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Decoding the conodont fossil record through analysis of function in ontogeny and phylogeny

Final Report Summary - CONODONT (Decoding the conodont fossil record through analysis of function in ontogeny and phylogeny)

Introduction:
Conodonts were a successful and important group of early fish, with an excellent fossil record spanning over 300 million years. Their skeleton formed the first vertebrate dental apparatus. As such, the conodont feeding apparatus is of great significance because of the insights it provides into the biology and function of the skeleton of the most primitive vertebrates.

The elements of this feeding apparatus display extensive variation. However, our understanding of how such morphological differences influenced their function as teeth is limited. Therefore, understanding the functional significance of those dental morphological changes can be used to interpret the evolutionary history of ecological interactions in the first vertebrate skeleton. Previous qualitative analyses of individual elements have revealed that their growth and basic occlusion differed fundamentally from the teeth of jawed vertebrates, reflecting independent origin. Moreover, at approximately 0.2-2 mm long, these elements are at least an order of magnitude smaller than many teeth and were not anchored in jaws. Thus, it is unclear how effectively elements could have function as teeth.

Based on the development of state of the art tomographic, computed functional, and microstructural analytic techniques, our aim in this project was to address these issues, to gain a better understanding of conodont functional morphology, and so unlock their potential for elucidating broader questions about dental evolution and ecology of the earliest representatives of our own linage.

The objectives can be summarized in:
1. To acquire the first clade-wide database of high resolution 3D models of conodont clusters, using the SLS (Switzerland) synchrotron.
2. To develop the application of finite element analysis (FEA) and Occlusal Fingerprint Analysis (OFA) based on these models, to investigate the conodont P1 element function.
3. To test the predictions of FEA using microwear, mesowear and microstructural data.
4. To determine whether conodont elements were shed and replaced or grew in alternating episodes with function.
5. To determine the functional and, therefore, ecological implications of phylogenetic and ontogenetic changes in conodont P1 element morphology.

Results
1. We have demonstrated that combined application of FE analysis, occlusal functional analysis and observations of wear and microstructure provides a useful and informative approach to understand conodont element function.
2. Using this approach, we have revealed that conodont crown tissue is adapted to dental function; supported by other functional data, this shows that conodonts optimised their dental microstructure to maximise their resistance to loads.
3. Based on this information, we have analysed the occlusal cycle of several species revealing a non-common pattern among the clade.
4. FE analysis applied for the first time to a single conodont phylogeny has demonstrated that the iterative morphological evolution of several clades is closely related with functional optimization to the dental function.
5. In addition, the study of several clusters belonging to different species in different ontogenetic stages, will allow us to evaluate how the function change during the ontogeny, establishing hypothesis of ecological preferences throughout the life of a single taxa.
5. Based on those previous evidence, variation in conodont element morphology reflect variation in dietary preferences and occlusal models.
6. We reveal that the explorations of functional morphological innovation (radiations), were linked to the exploitation of new trophic niches.
7. The synchrotron analysis of extremely well preserved specimens have shown inner patterns of wear and repair, demonstrating that the elements were not shed and replaced and therefore grew in alternating episodes with function.
8. In addition the ptychographic nanotomography at the cSAXS beamline has permited a detailed characterisation of the physical structure of proto-, para- and euconodont elements which will help us to test the hypothesis of relationships between paraconodonts and euconodonts, and the homology or analogy of conodont and vertebrate skeletal tissues, providing a direct insight into the evolutionary origin of the vertebrate skeleton.

Impact: Integration and analysis of these data have produced the first comprehensive analytical conodont dataset to provide a macroevolutionary prospectus of morphological diversity and function in conodonts for the first time. It will also provide the first quantitative description of conodont element function in a broad range of taxa, but also evaluating the different morphological trends occurred during the ontogeny and phylogeny of single taxa, allowing us to understand how the earliest dental apparatus functioned and evolved in unprecedented detail. The results of the project will form the foundation for all future analysis of conodont function and provides a fresh perspective on the evolution of teeth more generally, establishing conodonts as a phylogenetically independent comparative system for examining constraint, complexity and function within and between dental apparatuses.