Computational biomechanics, functional anatomy and the evolution of dinosaur quadrupedality

鸟颈目恐龙出现于晚三叠纪（约2.25亿年前），在白垩纪-第三纪边界（6500万年前）所有非鸟类恐龙最终灭绝之前，成为白垩纪生态系统中主要的食草动物。最早的鸟臀目是脚快的两足动物，但后来许多谱系采用了四足站立，这种步态在更原始的爬行动物中很普遍。鸟臀目动物在这方面是不寻常的：两足行走是一种罕见的进化创新（只在少数爬行动物和哺乳动物群体中出现），四足行走的二次逆转只在恐龙和鳄鱼中发现。事实上，鸟臀目动物在脊椎动物中是独一无二的，因为在这个群体中，二级四足动物在很多场合都有进化：有甲甲龙（甲龙和剑龙）、有角角龙和鸭嘴龙（“鸭嘴龙”）。然而，这些动物的运动生物力学，以及导致这些不同姿态/步态进化的选择压力，从未使用严格的分析/定量技术或在现代系统发育框架内进行过研究。在此，我们提出了一个多学科项目来研究鸟臀目四足动物的功能解剖学和进化，使用比较解剖学、系统发育学和计算生物力学等方法。使用这种方法，我们将测试一系列假设，这些假设属于两个广泛的目标：1)驱动鸟臀龙四足动物进化的生物力学因素是什么？2)从小型两足动物到多吨四足动物的进化转变是如何实现的？为了研究鸟臀目动物的生物力学，我们将在骨骼残骸的基础上，并与现存鸟类和爬行动物的肢体肌肉组织进行比较，对这些动物的肌肉骨骼系统进行新的重建。这些重建将用于约束肢体运动的范围，也将影响质心位置的计算。新获得的生物力学软件将用于生成包含这些解剖信息的两足和四足鸟臀目动物的虚拟模型。这些模型将用于一系列实验，以测试重型真皮甲（thyreophorans）或特别大的头骨（ceratopans）的发展是否与四足动物的进化有关，并测试以不同步态以不同速度行走/跑步的稳定性。最后，这些系统的进化将通过将这些解剖和功能形态学数据与系统发育信息相结合来研究这些运动复合体是如何随着时间的推移而进化的，以及不同鸟臀目群体独立获得四足动物是否涉及共同因素。该项目将代表恐龙运动研究的一个基准，并将展示一套在古生物学领域开创的生物力学技术，这些技术很容易适用于现存的分类群，但尚未广为人知。它将为研究动物运动、古生物学和进化生物学的工作者提供直接相关的结果，不仅对生命科学家，而且对研究生物系统工程和力学的物理科学家都有广泛的兴趣。

Ornithischian dinosaurs appeared in the Late Triassic period (approximately 225 million years ago) and became the dominant herbivores in Cretaceous ecosystems prior to the final extinction of all non-avian dinosaurs at the Cretaceous-Tertiary boundary (65 million years ago). The earliest ornithischians were fleet-footed bipeds, but many lineages subsequently adopted a quadrupedal stance, a gait that is prevalent among more primitive reptiles. Ornithischians are unusual in this respect: bipedality is a rare evolutionary innovation (seen only in a small number of reptile and mammal groups) and secondary reversals to quadrupedality are found only in dinosaurs and crocodilians. Indeed, ornithischians are unique among vertebrates, as secondary quadrupedality evolved on multiple occasions within this group: in the armoured thyreophorans (ankylosaurs and stegosaurs), the horned ceratopsians, and the hadrosaurs ('duck-billed dinosaurs'). However, the locomotor biomechanics of these animals, and the selective pressures that led to the evolution of such disparate stances/gaits, have never been examined using rigorous analytical/quantitative techniques or within a modern phylogenetic framework. Here, we propose a multidisciplinary project to investigate the functional anatomy and evolution of ornithischian quadrupedality, using methods encompassing comparative anatomy, phylogenetics and computational biomechanics. Using this approach we will test a series of hypotheses that fall under two broad objectives: 1) what were the biomechanical factors that drove the evolution of ornithischian quadrupedality?; and 2) how was the evolutionary transformation from a small biped to a multi-tonned quadruped achieved? In order to investigate ornithischian biomechanics we will generate new reconstructions of the musculoskeletal systems of these animals on the basis of skeletal remains and comparisons with the limb musculature of living birds and reptiles. These reconstructions will be used to constrain ranges of limb movement and will also affect calculations of the position of the centre of mass. Newly available biomechanical software will be used to generate virtual models of bipedal and quadrupedal ornithischians that incorporate this anatomical information. The models will be used in a series of experiments to test whether the development of heavy dermal armour (thyreophorans) or exceptionally large skulls (ceratopsians) were involved in the evolution of quadrupedality and also to test the stability of walking/running at different speeds with different gaits. Finally, the evolution of these systems will be investigated by combining this anatomical and functional morphological data with phylogenetic information to examine how these locomotor complexes evolved through time and whether common factors were involved in each of the independent acquisitions of quadrupedality by different ornithischian groups. This project will represent a benchmark for studies on dinosaur locomotion and will showcase a set of biomechanical techniques pioneered in palaeobiology that are readily applicable to living taxa, but which are not widely known. It will provide results of direct relevance to workers on animal locomotion, palaeobiology and evolutionary biology and will be of broad interest not only to life scientists, but also to physical scientists dealing with the engineering and mechanics of biological systems.