41 research outputs found
Changements environnementaux dans les Alpes carniques et le Nevada central au cours de lâOrdovicien terminal : une Ă©tude comparative
La comparaison entre les successions stratigraphiques et les enregistrements fauniques de lâOrdovicien terminal
des Alpes carniques (situés en paléolatitudes moyennes) et ceux du Nevada (en paléolatitudes tropicales) présente
certaines similitudes. Durant la majoritĂ© de lâintervalle glaciaire fini-ordovicien, les deux secteurs gĂ©ographiques 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 derniĂšre 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 caractĂ©risĂ© par la mise en place dâun dĂ©pĂŽt
sableux largement rĂ©pandu Ă la faveur de ruptures de pente et de talus. Les plates-formes des deux secteurs ont peut-ĂȘtre
subi une remontĂ©e isostatique et ont ensuite Ă©tĂ© exondĂ©s par des processus tectoniques aprĂšs lâĂ©pisode 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
Cyrtograptids from the Telychian (upper Llandovery) of Kinnekulle Mountain, southern Sweden
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EVOLUTIONARY AND GEOLOGIC CONSEQUENCES OF ORGANIC CARBON FIXING IN THE PRIMITIVE ANOXIC OCEAN
Revision of the Zone 13 graptolite biostratigraphy in the Marathon, Texas, standard succession and its bearing on Upper Ordovician graptolite biogeography
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