DIVERSITY ANALYSIS OF LOWER JURASSIC BIVALVES OF THE ANDEAN BASIN ANDTHE PLIENSBACHIAN-TOARCIAN MASS EXTINCTION

In the Andean Basin of southern South America marine bivalves show a m arked decrease in diversity across the Pliensbachian-Toarcian boundary . This observation is in contrast to earlier statements, according to which a drop in diversity of marine organisms at the boundary is restr icted to epicontinental seas of Western Europe. The decrease in bivalv e diversity within the Andean Basin is largely due to the extinction o f endemics and of some cosmopolitan species, less commonly to the Laza rus effect and local disappearances of taxa. As the only two regions f or which detailed data are available (Andean Basin and Western Europe) exhibit similar diversity patterns across the boundary, the extinctio n event is not a regional feature, but appears to be global. In both r egions the mass extinction correlates with sea-level highstand and wid espread oxygen-poor deep-shelf environments suggesting a causative rel ationship. As shallow, largely well-aerated shelf environments in the Andean Basin also exhibit, though less markedly, a drop in bivalve div ersity across the boundary, oxygen deficiency cannot be the sole cause of the mass extinction. Rather, in our model we assume sea-level chan ges and the resulting oceanographic and biotic changes to be the ultim ately controlling factors of species diversity patterns. The apparent global character of the Pliensbachian-Toarcian mass extinction fits th e periodicity hypothesis. However, as the mass extinction does not app ear to have been catastrophic and as there is no indication of impact( s) of extraterrestrial bodies, the oceanographic model proposed here i s regarded as a more plausible (because it is more parsimonious) hypot hesis.