Data from: The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents

Background. Powered flight is implicated as a major driver for the success of birds. Here we examine the effectiveness of three hypothesized pathways for the evolution of the flight stroke, the forelimb motion that powers aerial locomotion, in a terrestrial setting across a range of stem and basal avians: flap running, Wing Assisted Incline Running (WAIR), and wing-assisted leaping. Methods. Using biomechanical mathematical models based on known aerodynamic principals and in vivo experiments and ground truthed using extant avians we seek to test if an incipient flight stroke may have contributed sufficient force to permit flap running, WAIR, or leaping takeoff along the phylogenetic lineage from Coelurosauria to birds. Results. None of these behaviours were found to meet the biomechancial threshold requirements before Paraves. Neither was there a continuous trend of refinement for any of these biomechanical performances across phylogeny nor a signal of universal applicability near the origin of birds. None of these flap-based locomotory models appear to have been a major influence on pre-flight character acquisition such as pennaceous feathers, suggesting non-locomotory behaviours, and less stringent locomotory behaviours such as balancing and braking, played a role in the evolution of the maniraptoran wing and nascent flight stroke. We find no support for widespread prevalence of WAIR in non-avian theropods, but can’t reject its presence in large winged, small-bodied taxa like Microraptor and Archaeopteryx. Discussion. Using our first principles approach we find that “near flight” locomotor behaviors are most sensitive to wing area, and that non-locomotory related selection regimes likely expanded wing area well before WAIR and other such behaviors were possible in derived avians. These results suggest that investigations of the drivers for wing expansion and feather elongation in theropods need not be intrinsically linked to locomotory adaptations, and this separation is critical for our understanding of the origin of powered flight and avian evolution.

背景：动力飞行被认为是鸟类演化成功的核心驱动因素之一。本研究针对一系列干群鸟类与基干鸟类（stem and basal avians），在陆地环境中检验三种被提出的飞行扑动（flight stroke，即驱动空中运动的前肢动作）演化路径的有效性：拍跑（flap running）、翼辅助爬坡跑（Wing Assisted Incline Running, WAIR）以及翼辅助跳跃。
方法：本研究基于已被证实的空气动力学原理构建生物力学数学模型，并结合活体实验，同时以现存鸟类进行校准验证，旨在检验：在从虚骨龙类（Coelurosauria）到鸟类的系统发育谱系中，初期飞行扑动是否能够提供足够的力，以支持拍跑、WAIR或跳跃起飞行为。
结果：在副鸟类（Paraves）出现之前，上述三种行为均未达到生物力学阈值要求；在整个系统发育过程中，未观察到任何一种这类生物力学性能持续优化的趋势，在鸟类起源附近也未发现其普遍适用的信号。基于扑动的运动模型均未对正羽（pennaceous feathers）等飞行前特征的演化产生显著影响，这表明非运动行为以及平衡、制动等低要求的运动行为，在手盗龙类（Maniraptora）的翼与新生飞行扑动的演化中发挥了作用。本研究未发现非鸟类兽脚类（non-avian theropods）中普遍存在WAIR行为的证据，但无法排除在小盗龙（Microraptor）、始祖鸟（Archaeopteryx）这类翼展较大、体型较小的类群中存在该行为的可能性。
讨论：通过第一性原理研究方法，本研究发现“近飞行”运动行为对翼面积最为敏感，且非运动相关的选择机制很可能在衍生鸟类能够实现WAIR等行为之前，就已经推动了翼面积的扩张。本研究结果提示，针对兽脚类动物翼扩张与羽毛伸长的驱动因素研究，无需与运动适应性内在绑定，这种分离对于我们理解动力飞行的起源与鸟类演化至关重要。