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Imagerie de plantes fossiles par la tomographie synchrotron : cas de préservations exceptionnelles et de fleurs du Cénomanien de Charente-Maritime et du Gard

Abstract : The Lower Cretaceous-Upper Cretaceous transition (about 100 million years ago) stands as a crucial period for the evolution of some seed plants. It consists of the brutal radiation of flowering plants and an ecological reorganisation of Angiosperms and conifers. However, our knowledge of mid-Cretaceous Laurasian seed plants remains partial. During the last decade, synchrotron microtomography has successfully been employed for non-destructively imaging inner structures of various types of fossil remains. To date, the applications of this technique in palaeobotany are still limited. This thesis focuses on: (1) developing new tomographic imaging approaches at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), relevant to palaeobotany, and especially for investigating various kinds of plant preservations; (2) applying these approaches to improve our knowledge about the morpho-anatomy, the systematics, the diversity, and the ecology of Laurasian seed plants during the mid-Cretaceous. Hundreds of unique or rare mid-Cretaceous fossil plant specimens (e.g. inflorescences, flowers, fruits, and pollen grains of Angiosperms; cones and leafy axes of conifers) from French Cretaceous deposits were imaged and investigated using several new synchrotron tomographic protocols. For the first time, abundant and diverse fossil flowers are described from the Cenomanian deposits of Gard (southeastern France). Some flowers are exquisitely well-preserved in three dimensions. In some cases, all floral units of the perianth, the androecium, and the gynoecium are preserved. A new tomographic approach combining propagation phase contrast X-ray microtomography and nano-holotomography was developed to image those small and isolated plant specimens such as charcoalified/lignitized fossil flowers (up to 50 nm of voxel size). This allows the description of fossil flowers, from their gross morphology to the micromorphology of the exine of in situ pollen grains inside stamens. Some of these flowers show uncommon floral features. They are mainly and unambiguously ascribed to Lauraceae. In addition, large, opaque and dense silica-rich flint nodules containing plant inclusions are reported from the Cenomanian deposits of Charente-Maritime (western France). They contain abundant hidden conifer (e.g. Brachyphyllum, Frenelopsis, Geinitzia and Glenrosa). These Cenomanian conifers are exceptionally well-preserved in three dimensions and consist of siliceous permineralisation. Most of the tissues and cells are preserved. We propose a multiscale, high resolution and high energy tomographic protocol useful for the study of plant inclusions contained inside large rocky nodules. We study the partially unexplored extinct conifer Glenrosa, from the gross morphology to the histology of its vegetative and reproductive structures. We discuss systematics and palaeoecophysiology of the genus. Xeromorphic features of Glenrosa suggest that this conifer was adapted to tolerate harsh conditions induced by coastal marine environments. This work supports that during the Cenomanian and in coastal areas, on the one hand, palaeoenvironments which were highly influenced by marine inputs were probably dominated by conifers. On the other hand, innermost and protected palaeoenvironments showed Angiosperm-dominated floras. Finally, this doctoral research highly contributes to our knowledge of Laurasian Angiosperm and conifers during the mid-Cretaceous. In addition, the new tomographic approaches proposed in this work could be applied for future research on other floras or other palaeontological specimens from various geographical and stratigraphic origins.
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Submitted on : Wednesday, October 4, 2017 - 11:40:32 AM
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Jean-David Moreau. Imagerie de plantes fossiles par la tomographie synchrotron : cas de préservations exceptionnelles et de fleurs du Cénomanien de Charente-Maritime et du Gard. Paléontologie. Université de Rennes 1, 2014. Français. ⟨tel-01609973⟩



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