Diplodocus longus was named by
Marsh (1878). Its type specimen is YPM 1920, a partial skeleton, and it is a 3D body fossil. Its type locality is
Felch Quarry 1, Garden Park (YPM), which is in a Kimmeridgian/Tithonian channel sandstone/claystone in the Morrison Formation of Colorado. It is the type species of
Diplodocus.