Mass origination versus mass extinction: the biological contribution to the Pliensbachian-Toarcian extinction event

The Pliensbachian-Toarcian bivalve mass extinction in the Andean Basin of South America is characterized by a sharp drop in species diversity, caused mainly by extinction of endemics, and an extended lag phase. Whilst analysis of community attributes such as guild diversity, number of associations, and species richness of samples constituting associations did not yield any significant changes across the critical interval, rates of origination and extinction of species differed markedly between the various substages of the Early Jurassic. The most prominent feature is the drastic increase in the rate of origination of endemics culminating in the Early Pliensbachian. The time span between the peaks of rare of origination and extinction (5.7 Ma) closely corresponds to the longevity peak of Jurassic endemic bivalves (4-6 Ma). In addition to physico-chemical changes, we therefore explain the drop in species diversity as a result of the evolutionary dynamics of endemics, characterized by a preceding increase in the origination rate. Global mass extinctions are usually explained by one or more physico-chemical causes. We suggest that evolutionary factors, in particular species longevities, may substantially contribute to the phenomenon.