Genus: Diplodocus MARSH, 1878
Etymology: Greek, diplo-, “double”, and Greek, dokos, “Beam, rafter”, + us (refereeing to some of the caudal vertebrae): “Double beam”.
= Seismosaurus GILLETTE, 1991
Etymology: Greek, sesimos, “earthquake’, and Greek, sauros, “lizard”: “Earthshaker lizard”.

Species: hallorum (GILLETTE, 1991) (emend OLSHEVSKY, 1998)
Etymology: In honor of the late Reverend James W. Hall, former director of the Ghost Ranch Conference Center, Abiquiu, New Mexico, and his wife Ruth, honoring their support of paleontology in northern New Mexico spanning three decades.
= Seismosaurus halli GILLETTE, 1991

Holotype: NMMNH 3690

Locality: Seismosaurus Quarry, 16 km west of San Ysidro, Bernalillo County, Central New Mexico.

Horizon: Upper Brushy Basin Member, Morrison Formation.

Biostratigraphy: Zone 3.

Age: Upper Part, Comobluffian age, Kimmeridgian Stage, uppermost Malm Epoch, Late Jurassic.

Material: One isolated and seven articulated dorsal vertebrae, articulated ribs, both ilia and ischia, one pubis, sacrum, caudal vertebrae 1-8, 12-16, and 20-27, five chevrons, with 230 gastroliths.
Note: LUCAS, 2000, LUCAS, HECKERT, HUNT & SULLIVAN,2004 do not believe that these are gastroliths but are actually stream-deposited pebbles.
Note: Pathologic distal expansion on the ischium,LUCAS, HECKERT, HUNT & SULLIVAN,2004.