Ultrastructural evolution of conodont skeletal tissues - reconstruction using electron backscatter diffraction (EBSD)

脊椎动物矿化骨骼是生命史上最成功的创新之一。它的特性允许生物力学策略的惊人多样性的发展，包括运动、食物加工、捕食和防弹衣，刺激了主要的多样化事件。脊椎动物骨骼组织的复合羟基磷灰石-有机结构也为工程医疗材料提供了灵感。目前对这些组织的超微结构和功能特性之间关系的理解几乎完全来自哺乳动物的牙齿。然而，哺乳动物牙齿模型并不能代表最早的脊椎动物高矿化组织中存在的结构的全部宽度。为了检验关于它们的功能适应的假设，需要一个连接它们的结构和功能的模型。这反过来又需要一种方法来定量地表征单个晶体和晶体域。在钙质骨骼中，这已经通过电子背散射衍射（EBSD）实现，但迄今为止，将该技术应用于羟基磷灰石组织的尝试尚未成功。我们提出了一种研究方案，允许将EBSD应用于最早的脊椎动物高矿化组织，并使用原位和粉末x射线衍射进行交叉测试。我们的重点是牙形刺，这是脊椎动物中第一次出现高矿化骨骼组织的化石类群，与其他类群平行。该项目旨在验证其超矿化组织具有纳米颗粒复合结构的假设。这种结构最近被证明是大多数生物矿化动物门的共同模式，并有助于其特殊的材料特性，如抗裂纹扩展。我们还验证了先前提出的假设，即牙形牙冠组织显示出对食品加工功能的超微结构适应，这些适应中表现出的广泛超微结构变化是通过在几个组织水平上改变晶体的大小和取向以及整个晶体域而成为可能的。最后，我们的目的是通过实验来评估这些模式的成岩改变。

The vertebrate mineralized skeleton is among the most successful innovations in the history of life. Its properties allowed for the development of an astounding diversity of biomechanical strategies, including locomotion, food processing, predation, and body armour, stimulating major diversification episodes. The composite hydroxyapatite-organic structure of vertebrate skeletal tissues has also served as an inspiration for engineered medical materials. The understanding of the relationship between the ultrastructure and functional properties in these tissues is currently derived almost exclusively from mammal teeth. The mammalian teeth model, however, does not represent the full breadth of structures present in the earliest vertebrate hypermineralized tissues. A model linking their structure and function is needed in order to test hypotheses on their functional adaptations. This, in turn, requires a method allowing to characterise individual crystals and crystal domains quantitatively. In calcareous skeletons, this has been achieved using electron backscatter diffraction (EBSD), but attempts to employ this technique to hydroxyapatite tissues have been unsuccessful so far. We propose a research protocol allowing to apply EBSD to the earliest vertebrate hypermineralized tissues and cross-test it using in situ and powder X-ray diffraction. We focus on conodonts, a fossil group which has developed hypermineralized skeletal tissues for the first time among vertebrates and in parallel to other groups. The project aims to test the hypothesis that their hypermineralized tissues have a nanogranular composite structure. This structure has been recently demonstrated to be a common pattern in most biomineralizing animal phyla and contributes to their exceptional material properties such as resistance to crack propagation. We also test a previously proposed hypothesis that conodont crown tissues show ultrastructural adaptations to food-processing functions and the broad ultrastructural variation manifested in these adaptations is made possible through modifications of sizes and orientations of crystals and entire crystal domains at several levels of organization. Finally, we aim to assess diagenetic alteration of these patters experimentally.