Diplodocus

Diplodocus (/dɪˈplɒdəkəs/, /daɪˈplɒdəkəs/, or /ˌdɪploʊˈdoʊkəs/) is an extinct genus of diplodocid sauropod dinosaurs known from the Late Jurassic of North America. The first fossils of Diplodocus were 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 διπλός (diplos) "double" and δοκός (dokos) "beam", in reference to the double-beamed chevron bones located in the underside of the tail, which were then considered unique.

The genus lived in what is now mid-western North America, at the end of the Jurassic period. It is one of the more common dinosaur fossils found in the middle to upper Morrison Formation, with most specimens being found in rocks dated between about 151.88 and 149.1 million years ago, during the latest Kimmeridgian Age, although it may have made it into the Tithonian, with at least one specimen (AMNH FR 223) being potentially from among the youngest deposits of the formation. The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs, such as Apatosaurus, Barosaurus, Brachiosaurus, Brontosaurus, and Camarasaurus. 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 that they coexisted with Diplodocus.

Diplodocus is among the most easily identifiable dinosaurs, with its typical sauropod shape, long neck and tail, and four sturdy legs. For many years, it was the longest dinosaur known.

Among the best-known sauropods, Diplodocus were 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 cantilever bridges. In fact, D. carnegii is currently one of the longest dinosaurs known from a complete skeleton, with a total length of 24–26 meters (79–85 ft). Modern mass estimates for D. carnegii have tended to be in the 12–14.8-metric-ton (13.2–16.3-short-ton) range.

No skull has ever been found that can be confidently said to belong to Diplodocus, though skulls of other diplodocids closely related to Diplodocus (such as Galeamopus) are well known. The skulls of diplodocids were very small compared with the size of these animals. Diplodocus had small, 'peg'-like teeth that pointed forward and were only present in the anterior sections of the jaws. Its braincase was small, and the neck was composed of at least 15 vertebrae.

D. hallorum, known from partial remains, was even larger, and is estimated to have been the size of four elephants. When first described in 1991, discoverer David Gillette calculated it to be 33 metres (108 feet) long based on isometric scaling with D. carnegii. However, he later stated that this was unlikely and estimated it to be 39–45 meters (128–148 ft) long, suggesting that some individuals may have been up to 52 metres (171 feet) long and weighed 80 to 100 metric tons, making it the longest known dinosaur (excluding those known from exceedingly poor remains, such as Amphicoelias or Maraapunisaurus). The estimated length was later revised downward to 30.5–35 m (100–115 ft) and later on to 29–33.5 m (95–110 ft) based on findings that show that Gillette had originally misplaced vertebrae 12–19 as vertebrae 20–27. Weight estimates based on the revised length are as high as 38 metric tons (42 short tons) although more recently, and according to Gregory S. Paul, a 29 m (95 ft) long D. hallorum was estimated to weigh 23 metric tons (25 short tons) in body mass. A study in 2024 later found the mass of a 33 m (108 ft) D. hallorum to be only 21 metric tons (23 short tons), though the study suggested this only represents the average adult size and not the above average or maximum body size. The nearly complete D. carnegii skeleton at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, on which size estimates of D. hallorum are mainly based, also was found to have had its 13th tail vertebra come from another dinosaur, throwing off size estimates for D. hallorum even further. While dinosaurs such as Supersaurus were probably longer, fossil remains of these animals are only fragmentary and D. hallorum still remains among the longest known dinosaurs.

The estimated tail length of Diplodocus makes up approximately 55% of the total body length, with the tail sometimes hypothesized to be capable of functioning like a very long, tapering bullwhip. This extremely long tail is composed of about 80 caudal vertebrae, which are 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). Some speculation exists as to whether it may have had a defensive or noisemaking (by cracking it like a coachwhip) or, as more recently suggested, tactile function. The tail may have served as a counterbalance for the neck. The middle part of the tail had "double beams" (oddly shaped chevron 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. Chevron bones of this particular form were initially believed to be unique to Diplodocus; since then they have been discovered in other members of the diplodocid family as well as in non-diplodocid sauropods, such as Mamenchisaurus.

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.