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。它的特性使得生物力学策略的发展惊人的多样性，包括运动，食品加工，捕食和防弹衣，刺激主要的多样化事件。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学材料提供了灵感。这些组织的超微结构和功能特性之间的关系的理解，目前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。一个模型连接其结构和功能是必要的，以测试其功能适应的假设。这又需要一种允许定量地分析单个晶体和晶畴的方法。在钙质骨骼中，这已经使用电子背散射衍射（EBSD）实现，但迄今为止，将这种技术应用于羟基磷灰石组织的尝试尚未成功。我们提出了一个研究协议，允许应用EBSD最早的脊椎动物超矿化组织和交叉测试，它使用原位和粉末X射线衍射。我们专注于牙形刺，化石组已开发出高矿化骨骼组织的第一次在脊椎动物和平行于其他群体。该项目旨在测试他们的高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常见模式，并有助于其特殊的材料性能，如抗裂纹扩展。我们还测试了以前提出的假设，牙形石冠组织显示超微结构的适应食品加工功能和广泛的超微结构变化，这些适应表现在通过修改的大小和方向的晶体和整个晶体域在几个层次的组织。最后，我们的目标是评估这些图案的成岩蚀变实验。

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.