Opportunistic development of Ogmoconcha amalthei (ostracod) in the lower Liassic of Quercy (SW France): an indicator of sedimentary disturbance

Ostracod assemblages have been studied in the sedimentary units of the lower Liassic of Quercy (SW France), at the Carixian–Domerian boundary (Pliensbachian). Ogmoconchella transversa was found to decrease in frequency towards the base of the Domerian while at the same time the O. pseudospina population increased. It is also noteworthy that the occurrence of small-sized species coincides with dominance of species of the Ogmoconcha amalthei group. This large-sized species makes up half or more of the ostracod assemblages in the periods of environmental disturbance indicated by sequence stratigraphy. The biocoenotic indices (number of taxa, faunal turnover and equitability) seem to confirm environmental disturbance at these levels. Correspondence analysis (CA) distinguishes such samples from those in which Ogmoconchella and Polycope species are more common. The appearance or development in the Domerian of species that were rare or absent throughout the Carixian indicates a change of environmental conditions. The opportunism of species of the Ogmoconcha amalthei group can be used to determine periods of disturbance in sedimentary units.