We report in this paper two unusual cases of M1 agenesis recently discovered in Japanese macaques (Macaca fuscata). Molar agenesis is the congenital absence of molars. In general, molar agenesis occurs from the posterior molars. In most cases, missing teeth are third molars. Second molars may occasionally be absent in addition to the third molars. However, the congenital absence of first molars is hardly observed in humans, and to our knowledge, no case has been reported in other primates. In addition to the rareness of first molar agenesis, the present cases in M. fuscata are also quite unusual, because the posterior molars (M2 and M3) were completely preserved in contrast to the ordinary pattern of molar reduction. Interestingly, the two cases of the present study were observed in a single maternal lineage (a mother and her son), suggesting that M1 agenesis in these Japanese macaques is genetically inherited.
2011 The Anthropological Society of Nippon