Molluscs of the Lower Ordovician Cabrières Lagerstätte, France – diversity and distribution

The Cabrières Biota from the late Floian Landeyran Formation possesses a mollusc association comprising eleven identified taxa belonging to Bivalvia, Rostroconchia, Tergomya, and Cephalopoda. The fossil record of Cabrière molluscs is impoverished compared to mollusc associations from the entire Landeyran Formation, which represents a peak in molluscan diversity in the Montagne Noire. Bivalves and rostroconchs prevail in the mollusc assemblage in Cabrières Biota with dominant Redonia michelae and Ribeiria sp., whereas gastropods are not recorded. Generally, in the Cabrières Lagerstätte, the molluscs are preserved as external and internal moulds, indicating that their original shell was dissolved. The Cabrières bivalves Babinka prima, Coxiconchia guiraudi, Redonia michelae, Thoralia languedociana and rostroconchs Ribeiria sp. and Tolmachovia sp. nov. were infaunal or semi-infaunal filter feeders. Early Ordovician bivalve taxa, including the significant genera Redonia, Babinka and Coxiconchia, were rare, geographically restricted to Gondwana and peri-Gondwanan shelves and typically favouring inshore clastic sedimentary environments. Conversely, the rostroconchs with key elements Ribeiria and Tolmachovia exhibit a widespread global distribution and reached their peak diversity during this period. Tergomyans, represented by Thoralispira laevis and Carcassonnella vizcainoi were probably semi-infaunal herbivores. They are typical taxa of the Floian Montagne Noire succession. The cephalopod assemblage is dominated by bathmoceratids and longiconic orthocones, which are interpreted as pelagic, vertical migrants known from other Gondwanan and peri-Gondwanan shelf settings. Among the Cabrières mollusc fauna, bivalves and rostroconchs show close affinities with assemblages of comparable age occurring in Morocco as well as with Middle Ordovician faunas from Perunica, Iberia and Avalonia. Tergomyans on the other hand are only shared with coeval occurrences in Morocco and Avalonia.