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

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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.