Diplodocus was a plant eater and had a long neck reaching around 26 feet, but it could not hold its neck beyond 17 feet off the ground. It had a very long tail (around 46 feet) that was whip-like in appearance. Diplodocus had a head, with nostrils on top that was a little less than 2 feet long. The Diplodocus' had significant short front legs when compared to its back ones. Its legs featured feet with five toes each, very elephant-like in appearance. Each foot had one thumb claw, which is believed to have been a protection mechanism.

Sauropods such as Diplodocus were believed to have dual brains. Later, it was discovered that this so-called 2nd brain was nothing more than the spinal cord that had an extension/enlargement. This enlargement, found in the hip area was even larger than the Diplodocus' already small brain. The name Diplodocus means 'Double-beamed' and the dinosaur was named in 1878. This plant eater mainly ate conifers, which were common to its natural habitat. Diplodocus also fed on secondary food sources like gingkos, cycads, seed ferns, club mosses and horsetails.

Diplodocus had blunt teeth that were useful for nipping at and stripping foliage. The dinosaur was 90 feet long, 16 ft tall at the hips and weighed between 10 and 20 tons. It lived about 155 million years ago.