Diplodocus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Diplodocidae

Genus: Diplodocus

Species: Diplodocus longus

Introduction

Diplodocus is one of the most easily recognized of dinosaur species; its long neck and tail, small head and feet, and gigantic body have become the quintessential body shape many people instinctively associate with dinosaurs. One reason the animal is so well known is that in the late nineteenth and early twentieth centuries, the wealthy American entrepreneur Andrew Carnegie—a dinosaur enthusiast—had eleven copies of a complete Diplodocus skeleton made, sending them to important museums in Europe and the Americas. The original skeleton still stands in the Carnegie Museum of Natural History in Pittsburgh; it is affectionately called “Dippy.” It is 26 meters (85 feet) long and comprises 292 bones.

Classification

Under the traditional Linnaean system of classification, Diplodocus is considered a member of the Diplodocidae family, which also includes dinosaur species like Apatosaurus, Barosaurus, and Supersaurus. Diplodocids were, in turn, part of a larger group of dinosaurs known as sauropods. Sauropods were plant eaters that dominated the earth in the Late Jurassic and Early Cretaceous eras. Using a Linnaean system of classification, Diplodocus and Apatosaurus (which closely resembles Diplodocus, but is shorter, stockier, much heavier, and has a larger skull) are grouped together until the level of the genus.

A second tool scientists have of classifying species is known as cladistics. This approach analyzes the number of measurable physical characteristics two species have in common that are judged to be evolutionary novelties inherited from a common ancestor. The more shared evolutionary novelties they have, the more likely it is that they are closely related, placing them in the same clade. A cladistics-based approach to classifying Diplodocus places it in the clade Neosauropoda, a sub-group of sauropods that also includes Supersaurus. Among the important distinguishing features of neosauropods are the presence of teeth only at the front of the jaw and distinct curved vertebrae at the base of the tail.

Anatomy

Diplodocus was a long, large animal with a disproportionately small head, the skull of which ended in a blunt snout. It had a long tail that tapered towards the end, an elongated neck, and wide hips. It also had relatively narrow, compact feet. Some fossil impressions made by Diplodocus's skin suggest that the dinosaur may have had a row of narrow spines running all along its back from its hips to the tip of its tail; the spines would have been made of keratin (not bone) and would have tapered to a thin point. The evidence for this, however, is not conclusive, so some museum displays of Diplodocus depict the animal without these spines. The name Diplodocus comes from the unusual shape of the bones beneath its tail vertebrae, known as chevrons. Unlike most dinosaurs, Diplodocus had chevrons that were split in the middle, forming a structure that looked like two long beams connected by a point.

Intelligence

Diplodocus—with its tiny brain and massive body—had an EQ of about 0.05.

There have been changes in the way paleontologists calculate EQ over the years. Initially, scientists believed that dinosaurs had brains, like reptiles, that only took up about half of the space inside their skulls. Now the prevailing theory is that dinosaur brains are more like bird brains, which basically fill the skull cavity. Therefore, more recent estimates have placed Diplodocus EQ closer to 0.1, but this is still extremely low compared to many other dinosaurs. Triceratops, for instance, had a brain to body ratio that was about twice as high as Diplodocus, at 0.2, and the gigantic Tyrannosaurus may have had an EQ of about 1.

Reproduction and Population

In general, the fossil evidence relating to dinosaur reproduction is relatively scarce. However, discoveries of eggs and nests, rare as these are, have helped scientists to come to some basic conclusions about how these animals bred. For example, the nests of many carnivorous dinosaurs, such as dromaeosaurids and oviraptors, are typically found singly, with no other nests nearby. Such findings suggest that the species in question lived largely in isolation. In contrast, the nests of sauropods similar to Diplodocus have occasionally been uncovered in groups, or “colonies,” where entire layers of fossils are found filled with clusters of hundreds or thousands of eggs. This may indicate that it was a social animal that lived and mated in large numbers.

Diet

Like its fellow sauropods, including Camarasaurus and Apatosaurus, Diplodocus was an herbivore whose diet consisted of Jurassic-era plants, such as conifers, cycads, and ferns. Although Diplodocus probably did need to eat a lot of food in order to maintain its huge size, scientists also know that as animals get larger, their daily food requirements go down in relation to their overall weight. One estimate is that Diplodocus may have eaten about 41 kilograms (91 pounds) of food per day. If, however, dinosaurs were endothermic (warm-blooded) instead of exothermic (cold-blooded)—a question that has not yet been resolved—this figure would have been substantially higher.

