Diplodocus

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 diploos "double" and 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 150 to 147 million years ago, in what is now termed the Kimmeridgian and Tithonian stages. 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 Gillete calculated it may have been up to 54 m (177.05 ft) long, making it the longest known dinosaur (excluding those known from especially poor remains, such as Amphicoelias). Some weight estimates ranged as high as 113 (rather only 50) tonnes (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 Gillete 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. 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. Modern mass estimates 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). 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.

Like other sauropods, the manus (front "feet") of Diplodocus were highly modified, with the finger and hand bones arranged into a vertical column, horseshoe-shaped in cross section. Diplodocus lacked claws on all but one digit of the front limb, and this claw was unusually large relative to other sauropods, flattened from side to side, and detached from the bones of the hand. The function of this unusually specialized claw is unknown.

Discovery and species
Several species of Diplodocus were described between 1878 and 1924. The first skeleton was found at Como Bluff, Wyoming by Benjamin Mudge and Samuel Wendell Williston in 1878, and was named Diplodocus longus ('long double-beam'), by paleontologist Othniel Charles Marsh in 1878. Diplodocus remains have since been found in the Morrison Formation of the western U.S. States of Colorado, Utah, Montana and Wyoming. Fossils of this animal are common, except for the skull, which is often missing from otherwise complete skeletons. Although not the type species, D. carnegii is the most completely known and most famous due to the large number of casts of its skeleton in museums around the world.

The two Morrison Formation sauropod genera Diplodocus and Barosaurus had very similar limb bones. In the past, many isolated limb bones were automatically attributed to Diplodocus but may, in fact, have belonged to Barosaurus.

Valid species

 * D. longus, the type species, is known from two skulls and a caudal series from the Morrison Formation of Colorado and Utah.
 * D. carnegii (also spelled D. carnegiei), named after Andrew Carnegie, is the best known, mainly due to a near-complete skeleton collected by Jacob Wortman, of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania and described and named by John Bell Hatcher in 1901.
 * D. hayi, known from a partial skeleton discovered by William H. Utterback in 1902 near Sheridan, Wyoming, was described in 1924.
 * D. hallorum, first described in 1991 by Gillette as Seismosaurus halli from a partial skeleton comprising vertebrae, pelvis and ribs. George Olshevsky later attempted to emend the name as S. hallorum, citing incorrect grammar on the part of the original authors, a recommendation that has been followed by others, including Carpenter (2006). In 2004, a presentation at the annual conference of the Geological Society of America made a case for Seismosaurus being a junior synonym of Diplodocus. This was followed by a much more detailed publication in 2006, which not only renamed the species Diplodocus hallorum, but also speculated that it could prove to be the same as D. longus. The position that D. hallorum should be regarded as a specimen of D. longus was also taken by the authors of a redescription of Supersaurus, refuting a previous hypothesis that Seismosaurus and Supersaurus were the same.

Nomina dubia (doubtful species)

 * D. lacustris is a nomen dubium, named by Marsh in 1884, from remains of a smaller animal from Morrison, Colorado. These remains are now believed to have been from an immature animal, rather than from a separate species.

Paleobiology
Due to a wealth of skeletal remains, Diplodocus is one of the best-studied dinosaurs. Many aspects of its lifestyle have been subjects of various theories over the years.