Diversity dynamics, trophic, community and environmental changes in early cretaceous rudist bivalves (Hippuritida): New insights on the late Barremian “Agriopleura extinction event” and subsequent recovery

Abstract

The “Agriopleura event”, which expresses the regional extinction, in northwestern Europe, of the rudist genus Agriopleura, is associated with environmental and biological changes. This event marks the boundary between two distinctive late Barremian regional rudist assemblages, the pre-event Brouzet-les-Alès and post-event Orgon faunas. The succeeding Palorbitolina episode is coeval with a rudist eclipse. The ensuing Rustrel fauna identifies the phase of post-extinction recovery. The last occurrence of Agriopleura, the stem genus of the Radiolitidae, coincides with the extinction of 41% of the pre-event Requieniidae and Monopleuridae. The recovery phase is characterized by new species and a low extinction pattern. This phase is marked by a burst of speciation, in part indigenous and in part due to immigrants i.e. Caprinidae. The key morphotypes, clingers, elevators and recumbents record contrasting changes in specific diversity, clingers being dominant due to the substantial diversity of Requieniidae and Monopleuridae. During the extinction event elevators decrease and suffered a reduction in size, large size species being selectively eliminated. The entry of the Caprinidae identifies the recovery phase and accounts for size increase and an important contribution of recumbent. The main agent of the extinction, combines cooling, anoxia, platform exposure and a trophic factor assumed to account for the selective removal of the elevators. A dual partition in feeding behavior of clingers and elevators is suggested: elevators exploiting the phytoplankton flux and the re-suspended bottom biodeposits; and clingers feeding on the benthic boundary layer. Significant community changes are associated with Agriopleura extinction and its aftermath, which clearly respond to changes in taxonomic composition. Elevator dominated communities with the Agriopleura are replaced by polytaxic requieniid rich communities then by caprinids. The Palorbitolina episode, records a trophic peak, cooling and a deepening trend. The recovery phase mainly records an aragonitic pulse coupled with temperature increase.