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

摘要

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 biome-chanics 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 Yixia-nornis, 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.%对鸟类和那些在渐进性变化方面与它们关系更rn为密切的恐龙所做的三维数字化重建，使得rnJohn Hutchinson及其同事能够确定恐龙身体rn构造是怎样变成典型的鸟类身体构造的以及这rn种变化是什么时候发生的。鸟类形成一个独特rn的蹲伏状后肢姿势。对17种“祖龙”(包括中国rn的“迅猛龙”和“始祖鸟”两种鸟类化石的完整rn骨架和身体)所做的重建表明，在兽脚类演化rn过程的大部分时间，更接近蹲伏状的肢体姿势rn是逐渐地、分步地获得的，其中“手盗龙”(真rn正的鸟类和与它们最接近的“恐爪龙”)所发rn生的变化更快。骨骼变化表明，上肢变化在改rn变身体的机械平衡方面是重要的，因而在鸟类rn两个关键行为(“二足性”和飞行)的转变中也rn是重要的。