Data from: Sexual dimorphism and sexual selection in cytheroidean ostracodes from the Late Cretaceous of the US Coastal Plain

Sexual dimorphism is common in many extant animals, but it is difficult to demonstrate in fossil species. Working with material from the Late Cretaceous of the U.S. Coastal Plain, we herein analyze sexual dimorphism in ostracodes from the superfamily Cytheroidea, a group whose extant members have males that are relatively more elongate than females. We digitized outlines of more than 6000 individual ostracode valves or carapaces, extracted size (area) and shape (length-to-height ratio) information, and used finite mixture models to assess hypotheses of sexual dimorphism. Male and female clusters can be discerned in nearly all populations with sufficient data, resulting in estimates of size and shape dimorphism for 142 populations across 106 species; an additional nine samples are interpreted to consist only of females. Dimorphism patterns varied across taxa, especially for body size: males range from 30% larger to 20% smaller than females. Magnitudes of sexual dimorphism are generally stable within species across time and space; we can demonstrate substantial evolutionary changes in dimorphism in only one species, Haplocytheridea renfroensis. Several lines of evidence indicate that patterns of sexual dimorphism in these ostracodes reflect male investment in reproduction, suggesting that this study system has the potential to capture variation in sexual selection through the fossil record.

性二态性（sexual dimorphism）在多数现生动物类群中普遍存在，却难以在化石物种中得到证实。本研究依托美国沿海平原晚白垩世地层的化石材料，对花介超科（Cytheroidea）介形类的性二态性展开分析——该类群现生成员的雄性个体相较于雌性更为修长。我们对逾6000个单个介形类壳瓣或介壳的轮廓进行数字化处理，提取了个体大小（面积）与形态（长径比）信息，并采用有限混合模型（finite mixture models）检验性二态性相关假说。在数据量充足的绝大多数种群中，均可辨识出雄性与雌性的聚类簇，据此我们为106个物种的142个种群估算出了大小与形态的性二态性参数；另有9个样本被判定仅包含雌性个体。不同类群的性二态性模式存在差异，其中体尺维度的差异尤为显著：雄性个体大小相较于雌性，可从大于雌性30%到小于雌性20%不等。多数物种的性二态性程度在不同时空尺度下整体保持稳定；我们仅在Haplocytheridea renfroensis（肾形真花介）中观测到了性二态性的显著演化变化。多项证据表明，这些介形类的性二态性模式反映了雄性在繁殖过程中的资源投入，这意味着本研究体系具备通过化石记录捕捉性选择（sexual selection）变异的潜力。