Abstract: Sivapithecus lufcngensis shows a great magnitude of sexual dimorphism, as justified previously on the basis of abundant materials. The sexual dimorphism of Laccopithecus robustus in Lufeng is also markedly expressed. Reexamination of the specimens reveals the facts that: 1) diastemata between maxillary second incisors and canines are evidently present on both sides in one specimen (PA 876) , while absent in another (PA 860) ; 2) lower canines show a striking difference in size which could not be considered to represent different individuals of the same sex. Because of the small sample size we do not adhere to the average magnitude of the sexual dimorphism we have determined. Nevertheless, this marked expression of sex difference is definitely not merely an accident of small sample size. The reduction of sexual dimorphism is revealed in an additional primate lineage-gibbon lineage.
Comparative studies indicate that Sivapithecus lufengensis is sexually more dimorphic in dental measurements than those of chimpanzees, gorillas and orangutans. The average and the coefficient of variation of the reductions of the dimorphism indices from Sivapithecus lufengensis to each kind of extant great apes are used to describe the extent and the nature of dimorphism change. Chimpanzees have the largest average and the smallest coefficient of variation in extant great apes. That seems to say, in chimpanzees, the reduction of sexual dimorphism from the condition of Sivapithecus lufengensis is most marked and, more importantly, most proportional in dental measurements. This fact strengthens the claim on the close phylogenetic relationship between Sivapithecus lufengensis and extant chimpanzees.

Key words: Sivapithecus lufengensis; Laccopithecus robustus; Sexual dimorphism