Using tooth wear to impose ecological constraints on isotopic and biostratigraphic analyses based on conodonts

这项研究将检验牙形石牙齿化石的磨损模式因物种是生活在海底还是海底之上而不同的假设。如果磨损确实不同，就像鱼牙齿一样，这将为增加牙形石在地质和深部环境分析中的可靠性提供一种新的方法。牙形刺是一种小型的、类似鳗鱼的原始鱼类。它们自三叠纪末就灭绝了，但它们的显微化石牙齿的3亿年记录异常完整，而且它们很容易通过在弱酸中溶解石灰石而大量获得。因此，牙形刺是地质年代测定的最重要工具之一-确定岩石何时被放下以及地质事件何时发生。它们也日益成为研究地质时期的古气候、古温度和古海洋学的重要工具，这些研究对于了解当前气候变化的背景十分重要。例如，对牙形石牙齿中氧同位素的研究为数亿年前的冰川、海平面和海水温度提供了新的见解。牙形刺特别适合此类研究，因为它们牙冠的结构和磷酸钙成分最大限度地提高了牙齿中记录的化学特征反映动物活着时海洋条件的机会，并最大限度地减少了牙形刺化过程引起的变化。最近的工作表明，基于牙形石冠组织的同位素分析比任何其他可比年龄的化石牙齿或贝壳的分析提供更可靠的结果。然而，要实现牙形刺的全部潜力，我们需要限制它们的生态和生活方式。区分生活在海底的底栖生物分类群和生活在远离海底的表层沃茨的浮游生物分类群特别重要。地质年代测定的最佳分类群是远洋的，因为远洋分类群有更广泛的（潜在的全球）分布。它们也更快地分散，因此新进化的远洋物种首次出现在不同地点的化石记录中的时间更有可能是同步的（这对确定岩石序列的年龄相当很重要）。解释牙形石齿冠化学成分随温度或海平面变化的分析必须排除样本之间的差异反映了样本牙形石物种生活的深度栖息地差异的可能性（更深的水更冷）。不幸的是，牙形刺的生活方式受到很好的限制，这就造成了问题。我们知道，它们是活跃的游泳动物，从浅海近岸到深海环境，但很难确定一个特定的物种是否占据了底栖或远洋生态位。目前的方法是基于牙形刺沿着深度梯度的假设分布，这种方法相当粗糙，而且通常不可靠。该建议旨在开发一种新的方法来限制牙形石类群的深度生境。研究人员最近的工作发现，底栖摄食鱼类的牙齿磨损模式与中上层摄食鱼类的牙齿磨损模式不同，可用于研究化石鱼类摄食的变化。牙形刺也是如此吗？为了找出答案，我们将进行第一次系统的牙形石牙齿磨损分析和测试的假设，浮游动物和底栖动物喂养的物种表现出不同的磨损模式。这将基于对数百颗牙形石牙齿的显微镜调查和对其表面保留的磨损模式的详细统计分析。与大多数牙形刺不同，这些牙齿将取自该物种的古生态学受到很好限制的样本。如果发现差异，同位素分析将提供关于温度/深度生境的独立数据。

This study will test the hypothesis that wear patterns on fossil conodont teeth differ according to whether species lived on the sea floor or above it. If wear does differ, as it does in fish teeth, this will provide a new way of increasing the reliability of conodonts in geological and deep-time environmental analysis. Conodonts were small, eel-like primitive fishes. They have been extinct since the end of the Triassic, but the 300 million year record of their microscopic fossil teeth is exceptionally complete, and they are easy to obtain in large numbers by dissolving limestone in weak acid. Consequently, conodonts are among the most important tools for geological dating - determining when rocks were laid down and when geological events occurred. They are also increasingly important tools for investigating the palaeoclimate, palaeotemperature and palaeooceanography of geological periods in deep time, studies which are important for understanding the context of current climate change. For example, investigations of the oxygen isotopes in conodont teeth are providing new insights into glaciations, sea level and sea temperature hundreds of millions of years ago. Conodonts are particularly suited to such studies because the structure and calcium phosphate composition of their tooth crown maximises the chances that the chemical signatures recorded in the teeth reflect ocean conditions at the time the animal was alive, and minimises changes caused by the process of fossilization. Recent work indicates that isotopic analyses based on conodont crown tissue give more reliable results than analyses of any other fossil teeth or shells of comparable age. Realising the full potential of conodonts, however, requires that we can constrain their ecology and mode of life. Differentiating between benthic taxa, which lived on the sea floor, and pelagic taxa, which lived away from the sea floor and in surface waters, is particularly important. The best taxa for geological dating are pelagic, because pelagic taxa have broader (potentially global) distributions. They also disperse more rapidly, so the time at which a newly evolved pelagic species first appears in the fossil record in different locations is more likely to be synchronous (important for establishing the age equivalence of rock sequences). Analyses that interpret shifts in the chemical composition of conodont tooth crowns in terms of temperature or sea level must exclude the possibility that differences between samples reflect differences in the depth habitat at which the sampled conodont species lived (deeper water is cooler). Unfortunately, the mode of life of conodonts is poorly constrained, and this causes problems. We know that they were active swimming animals that ranged from shallow nearshore through to deep ocean environments, but determining whether a particular species occupied a benthic or a pelagic niche is difficult. Current methods, based on hypothetical distributions of conodonts along depth gradients, are rather crude and generally unreliable. This proposal aims to develop a new approach to constraining the depth habitats of conodont taxa. Recent work led by the investigator discovered that patterns of tooth wear in benthic feeding fish differ from those of pelagic feeding fish and can be used to study changes in feeding in fossil fish. Does the same apply to conodonts? In order to find out we will conduct the first systematic analysis of conodont tooth wear and test the hypothesis that pelagic feeding and benthic feeding species exhibit different wear patterns. This will be based on microscopic investigation of hundreds of conodont teeth and detailed statistical analysis of the patterns of wear preserved on their surfaces. These teeth will be taken from samples where, unlike most conodonts, the palaeoecology of the species is well constrained. If differences are detected, isotopic analysis will provide independent data concerning temperature/depth habitat.