Ammonoid multi-extinction crises during the Late Pliensbachian – Toarcian and carbon cycle instabilities

Abstract. Five crises affecting ammonite evolution occurred during the Late Pliensbachian to Late Toarcian stages (Early Jurassic). The first two (Gibbosus and end-Spinatum zones) occurred during highly cold and regressive conditions which were followed by a global anoxic event generated during a supergreenhouse warm event (Levisoni subzone) concomitant with a worldwide transgressive event. The last two (Late Variabilis and Late Insigne zones) are related to regressive events. We present new carbon isotope data from Southern Peru that demonstrate that most of the major Toarcian crises affecting ammonite evolution can be correlated with the variations affecting the δ¹³C during that unstable period. The morphogenetic reactions of the ammonites during the five different upper Liassic critical episodes are analysed, showing how this group can be used as stress indicators.