Diplodocus

Credit: Fred Wierum.

*Diplodocus* is an extinct genus of diplodocid sauropod dinosaur that lived in North America during the Late Jurassic.

## History

The first specimen was found in Cañon City by Benjamin Mudge and Samuel Wendell Williston (1877), which kicked off multiple species being named between 1878 and 1924. This species was named *D. longus* by O.C. Marsh (1878). However, *D. carnegii* is the most iconic species despite not being the type, due to having many skeletal mounts worldwide. It is known from the Morrison Formation where it is common. *Diplodocus* and *Barosaurus* have similar limbs, with many accidentally referring to *Diplodocus* due to their coexistence and similarity. Most *Diplodocus* are from stratigraphic zone 5 of the Morrison.

## Description

*Diplodocus* is one of the best known sauropods, being extremely large, quadrupedal, having long necks and whip-like tails. The forelimbs are slightly shorter then the hind, which causes a mostly horizontal posture. The limbs are sturdy and their body is balanced by the elongate tail and neck, which leads to them looking similar to a suspension bridge. *D. carnegii* represents the longest dinosaur with a complete skeleton, 24 meters long. *D. hallorum* is known from partial, larger remains. David Gillette (1991) discovered it, finding a length of 52 meters, which would make it the longest dinosaur excluding fragmentary taxa such as *Amphicoelias*. Gilette had misplaced vertebrae 12-19 as 20-27, which led to massive size estimates now found incorrect. The *D. carnegii* skeleton at The Carnegie Museum also had the 13th vertebrae borrowed from another taxon, which also affected the estimates. *Supersaurus* was probably longer, but it is now seen as a synonym of *Diplodocus* and too fragmentary to be sure. The tail is made from 80 caudals, which is nearly double the length of most sauropods. Some think it may have been defensive or could have snapped the sound barrier when cracked. However, it may have also been a counterbalance and was probably not for defense, as it would have been incredibly painful for the animal. The mid-caudals have double beams, which it is named for, comprised of chevrons. These supported the vertebrae and maybe even prevented blood vessels from crushing it the tail touched the ground. Chevrons like these are also seen in many dinosaurs, but were initially assumed to be an autapomorphy before the rest of Diplodocidae was found. The forefeet were heavily modified into a skeletal column which the fingers and hand bones lay. They were shaped like a horseshoe in cross section, with ever digit lacking a claw minus one, where an unusually large (for diplodocids) claw flattened from side-to-side and disarticulated from the manus lay. It is unknown what function this had. Their skulls are small with peg-like teeth that project forward present in the anterior of the jaws. The braincase is small. The neck is made of 15 cervicals, which may have been held parallel to the ground and unable to orient horizontal.

Diplodocid skin impressions from 1990 show that some species had narrow, pointed keratinous spines similar to iguanas that reached up to 18 centimeters long. These sad on the end of the tail, and possibly along the neck and back. They noted the impressions came from an unnamed taxon similar to Diplodocus and Barosaurus, but these have since been referred to *Kaatedocus* and *Barosaurus sp.*. *D. sp.* skin from Mother's Day Quarry show rectangular, polygonal, pebble, ovoid, dome and globular scales that range in size and shape in different locations, the smallest being 1 millimeters and the largest 10. Their orientations may indicate their position on the body. Clustering scales similar to some reptiles are probably dorsal-most. Arching rows of square scales that interrupts polygonal patterns are also seen, and are similar to crocodilian limb scales, which suggests they come from the limb region. The skin is small, 70 centimeters long, but has high diversity and being smaller than other impressions. Thus, it may be of a juvenile.

## Classification

*Diplodocus* is the type of Diplodocidae, with Diplodocinae erected to include it and its closest relatives.

## Paleobiology

*Diplodocus* is one of the most studied dinosaurs due to how well it is preserved. Sclerotic rings suggests it was cathemeral. Marsha and Hatcher assumed it was aquatic because the nostrils were positioned at the apex of the skull, which was an assumed aquatic trait. However, the pressure of the water would have crushed their chest wall and made breathing impossible.

### Posture

Oliver P. Hay (1910) first depicted *Diplodocus* with splayed limbs, which was supported by Gustav Tornier. William Jacob Holland countered this by stating *Diplodocus* would have needed to hunch over a trench in order to haul its abdomen without touching the ground. Footprints found in the 1930s put the sprawling theory to rest. Later, they were depicted with upright necks, but many biomechanics studies recover a horizontal neck posture. A nuchal ligament may have held held the neck straight. One 2009 study finds that all tetrapods may hold the base of their necks at the maximum vertical extension when in the normal posture. However, it is unknown if sauropods bore unique neck soft tissue that would separate them.. Others argue they would not have been able to pump blood to the head at a horizontal posture, and others counter saying the horizontal position would eliminate the issue entirely.

### Diet

Diplodocines have teeth with long, slender crowns that are elliptical in cross section, having a blunt apex and a triangular point. Wear facets are most common at the apex, with wear patterns on the labial surface of the upper and lower teeth, which implies a radically different feeding mechanism. They likely used unilateral branch stripping, since it explains the wear patterns caused by effective stripping methods.Young et al. (2012) examined diplodocine performance, finding they could not have stripped bark due to extreme stresses.. However, branch stripping and precision biting were plausible. Their teeth replaced within less than 35 days (D'Emic et al.). As many as 5 replacement teeth developed within one socket. It preferred different food than the other Morrison sauropods, which could have allowed them to coexist without competition. It could browse 4 meters from the ground. *Diplodocus* could rear into a vertical posture with ease since the center of mass was nearer to the hips, using its tail as a tripod. I could then browse up to 11 meters. It has also been speculated it could have browsed from aquatic plants, with posture and the peg-like teeth supporting this. Matthew Cobley et al. (2013) find that muscles and cartilage would have restricted the neck, noting that then their range of motion would be lesser, moving their entire bodies to access different vegetation. Thus, they may have foraged more to meet their energy needs. However, MIke Taylor (2014) disputes this, finding a very flexible neck. Whitlock et al. (2010) describe CM 11255, a juvenile skull, that differs greatly from adults. The snout is blunt with teeth not confined to the front of the jaws. This suggests they fed differently. Such an ecological difference was not observed in previous sauropodomorphs.

### Ontogeny

It is suggested the neck may have been used as sexual selection. Based on *Saltasaurus*, they may have laid their eggs in shallow pits communally over large areas, where each pit was covered with vegetation.. Their eggs were probably quite small, which was likely a predation adaptation, since larger eggs would have taken longer to develop. They likely grew quickly over the span of a decade, likely a little over, and then growing for the rest of their lives.

## Paleoecology

*Diplodocus* lived in the Morrison Formation alongside large stegosaurs, allosaurs and other large sauropods, among others.

## References

* https://en.wikipedia.org/wiki/Diplodocus
* https://www.nature.com/articles/s41598-022-21633-2