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)....尽管非鸟类恐龙具有极高的物种多样性与形态分异度，且各类脊椎动物支系中普遍存在性二态现象，但目前关于非鸟类恐龙的性二态证据却极度匮乏。本研究选取系统发育上分别处于恐龙类群两侧的现生类群——密西西比短吻鳄（Alligator mississippiensis）与美洲鸵（Rhea americana）的体型数据为参照，证明即使某一物种存在显著的性二态，若对具有持续生长特征的种群（如短吻鳄与多数恐龙）进行随机采样，也会导致该性二态信号丢失。性二态现象可能在化石类群中广泛存在，但若无个体发育年龄控制与大样本量支撑，便极难被检测到；而化石记录的固有局限性使得这两项条件均难以满足。Ⅲ型存活曲线（Type III survivorship）会进一步阻碍该信号的检出：该曲线下幼体死亡率较高，理论上会提升幼体个体的采样概率，除非存在针对小型个体的捕食偏好或埋藏学偏差抵消了这一效应。