Data from: Linking the evolution of body shape and locomotor biomechanics in bird-line archosaurs

Locomotion in living birds (Neornithes) has two remarkable features:
feather-assisted flight, and the use of unusually crouched hindlimbs for
bipedal support and movement. When and how these defining functional
traits evolved remains controversial. However, the advent of computer
modelling approaches and the discoveries of exceptionally preserved key
specimens now make it possible to use quantitative data on whole-body
morphology to address the biomechanics underlying this issue. Here we use
digital body reconstructions to quantify evolutionary trends in locomotor
biomechanics (whole-body proportions and centre-of-mass position) across
the clade Archosauria. We use three-dimensional digital reconstruction to
estimate body shape from skeletal dimensions for 17 archosaurs along the
ancestral bird line, including the exceptionally preserved, feathered taxa
Microraptor, Archaeopteryx, Pengornis and Yixianornis, which represent key
stages in the evolution of the avian body plan. Rather than a discrete
transition from more-upright postures in the basal-most birds (Avialae)
and their immediate outgroup deinonychosauria, our results support
hypotheses of a gradual, stepwise acquisition of more-crouched limb
postures across much of theropod evolution, although we find evidence of
an accelerated change within the clade Maniraptora (birds and their
closest relatives, such as deinonychosaurs). In addition, whereas
reduction of the tail is widely accepted to be the primary morphological
factor correlated with centre-of-mass position and, hence, evolution of
hindlimb posture, we instead find that enlargement of the pectoral limb
and several associated trends have a much stronger influence.
Intriguingly, our support for the onset of accelerated morpho-functional
trends within Maniraptora is closely correlated with the evolution of
flight. Because we find that the evolution of enlarged forelimbs is
strongly linked, via whole-body centre of mass, to hindlimb function
during terrestrial locomotion, we suggest that the evolution of avian
flight is linked to anatomical novelties in the pelvic limb as well as the
pectoral.

现生鸟类（Neornithes）的运动系统具备两项标志性特征：依靠羽毛辅助的飞行能力，以及采用异常屈膝的后肢完成双足支撑与移动。这些具有界定意义的功能特征究竟何时、以何种方式演化而来，至今仍存在争议。不过，随着计算机建模方法的兴起，以及一批保存异常完好的关键标本的发现，如今我们已可借助全身体态形态的量化数据，探究这一问题背后的生物力学机制。
本研究通过数字化身体重建，量化主龙类（Archosauria）演化支中沿鸟类祖先谱系的运动生物力学演化趋势，涵盖全身体例比例与质心位置两项指标。我们基于骨骼尺寸，通过三维数字化重建手段，对17个沿鸟类祖先谱系分布的主龙类物种的身体形态进行估算，其中包括保存极为完好的带羽毛类群：小盗龙（Microraptor）、始祖鸟（Archaeopteryx）、鹏鸟（Pengornis）与义县鸟（Yixianornis），这些类群代表了鸟类身体构型演化的关键阶段。
我们的研究结果并不支持“最基干的鸟类（鸟翼类Avialae）及其近缘外类群驰龙类（deinonychosauria）存在从更直立姿态到蹲伏姿态的离散过渡”这一假说，反而支持“在兽脚类恐龙演化的大部分历程中，蹲伏的后肢姿势是逐步、渐进式演化而来”的观点；同时我们也发现，手盗龙类（Maniraptora，鸟类及其近缘类群，如驰龙类）演化支内部存在演化速率加快的证据。
此外，尽管学界普遍认为尾部缩减是与质心位置相关、进而影响后肢姿势演化的首要形态学因素，但我们的研究发现，前肢的增大以及若干相关演化趋势的影响更为显著。
耐人寻味的是，我们关于手盗龙类内部形态功能演化速率加快的结论，与飞行的演化进程密切相关。鉴于我们发现前肢的增大通过影响全身体质心，与陆地运动时的后肢功能紧密关联，因此我们提出：鸟类飞行的演化，不仅与前肢的解剖学创新有关，同样也与后肢的解剖学革新密不可分。