Diplodocus is a genus of diplodocid sauropod dinosaur whose fossils were first discovered in 1877 by S. W. Williston. The generic name, coined by Othniel Charles Marsh in 1878, is a Neo-Latin term derived from Greek διπλόος (diploos) "double" and δοκός (dokos) "beam", in reference to its double-beamed chevron bones located in the underside of the tail. These bones were initially believed to be unique to Diplodocus; however, they have since then been discovered in other members of the diplodocid family and in non-diplodocid sauropods such as Mamenchisaurus.
It lived in what is now western North America at the end of the Jurassic Period. Diplodocus is one of the more common dinosaur fossils found in the Upper Morrison Formation, a sequence of shallow marine and alluvial sediments deposited about 155 to 148 million years ago, in what is now termed the Kimmeridgian and Tithonian stages (Diplodocus itself ranged from about 154 to 150 million years ago). The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Barosaurus, Apatosaurus and Brachiosaurus.
Diplodocus is among the most easily identifiable dinosaurs, with its classic dinosaur shape, long neck and tail and four sturdy legs. For many years, it was the longest dinosaur known. Its great size may have been a deterrent to the predators Allosaurus and Ceratosaurus: their remains have been found in the same strata, which suggests they coexisted with Diplodocus.
Description
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. In fact, Diplodocus is the longest dinosaur known from a complete skeleton. The partial remains of D. hallorum have increased the estimated length, though not as much as previously thought; when first described in 1991, discoverer David Gillette calculated it may have been up to 54 m (177 ft) long, making it the longest known dinosaur (excluding those known from exceedingly poor remains, such as Amphicoelias). Some weight estimates ranged as high as 113 tons (125 US short tons). This review was based on recent findings that show that the giant tail vertebrae were actually placed further forward on the tail than Gillette originally calculated. The study shows that the complete Diplodocus skeleton at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, on which estimates of Seismosaurus were based, had its 13th tail vertebra come from another dinosaur, throwing size estimates for Seismosaurus off by up to 30%. While dinosaurs such as Supersaurus were probably longer, fossil remains of these animals are only fragmentary. Modern mass estimates for Diplodocus (exclusive of D. hallorum) have tended to be in the 10 to 16 tonne (11–17.6 ton) range: 10 tonnes (11 tons); 11.5 tonnes (12.7 tons); 12.7 tonnes (14 tons); and 16 tonnes (17.6 tons).
The skull of Diplodocus was very small, compared with the size of the animal, which could reach up to 35 m (115 ft), of which over 6 m (20 ft) was neck. Diplodocus had small, 'peg'-like teeth that pointed forward and were only present in the anterior sections of the jaws. 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.
Diplodocus had an extremely long tail, composed of about 80 caudal vertebrae, 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 or noisemaking (by cracking it like a coachwhip) function. 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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