Diplodocus was one of the longest animals ever to walk the earth.

One of the best known sauropod dinosaurs, Diplodocus had a more slender body and longer neck and tail than its close relative Apatosaurus. The distinctive skull of Diplodocus has the bony nostrils placed on the top of the head. The peg-like teeth are restricted to the front of the jaws.

The specimen forming most of the reconstructed skeleton on exhibit, CM 84, was found in 1899 and was vital in establishing Carnegie Museum's place in the collection and study of dinosaurs. Andrew Carnegie was so delighted with this find that he presented plaster casts of the skeleton to major museums around the world. Today, casts of Carnegie Museum's Diplodocus stand in museums on four continents.

During the first excavations at what is now Dinosaur National Monument, Carnegie Museum collectors were puzzled. They had found bones of several different sauropod dinosaurs, but which skull belonged with the bones of Diplodocus? Dinosaur Hall presents a lucky find, shown as originally preserved: a set of neck vertebrae associated with the skull, neatly solving the puzzle.