Abstract: The transformation from a long reptilian tail to a shortened tail ending in a pygostyle and accompanied by aerodynamic fanning rectrices is one of the most remarkable adaptations of early avian evolution. However, no fossils directly capture this transition, and information regarding the structural morphology and the early evolution of the pygostyle in Mesozoic birds and their integuments is relatively limited. Here we provide a review of the pygostyle morphology of Early Cretaceous birds with comparison to the structure in living birds. This study emphasizes the convergent evolution of distally co-ossified caudal vertebrae in non-avian maniraptorans and early birds. There further exist distinct differences in pygostyle morphology between Sapeornithiformes, Confuciusornithiformes, Enantiornithes, and Ornithuromorpha. The morphology of the pygostyle and rectrices in early ornithuromorphs appear similar to that of extant birds, whereas the pygostyle in more primitive birds does not appear morphologically capable of supporting the rectricial bulbs and musculature necessary to control an aerodynamic fan-shaped tail. The rectricial bulbs and rectricial fan appear to have coevolved with the plough-shaped pygostyle early in the evolution of the Ornithuromorpha. This study also shows that the confuciusornithiform pygostyle was more similar to that of enantiornithines than previously recognized, consistent with the presence of nearly identical ornamental tail feathers in both groups.

摘要： 从兽脚类恐龙中爬行类的骨质长尾，到以尾综骨为终端，并附着具有空气动力学功能的扇状尾羽的短巧尾部，是早期鸟类演化中最显著的适应性转变之一。但能直接反映这一转变的化石记录匮乏，而且对中生代鸟类尾部形态结构，以及尾综骨和尾羽早期演化的认知也相对不足。在此对早白垩世鸟类的尾综骨形态予以概述并将其与现生鸟类尾部结构类比。本研究强调了非鸟手盗龙类中尾椎的联合骨化与早期鸟类的尾综骨实属趋同演化。本研究表明，会鸟形类、孔子鸟形类、反鸟类和今鸟型类的尾综骨结构存在明显差异。今鸟型类尾综骨和尾羽(舵羽)与现代鸟类的相似，而相对更原始的鸟类的尾综骨，从形态来看，并不能支持舵羽球状膨大和必要的肌肉附着来操控具有空气动力学功能的扇状尾羽。由此可见，舵羽球状膨大、舵羽扇面与犁铧状的尾综骨是在今鸟型类演化早期相伴相生的。相对于从前的认知，本研究还发现孔子鸟类的尾综骨与反鸟类的有更多相似之处，与二者都具有的几乎相同的装饰性尾羽相符合。