Diplodocus herd, artwork. Diplodocus was a large (up to 35 metres long) sauropod dinosaur that lived during the Late Jurassic (160-145 million years ago). The neck posture of sauropod dinosaurs have long been a matter of debate. The depiction here shows Diplodocus with an upright neck posture, a position that some scientists believe can be inferred by examining modern terrestrial animals. Other scientists, using computer models, believe that sauropods held their necks horizontally.