在恐龙向鸟类演化的过程中，尾骨变化最为显著，由于缺少过渡环节的化石，这一演化过程并不清楚。现代鸟类的尾骨缩短并愈合成犁状尾综骨，其上附有扇状尾羽而构成飞行器官的重要组成部分。学术界普遍认为犁状尾综骨和扇状尾羽是同步演化的。中国科学院古脊椎动物与古人类研究所王敏等报道了在河北白垩纪地层中发现的1.3亿年前的反鸟类化石，显示其具有与现代鸟类相似的犁状尾综骨，但却并不具有扇状尾羽。本研究组综合形态学、骨组织学和扫描电镜等方法，提出"犁状尾综骨与扇状尾羽在早期鸟类中相互独立"的假设，挑战了此前长期占主流的学术观点。相较于犁状尾综骨在今鸟型类的广泛发育，这一结构在部分反鸟类中的出现则是平行演化的结果，结合越来越多的其他化石证据，表明在鸟类演化早期，平行演化是一个非常普遍的现象，这也为依据骨骼形态讨论中生代鸟类的系统发育关系提出了挑战。
During the dinosaurs-birds transition, the most conspicuous morphological change is the abbreviation of the long bony tail. Due to the lack of transitional fossils, little is known about how that modification took place. In living birds, the tail ends in a compound ploughshaped element termed pygostyle that attaches the fan-shaped tail feathers, forming an indispensable flight apparatus. Based on comparative anatomical, histological and electrical scanning analyses, Wang et. al. suggested that the plough-shaped pygostyle and the tail fanning is evolutionally decoupled in the early avian history, which challenges the long-standing theory that these two features are coevolved. A plough-shaped pygostyle is distributed widely in Ornithuromorpha. Therefore, the rare occurrence of this structure in some enantiornithines is the result of convergence, as confirmed by a few other fossil birds, further highlighting that the early avian evolution is characterized by parallelism.
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