Paléoenvironnements et dynamique de faunes de Trilobites dans l'Ordovicien (Llanvirn Supérieur-caradoc basal) du Massif Armoricain (France)

Abstract

During the Ordovician, the Armorican Massif is generally considered as a platform domain, a shallow marine environment characterized by endemic invertebrate faunas. However, approximately from the late Llanvirn to the end of the Llandeilo, different trilobite associations develop. Their geographic distribution seems to be mainly related to bathymetric variations. The interpretations proposed in this paper are supported by lithological and sedimentological data and comparisons with living marine crustaceans. The possible mode of life of some trilobites has also been taken into consideration; thus, at the eastern end of the Armorican median synclinorium, and south of Rennes (Martigné-Ferchaud synclinorium), the presence of Parabarrandia, a mesopelagic trilobite from open shelf, indicates amongst other things a marked increase of depth from west to east and from north to south. In the southeastern part of the Armorican Massif (Ancenis region), the development of a cyclopygid biofacies indicates the beginning of a slope opening onto a deep marine environment (South-Armorican ocean?). In some cases ( Uralichas, Ectillaenus, Panderia), the exceptionally large dimensions of the dorsal exoskeletons would result from the low water temperature at high latitudes, which agrees well with the existence of a Gondwanan glaciation at the end of the Ordovician.