Applying synchrotron X-ray microtomography to a study of Morganucodon and Kuehneotherium: two of the earliest stem mammals.

虽然恐龙灭绝后的时代通常被称为哺乳动物时代，但哺乳动物的历史实际上可以追溯到大约220 Ma前的三叠纪晚期。它们的中生代化石记录少得令人沮丧，哺乳动物在中生代生态系统中扮演的角色也是粗略的。然而，格拉斯哥甘郡的裂缝填充物确实产生了大量的牙齿和脱节的骨头。摩尔古齿龙和昆尼奥特龙是来自这些裂隙的微小祖先哺乳动物，这项拟议的研究将使用严格量化的建模方法，揭示关于这些哺乳动物祖先的生态和进化的新信息。我们从哺乳动物的祖先犬齿动物那里了解到了很多关于哺乳动物过渡的某些方面，特别是颌骨铰链的部分如何转变为中耳的机制；这是脊椎动物进化形态的伟大案例之一。有中间状态的基础哺乳动物，如摩根齿龙和库赫尼奥瑟氏菌，帮助理解了这种转变。然而，关于这些最早的哺乳动物，我们仍然有许多不了解的地方。它们是完全恒温的吗，所以很可能是毛茸茸的，母亲给婴儿提供牛奶了吗？我们可以通过对牙齿和颌骨的研究，通过评估有效喂养的适应性和观察牙齿的替换模式来获得这些问题的一些答案。爬行动物一生中不断更换牙齿，但哺乳动物只有两组牙齿。这使上牙和下牙保持在适当的位置，以便有效地处理食物，并表明哺乳动物的生理和生长模式。古生物学家正在使用许多新的技术应用来帮助他们解释化石。使用同步辐射可以产生详细的高分辨率扫描。小尺寸的摩根鱼和Kuehnetherium的牙齿大小在1-2 mm之间，这是一个优势，因为它们可以容纳在苏黎世附近瑞士光源(SLS)的高分辨率设施中。破译颌骨功能线索的一种方法是工程技术有限元分析(FEA)。研究人员建立了他们想要测试的结构的数字模型(即桥梁、摩根牙鲨下巴)，施加弹性特性和加载力，以模拟待测试的行为(即咬合)。该分析计算结构内的应变和应力，以响应这些加载力。非侵入性技术，如计算机断层扫描，可用于生成精确的结构三维图像，如颌骨，这是形成准确有限元模型的基础所必需的。微小的早期哺乳动物被认为是食虫动物，但这涵盖了非常广泛的可能食物。这项研究将使用有限元分析来测试摩根齿龙和夸尼奥瑟姆的下巴是否适合咬甲虫或咬路过的飞蛾。它将估计它们被咬的力量/例如它们能穿透昆虫的角质层吗？这一点很重要，因为它将为摄食专业化和生态位提供信息。摩尔古齿龙和双颌龙都有一个中间的双颌关节，在向哺乳动物颌关节过渡的过程中，以前没有研究过的一个方面是，在个体的生长过程中，这两个关节的优先顺序是否发生了变化。从不同颌骨的相对大小来看，有没有某种迹象表明这一点？如果是这样的话，这将有助于我们理解过渡的机制。高分辨率同步加速器扫描还将提供这些基础哺乳动物牙齿替换模式的详细信息，从而告诉我们更多关于哺乳动物如何过渡到牙齿的信息。一些祖先的更替模式仍在发生，但我们将能够通过这些内部扫描以前所未有的细节研究其机制。

Although the era following the extinction of the dinosaurs is often referred to as 'The Age of Mammals', mammalian history actually stretches back three times as far to the Late Triassic, about 220Ma ago. Their Mesozoic fossil record is frustratingly sparse, and the part that mammals played in Mesozoic ecosystems is sketchy. Exceptionally though, the fissure fillings of Glamorganshire have yielded truly enormous collections of teeth and disarticulated bones. Morganucodon and Kuehneotherium are tiny ancestral mammals from these fissures and this proposed study will reveal new information about the ecology and evolution of these mammalian ancestors, using a rigorously quantified modelling approach. We know a lot about certain aspects of the mammalian transition from their ancestors, the cynodonts, particularly how parts of the jaw hinge transformed to the mechanism of the middle ear; one of the great case studies of vertebrate evolutionary morphology. Basal mammals such as Morganucodon and Kuehneotherium, which have an intermediate condition, have helped in the understanding of this transition. There are still many things that we do not understand about these earliest mammals though. Were they fully homeothermic, and so probably hairy, and did the mother provide milk for the babies? We can get some answers to these questions by a study of the teeth and jaws, by assessing adaptations for efficient feeding and looking at the replacement pattern of the teeth. Reptiles continue to replace their teeth throughout life but mammals have only two sets of teeth. This keeps the upper and lower teeth in position to process the food efficiently and indicates a mammalian physiology and growth pattern. Palaeontologists are using many new technological applications to help them interpret fossils. Detailed high resolution scans can be produced using synchrotron radiation. The small size of Morganucodon and Kuehneotherium, with teeth between 1-2mm in size, is here an advantage as they can be accommodated in the high resolution facilities at the Swiss Light Source (SLS) near Zurich. One method to decipher functional clues in the jaw is the engineering technique Finite Element Analysis (FEA). Researchers build a digital model of the structure they wish to test (i.e. bridge, Morganucodon jaw), apply elastic properties and loading forces that mimic the behaviour (i.e. biting) to be tested. The analysis calculates strain and stress within the structure in response to these loading forces. Non-invasive techniques such as computed tomography scans can be used to generate accurate three-dimensional images of structures, such as jaws, which are required to form the basis of an accurate finite element model. Tiny early mammals are assumed to be insectivores but this covers a very wide range of possible food. This study will use FEA to test whether the jaws of Morganucodon and Kuehneotherium are adapted for biting beetles or snapping at passing moths. It will estimate the force of their bite / e.g. could they penetrate insect cuticles? This is important, as it will inform on feeding specialization and ecological niches. Morganucodon and Kuehneotherium both have an intermediate double jaw joint and one aspect of the transition to the mammalian jaw joint which has not been previously investigated is whether there is a change in priority of the two joints during the growth of an individual. Is there some indication for this from the relative size of the different jaw bones? If so it will help us to understand the mechanism of the transition. High resolution synchrotron scans will also provide detailed information about the tooth replacement pattern in these basal mammals and so tell us more about how the transition to a mammalian dentition occurred. Some ancestral replacement patterns still occurred but we shall be able to study the mechanism with these internal scans in unprecedented detail.