and E. proditor no age variation was found on the posterior loop, thus resembling Microtus pennsylvanicus. E. smithii shows a little age variation in the enamel patterns, the variation of which is of an intermediate type.