Data from: Sexual shape dimorphism and selection pressure on males in fossil ostracodes

Sexual dimorphism is thought to have evolved via selection on both sexes. Ostracodes display sexual shape dimorphism in adult valves; however, no previous studies have addressed temporal changes on evolutionary timescales or examined the relationships between sexual shape dimorphism and selection pressure and between sexual shape dimorphism and juvenile shape. Temporal changes in sexually dimorphic traits result from responses of these traits to selection pressure. Using the Gaussian mixture model for the height/length ratio, a valve-shape parameter, we identified sexual differences in the valve shape of Krithe dolichodeira s.l. from deep-sea sediments of the Paleocene (62.6–57.6 Ma) and estimated the proportion of females in the fossil populations at 11 time intervals. Because the proportion of females in a population is altered by the mortality rate of adult males, it is reflective of selection pressure on males. We attempted to correlate the height/length ratios between the sexes with the proportion of females, taking into consideration that the valve shape was not linked with the selection pressure on males. In time-series data of the height/length ratio, both sexes indicate no significant changes on evolutionary timescales, even though the sex ratio of the population changed from female skewed to male skewed during the late Paleocene. The sexual shape dimorphism was not driven by sexual selection. The static allometry between the height/length ratio and length indicates that the sexual shape dimorphism did not function for sexual display. The absence of change over time in the female allometric slope suggests that the evolution of valve shape was constrained by stasis.

性别二态性（sexual dimorphism）被认为是通过对两性的选择作用演化而来的。介形类（Ostracodes）的成体壳瓣存在形态性别二态性，但此前尚无研究探讨演化时间尺度上的时序变化，也未考察形态性别二态性与选择压力之间的关联，以及形态性别二态性与幼体形态之间的关系。性二态性状的时序变化源于这些性状对选择压力的响应。本研究以壳瓣形态参数——高长比——构建高斯混合模型（Gaussian mixture model），识别了采自古新世（62.6~57.6 Ma）深海沉积物中的长刺克丽特介广义种（Krithe dolichodeira s.l.）壳瓣形态的性别差异，并估算了11个时间间隔内化石种群的雌性个体占比。由于种群内雌性占比会因成年雄性的死亡率发生改变，因此该占比可反映雄性所受的选择压力。我们尝试将两性的高长比与雌性占比进行关联分析，同时考虑到壳瓣形态与雄性所受选择压力并无关联这一前提。在高长比的时序数据中，尽管种群性比在古新世晚期从雌性偏倚转变为雄性偏倚，但两性的高长比在演化时间尺度上均未出现显著变化。本研究结果表明，形态性别二态性并非由性选择驱动。高长比与壳瓣长度之间的静态异速生长（static allometry）关系显示，形态性别二态性并未起到性展示的功能。雌性异速生长斜率未随时间发生改变，这表明壳瓣形态的演化受到稳定停滞（stasis）的约束。