Data from: Protracted growth impedes the detection of sexual dimorphism in non-avian dinosaurs

Evidence for sexual dimorphism is extremely limited in the non-avian dinosaurs despite their high diversity and disparity, and despite the fact that dimorphism is very common in vertebrate lineages of all kinds. Using body-size data from both Alligator mississippiensis and Rhea americana, which phylogenetically bracket the dinosaurs, we demonstrate that even when there is strong dimorphism in a species, random sampling of populations of individuals characterized by sustained periods of growth (as in the alligator and most dinosaurs) can result in the loss of this signal. Dimorphism may be common in fossil taxa but very hard to detect without ontogenetic age control and large sample sizes, both of which are hampered by the limitations of the fossil record. Signal detection may be further hindered by Type III survivorship, whereby increased mortality among the young favours the likelihood that they will be sampled (unless predation or taphonomic bias against small size acts against this). These, and other considerations relating to behaviour and ecology, provide powerful reasons to suggest that sexual dimorphism in dinosaurs may be very difficult to detect in almost all currently available samples. Similar issues are likely also to be applicable to many fossil reptiles, or animals more generally.

尽管非鸟类恐龙（non-avian dinosaurs）具有极高的物种多样性与形态分异度，且两性异形（sexual dimorphism）在各类脊椎动物支系中均极为普遍，但目前关于非鸟类恐龙两性异形的实证证据却极为有限。本研究以系统发育位置紧邻恐龙类群的现生类群——密西西比短吻鳄（Alligator mississippiensis）与美洲鸵（Rhea americana）的体型数据为参照，证实即便某一物种存在显著的两性异形，若对具有持续生长模式（如短吻鳄与多数恐龙）的种群个体进行随机采样，仍可能丢失这一性状信号。两性异形或许在化石类群中普遍存在，但若无个体发育年龄控制（ontogenetic age control）与足够大的样本量，便极难被检测到——而这两项条件均受限于化石记录的固有局限性。III型存活曲线（Type III survivorship）会进一步干扰信号检测：该曲线下幼体死亡率较高，理论上会提升幼体个体被采样到的概率，除非存在针对小型个体的捕食偏好或埋藏学偏差（taphonomic bias）抵消了这一效应。上述因素，以及其他与行为、生态学相关的考量，均强有力地表明：在当前几乎所有已有的化石样本中，恐龙的两性异形信号极难被识别。类似的问题或许也适用于诸多化石爬行动物类群，乃至更广泛的史前动物类群。