Recovery of the marine fauna in Europe after the end-Triassic and Early Toarcian mass Extinctions

Abstract

Abstract A detailed analysis of species diversity increase, zone by zone up the Liassic section in northwest Europe, has been undertaken for the six fossil groups for which adequate data are available: bivalves, ammonites, rhynchonellid brachiopods, crinoids, foraminifera and ostracods. The general pattern is of a rapid increase through the Hettangian from a very low level after the end-Triassic mass extinction, followed by a slower rate of increase until the late Pliensbachian. Thereafter there was a drastic fall in the early Toarcian as a consequence of the mass extinction of that time, followed by a further rise continuing into the Middle Jurassic. A more general study takes into account data from southern Europe and extra-European localities, the best of which for the Liassic being in Argentina and Chile. The best data come from brachiopods and bivalves. The brachiopod pattern is similar to that described above for a more limited geographical region, with genera already occurring in the Hettangian and Sinemurian of southern Europe spreading into northwest Europe in the Pliensbachian. The bivalve data show that genera present in the Sinemurian and Pliensbachian of South America did not reach Europe until after the late Toarcian and early Middle Jurassic. Reef ecosystems, which were drastically affected by the end-Triassic mass extinction, did not re-establish themselves until the Pliensbachian; the early Toarcian event had a similarly deleterious effect. There is a good correlation between the Hettangian to Pliensbachian diversity rise and rise of sea level, with the very low diversity values of the early Hettangian being associated with widespread dysoxic and anoxic conditions. The low diversities of the early Toarcian are also associated with anoxia correlated with a rapid sea-level rise, with the immigration into Europe of South American taxa taking place subsequently when environmental conditions ameliorated and the higher sea level permitted freer communication, most probably through the Hispanic Corridor across the present Central Atlantic region.