This thesis addresses graptolitic mudrocks in an Earth system context and examines issues concerning the preservation of organic carbon. It investigates the palaeoenvironmental and paleoecological aspects of these mudrocks. It relates their deposition to climate modulation in an hitherto-unrecognised Early Palaeozoic Icehouse, and reassesses the fossils of the problematic genus Dawsonia Nicholson. This work identifies seven glacial maxima in the Late Ordovician and Early Silurian recognised by the occurrence of well-dated glacial deposits coincident with stable isotope excursions and eustatic regressions. Comparison of these data with the occurrence of graptolitic shales reveals that deglacial transgressions led to increased stratification of the water column and the onset of widespread marine anoxia. The burial of organic carbon in these deglacial anoxic events may have served as a negative feedback mechanism by drawing down sufficient atmospheric carbon dioxide to prevent runaway warming and stabilise this long-lived Early Palaeozoic Icehouse. This study suggests that graptolites are best viewed as a mixed-layer zooplankton and that their occurrence in anoxic mudrocks should be regarded as representing the conditions they were preserved in rather than those in which they lived. Comparison of published data on graptolite diversity with the oxic-anoxic stratigraphy for the EPI shows there is no strong link between graptolite diversity and marine anoxia. Meanwhile, the general absence of graptolites from bioturbated facies may reflect enhanced decay and scavenging related to well-oxygenated conditions; whilst the documentation of rare occurrences of graptolites in oxic facies and those from above the storm wave base shows that they could live in well-oxygenated waters. Though anoxia alone is insufficient to explain the fossilisation of organic-walled fossils and conversion of subfossil cuticle to a more stable biopolymer may have inhibited degradation, it is clear that fossilisation does not render such fossils entirely inert or homogeneous. Analysis of multifaunal assemblages, such as those preserving Dawsonia Nicholson (shown to be brachiopods, crustacean tailpieces and an organic-walled problematicum), where graptolites co-occur with shelly fossils and diagenetic pyrite, shows that graptolites acted as a key site for phyllosilicate authigenesis in very low-grade metamorphism. Petrographic evidence and comparison with white mica crystallinity data suggests that the expulsion of volatiles in maturation may have catalysed the formation of phyllosilicates on these fossils. This mode of phyllosilicate formation may also account for the formation of phyllosilicate films on Burgess Shale fossils.

Sur la base des observations macro- et microscopiques détaillées, sur du matériel comparatif aussi bien fossile que moderne, les résultats de cette étude permettent d’avancer dans la compréhension de la taphonomie, de la paléobiologie et de la paléoécologie des fossiles macroscopiques de l’Ediacarien qui n’ont pas été affectés par la crise Kotlinienne. Premièrement, les analyses taphonomiques des palaeopascichnidés préservés dans des roches carbonatées issues de l’Olenek Uplift établissent la nature agglutinée possible de leur squelette et une organisation semblable aux protozaires. Ces observations permettent également un regard sur leur paléoécologie opportuniste et distribution biogéographique globale malgré la crise kotlinienne. Deuxièmement, leur étude et révision taxonomique a permis d’améliorer leur classification systématique et d’introduire le groupe Palaeopascichnida, composé de deux genres révisés : Palaeopascichnus and Orbisiana. L’étude pluridisciplinaire des possibles analogues modernes des organismes macroscopiques édiacariens, tels que les arumberiamorphes et les morphostructures biologiques modernes, observées sur la surface des tapis microbiens formés dans les salines de Guérande, m’ont aidé à comprendre les processus taphonomiques, la distribution locale et les contextes environnementaux de morphostructures édiacariennes comparables. Avec toutes ces informations en main et en utilisant des approches sédimentologiques, stratigraphiques, paléontologiques et paléoécologiques on peut maintenant élargir notre compréhension de la crise kotlinienne et ses effets sélectifs sur certains représentants des communautés édiacariennes. / Based on detailed macro- and microscopic observations of both fossil and modern comparative material the results of this study improve our understanding of the taphonomy, palaeobiology and palaeoecology of Ediacaran macroscopic fossils that survived the Kotlinian Criss. Firstly, taphonomic analyses of the carbonate-hosted palaeopascichnids from the Olenek Uplift establish the possible agglutinated nature of their skeleton and protozoan organization. They also provide valuable insights to their opportunistic paleoecology and global biogeographic distribution in spite of the Kotlinian Crisis. Secondly, their taxonomic study and revision allowed to improve their systematic classification and to introduce the group Palaeopascichnida consisted of two revised genera: Palaeopascichnus and Orbisiana. The multidisciplinary study of possible modern analogues to Ediacaran macroscopic organisms, such as the arumberiamoprhs he and discoidal biological morphostructures, encountered on the surface of microbial mats formed in the Guérande Salinas, helped me understand the taphonomic processes, local distribution and environmental settings of comparable Ediacaran morphostructures. With all this new information in hand, utilising sedimentological, stratigraphical, palaeontological and palaeoecological methods and achievements, now we can significantly broaden our picture of the Kotlinian crisis and its selective effect on certain representatives of the Ediacaran biota.