One of the best known sauropods, Diplodocus was a very large long-necked quadrupedal animal, with a long, whip-like tail. Its forelimbs were slightly shorter than its hind limbs, resulting in a largely horizontal posture. The long-necked, long-tailed animal with four sturdy legs has been mechanically compared with a suspension bridge.[3] In fact, Diplodocus is the longest dinosaur known from a complete skeleton.[3] While dinosaurs such as Supersaurus were probably longer, fossil remains of these animals are only fragmentary.[4]
The skull of Diplodocus was very small, compared with the size of the animal, which could reach up to 27 metres (90 feet), of which 6 metres (20 feet) was neck.[5] Diplodocus had small, 'peg'-like teeth only at the anterior part of the jaws, which were distinctly procumbent.[6] Its braincase was small. The neck was composed of at least fifteen vertebrae and is now believed to have been generally held parallel to the ground and unable to have been elevated much past horizontal.[7] Modern mass estimates have tended to be in the 10 to 16 tonne (11–17.6 ton) range: 10 tonnes (11 tons);[8] 11.5 tonnes (12.7 tons);[9] 12.7 tonnes (14 tons);[10] and 16 tonnes (17.6 tons).[11]
Diplodocus had an extremely long tail, composed of about eighty caudal vertebrae,[12] which is almost double the number some of the earlier sauropods had in their tails (such as Shunosaurus with 43), and far more than contemporaneous macronarians had (such as Camarasaurus with 53). There has been speculation as to whether it may have had a defensive[13] or noisemaking function.[14] The tail may have served as a counterbalance for the neck. The middle part of the tail had 'double beams' (oddly-shaped bones on the underside, which gave Diplodocus its name). They may have provided support for the vertebrae, or perhaps prevented the blood vessels from being crushed if the animal's heavy tail pressed against the ground. These 'double beams' are also seen in some related dinosaurs.
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