Wu Qian, O'Connor Jingmai K, Wang Shiying, Zhou Zhonghe
University of Chinese Academy of Sciences, Beijing, China.
CAS Center for Excellence in Life and Paleoenvironment, Beijing, China.
PeerJ. 2024 Feb 29;12:e16960. doi: 10.7717/peerj.16960. eCollection 2024.
Important transformations of the pectoral girdle are related to the appearance of flight capabilities in the Dinosauria. Previous studies on this topic focused mainly on paravians yet recent data suggests flight evolved in dinosaurs several times, including at least once among non-avialan paravians. Thus, to fully explore the evolution of flight-related avian shoulder girdle characteristics, it is necessary to compare morphology more broadly. Here, we present information from pennaraptoran specimens preserving pectoral girdle elements, including all purportedly volant taxa, and extensively compare aspects of the shoulder joint. The results show that many pectoral girdle modifications appear during the evolution from basal pennaraptorans to paravians, including changes in the orientation of the coracoid body and the location of the articulation between the furcula and scapula. These modifications suggest a change in forelimb range of motion preceded the origin of flight in paravians. During the evolution of early avialans, additional flight adaptive transformations occur, such as the separation of the scapula and coracoid and reduction of the articular surface between these two bones, reduction in the angle between these two elements, and elongation of the coracoid. The diversity of coracoid morphologies and types of articulations joining the scapula-coracoid suggest that each early avialan lineage evolved these features in parallel as they independently evolved more refined flight capabilities. In early ornithothoracines, the orientation of the glenoid fossa and location of the acrocoracoid approaches the condition in extant birds, suggesting a greater range of motion in the flight stroke, which may represent the acquisition of improved powered flight capabilities, such as ground take-off. The formation of a new articulation between the coracoid and furcula in the Ornithuromorpha is the last step in the formation of an osseous triosseal canal, which may indicate the complete acquisition of the modern flight apparatus. These morphological transitions equipped birds with a greater range of motion, increased and more efficient muscular output and while at the same time transmitting the increased pressure being generated by ever more powerful flapping movements in such a way as to protect the organs. The driving factors and functional adaptations of many of these transitional morphologies are as yet unclear although ontogenetic transitions in forelimb function observed in extant birds provide an excellent framework through which we can explore the behavior of Mesozoic pennaraptorans.
肩带的重要转变与恐龙飞行能力的出现有关。此前关于这一主题的研究主要集中在近鸟类，然而最近的数据表明飞行在恐龙中多次独立演化，至少在非鸟翼类近鸟类中出现过一次。因此，为了全面探究与飞行相关的鸟类肩带特征的演化，有必要更广泛地比较形态学特征。在这里，我们展示了保存有肩带元素的近鸟类标本的信息，包括所有据称会飞的类群，并广泛比较了肩关节的各个方面。结果表明，从基础近鸟类到近鸟类的演化过程中出现了许多肩带的变化，包括喙骨体的方向变化以及叉骨与肩胛骨之间关节的位置变化。这些变化表明在近鸟类飞行起源之前，前肢的运动范围发生了改变。在早期鸟翼类的演化过程中，出现了更多与飞行适应相关的变化，比如肩胛骨与喙骨的分离以及这两块骨头之间关节面的减小、这两个元素之间夹角的减小以及喙骨的延长。喙骨形态的多样性以及连接肩胛骨 - 喙骨的关节类型表明，每个早期鸟翼类谱系在独立演化出更精细的飞行能力时，这些特征是平行演化的。在早期鸟胸类中，肩臼窝的方向和肩峰喙骨的位置接近现存鸟类的情况，这表明飞行冲程中的运动范围更大，这可能代表着获得了更好的动力飞行能力，比如地面起飞。鸟尾类中喙骨与叉骨之间新关节的形成是骨化三骨管形成的最后一步，这可能表明现代飞行装置的完全形成。这些形态转变使鸟类具有更大的运动范围、增加且更高效的肌肉输出，同时以保护器官的方式传递由越来越有力的扑翼运动产生的增加的压力。尽管现存鸟类前肢功能的个体发育转变为我们探索中生代近鸟类的行为提供了一个绝佳的框架，但许多这些过渡形态的驱动因素和功能适应仍不清楚。
