Diplodocus, duh PLOD uh kuhs, was an extremely long, slender, plant-eating dinosaur that lived about 150 million years ago in what is now the Western United States. It belonged to a group of giant, plant-eating dinosaurs known as sauropods. The first Diplodocus bones were found in 1877 near Morrison, Colo. The name Diplodocus means double beam and describes the beams of bone in the underside of the animal's tail.