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

Sexual dimorphism is considered to have evolved by selection on both sexes. Ostracodes display sexual shape dimorphism in adult valves; however, no previous studies have addressed temporal changes in evolutionary time scales or examined the relationships between sexual shape dimorphism, selection pressure, and juvenile shape. Temporal changes in sexually dimorphic traits result from responses of the 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 from deep-sea sediments of the Paleocene age (62.6–57.6 Ma) and estimated the ratio of females in the fossil populations at eleven time intervals. As the female ratio in a population is altered by the mortality rate of adult males, it is reflective of the selection pressure on males. We attempted to correlate the height/length ratios between the sexes with the female ratios and considered that the valve shape did not link with the selection pressure on males. In time series data of the height/length ratio, both sexes indicate no real changes in the evolutionary time scales, although the population changed the sex ratios from female-skewed into male-skewed during the late Paleocene. The sexual shape dimorphism was not under sexual selection. The static allometry between the height/length ratio and length indicates that the sexual shape dimorphism was not functional for sexual display. The temporal change in female allometric slope indicates no real changes, suggesting that the evolution of the valve shape is constrained by static allometry.

性二态性（sexual dimorphism）通常被认为是通过对两性的选择作用演化而来。介形类（Ostracodes）的成体介瓣存在形态性二态性，但此前尚无研究在演化时间尺度上探讨其时间动态，也未厘清形态性二态性、选择压力与幼体形态之间的关联。性二态性状的时间变化源于该性状对选择压力的响应。本研究以介瓣形态参数——高长比——构建高斯混合模型（Gaussian mixture model），在古新世（Paleocene，62.6–57.6 Ma）深海沉积物中鉴定出长形克丽介（Krithe dolichodeira）介壳形态的两性差异，并估算了11个时间间隔内化石种群的雌性占比。由于种群内雌性占比会受成年雄性死亡率的影响，因此该占比可反映作用于雄性的选择压力。我们尝试将两性的高长比与雌性占比进行关联分析，结果表明介瓣形态与雄性所受选择压力并无关联。在高长比的时间序列数据中，尽管种群的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真实变化。该形态性二态性并未受到性选择作用。高长比与体长之间的静态异速生长（static allometry）分析表明，该形态性二态性并不具备性展示的功能。雌性异速生长斜率的时间变化并未显现真实差异，这提示介瓣形态的演化受到静态异速生长的约束。