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

脊椎动物的矿化骨骼是生命史上最成功的创新之一。它的特性允许开发出令人震惊的多样化的生物力学策略，包括运动、食物加工、捕食和防弹衣，刺激了重大的多样化情节。脊椎动物骨骼组织的羟基磷灰石-有机复合结构也为工程医学材料提供了灵感。目前对这些组织的超微结构和功能特性之间的关系的了解几乎完全来自哺乳动物的牙齿。然而，哺乳动物的牙齿模型并不能代表最早的脊椎动物超矿化组织中存在的全部结构。需要一个将它们的结构和功能联系起来的模型，以检验关于它们功能适应的假说。这反过来又需要一种方法，能够定量地表征单个晶体和晶区。在钙质骨骼中，这已经通过电子背散射衍射(EBSD)实现，但将这一技术应用于羟基磷灰石组织的尝试迄今尚未成功。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超矿化组织，并使用原位和粉末X射线衍射进行交叉测试。我们专注于牙形刺，这是一个化石群体，它在脊椎动物中首次形成了高矿化的骨骼组织，并与其他群体平行。该项目旨在验证他们的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中的一种常见模式，并有助于它们具有特殊的材料性能，如抗裂纹扩展。我们还检验了先前提出的一个假设，即牙形刺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的超微结构变化是通过在几个组织水平上改变晶体和整个晶体结构域的大小和取向来实现的。最后，我们的目标是从实验上评估这些模式的成岩改造作用。

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.