Changements environnementaux dans les Alpes carniques et le Nevada central au cours de l’Ordovicien terminal : une étude comparative

La comparaison entre les successions stratigraphiques et les enregistrements fauniques de l’Ordovicien terminal des Alpes carniques (situés en paléolatitudes moyennes) et ceux du Nevada (en paléolatitudes tropicales) présente certaines similitudes. Durant la majorité de l’intervalle glaciaire fini-ordovicien, les deux secteurs géographiques sont représentés par des plates-formes profondes dont la sédimentation dominante était composée de « debris flow », transportés depuis des environnements de plate-forme interne ou peu profonde. La dernière phase glaciaire et la chute finale du niveau de la mer ont favorisé une intense karstification dans les domaines de plates-formes, internes ou peu profondes. Dans les environnements profonds des plates-formes, cet intervalle est caractérisé par la mise en place d’un dépôt sableux largement répandu à la faveur de ruptures de pente et de talus. Les plates-formes des deux secteurs ont peut-être subi une remontée isostatique et ont ensuite été exondés par des processus tectoniques après l’épisode glaciaire fini-ordovicien

The position of graptolites within Lower Palaeozoic planktic ecosystems.

An integrated approach has been used to assess the palaeoecology of graptolites both as a discrete group and also as a part of the biota present within Ordovician and Silurian planktic realms. Study of the functional morphology of graptolites and comparisons with recent ecological analogues demonstrates that graptolites most probably filled a variety of niches as primary consumers, with modes of life related to the colony morphotype. Graptolite coloniality was extremely ordered, lacking any close morphological analogues in Recent faunas. To obtain maximum functional efficiency, graptolites would have needed varying degrees of coordinated automobility. A change in lifestyle related to ontogenetic changes was prevalent within many graptolite groups. Differing lifestyle was reflected by differing reproductive strategies, with synrhabdosomes most likely being a method for rapid asexual reproduction. Direct evidence in the form of graptolithophage 'coprolitic' bodies, as well as indirect evidence in the form of probable defensive adaptations, indicate that graptolites comprised a food item for a variety of predators. Graptolites were also hosts to a variety of parasitic organisms and provided an important nutrient source for scavenging organisms

Late Silurian and early Devonian graptolites from North Greenland

The precise age of the youngest part of the geosynclinal fill of the North Greenland fold belt has been the subject of important discussion, particularly with regard to the problem of dating the Palaeozoic diastrophism (Kerr, 1967; Dawes, 1971). Since Lauge Koch's field work between 1916 and 1923 it has been known that strata bearing Monograptus priodon were involved in the folding (Koch, 1920), indicating the presence of Silurian of Llandovery-Wenlock age. In addition, Poulsen (1934) identified Cyrtograptus cf. C. multiramus and Monograptus bohemicus in collections made by Koch from unfolded shales on the platform, to the south of the fold belt, which demonstrated that the section included Wenlock and early Ludlow strata.</jats:p