Diplodocus longus was a sauropod dinosaur. It was a long-necked massive herbivore. It lived in the Late Jurassic of North America, approximately 154 to 150 million years ago. It was much larger than any land animals alive today, at 25 m long and weighing approximately 12,000 kg (1). Diplodocus is one of the best known sauropod dinosaurs, perhaps because it is the most common member of this group to be displayed in museums throughout the world. One of the most striking features of this dinosaur is its long and whip-like tail (3).
Diplodocus belongs to the family Diplodocidae. Though it lends its name to this family, many more species are a part of this grouping, such as Apatosaurus and Supersaurus. In Diplodocidae, Diplodocus is considered a highly derived member. It is closely related to Barosaurus, another whip-tailed and long-necked sauropod dinosaur (1). Diplodocidae itself is considered a closely-related sister group, to the other sauropod group of Macronaria. Macronaria contains other famous sauropod species, such as Brachiosaurus and Titanosaurus (2).
Diplodocus longus lived in the Late Jurassic, approximately 154 to 150 million years ago. It has been found in the Morrison Formation, in Colorado and Utah of the United States. The habitat represented by the Morrison Formation was marked by periodic wet seasons in an otherwise semiarid climate. The landscape was a mixture of floodplain prairies and riverine forests. Many different dinosaurs shared this environment, including predators such as Allosaurus and Torvosaurus, which were potential predators of D. longus. Many other long-necked sauropod dinosaurs inhabited this habitat as well, such as Camarasaurus and Brachiosaurus (1).
Though similar ecosystems exist today, the species composition was very different from those seen in modern times. Mammals played only a small role in the Late Jurassic and those co-existing with D. longus still laid eggs and were no larger than rats. Flowering plants had not yet evolved, so grasses and many modern tree groups were absent. Insect life, however, existed in similar varieties to those seen today (2).
Diplodocus longus was an herbivorous dinosaur and many aspects of its biology supported this lifestyle. Its teeth were long, slender, and pencil-shaped. Unlike other dinosaurs, it had teeth only in the front of its jaws and none along the sides. It had small jaws. Skull morphology suggests that Diplodocus had weak jaw muscles, indicating that Diplodocus fed on soft plants such as fresh conifer growth and ferns (1). Tooth wear patterns support this interpretation as well (2).
D. longus, like other diplodocid dinosaurs, had a very long neck. This long neck likely aided it in gathering food. It could have gathered foods at levels where other herbivorous dinosaurs could not reach. Previously, paleontologists identified another possible part of the sauropod digestive system, gastroliths. Gastroliths are stones swallowed by birds and other animals that help grind food. Paleontologists found smooth stones in the abdominal cavity of sauropod skeletons and concluded that they were gastroliths and helped grind the plant material ingested by the dinosaurs. Diplodocus was one of the dinosaurs thought to use stones in this method (1). However, recent comparisons between the stones found in body cavities of sauropods and gastroliths found in modern birds show that this interpretation is unlikely to be correct. Instead of using gastroliths to process food, sauropod dinosaurs such as D. longus most likely kept food within their digestive system for longer periods of time (3).
In the early days of dinosaur discovery, paleontologists thought that these animals had similar posture to modern day reptiles. Early artwork and skeletal reconstructions reflected this, with Diplodocus shown with a splay-legged sprawling posture. This posture is now recognized to be inaccurate. By studying preserved trackways, paleontologists now understand that Diplodocus walked with legs erect under the body, much more similar to a modern elephant than to any reptile. They did not drag their tails as earlier hypothesized either, instead holding them horizontally off the ground. Held this way, the tail counterbalanced the long neck of Diplodocus while walking (1).
The very long tail of Diplodocus placed its center of gravity close to its hip joints, further back than in relatives such as Brachiosaurus. The hind legs of Diplodocus must have supported the bulk of its weight based on this arrangement (2).
Diplodocus longus is distinguished by an exceptionally long neck, likely held horizontal. The structure and support of its neck has been a subject of interest for paleontologists. Such a long neck would have required specialized support systems. Paleontologists have identified notches in the vertebrae, or backbone, of Diplodocus that appear to have housed a ligament. Modern cattle have neck ligaments to support the weight of their necks and heads. It is likely that Diplodocus had a similar ligament running through its neck and providing support through tension (1).
In cattle, this ligament is made up of elastic proteins that allow for stretching without permanent deformation. Diplodocus’ ligament could have had similar properties. The combined mass of the head and neck of Diplodocus has been estimated at 1300 kg (the weight of a small rhinoceros). If the neck ligament in Diplodocus was composed of similar materials to that of a cow’s neck ligament, it would have barely been strong enough to support the weight (1).
As Diplodocus has been extinct for millions of years, there are many aspects of its biology that are unknown and likely will forever remain so. This has lead to a wide amount of speculation on aspects of its morphology and behavior, leading to some rather fanciful interpretations. One of the earliest assumptions about Diplodocus was that it had a sprawling gait similar to that of modern reptiles (1). These paleontologists neglected to take the deep chest of Diplodocus into account in this assumption. If Diplodocus truly had walked with a sprawled posture, it would have required trenches to move its chest through, as the level of its chest would dip below the level of the ground if its limbs were held horizontally (2).
The position of the nostrils of Diplodocus has caused yet more speculation. Diplodocus had nostrils positioned on top of its skull. Originally, paleontologists thought that Diplodocus lived in the water and breathed through these dorsal nostrils (3). This hypothesis has since been invalidated through studies showing that the Diplodocus would have been damaged by water pressure if it kept its body submerged. It is now understood that Diplodocus was a terrestrial animal (4). Other scientists saw similarities between the skull nasal openings of Diplodocus and modern elephants. They thought this might suggest that Diplodocus had a trunk (5), though later research showed this interpretation to be very unlikely (6).
Often described as “whip-like,” the tail of Diplodocus is the subject of a final interesting hypothesis. Some scientists have argued that Diplodocus could have lashed their tails at supersonic speeds, creating a sound similar to the cracking of a whip. This could have been used either for defensive measures or sexual selection. This hypothesis has not been supported by any further research (7).
