Diplodocus, meaning “double beam,” is a genus of diplodocid sauropod dinosaur from the Late Jurassic Period of what is now western North America (about 150 million years ago). The first fossils of this dinosaur were discovered in 1877 by S.W. Williston. Its generic name was coined by Othniel Charles Marsh in 1878.

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 million years ago in what is now known as the Kimmeridgian and Tithonian stages. The Morrison Formation also includes fossils from other gigantic sauropods such as Camarasaurus, Barosaurus, Apatosaurus and Brachiosaurus.

Several subspecies of Diplodocus have been described between 1878 and 1924. The first fossil was discovered in Como Bluff, Wyoming in 1878. Marsh gave it the species name D. longus in 1878. Diplodocus remains have since been found in Colorado, Utah and Montana. Fossils are pretty common, except for the skull, which is often missing from otherwise complete skeletons. The most completely known Diplodocus is the subspecies D. carnegii.

There are several valid species of Diplodocus. The type species, D. longus, is known from two skulls and some caudal (hind) vertebrae. D. carnegii, named after Andrew Carnegie, is known from a nearly-complete fossil collected by Jacob Wortman of the Carnegie Museum of Natural History in Pittsburgh, PA. It was named and described by John Bell Hatcher in 1901.

D. hayi is known from a partial skeleton discovered in 1902 near Sheridan, Wyoming, and was described in 1924. D. hallorum is known from a partial skeleton first described in 1991 by Gillette as Seismosaurus halli. The species was later renamed S. hallorum. Then in 2004, a case was made for Seismosaurus to be considered a junior synonym of Diplodocus. It was then renamed D. hallorum in 2006, based on further evidence. Some scientists also speculate that D. hallorum should be regarded as a specimen of D. longus.

Another species, D. lacustris, named by Marsh in 1884, is believed to be an immature specimen of an already named subspecies, rather than a separate species.

Based on numerous skeletal remains, Diplodocus is one of the best-studied dinosaurs. Many aspects of its lifestyle have been subjects of various theories over the years.

Diplodocus is among the most easily identifiable dinosaurs, with its classic dinosaur shape, long neck and tail, along with four sturdy legs. For many years, it was the longest dinosaur known. Its massive size may have been a deterrent to predators such as Allosaurus and Ceratosaurus.

Diplodocus was a very large long-necked quadrupedal sauropod, with a long, whip-like tail. Its forelimbs were slightly shorter than its hind limbs, resulting in a horizontal posture. The Diplodocus, with its long neck and tail and sturdy legs, has been mechanically compared to a suspension bridge. Diplodocus is the longest dinosaur known from a complete skeleton. Based on the subspecies D. hallorum, described in 1991, Diplodocus may have been up to 177 feet long. Some weight estimates range it as high as 125 tons, but a more well-established weight is between 10 and 25 tons.

Despite its large features overall, its skull was very small compared to the rest of its body. It had small, peg-like teeth that pointed forward and were only present in the anterior sections of the jaws. Its braincase was also small. The neck, which comprised of up to 20 feet of the overall length of the dinosaur, was composed of at least fifteen vertebrae. It is now believed that the neck was held generally parallel to the ground and was unable to be lifted much past horizontal.

The tail of the Diplodocus was also extremely long with about 80 caudal vertebrae, which is nearly double the number of vertebrae found in earlier sauropod tails. The dinosaur may have used the long tail as a defensive tool and/or noisemaking tool, cracking it like a whip, using it to deter potential predators. The middle part of the tail had ‘double beams’ which Diplodocus is named for. They may have provided support for the vertebrae, or perhaps prevented the blood vessels from being crushed when the tail pressed against the ground.

Like in other sauropods, the 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 claw is unknown.

The depiction of the posture of Diplodocus has changed considerably through the years. A 1908 reconstruction by Oliver P. Hay depicts Diplodocus with splayed lizard-like limbs. He argued that Diplodocus had a sprawling, lizard-like gait with widely splayed legs. This depiction was supported by at least one of his colleagues, but was later contested by WJ Holland, who demonstrated that a sprawling Diplodocus would have needed a trench to pull its belly through. Discovery of sauropod footprints in the 1930s disproved Hay’s theory.

