Named By: Othniel Charles Marsh - 1878.
Synonyms: Diplodocus lacustris, Seismosaurus hallorum.
Classification: Chordata, Reptilia, Dinosauria, Saurischia, Sauropodomorpha, Sauropoda, Diplodocidae, Diplodocinae.
Species: D. longus (type), D. carnegii, D. hayi, D. hallorum.
Size: Estimated at up to 35 meters long.
Known locations: USA, Colorado, Montana, Utah, Wyoming - Morrison Formation.
Time period: Kimmeridgian to Tithonian of the Jurassic.
Fossil representation: Many specimens, although some are partial they together reveal the full shape of the dinosaur. Footprints are also known.
The discovery, naming and
reconstruction of Diplodocus
Two factors have helped make Diplodocus famous all over the world. The first is that for a long time Diplodocus was thought to be the largest (at least in terms of length) dinosaur known to walk the Earth. The second is the large number of skeletal casts of Diplodocus carnegii that were donated by Andrew Carnegie to numerous museums around the world. While these casts still exist to be seen and studied by all however, discoveries of new dinosaurs have since displaced Diplodocus from any claim it had to the title of biggest dinosaur.
The first Diplodocus remains were found in 1877 by Benjamin Franklin Mudge and Samuel Wendell Williston at Caņon City, Colorado. The remains were subsequently studied and named by Othniel Charles Marsh in a period of American palaeontological history dubbed the ‘bone wars’. Marsh paid special note of the two long bone growths that were on the underside of the caudal (tail) vertebra and coined the name Diplodocus longus, 'long double beam’. However the vertebrae that Diplodocus is named for are now known to not be unique to this genus, and can be found in many other sauropod genera.
Diplodocus has the basic sauropod body shape of a large round body on four legs with a long tail and neck. Much of the length of Diplodocus was actually neck (as much as six meters long) and tail and even so this granted Diplodocus an incredible body length, the actual form would have been quite lightly built. As such even when Diplodocus was thought to be the longest dinosaur most considered others such as Brachiosaurus to be larger on the grounds of greater body weight. Today neither of these dinosaurs is considered the largest in the light of later discoveries such as Argentinosaurus which is thought to have been heavier than either one.
The earliest artistic reconstructions that depicted Diplodocus in its presumed habitat of swamps show a sauropod that has a serpentine neck that is held up almost vertical, a tail that hangs limp across the ground and legs that sprawl out to the sides like a lizards. For a start the limbs sprawling out to the sides is an obvious mistake, something that was pointed out by W. J. Holland. These sprawling limbs could not have lifted the massive body off the ground, and the only way that they could have done so was if they were placed below the body and acted like pillars. Today this seems an obvious conclusion, but it was not universally accepted until the 1930‘s when Diplodocus footprints were found that proved without doubt that Diplodocus had legs that were under the body.
The tail is now known to have been carried off the ground and roughly horizontally level. The whip like ending however still causes some confusion to its purpose, either being a defence to lash out at attacking predators, a signalling device which could be used to create a loud ‘crack’ sound to signal to other Diplodocus, or completely unknown function. The neck is also usually envisioned as being carried horizontal to the ground except when feeding, although new interpretations suggest that it may have been carried at a higher angle when not feeding. This is based upon the observation that all tetrapods (four limbed vertebrates such as amphibians, bird, mammals and reptiles) carry their heads as high as they can when they are not preoccupied with other things such as looking for food. There is no reason to assume that as reptiles the dinosaurs would be any different, and a 2000 study by Upchurch et al. suggested that Diplodocus may have held its neck at up to a forty-five degree angle. Still the neck of Diplodocus was not capable of being bent in S-shaped curves, and regardless of what direction it was pointing in, up or down, left or right, it would almost certainly have been kept near straight.
Like other sauropods Diplodocus is thought to have been a fully terrestrial dinosaur that inhabited plains and areas of sparse woodland on the fringes of denser forests that may have been too overgrown for such a large creature to move around in. This is very different to the earliest depictions of this dinosaur that saw it wallowing around in swamps and lakes to avoid predators and rely upon the water to support its weight. However when you think about how the water pressure pressing on the body would have made breathing for Diplodocus difficult as well as the fact that the majority of sauropod remains are known from dry environments, you end up with a better idea as to the kind of habitats that dinosaurs like Diplodocus lived in.
Diplodocus is thought to have been a ‘leaf stripper’ of vegetation. This is suggested by the peg like teeth that point forwards, as well as the fact that in adults the teeth are only present in the forward portion of the mouth. Diplodocus had a very particular wear pattern in that the teeth are more worn along their sides than on the ends like you would expect with animals that bit down on food. This means that Diplodocus fed by closing its mouth around vegetation so that the stems of the plant slotted in between the teeth. Diplodocus would then pull its head back so that as the stems ran between the teeth the softer foliage was stripped off and collected inside the mouth. A further adaptation was the snout of Diplodocus that was proportionately longer than other kinds of sauropod, which in the simplest terms means that Diplodocus could fit a greater length of plants in its mouth so that it ended up with a larger amount of vegetation. Juvenile Diplodocus had teeth in the back of the mouth as well, and this may indicate that they pulled their heads to the side rather than just back. This may have made feeding much easier for them given their smaller size, and when they grew so large they would almost always be feeding from above they would only have need for the front teeth.
