Bagualia (meaning "wild horse") is an extinct genus of eusauropod dinosaur from the Early Jurassic (middle Toarcian) Cañadón Asfalto Formation in what is now the Chubut Province of Argentina. The type species, B. alba, was formally described in 2020. Bagualia represents the oldest known definitive eusauropod, and due to the completeness of its material, it represents one of the most important taxa for understanding the early evolution of the group.

The Bagualia fossil material was discovered in Bagual Canyon, approximately 4.3 kilometres (2.7 mi) from Cerro Cóndor in Chubut, Argentina within the Early Jurassic deposits of the Cañadón Asfalto Formation. The fossils were excavated by the Museo Paleontológico Egidio Feruglio during fieldwork in 2007 and 2009. The remains were embedded in a dark grey pelitic matrix rich in organic matter. This layer, dated precisely to around 179 million years ago, formed in a lacustrine environment beneath a basaltic layer. The fossils include the holotype, MPEF-PV 3301 (a partial skull with cervical vertebrae), and additional remains from at least three individuals (MPEF-PV 3305–3348).

The generic name, Bagualia comes from "bagual," the Spanish word for "wild horse," referencing the specimens' discovery in the Bagual Canyon ("Cañadón Bagual"). The specific name, alba, is a Spanish word meaning "dawn," highlighting the dinosaur's early age in the sauropod lineage.

Bagualia is known from many bones from three individuals, including vertebrae from the neck, back, and tail, limb and girdle bones, as well as skull and teeth fragments. The size of Bagualia was likely brought on by a newly formed ecosystem and climate shifts, which were all caused by volcanic activity in the Southern Hemisphere during the Early Jurassic. While the harsh climate and ashes drove most sauropodomorphs to extinction, Bagualia was able to adapt to newly sprouted conifers and plants, using its long neck to snip plant matter from them while staying in place, conserving energy. Its teeth are surrounded by a thick layer of enamel, roughly 7x thicker than other extinct herbivores, enabling the animal to better shear conifer leaves. The digestive system of Bagualia was also a likely reason why it grew to such a large size, and another function of its long neck has been proposed: it may have dissipated heat in a similar fashion to how elephants use their ears.

The skull of Bagualia is relatively complete. The premaxilla is robust and nearly complete with a tall, lateromedially compressed structure, with a smooth lateral surface with several foramina, and its anterior margin lacks a step. A beak-like process is present at the anteroventral end, unique to Bagualia, which may have supported a larger keratinous structure. The maxilla of Bagualia shows 13 alveoli, with some teeth preserved in varying stages of eruption. It has a prominent premaxillary process and a deep narial fossa. Although the ascending process of the maxilla is missing, the antorbital fenestra and lacrimal processes are well-defined. The maxilla articulates with the jugal and palatine, but lacks contact with the ectopterygoid. The right nasal has a broad but damaged articulation with the frontal and a thinner connection with the prefrontal. The lacrimal is robust and dorsoventrally tall, with distinct articulations for the jugal, maxilla, prefrontal, and nasal, similar to other sauropods like Camarasaurus and Turiasaurus. The rhomboid-shaped left prefrontal features prominent articular facets for the lacrimal, nasal, and frontal bones, characterized by its elongated shape and triangular cross-section. The postorbital forms the posterior and posterodorsal boundaries of the orbit, featuring a slender ventral process typical of early sauropodomorphs. The robust squamosals have four articulating processes, with the longest ventral process contributing to an 'S'-shaped profile.

The braincase is nearly complete and ossified, showcasing a robust and tall structure similar to other eusauropods. It features limited cranial pneumaticity and lacks certain recesses, with an elliptical foramen magnum dorsoventrally oriented, contrasting with the circular shape seen in many non-sauropod sauropodomorphs. The paroccipital process is laterally projected, with a unique morphology that differs from other sauropodomorphs. In the Endocranium, pituitary gland hypertrophy supports its link to increasing body size, aligning with trends in later large-bodied sauropods. The floccular lobe is reduced in Bagualia, consistent with a shift to low-agility, quadrupedal locomotion, shared with later eusauropods. Its inner ear anatomy, particularly the shortened and widened semicircular canals, reflects reduced agility and a stable posture. Bagualia shows a moderate sense of smell, more developed than in latter sauropods but less than in predatory sauropodomorphs. Its encephalization quotient is similar to Spinophorosaurus, suggesting moderate brain activity, possibly aided by a well-developed venous system.

The right dentary has 16 alveoli and shows an emerging tooth, while the left dentary has 14 alveoli with five partially erupted teeth. Notably, these dentaries exhibit a U-shaped configuration characteristic of eusauropods, featuring unique structural traits, including well-developed alveoli and a prominent coronoid process on the surangular. The teeth are spoon-shaped with heavily wrinkled enamel, displaying asymmetrical mesial and distal margins, characteristic of many sauropodomorphs, with notable features like a medial convex area and a procumbent arrangement typical of eusauropods. Numerous small pores on the ventral margin, along with little wear on the first erupted tooth, may indicate a vascular function, possibly supporting a keratin-like covering.

Bagualia's appendicular skeleton provides key insights into the evolution of Eusauropoda. It exhibits a well-developed acromion process, deeper acromion fossa, and a straight humerus with a low deltopectoral crest, indicating adaptations for a graviportal lifestyle. The humerus and femur show increased eccentricity, reinforcing resistance to bending forces in large-bodied sauropods. A deep radial fossa in the ulna and changes in metacarpal proportions highlight the transition to obligate quadrupedalism. The tibia's laterally projecting cnemial crest and a wedge-shaped astragalus suggest refinements in locomotion. These features mark Bagualia as a pivotal taxon in the shift towards the massive, quadrupedal build of later sauropods.