Cranial functional morphology of Archaeopteryx and the biomechanical triggers of avian evolution

石印始祖鸟是著名的“缺失环节”。在达尔文的《物种起源》出版仅两年后，这块著名的半鸟半爬行动物化石就引起了世界科学界的注意，被誉为进化论的终极证据。这里提出的研究将使用严格的量化建模方法，揭示有关这种神秘化石的生态学和进化的新信息。在某些方面，我们对始祖鸟了解得很多。它是最基础的鸟类，是兽脚亚目恐龙的后代。它具有鸟类和兽脚亚目恐龙的特征：像鸟类一样的大脑，能够控制复杂的运动，翅膀上有不对称的空气动力学飞行羽毛，但有爪状的手，细长的尾巴和牙齿排列的下颌，而不是喙。一个半世纪过去了，我们对始祖鸟还有很多不了解的地方。它能飞得多好？它是陆生的，从地面起飞，还是生活在树上？它能像鹧鸪那样扇动翅膀跑上树干吗？颅骨的变化是否反映了骨骼功能的变化？颅骨的进化与骨骼的其他部分同步吗？始祖鸟头骨的功能是鸟类还是恐龙？骨骼通常会适应它们所承受的负荷，由此产生的应力和应变可以决定骨骼的形状。尽管位于一个主要辐射的底部，我们对始祖鸟的头骨形态与它的功能之间的关系一无所知。推断出这一点将有助于了解其进食行为、生态和驱动颅骨进化的选择压力。这项研究的目的是传递这些知识。破译骨架功能线索的一种方法是工程技术有限元分析（FEA）。用户建立他们希望测试的结构的数字模型（即桥梁，始祖鸟头骨），应用弹性特性和加载力来模拟要测试的弹性和行为（即咬）。该分析计算了响应这些荷载力的结构内的应变和应力。本研究将使用有限元分析来测试始祖鸟的头骨是否适合啄或咬和撕裂，并估计其咬的力量，例如它是否可以穿透昆虫角质层或坚硬的种子。这很重要，因为它将告诉我们最早的飞行动物的生态位，以及它们主要是陆生的还是树栖的。这项研究将比较始祖鸟在功能上是否与鸟类或非鸟类兽脚亚目动物相似，并测试始祖鸟具有鸟类头骨灵活性（运动能力）的说法。这对于理解鸟类特征获得的顺序和时间、头骨是否与剩余骨骼一起快速进化以及早期鸟类的头骨是否比它们的非鸟类兽脚亚目祖先在形态上更具多样性至关重要。有限元分析的问题是，我们对模型结果反映现实的程度知之甚少，也不知道哪个输入参数最重要。因此，我会先验证FEA的准确性。我将在一个死的，去皮的鸵鸟头骨上加载精心量化的力，并通过实验测量骨张力。我将测量颅骨的弹性，内收肌施加的力，并使用CT扫描在fe软件中准确重建鸵鸟头骨。然后，我将对有限元模型施加相同的载荷，并量化实验应变与有限元模型导出应变的方向和大小差异。通过在灵敏度分析中改变弹性特性、载荷和肌肉力，我将看到哪些参数对模型结果影响最大。由此产生的信息可用于根据我们所知道的输入参数尽可能严格地制作灭绝动物有限元模型，结果对所有对生物学中有限元分析感兴趣的人都很重要。

Archaeopteryx lithographica is the famous 'missing-link'. Brought to the attention of the world's scientific community only two years after publication of Darwin's On the Origin of the Species, the famous half-bird, half-reptile fossil was hailed as the ultimate evidence for evolution. The study proposed here will reveal new information about the ecology and evolution of this enigmatic fossil, using a rigorously quantified modelling approach. In some respects we know a good deal about Archaeopteryx. It is the most basal bird, and descended from within the theropod dinosaurs. It has a mosaic of bird and theropod characters: a bird-like brain capable of complex motor control, wings with asymmetrical, aerodynamic flight feathers but clawed hands, an elongated tail and jaws lined with teeth rather than a beak. A century and a half later there are still many things that we don't know about Archaeopteryx. How well could it fly? Was it terrestrial and took off from the ground up, or did it live in the trees? Could it run up tree trunks flapping its wings like partridges do? Are changes in function of the skeleton mirrored by changes in the skull? Did cranial evolution keep pace with the rest of the skeleton? Was the Archaeopteryx skull functioning as a bird or a dinosaur? Bones are generally adapted to the loads they experience, and the resulting stress and strain can dictate the shape of the skeleton. Despite lying at the base of a major radiation, we know nothing concrete about how the skull morphology of Archaeopteryx relates to its function. Deducing this would inform on its feeding behaviour, ecology and the selective pressures driving cranial evolution. The aim of this study is to deliver this knowledge. One method to decipher functional clues in the skeleton is the engineering technique Finite Element Analysis (FEA). Users build a digital model of the structure they wish to test (i.e. bridge, Archaeopteryx skull), apply elastic properties and loading forces that mimic the elasticity and behaviour (i.e. biting) to be tested. The analysis calculates strain and stress within the structure in response to these loading forces. This study will use FEA to test whether the skull of Archaeopteryx is adapted for pecking or biting and tearing, and estimate force of its bite, could it penetrate insect cuticle or hard seeds for example? This is important, as it will inform on ecological niches of the earliest flier and if these were predominately terrestrial or arboreal. The study will compare whether Archaeopteryx was functionally similar to a bird or a non-avian theropod, and test assertions that avian skull flexibility (kinesis) was present in Archaeopteryx. This is crucial to understanding the sequence and timing of the acquisition of avian characters, whether the skull evolves apace with the remaining skeleton and whether the skulls of early birds were more morphologically diverse than their non-avian theropod ancestors. The problem with FEA is that we know little of how well model results reflect reality, and which input parameters matter the most. Because of this, I will verify the accuracy of FEA first. I will load a dead, defleshed ostrich skull with carefully quantified force and experimentally measure bone strain. I will measure the elasticity of cranial bone, force exerted by the adductor muscles and use CT scans to accurately reconstruct the ostrich skull in FE-software. I will then apply the same loads to the FE-model and quantify the difference in orientation and magnitude of the experimental versus FE-model derived strain. By altering elastic properties, loads and muscle force in a sensitivity analysis, I will see which parameters influence model results the most. The resulting information can be used to make extinct animal FE-models as rigorous as possible based on what we know of the input parameters, the results being of importance to all those interested in FEA in biology.