During the Early Jurassic the southwestern margin of Gondwana was aﬀected by a voluminous magmatic episod related to the emplacement of t heKaroo-Ferrar-Chon Aike Large Igneous Provinces (LIPs). That pulse of volcanism led to global warming, with at least four times the present level of atmospheric CO2, and ocean acidification that resulted in a mass extinction of marine invertebrates and turnover among groups of marine plankton. The period is also characterized by a floristic turnover and the diversification of the conifers, especially modern families, with small coriaceous leaves. Biostratigraphic and high-precision geochronologic results indicate that a major faunal turnover of the sauropodomorph dinosaurs took place in the Early Jurassic, which led to the rise of the eusauropods. Cañadón Asfalto Basin (part of the Chon Aike Igneous Province of Patagonia) in the Chubut Province of Argentina preserves an extraordinary record of Jurassic fauna and flora that marks key events in the evolution of Mesozoic life. The recently described Bagualia alba, recovered from the base of the Cañadón Asfalto Formation, lived 179 million years ago and is the oldest known eusauropod. The name of the new specimen refers to Cañadón Bagual, the site where the fossil was found, and alba (dawn, in Spanish), for its early age.
Discovered in 2007 by an international team of researchers led by Argentinean paleontologist Diego Pol, the holotype of Bagualia alba (MPEF PV 3301) consists of a posterior half of a skull found in articulation with seven cervical vertebrae. It was found in close association with multiple cranial and postcranial remains belonging to at least three individuals. Body mass estimated suggests that Bagualia weighted 10 tons, approximately the size of two African elephants. Among the characters that distinguish Bagualia from other early sauropods area pointed process on the anteroventral end of the premaxilla and anterodorsal end of the dentary; orbital margin of the frontal with a close V-shape pointed medially that results in a short contribution to the orbit; supratemporal fenestra about as anteroposteriorly long as lateromedially wide; and strongly marked proatlantal facets on the laterodorsal margin of the foramen magnum. The teeth have a D-shaped cross section, apical denticles, and buccal and lingual grooves. But the most striking feature of Bagualia is the enamel layer which is extremely thick, seven times that of other pre-volcanic herbivores, and is heavily wrinkled on its outer surface.
In Patagonia, prior to the Toarcian palaeoenvironmental crisis the plant assemblage consisted of sphenophytes, dipteridacean ferns, conifers, seed ferns, Bennetitales and cycads. This diversity is indicative of more humid conditions. By contrast, the less diverse palynologycal assemblage postdating the volcanic event is indicative of seasonally dry and warm conditions, and are largely dominated by the conifers Araucariaceae, Cheirolepidiaceae and Cupressaceae. These large conifers with coriaceous leaves as the dominant trees likely acted as a strongly selective regime favouring the survival and success of eusauropods, which had powerful jaws and an oversized gut. Conversely, the disappearance of many elements of the diverse pre-Toarcian flora could have influenced the extinction of the diverse lineages of smaller non-sauropods, which lacked adaptations to high-fibre herbivory as their gracile skulls and mandibles were less mechanically efficient and their teeth were small, with thin enamel (less than 200 μm) and lacked tooth–tooth occlusion.