Diplodocus (//, //, or //) is an extinct genus of diplodocid sauropod dinosaurs whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a neo-Latin term derived from Greek διπλός (diplos) "double" and δοκός (dokos) "beam", in reference to its double-beamedchevron bones located in the underside of the tail. Chevron bones of this particular form were initially believed to be unique to Diplodocus; however, they have since then been discovered in other members of the diplodocid family and in nondiplodocid sauropods such as Mamenchisaurus. It is now common scientific opinion that Seismosaurus hallorum is a species of Diplodocus.
This genus of dinosaurs lived in what is now western North America at the end of the Jurassic period. Diplodocus is one of the more common dinosaur fossils found in the middle to upper Morrison Formation, between about 154 and 152 million years ago, during the late Kimmeridgian age. The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Apatosaurus, Barosaurus, Brachiosaurus, Brontosaurus, andCamarasaurus.