Dinosaur Diplodocus is an extinct genus of diplodocid sauropod dinosaurs whose fossils were first discovered in 1877 by S. W. Williston. Diplodocus grew longer than a tennis court, yet weighed no more than two large elephants. Diplodocus was very large, long-necked, quadrupedal animals, with long, whip-like tails. Their forelimbs were slightly shorter than their hind limbs, resulting in a largely horizontal posture. The skeletal structure of these long-necked, long-tailed animals supported by four sturdy legs have been compared with suspension bridges.In fact, Diplodocus carnegii is currently one of the longest dinosaurs known from a complete skeleton, with a total length of 25 meters (82 ft). Modern mass estimates for Diplodocus carnegii have tended to be in the 10-to-16-metric-ton (11-to-18-short-ton) range.