The scleral rings (bone that supported the shape of the eye) of Diplodocus indicate that it was cathemeral. This means that Diplodocus was active for short periods throughout both the day and the night, and it’s possible that it adapted to such a way of life to ensure that it had a steady intake of food to keep its massive body going.
When it comes down to feeding Diplodocus is usually envisioned as being a low browser that kept its neck close to the ground for browsing. The shorter front legs support this idea as Diplodocus would have been in a more comfortable posture for feeding. It has been considered however that Diplodocus may have been able to rear up on its hind legs with its tail acting as a third support so that it could reach up and feed from the tree canopy as well. If true then this would make Diplodocus, and possibly similarly built sauropods, generalists that could adapt their browsing in accordance to the available plants of an area. This adaptability would have taken Diplodocus out of direct competition with other low browsing herbivores such as Stegosaurus which is also known from the same Morrison Formation at this time. A final feeding theory is that Diplodocus may have fed upon submerged aquatic plants by standing on the sides of a lake or large river and using its long neck to dip its head beneath the surface to pull up a mouthful of soft water weeds.
Anatomical details of Diplodocus looked in life are as in other dinosaurs being continuously looked at. Usually the head Diplodocus is recreated with a thin covering of skin and tissue so that it looks like a simple fleshed-out skull. It’s a plausible idea though that it may have had some kind of soft tissue growth that did not preserve although the Diplodocus skull does not have the large high arcing supports like some sauropods have. Still reconstructions exist which show everything from enlarged resonating chambers that would increase the sound of calls to small trunks like an elephant. Such reconstructions are of course pure speculation but they are still valuable tools in pushing the boundaries of our understanding of dinosaurs in general.
Later fossil finds have brought the possibility that Diplodocus may have had keratinous spines that ran down the back for the entire length of the animal. Because they were made from keratin this spikes would not have readily preserved like bones do, but similar spines can be seen in some other sauropods. In fact the distantly related diplodocid Amargasaurus from South America seems to have taken these spines a step further by having incredibly long spines rising up from the back of the neck.
How large sauropods like Diplodocus managed to lay eggs without them breaking has been the subject of considerable study but clear indication to how remains elusive. Some sauropods are thought to have built nests of soft vegetation that cushioned the fall of the egg, while others may have managed to squat down. Another theory is that of an ovipositor which would be a tube of soft tissue like muscles that an egg passed down on its way to the ground so that it had a controlled rate of descent and hit the ground softly. This method was depicted in the BBC series Walking With Dinosaurs which is why it is often associated with Diplodocus. Unfortunately no soft tissue remains exist to support this, and so it remains as a theory only.
How much care Diplodocus took in rearing young is also unknown. Saltasaurus, a South American titanosaur seems to have built nests as indicated by large scale fossil remains found associated with the species. Another sauropod called Camarasaurus which lived at the same time and location as Diplodocus however seems to have laid eggs in rows rather than building nests. If the difference is down to ecological factors then Diplodocus may have been similar to Camarasaurus in its rearing behaviour, but as always the answer will be uncertain without fossil evidence to back up one theory over the other.
After hatching young sauropods are thought to have headed into dense undergrowth where they could remain hidden from the eyes of predators. It’s possible that young Diplodocus did the same while adults moved on to fresh feeding grounds and their back teeth suggest a more open diet. Also the broad snout did not develop until adulthood when only the front teeth were present, something that again suggests that young Diplodocus were not as fussy in their browsing. Study of Diplodocus bones has shown that the most rapid rate of growth took place during the first ten years of life at the end of which the individual would reach sexual maturity. However even though Diplodocus could reproduce at this early age fossils show that it continued to grow for several years, although at a much slower rate.
How Diplodocus and other sauropods placed in their ecosystems food webs is an interesting subject. When fully grown a Diplodocus would have little in the way of natural predators, but first a Diplodocus would have to survive long enough to grow up to adulthood. Juveniles would have had to of been on a constant look out for predators from the moment of hatching, and with predators like Ceratosaurus and the even larger Allosaurus active in the same area juvenile mortality to predators may have been incredibly high. This might infer an r-strategy method of survival for Diplodocus where a large number of eggs are laid and hatch out, but a lack of constant parental care means that many are lost to predators so that only a handful at most survives to adulthood. This could suggest that juvenile Diplodocus (and other sauropods) could have formed an important portion of the prey available to the predators of Jurassic North America. A pelvis of a Camarasaurus proves that sauropods were eaten by predators like Allosaurus, but it does not prove that it was killed by one.
How invulnerable adult Diplodocus was to attack is also a matter of question. The huge size of Diplodocus was once thought to be all the protection that it needed, but eventually animals including Diplodocus would grow old, as well as occasionally getting sick and possibly even injured from something like a fall. These are all things that predators look for when tracking targets that could be their next meal. While predators like Allosaurus were much smaller than Diplodocus they obviously had no fear of a challenge, something which is indicated by fossils that prove Allosaurus would attack Stegosaurus, a herbivorous dinosaur that had the weaponry to kill an Allosaurus. Additionally fossil material from the Morrison Formation has been interpreted as revealing the presence of eleven to twelve meter long theropods that were even bigger than Allosaurus (although some think the material does belong to Allosaurus which suggests that some species could grow much bigger than previously thought). Add on new theropod hunting theories such as mobbing behaviour, where several theropods that are normally solitary mob and attack an already weakened individual going their separate ways after the kill, and you have an ecosystem where Diplodocus faced a day to day struggle throughout its life.