Later, Diplodocus was often portrayed with their necks held high in the air, allowing them to graze on tall trees. But current computer models have shown that neutral posture of the neck was horizontal rather than vertical. Kent Stephens, et. al., have used these computer models to argue that sauropods did not raise their heads much above shoulder level. However, subsequent studies demonstrated that all tetrapods appear to hold their necks at the maximum possible vertical extension when in normal, alert posture. These studies also argued that the same would hold true for sauropods barring any unknown, unique characteristics that set the soft tissue anatomy of their necks apart from other animals. One model shows Diplodocus holding its neck at about a 45 degree angle with the head pointed downwards in a resting posture.

The very long neck of the Diplodocus is the source of much controversy among scientists. A 1992 Columbia University study of the Diplodocus’ neck structure indicated that the longest necks would have required a 1.6 ton heart. The study proposed that animals like these would have had a rudimentary auxiliary ‘heart’ in the neck, whose only purpose was to pump blood up to the next ‘heart’.

Another study suggests that the oversized neck of the Diplodocus and its relatives may have been primarily used as a sexual display, with feeding benefits coming second.

Recent discoveries have suggested that Diplodocus and other diplodocids may have had narrow, pointed keratinous spines lining their back, much like those on an iguana. This radically different look has been incorporated into some recent reconstructions. But it remains unknown how many, if any, of these diplodocids had this trait, and whether it was present in other sauropods.

The habitat of the Diplodocus was of aquatic nature, assumed Marsh and, later, Hatcher. They based this assertion on the position of its nasal openings at the apex of the cranium. Similar aquatic-like features have been commonly depicted in other large sauropods as well, including Brachiosaurus and Apatosaurus. However, a 1951 study by Kenneth A. Kermack indicates that sauropods could not have breathed through their nostrils when the rest of the body was submerged, as the water pressure on the chest wall would be too great. Diplodocus has been labeled a firmly terrestrial animal since the 1970s. Although, a more recent theory suggest a likely riparian habitat for Diplodocus.

The highly unusual teeth of the Diplodocus, compared to those of other sauropods, would have given it a much different feeding mechanism. It is likely that diplodocids displayed unilateral branch-stripping techniques when feeding. The evidence of this is found in the unusual wear patterns on fossilized teeth of the Diplodocus. In unilateral branch-stripping, one tooth row would have been used to strip foliage from the stem, while the other would act as a guide and stabilizer. With the longer pre-orbital region of the skull, longer stems could be stripped in a single action.

With a laterally and dorsoventrally flexible neck, and the possibility of using its tail and rearing up on its hind limbs, Diplodocus could have had the ability to browse at many different levels, up to 33 feet off the ground. The neck’s range of movement would have also allowed the head to graze below the level of the body, leading some scientists to speculate on whether Diplodocus grazed on submerged aquatic plants, from the waters edge. This feeding behavior has been widely supported by members of the scientific community.

While there is no evidence of Diplodocus’ nesting habits, other sauropods have been associated with nesting sites. It is possible that Diplodocus may have laid their eggs in communal sites over a large area in many shallow pits, each covered with vegetation, such is the case with some other sauropods, including the Saltasaurus. The documentary ‘Walking with Dinosaurs’ portrayed a mother Diplodocus using an ovipositor to lay eggs, but that was pure speculation.

Based on a number of bone histology studies, Diplodocus grew at a very fast rate, reaching sexual maturity at just over ten years, though continuing to grow throughout their lives. Previous theories suggested that it took decades for the Diplodocus to reach sexual maturity.

Diplodocus is famous as being one of the most-depicted dinosaurs and has been on display in more places than any other sauropod dinosaur. Casts of Diplodocus skeletons are still displayed in many museums around the world.