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 boundar y (Pliensbachian). Ogmoconchella transversa was found to decrease in f requency towards the base of the Domerian while at the same time the O . pseudospina population increased. It is also noteworthy that the occ urrence 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 di sturbance indicated by sequence stratigraphy. The biocoenotic indices (number of taxa, faunal turnover and equitability) seem to confirm env ironmental disturbance at these levels. Correspondence analysis (CA) d istinguishes such samples from those in which Ogmoconchella and Polyco pe species are more common. The appearance or development in the Domer ian of species that were rare or absent throughout the Carixian indica tes a change of environmental conditions. The opportunism of species o f the Ogmoconcha amalthei group can be used to determine periods of di sturbance in sedimentary units. (C) 1998 Elsevier Science B.V. All rig hts reserved.