Faunal distribution and palaeoenvironmental trends in the Lower Ordovician of the Montagne Noire, France: insights into the Cabrières Biota

In the southern Montagne Noire, France, the upper Floian Cabrières Lagerstätte yielded an atypical and diverse marine assemblage dominated by algae and sponges. It contrasts with the Fezouata Shale, Morocco, the only other known high-latitude Lower Ordovician Lagerstätte where non-trilobite arthropods and echinoderms constitute the two main components of the communities. This paper focuses on the palaeoenvironmental context of the Cabrières Lagerstätte and extends to the entire southern Montagne Noire, where several groups of organisms with high potential for palaeoenvironmental reconstructions, including acritarchs (phytoplankton), as well as numerous echinoderms and trilobites (benthos), combined with sedimentological studies, allow for the analysis of the environmental context of the Cabrières Biota. In the western southern Montagne Noire, the composition of acritarch assemblages recorded from three sections documents an ecological shift from inshore, proximal settings to more distal, open shelf environments. Distinct trilobite and echinoderm assemblages are identified in the Lower Ordovician of the Montagne Noire. Their distribution and taxonomic composition are mainly controlled by depth, as well as possibly by salinity and oxygenation. In most cases, echinoderms were minor components of the diverse benthic communities, often dominated by brachiopods, molluscs, and trilobites. In the Lower Ordovician of the Montagne Noire, echinoderm-dominated assemblages seem to be restricted to unstable, high-energy environments. Trilobites and echinoderms of the Cabrières Biota inhabited soft, poorly-oxygenated substrates in distal and low-energy shelf environments, below the storm wave base.