Sauropods, the giant long-necked dinosaurs, became the dominant group of large herbivores in terrestrial ecosystems after multiple related lineages became extinct towards the end of the Early Jurassic (190–174 Ma). The causes and precise timing of this key faunal change and as well as the origin of eusauropods (true sauropods) have remained ambiguous mainly due to the scarce dinosaurian fossil record of this time. The Cañadón Asfalto basin in Patagonia (Argentina) contains terrestrial sequences that document this critical interval of dinosaur evolution. Here we report a new dinosaur from this basin that is the oldest eusauropod known with a nearly complete skull and provide precise U-Pb radioisotopic dates that time the rise of eusauropods in Patagonia. We show eusauropod-dominance was established after a massive magmatic event impacting southern Gondwana (180–184 Ma) and coincided with severe perturbations to the climate and a drastic decrease in the floral diversity characterized by the rise of conifers with small scaly leaves. Floral and faunal changes from other regions suggest these were global changes that affected terrestrial ecosystems during the Toarcian warming of climate and formed part of a second-order mass extinction event.