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. 

摘要： 性双型的特征通常被认为产生于种内争夺交配优先权的斗争。例如，现生和化石的雄性长鼻类动物具有较大的体型和较粗壮的上门齿。本研究阐释了如下现象：化石象型类动物(Elephantiformes, 长鼻类的主要类群)一些性双型特征与其进化历史具有相关性，而与性别竞争并非直接相关。在中新世的葛氏铲齿象(Platybelodon grangeri)和狭齿嵌齿象(Gomphotherium angustidens)中，雄性比雌性倾向于具有进化中更进步的特征，如同雄性在进化中领先雌性一步。这种现象可能与雌性偏好的机制相耦合。在象型类动物进化的早期(繁荣期)，性别选择压促使雄性比雌性产生更加显著的进步特征；然而，在它们进化的晚期(衰退期)，性别选择压似乎减弱，性别的异时进化也减少。这种新的发现或许在大型有蹄类的演化过程中有一定的普遍意义，因为那些繁荣的类群中通常性双型显著，如鹿科和牛科；而衰落的类群中通常性双型不显著。