Illustration of Sauropod dinosaurs. A herd of sauropod dinosaurs, Seismosaurus (Diplodocus) Hallorum, migrating to new feeding areas during the seasonally dry environment of western North America's late Jurassic Period. Known from a huge, forty-percent-complete skeleton, Seismosaurus was discovered in the Brushy Basin Member of New Mexico's Morrison Formation by Dr. David Gillette and Wilson Bechtel. Associated with the skeleton were Gastroliths, or "stomach stones," that may have aided digestion.