Plant diversity turnovers in the Triassic-Jurassic transition: evidence from the paleobotanical record of Argentina

Abstract

The Triassic–Jurassic transition was an important interval in Earth’s history, which encompassed two great biotic crises: the End-Triassic mass extinction ant the Early Toarcian Oceanic Anoxic Event. The response of the terrestrial vegetation to these crises has been the subject of debate. In Argentina, a floristic turnover occurring in the Triassic-Jurassic boundary has been suggested, which led to the disappearance of corystoperms and peltasperms, and to a greater influence of ferns, bennettitaleans and conifers. Recent findings have thrown into question the previous evolutionary models. With the purpose of understanding the terrestrial plant turnovers during the Triassic-Jurassic transition in Argentina, we calculated diversity indexes, survivorship proportions, and extinction rates, based on megafossil record. The results indicated that, despite only one lineage disappears in the Late Triassic (Pleuromeiaceae), an important but gradual vegetation change in the Triassic-Jurassic transition. The extinction rates indicate a peak at the end of Rhaetian and the diversity declines in the Hettangian. Late Triassic diagnostic lineages pass through the Jurassic but most of them with few occurrences and going extinct in the Early Jurassic. The Osmundaceae, Dipteridaceae, and Bennettitales diversify in the Sinemurian. In the Toarcian a taxa richness increase is recognized, marked by a conifer diversification. In the Bathonian-Callovian, the conifers and bennetitaleans acquire greater importance. The results show a high correlation between diversity and occurrences. The dataset can be improved in the future by intensifying fossil sampling.