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Riojasaurus was a herbivorous prosauropod dinosaur. It was one of the earliest of the large, plant-eating dinosaurs. Riojasaurus lived during the Upper Triassic, roughly 225 to 219 million years ago. Fossils have been found in La Rioja Province in Argentina. There are include incomplete skeletons from about 20 individuals.

Riojasaurus had a heavy body, bulky legs, and a long neck and tail. Its leg bones were dense and massive for a prosauropod. By contrast, its vertebrae were lightened by hollow cavities, and unlike most prosauropods, Riojasaurus had four sacral vertebrae instead of three.