Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, U.K.
Biol Rev Camb Philos Soc. 2014 Aug;89(3):588-617. doi: 10.1111/brv.12071. Epub 2013 Nov 19.
Ornithischian dinosaurs were primitively bipedal with forelimbs modified for grasping, but quadrupedalism evolved in the clade on at least three occasions independently. Outside of Ornithischia, quadrupedality from bipedal ancestors has only evolved on two other occasions, making this one of the rarest locomotory transitions in tetrapod evolutionary history. The osteological and myological changes associated with these transitions have only recently been documented, and the biomechanical consequences of these changes remain to be examined. Here, we review previous approaches to understanding locomotion in extinct animals, which can be broadly split into form-function approaches using analogy based on extant animals, limb-bone scaling, and computational approaches. We then carry out the first systematic attempt to quantify changes in locomotor muscle function in bipedal and quadrupedal ornithischian dinosaurs. Using three-dimensional computational modelling of the major pelvic locomotor muscle moment arms, we examine similarities and differences among individual taxa, between quadrupedal and bipedal taxa, and among taxa representing the three major ornithischian lineages (Thyreophora, Ornithopoda, Marginocephalia). Our results suggest that the ceratopsid Chasmosaurus and the ornithopod Hypsilophodon have relatively low moment arms for most muscles and most functions, perhaps suggesting poor locomotor performance in these taxa. Quadrupeds have higher abductor moment arms than bipeds, which we suggest is due to the overall wider bodies of the quadrupeds modelled. A peak in extensor moment arms at more extended hip angles and lower medial rotator moment arms in quadrupeds than in bipeds may be due to a more columnar hindlimb and loss of medial rotation as a form of lateral limb support in quadrupeds. We are not able to identify trends in moment arm evolution across Ornithischia as a whole, suggesting that the bipedal ancestry of ornithischians did not constrain the development of quadrupedal locomotion via a limited number of functional pathways. Functional anatomy appears to have had a greater effect on moment arms than phylogeny, and the differences identified between individual taxa and individual clades may relate to differences in locomotor performance required for living in different environments or for clade-specific behaviours.
鸟脚亚目恐龙最初是两足行走，前肢用于抓握，但四足行走在该类群中至少独立进化了三次。除了鸟脚亚目之外，从两足祖先进化而来的四足行走仅在另外两种情况下发生过，这使得四足动物进化史上的这种运动方式转变成为最罕见的一种。这些转变相关的骨骼和肌肉变化最近才被记录下来，而这些变化的生物力学后果仍有待研究。在这里，我们回顾了以前理解已灭绝动物运动的方法，这些方法大致可以分为基于现生动物的类比、肢体骨骼比例和计算方法的形态-功能方法。然后，我们首次系统地尝试量化两足和四足鸟脚类恐龙的运动肌肉功能的变化。我们使用主要骨盆运动肌肉力臂的三维计算建模，检查了个体分类群之间、四足和两足分类群之间以及代表三个主要鸟脚类恐龙谱系（兽脚亚目、鸟脚亚目、头饰龙下目）的分类群之间的相似性和差异性。我们的结果表明，角龙科的尖角龙和禽龙科的秀颌龙具有相对较低的力臂，用于大多数肌肉和大多数功能，这可能表明这些分类群的运动性能较差。四足动物的外展肌力臂比两足动物高，我们认为这是由于建模的四足动物整体身体更宽所致。与两足动物相比，四足动物在髋关节伸展角度较大时的伸肌力臂较高，而内旋肌力臂较低，这可能是由于后肢更柱状以及失去了内旋，作为四足动物的一种侧向肢体支撑形式。我们无法确定整个鸟脚类恐龙的力臂进化趋势，这表明鸟脚类恐龙的两足祖先并没有通过有限的功能途径限制四足运动的发展。功能解剖学似乎对力臂的影响大于系统发育，而在个体分类群和个体进化枝之间发现的差异可能与生活在不同环境或特定于进化枝的行为所需的运动性能有关。
