Computational biomechanics, functional anatomy and the evolution of dinosaur quadrupedality

鸟臀目恐龙出现在晚三叠世时期（约2.25亿年前），并在白垩纪生态系统中成为主要的食草动物，之后所有非鸟类恐龙在白垩纪-第三纪边界（6500万年前）灭绝。最早的鸟臀目是快速的两足动物，但许多谱系随后采用了四足站立，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中发现），而四足动物的次级逆转只在恐龙和鳄鱼中发现。事实上，鸟臀目在脊椎动物中是独一无二的，因为次级四足动物在这个群体中进化了很多次：在装甲甲锥龙（甲龙和剑龙），角角龙和鸭嘴龙（“鸭嘴龙”）中。然而，这些动物的运动生物力学，和选择性的压力，导致这种不同的立场/步态的演变，从来没有被检查使用严格的分析/定量技术或在现代系统发育框架。在这里，我们提出了一个多学科的项目，调查的功能解剖学和进化的鸟臀目四足动物，使用的方法包括比较解剖学，遗传学和计算生物力学。使用这种方法，我们将测试一系列的假设，属于两个广泛的目标：1）是什么生物力学因素，推动了鸟臀目四足动物的进化？2）如何从小型四足动物进化为多吨级的四足动物？为了研究鸟臀目的生物力学，我们将在骨骼遗骸的基础上，并与现存鸟类和爬行动物的四肢肌肉组织进行比较，对这些动物的肌肉骨骼系统进行新的重建。这些重建将用于限制肢体运动的范围，也将影响质心位置的计算。新的生物力学软件将用于生成包含这些解剖信息的双足和四足鸟臀目动物的虚拟模型。这些模型将被用于一系列实验，以测试重皮甲（thyreophorans）或超大头骨（ceratopsians）的发育是否参与了四足动物的进化，并测试以不同速度和不同步态行走/跑步的稳定性。最后，这些系统的进化将研究相结合的解剖和功能形态学数据与系统发育信息，以研究这些运动复合体如何随着时间的推移而演变，以及是否共同的因素参与了每个独立收购的四足动物由不同的鸟臀目群体。该项目将成为恐龙运动研究的基准，并将展示一套在古生物学中开创的生物力学技术，这些技术很容易适用于活体分类群，但并不广为人知。它将提供与动物运动、古生物学和进化生物学工作者直接相关的结果，不仅对生命科学家，而且对处理生物系统的工程和力学的物理科学家都具有广泛的兴趣。

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.