Abstract: Sexually dimorphic characters are usually thought to enhance copulatory success by intraspecific competition; for example,larger body size and stronger tusks are sexually dimorphic charactersin fossil and extant male proboscideans. Here, we show that some sexually dimorphic characters in fossil Elephantiformes, the largest group of proboscideans, are strongly correlated with the evolution of this group rather than direct sexual competition. In MiocenePlatybelodon grangeri and Gomphotherium angustidens, malestended to initially possess evolutionarily more derived characters thanfemales, and females then evolved similar variation. Thisphenomenon may have occurred as a result of female preference.During the early evolutionary stage (thriving stage) of Elephantiformes, sexual selection pressure promoted development ofmore prominent derived characters in males than females. However,during their late evolutionary stage (declining stage), sexual selection pressure seems to have weakened; thus, the asynchrony between the two sexes diminished. This new discovery may help explain a common mechanism of large ungulate evolution and extinction, because substantial sexual dimorphism is often displayed in thriving groups, such as Cervidae and Bovidae, in contrast to little sexual dimorphism in declining groups, such as extant taxa of Equidae,Rhinoceridae, and Giraffidae.