Abstract

Sauropod dinosaurs were the dominant large-bodied herbivores in many Mesozoic terrestrial ecosystems. Such predominance took place after a faunal replacement event linked to a global environmental change during the Early Jurassic (Pliensbachian-Toarcian), when the smaller basal sauropodomorphs were replaced by giant forms in the subclade Eusauropoda. Two main characteristics of this latter group are large body size (exceeding 10 tons) and a remarkably long neck. New data is presented on the axial anatomy of one of the earliest known eusauropods, Bagualia alba from the Toarcian levels of the Cañadón Asfalto Formation (Chubut Province, Argentina), which bear on the origins of these characteristics. Optimizing relevant axial characters on sauropodomorph phylogeny highlights important changes in cervical anatomy in the evolution of Sauropoda: a greater elongation of the neck due to both an increase in the vertebral count and the elongation of each cervical vertebra, and the appearance of pneumatic structures, first on the external surface (e.g., pleurocoels, laminae within the pleurocoels, accessory laminae of the neural arch) and subsequently invading the internal body of the vertebrae. These two changes have been considered essentials for body mass increase in Eusauropoda. The Toarcian age of Bagualia indicates that several important modifications in the axial skeleton and especially the neck occurred early in eusauropod evolution. These modifications are of ecomorphological importance and likely influenced the success of the eusauropods during the climatic changes surrounding the Pliensbachian-Toarcian crisis.