Computational biomechanics, functional anatomy and the evolution of dinosaur quadrupedality

鸟腿类恐龙出现在晚三叠世(约2.25亿年前)，并在白垩纪-第三纪界线上所有非鸟类恐龙最终灭绝(6500万年前)之前成为白垩纪生态系统中的主要食草动物。最早的鸟臀目动物是脚步敏捷的两足动物，但后来许多谱系采用了四足站姿，这种步态在更原始的爬行动物中很常见。鸟臀目动物在这方面是不寻常的：两足动物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)，而二次进化到四足动物的逆转只在恐龙和鳄鱼身上发现。事实上，鸟臀目动物在脊椎动物中是独一无二的，因为在这个类群中，次级四足动物在多种情况下进化：装甲甲壳动物(甲龙和剑龙)、有角的角龙和鸭嘴龙(鸭嘴龙)。然而，这些动物的运动生物力学以及导致这种不同姿势/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研究。在这里，我们提出了一个多学科的项目，利用比较解剖学、系统发育学和计算生物力学的方法来研究鸟臀部四足畸形的功能解剖和进化。使用这种方法，我们将检验一系列假设，这些假设归结于两个广泛的目标：1)推动鸟臀类四足动物进化的生物力学因素是什么？2)如何实现从小型两足动物到多音调四足动物的进化转变？为了研究鸟爪动物的生物力学，我们将在骨骼遗骸的基础上重建这些动物的肌肉骨骼系统，并与现存鸟类和爬行动物的肢体肌肉系统进行比较。这些重建将用于限制肢体运动的范围，还将影响重心位置的计算。最新可用的生物力学软件将被用来生成包含这些解剖信息的两足和四足鸟臀目的虚拟模型。这些模型将用于一系列实验，以测试厚重的真皮盔甲(甲状旁腺)或异常大的头骨(角足类)的发育是否参与了四足畸形的进化，并测试以不同速度和不同步态行走/跑步的稳定性。最后，这些系统的进化将通过结合解剖学和功能形态数据与系统发育信息来研究，以检验这些运动复合体是如何随着时间的推移而进化的，以及不同鸟腿类群独立获得四足动物的过程中是否涉及共同的因素。该项目将代表恐龙运动研究的基准，并将展示一套在古生物学中首创的生物力学技术，这些技术很容易适用于活着的类群，但并不广为人知。它将提供与动物运动学、古生物学和进化生物学工作者直接相关的结果，并将不仅引起生命科学家的广泛兴趣，而且还将引起从事生物系统工程和机械的物理学家的广泛兴趣。

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.