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.

羽枝的几何特征（羽枝长度与羽枝角度）决定了羽片的不对称性与刚性，这两者对羽毛的气动性能至关重要。这里，我们描述了不对称飞行羽毛演化史上羽枝几何特征与气动功能之间的关系——样本涵盖现代鸟类多样性之外的中生代类群（小盗龙、始祖鸟、热河鸟、孔子鸟及反鸟类始鹏鸟），以及大量现代鸟类样本。与此前假设相反，我们发现羽枝角度与羽片宽度不对称性无关；相反，羽枝角度随羽片功能变化，而羽枝长度的差异决定了羽片不对称性。我们证实，飞行羽毛羽片的不同功能区域之间羽枝几何特征存在显著差异；且尖端前缘羽片占据形态空间中的独特区域，其特征为羽枝角度较小。这种尖端前缘羽片形态在系统发育与功能多样性均丰富的现代鸟类及中生代基干鸟类样本中广泛存在，揭示了自晚侏罗世以来持续存在的一种基础气动适应。然而，在今鸟类（Ornithurae）与反鸟类之外的中生代基干类群中，后缘羽片的羽枝几何特征与现代鸟类存在显著差异。在现代鸟类与反鸟类中，后缘羽片的羽枝角度均大于始祖鸟等相对基干的类群——后者的后缘羽片羽枝角度较小。这一发现揭示了飞行羽毛形态演化中此前未被认知的过渡阶段，对始祖鸟与小盗龙等早期带羽毛兽脚类恐龙的飞行能力具有重要意义。我们的研究结果表明，完全现代的鸟类飞行羽毛——或许还包括现代动力飞行能力——是在孔子鸟之后的冠群中演化而来的，这一时间远晚于不对称飞行羽毛的起源，也比此前认知的要晚得多。