Data from: Barb geometry of asymmetrical feathers reveals a transitional morphology in the evolution of avian flight

The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.羽枝（barb）的几何形态——包括羽枝长度与羽枝角度——决定了羽毛羽片的不对称性与羽片刚度，二者均对羽毛的空气动力学性能至关重要。本研究系统梳理了不对称飞行羽毛演化历程中，羽枝几何形态与空气动力学功能之间的关联，研究样本覆盖中生代非现生鸟类类群（小盗龙、始祖鸟、神州鸟、孔子鸟以及反鸟类（enantiornithine）始鹏鸟），以及涵盖范围极广的现生鸟类群体。与既往研究假设相悖，我们发现羽枝角度与羽片宽度不对称性并无关联；相反，羽枝角度随羽片功能需求发生变化，而羽枝长度的差异才是决定羽片不对称性的核心因素。本研究证实，飞行羽毛羽片的不同功能区域间，羽枝几何形态存在显著差异；位于羽片前缘的尖端区域，占据了形态空间（morphospace）中以小羽枝角度为特征的独特区间。这种前缘尖端羽片形态在系统发育与功能均具多样性的现生鸟类及中生代基干鸟类样本中普遍存在，揭示了自晚侏罗世便延续至今的一项核心空气动力学适应特征。然而，在今鸟亚纲与反鸟类的干群类群中，羽片尾缘的羽枝几何形态与现生鸟类存在显著区别。无论是现生鸟类还是反鸟类，其尾缘羽片的羽枝角度均高于始祖鸟这类相对干群的类群——后者的尾缘羽枝角度极小。这一发现揭示了此前未被认知的飞行羽毛形态演化过渡过程，对始祖鸟、小盗龙等早期带羽毛兽脚类（theropod）的飞行能力研究具有重要参考价值。我们的研究结果表明，完全现生化的鸟类飞行羽毛，乃至现代动力飞行能力，均在孔子鸟之后的冠群类群中演化形成，这一时间晚于不对称飞行羽毛的起源节点，且远晚于此前的学术认知。