Data from: From baby birds to feathered dinosaurs incipient wings and the evolution of flight

Reconstructing the tree of life requires deciphering major evolutionary transformations and the functional capacities of fossils with "transitional" morphologies. Some of the most iconic, well-studied fossils with transitional features are theropod dinosaurs, whose skeletons and feathered forelimbs record the origin and evolution of bird flight. However, in spite of over a century of discussion, the functions of forelimb feathers during the evolution of flight remain enigmatic. Both aerodynamic and non-aerodynamic roles have been proposed, but few of the form-function relationships assumed by these scenarios have been tested. Here, we use the developing wings of a typical extant ground bird (Chukar Partridge) as possible analogues/homologues of historical wing forms to provide the first empirical evaluation of aerodynamic potential in flapping theropod "protowings." Immature ground birds with underdeveloped, rudimentary wings generate useful aerodynamic forces for a variety of locomotor tasks. Feather development in these birds resembles feather evolution in theropod dinosaurs, and reveals a predictable relationship between wing morphology and aerodynamic performance that can be used to infer performance in extinct theropods. By spinning an ontogenetic series of spread-wing preparations on a rotating propeller apparatus across a range of flow conditions and measuring aerodynamic force, we explored how changes in wing size, feather structure, and angular velocity might have affected aerodynamic performance in dinosaurs choosing to flap their incipient wings. At slow angular velocities, wings produced aerodynamic forces similar in magnitude to those produced by immature birds during behaviors like wing-assisted incline running. At fast angular velocities, wings produced forces sufficient to support body weight during flight. These findings provide a quantitative, biologically relevant bracket for theropod performance and suggest that protowings could have provided useful aerodynamic function early in maniraptoran history, with improvements in aerodynamic performance attending the evolution of larger wings, more effective feather morphologies, and faster angular velocities.

重建生命之树，需要解析重大演化转变事件，以及具有"过渡性"形态的化石的功能属性。部分最具标志性且研究充分的过渡特征化石为兽脚类恐龙，其骨骼与带羽毛前肢记录了鸟类飞行的起源与演化历程。然而，尽管已有百余年的研究讨论，飞行演化过程中前肢羽毛的功能仍未明确。学界已提出空气动力学与非空气动力学两类功能假说，但相关场景中所假设的形态-功能关联，鲜少得到实证检验。本研究以典型现生地栖鸟类——石鸡（Chukar Partridge）的发育中翼作为历史翼形态的潜在类比物/同源物，首次对振翅兽脚类"原翼"的空气动力学潜力开展实证评估。前肢发育不完善、翼结构原始的幼年地栖鸟类，可在多种运动行为中产生具有实用价值的空气动力。这类鸟类的羽毛发育过程与兽脚类恐龙的羽毛演化历程相似，揭示了翼形态与空气动力学性能之间的可预测关联，可用于推断已灭绝兽脚类的飞行性能。本研究通过在不同流场条件下，将一系列个体发育阶段的展翅标本置于旋转螺旋桨装置中，并测量其产生的空气动力，探究了翼尺寸、羽毛结构与角速度的变化，可能如何影响选择振翅初期翼的恐龙的空气动力学性能。在较低角速度下，原翼产生的空气动力幅值，与幼年鸟类在翼辅助爬坡跑等行为中产生的动力幅值相近。在较高角速度下，原翼产生的动力足以支撑飞行中的体重。本研究结果为兽脚类的飞行性能提供了量化且符合生物学意义的参照区间，表明原翼在手盗龙类演化早期便可提供具备实用价值的空气动力学功能；随着翼尺寸增大、羽毛形态更趋优化以及角速度提升，空气动力学性能也随之改善。