Data from: Qualitative skeletal correlates of wing shape in extant birds (Aves: Neoaves)

Background: Among living fliers (birds, bats, and insects), birds display relatively high aspect ratios, a dimensionless shape variable that distinguishes long and narrow vs. short and broad wings. Increasing aspect ratio results in a functional tradeoff between low induced drag (efficient cruise) and increased wing inertia (difficult takeoff). Given the wide scope of its functional effects, the pattern of aspect ratio evolution is an important factor that contributes to the substantial ecological and phylogenetic diversity of living birds. However, because the feathers that define the wingtip (and hence the wingspan and aspect ratio) often do not fossilize, resolution in the pattern of avian wing shape evolution is obscured by missing information. Here I use phylogenetic constrained ordination to investigate the relationship between skeletal proxies of flight feather attachment, aspect ratio, and body mass. The resulting model is used to infer aspect ratios for two fossil avian taxa with preserved wing feathers and the Cretaceous ornithurine bird Ichthyornis. Results: An accessory lobe of the internal index process of digit II-1, a bony correlate of distal primary attachment, shows significant relationships to aspect ratio and body mass independent of the morphology of the rest of the forelimb skeleton. The dorsal phalangeal fossae of digit II-1, which house the follicles of distal primaries VIII and IX, also show a trend of increased prominence with higher aspect ratio. Quill knobs on the ulna are examined concurrently, but do not show consistent signal with respect to wing shape. Conclusions: Although quill knobs are most frequently cited as skeletal correlates of flight performance in birds, their relationship to wing shape is inconsistent among extant taxa, and may reflect diverging selection pressures acting on a conserved architecture. In contrast, correlates of distal primary feather attachment on the major digit show convergent responses to increasing aspect ratio within Neoaves.研究背景：在现存飞行类群（鸟类、蝙蝠与昆虫）中，鸟类展现出相对较高的展弦比（aspect ratio）——这是用于区分长窄翼与短宽翼的无量纲形态参数。提升展弦比会引发功能权衡：一方面可降低诱导阻力（巡航效率更高），另一方面会增大翼惯性（起飞难度提升）。鉴于展弦比对鸟类功能的影响范围广泛，其演化模式是促成现存鸟类丰富生态与系统发育多样性的关键因素之一。然而，界定翼尖（进而决定翼展与展弦比）的羽毛通常难以形成化石，因此鸟类翼形演化的模式因信息缺失而难以清晰还原。本文采用系统发育约束排序法（phylogenetic constrained ordination），探究与飞羽附着相关的骨骼替代指标、展弦比与体重之间的关联。基于所得模型，本文对两件保存有翼羽的化石鸟类类群，以及白垩纪今鸟亚纲的鱼鸟（Ichthyornis）的展弦比进行了推断。 研究结果：第二指节II-1的内指突附属叶——作为初级飞羽远端附着的骨骼对应结构——与展弦比和体重存在显著关联，且该关联不受前肢骨骼其余部分形态的影响。第二指节II-1的背侧指骨凹窝（容纳第VIII、IX枚初级飞羽的羽囊）同样呈现出随展弦比升高而愈发突出的趋势。本文同时考察了尺骨上的羽结节（quill knobs），但并未发现其与翼形存在一致的关联信号。 研究结论：尽管羽结节常被视为鸟类飞行性能的骨骼对应结构，但现存类群中其与翼形的关联并不一致，这可能反映了保守架构下存在趋异的选择压力。相反，主指上与初级飞羽远端附着相关的结构，在新鸟类（Neoaves）中随展弦比升高呈现出趋同的演化响应。