Prosauropoda are an extinct group of early herbivorous dinosaurs that lived during the Triassic and early Jurassic periods. They were frequently the predominant herbivore in their environment, and quickly reached large size, from 6 to 10 metres (20 to 33 ft). All prosauropods had a long neck and small head, forelimbs shorter than the hindlimbs, and a very large thumb claw (inherited from the thecodontosaurs) for defense. Most were semi-bipedal, although at least one large form (Riojasaurus) was fully quadrupedal. They were originally thought to be the ancestors of the sauropods, but are now considered a parallel lineage.