Diplodocus was a "lizard-hipped" dinosaur. It was a sauropod, a group which were especially abundant in the Late Jurassic period. It was the longest of the land animals but not the heaviest. Much of its length was taken up with its strong whip-like tail. It had a brain the size of a fist, and a concentration of nerves in the base of its spine. This helped it to cope with it's enormous size, and control its hind legs and tail.

Diplodocus had simple peg-like teeth for stripping soft foliage like ferns but it could not chew. It swallowed stones which it held in its stomach. These gastroliths, as they are called, helped by grinding down the plant material.

The largest Diplodocus is usually noted as weighing 15 tonnes and being 27 metres long. Fossil evidence of a huge animal called Seismosaurus was found in New Mexico and many palaeontologists believe it is really an old Diplodocus. It weighed 30 tonnes and was 45 metres long.

So far four distinct species have been identified from fossils in the Morrison Formation of Colorado and Wyoming, in the USA. These include five near complete Diplodocus skeletons. In each case the skull has been missing. Skulls from partial skeletons have been used to complete the reconstructions.