Osteology of the appendicular skeleton of Bagualia alba (Dinosauria, Eusauropoda) from the Lower Jurassic of Patagonia and the macroevolutionary history of early eusauropods.

Since their origin, sauropodomorphs have undergone numerous anatomical changes from small and bipedal early sauropodomorphs towards massive-bodied and quadrupedal sauropods. However, the timing of these changes in the evolution of the group is unclear. Here, we describe the appendicular skeleton of the early diverging eusauropod Bagualia alba from the late Early Jurassic of Patagonia, Argentina, and conduct a morphological disparity analysis based on a phylogenetic dataset of Sauropoda. The results reveal a change in morphospace occupation between the pre-Toarcian and Toarcian-Middle Jurassic sauropodomorphs and between the latter and Late Jurassic forms. The first shift corresponds with the extinction of non-sauropodan sauropodomorphs and the diversification of sauropods, while the second corresponds with the diversification of Neosauropoda and closely related eusauropods (mamenchisaurids, turiasaurians) in the Late Jurassic, leading to a substantial shift and increase in morphological disparity. Finally, we found that body mass is significantly correlated with the first principal coordinate axis of the morphospace in two-thirds of a random sample of optimal trees, which suggests that body size played a role in shaping the evolution of sauropod morphology. In this context, Bagualia provides insights into the evolution of Sauropoda, particularly regarding changes that occurred during the Early to Middle Jurassic.