Absence of premolars in Urusus arctos yesoensis and Selenarctcs thibetanus japonicus, and reduction of teeth in Ursidae

Abstract

The difference in the absence of premolars between Ursus arctos and Selenarctos thibetanus was observed with special examination of differences in reduction of premolars in Ursidae. For this study, 151 specimen of U. arctos and 36 of S. thibetanus were investigated. The condition of the premolars was categorized by microscopic observation and X-ray investigation in accordance to the following 4 categories: normal, fractured, impacted and absent teeth. First the age of specimens were determined by the annual layers in the cementum of canine teeth, then the variation in the premolar conditions was examined according to the related age. The fractured or impacted premolars increased with age for both species and the absence of premolars was significantly higher in U. arctos than in S. thibetanus. We think that the difference in frequency of premolar absence probably shows the orthogenesistic reduction of premolars in Ursidae.