Avian wings as organs for aerial locomotion are furnished with a highly specialized musculoskeletal system compared with the forelimbs of other tetrapod vertebrates. Among the specializations, the propatagium, which accompanies a skeletal muscle spanning between the shoulder and wrist on the leading edge of the wing, represents an evolutionary novelty established at a certain point in the lineage toward crown birds. However, because of the rarity of soft-tissue preservation in the fossil record, the evolutionary origin of the avian propatagium has remained elusive. Here we focus on articulated skeletons in the fossil record to show that angles of elbow joints in fossils are indicators of the propatagium in extant lineages of diapsids (crown birds and non-dinosaurian diapsids), and then use this relationship to narrow down the phylogenetic position acquiring the propatagium to the common ancestor of maniraptorans. Our analyses support the hypothesis that the preserved propatagium-like soft tissues in non-avian theropod dinosaurs (oviraptorosaurian Caudipteryx and dromaeosaurian Microraptor) are homologous with the avian propatagium, and indicate that all maniraptoran dinosaurs likely possessed the propatagium even before the origin of flight. On the other hand, the preserved angles of wrist joints in non-avian theropods are significantly greater than those in birds, suggesting that the avian interlocking wing-folding mechanism involving the ulna and radius had not fully evolved in non-avian theropods. Our study underscores that the avian wing was acquired through modifications of preexisting structures including the feather and propatagium.

中文翻译：

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非鸟类恐龙中繁殖体的起源

与其他四足动物脊椎动物的前肢相比，作为空中运动器官的鸟类翅膀配备了高度专业化的肌肉骨骼系统。在这些特化中，propatagium 伴随着翅膀前缘跨越肩部和腕部的骨骼肌，代表了在冠鸟谱系中某个点建立的进化新奇。然而，由于化石记录中软组织保存的稀有性，鸟类繁殖体的进化起源仍然难以捉摸。在这里，我们关注化石记录中的关节骨骼，以表明化石中肘关节的角度是现存双孔类（冠鸟和非恐龙双孔类）谱系中繁殖体的指标，然后利用这种关系将获得繁殖体的系统发育位置缩小到手盗龙的共同祖先。我们的分析支持以下假设，即非鸟类兽脚类恐龙（偷蛋龙类尾翼龙和驰龙类小盗龙）中保存的类似前肢的软组织与鸟类前肢同源，并表明所有手盗龙类恐龙甚至可能在飞行起源之前就拥有前肢。另一方面，非鸟类兽脚类动物腕关节的保留角度明显大于鸟类，这表明涉及尺骨和桡骨的鸟类互锁机翼折叠机制在非鸟类兽脚类动物中尚未完全进化。