Although Diplodocus used its teeth to bite off vegetation, it could not chew with them, because they were only located in the front part of its jaw. In addition, it had only thin, crayon-like teeth, not the broad specialized grinding teeth most herbivores use to crush tough leaves. Instead of chewing, scientists think that Diplodocus relied on a large digestive tract that broke down the vegetation by chemical processes, possibly like the chemical digestion seen in some large, living ungulate mammals.

Behavior

Despite its tremendous size, scientists believe Diplodocus was probably a gentle animal without many aggressive tendencies. If it were to be attacked by a predator, some speculate it may be likely that it used its long, powerful tail as a kind of whip to defend itself.

There is some evidence that Diplodocus and other large sauropods like Apatosaurus and Camarasaurus were not solitary animals but traveled in herds like modern-day buffalo or elephants. For instance, many so-called “trackways” have been found that seem to show fossil footprints from multiple dinosaurs moving together in a line. In addition, sauropod bones are often discovered in enormous “bone beds,” where the skeletons of dozens of individual dinosaurs are found to be preserved in the same place. Though it is compelling, the hypothesis that Diplodocus and other sauropods were herd animals remains somewhat speculative.

Habitat and Other Life Forms

Diplodocus lived in what is now the Western United States (specimens have been found in New Mexico, Colorado, Utah, Wyoming, and Montana) between 150 and 145 million years ago. At that time, the climate in these areas was warm and humid, resembling the tropical climate of countries around the equator today. The landscape was a mixture of arid, sandy plains and wet, swampy regions. At one time, scientists believed that Diplodocus was largely a swamp-dweller, relying on water to help buoy its tremendous weight. Now, they generally agree that the dinosaur was probably able to travel on dry land.

Fortunately for the herbivorous Diplodocus, the Late Jurassic was a time when plants like cycads, mosses, and conifers were plentiful. Interestingly, though it coexisted with several other sauropods, most notably Camarasaurus , competition for food resources did not lead either species to extinction. Based on differences in the wear observed in their teeth, as well as other anatomical differences, scientists think it is possible that the two creatures either ate different types of plants or browsed for vegetation at different levels.

Research

Over the years since the species was first identified, a lively debate has gone on in the research world over how sauropods like Diplodocus carried their necks and heads. Initially, the skeletal evidence led scientists to believe that, like modern elephants or giraffes, these animals were tree-top grazers that could have fed on leaves at heights of up to 6.3 to 11.6 meters (about 20 to 40 feet) off the ground.

But in the late twentieth and early twenty-first centuries, findings have surfaced (such as one paper claiming that the animal's heart would not have been powerful enough to pump blood up to its head if held so high) suggesting that the animal may have held its neck in a more horizontal position. Later research, based on a careful analysis of sauropod vertebrae compared with those of living mammals with upright neck postures, has swung some scientific opinion back to the earlier theory.

The first complete immature diplodocid skull was found in 2010 and described in 2018. The narrow skull was about twenty-four centimeters long, the smallest found to date, and came from an individual estimated to be two to four years old. Its thirteen teeth included both peg-like teeth in the front and spoon-shaped teeth in the back, which Cary Woodruff and his colleagues posit may have allowed the young animal to eat selectively from tender and tough plants. The skull was found in Montana amid fifteen other diplodocines all apparently aged two to six years, likely an age-segregated herd, which might indicate a lack of parental care for sauropod young.

Teeth were a focus of a great deal of Diplodocus research throughout the 2010s. By counting daily dentin lines and computed the time needed to form new teeth, Michael D'Emic and his colleagues determined Diplodocus replaced teeth frequently—every thirty-five days—perhaps because of browsing low-lying, grittier plant matter. Similarly, through computed tomography (CT) scanning and a 3-D skull model, M. T. Young's team concluded Diplodocus likely engaged in branch stripping or biting, as bark stripping would have been more destructive to its teeth. John Whitlock's comparative study in the Journal of Vertebrate Paleontology found diplodocids likely rotated their jaws when eating, whether through branch-stripping or cropping action. According to Kayleigh Wiersma and Martin Sander, who assessed wear patterns and soft-tissue impressions, sauropods like Diplodocus may have had a beak-like keratin structure to protect their rows of teeth.

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