Dinosaurs

The word “dinosaur,” which is derived from the Greek term for “terrible lizard,” is the popular name for a group of land-dwelling reptiles active for millions of years. They were the dominant vertebrate animals during most of the Mesozoic era, which began 225 million years ago and ended 65 million years ago. Although dinosaurs were long considered extinct, and indeed most species were wiped out after the extinction event marking the Cretaceous-Palogene boundary, modern research has indicated that birds are, in fact, living examples of dinosaurs. Among the dinosaurs were the largest animals that ever walked the earth, although some of the earliest dinosaurs were very small.

The Mesozoic era is divided into three periods, Triassic, Jurassic, and Cretaceous, of approximately equal length. Dinosaurs first appeared in the later third of the Triassic period. Experts believe that dinosaurs developed from a group of archosauromorph reptiles, such as Marasuchus, which was a lightly built flesh eater about 1.3 meters long. It was clearly a biped, running on its hind legs, and the long tail was presumably used as a balancing organ.

Dinosaurs are divided into two separate orders, depending on the arrangement and shape of the hip bones, which determine the way an animal walks and holds its body. The saurischians, or “reptile hips,” as they are commonly called, arose in the early part of the Late Triassic; the ornithischians, or “bird hips,” arose toward the end of the Triassic period.

Saurischians

The earliest dinosaurs were saurischians, which are best known from the Ischigualasto Formation of Argentina. The order Saurischia may be divided into two major suborders: the theropods, or “beast-footed dinosaurs,” and the sauropods, or “reptile-footed dinosaurs.” The theropods, which were more primitive than the sauropods, were primarily bipedal, although many of them probably used all four feet when walking or resting. The hind legs were strong and bore birdlike feet, while the forelimbs bore sharp, curved claws for seizing and holding prey. All theropods had long tails that functioned as stabilizers. The head was large, and the jaws of most of the theropods contained sharp teeth.

The theropods are divided into two major groups. A basal group, the ceratosaurs, includes such dinosaurs as Coelophysis, a small, agile carnivore with a long, narrow skull represented by many hundreds of specimens from the Ghost Ranch Quarry in New Mexico. However, larger dinosaurs, such as Ceratosaurus, are included within this group. The remaining theropods, termed tetanurans, include the Maniraptori formes, which share many advanced characteristics with birds. The largest of these is Tyrannosaurus rex, from the Late Cretaceous period of North America, which grew to a weight of 4,500 kilograms, a height of 6 meters, and a length of 15 meters. It was theropod dinosaurs that evolved feathers and gradually decreased in size over about 50 million years to give rise to birds, including the early species Archaeopteryx. Scientists first speculated about the connection between birds and dinosaurs as far back as the mid-1800s, but it was only in the 1990s and beyond that most researchers concluded that birds were, indeed, surviving dinosaurs rather than a separate classification.

The sauropods, which appeared slightly later in the Triassic than the theropods, have come to stand as a symbol of gigantism in land animals. They were all quadrupeds and vegetarians. They had small skulls, long necks and tails, large barrel-shaped bodies, padded feet, and large claws on the innermost toe of the forefoot and the innermost toe of the hind foot. The ancestral stock of the sauropods were the prosauropods, which were much smaller than the sauropods. Like most prosauropods, Plateosaurus had blunt, spatulate teeth, was an herbivore, and was quadrupedal, although it was capable of bipedal posture and gait.

The later sauropods had longer necks, and their skulls were relatively small. The limb bones became solid and pillarlike to support their great weight. This category contained many extremely large dinosaurs, including Brachiosaurus, which is estimated to have weighed 73,000 kilograms. Among the best-known sauropods are Brontosaurus and Diplodocus, from the Late Jurassic period of North America. The group known as titanosaurs were even larger than the brachiosaurids; Argentinosaurus is often considered both the heaviest and longest land animal known, although the fragmentary nature of fossil evidence means that measures are only estimates and many other contenders have been suggested. Although it was once assumed that these huge beasts had to live in swamps where the water could support their great weight, it is now clear that they were terrestrial animals that used their long necks to eat from trees.

Ornithischians

The sauropods reached their zenith in the Late Jurassic; the ornithischians replaced them as the dominant herbivores in the Cretaceous period. The expansion of this group was associated with the advent of the flowering plants during the Cretaceous period. Characteristically, a horny beak was developed at the front of the mouth, and the toes ended in rounded or blunt hooves instead of claws.

The earliest ornithischians were the ornithopods. A typical example is Hypsilophodon, a small, swift dinosaur with a long, slender tail and long, flexible toes. The most specialized of the ornithopods were the “duck-billed dinosaurs,” also known as hadrosaurs. Although they had flat beaks and no anterior teeth, the cheek region had rows of grinding teeth. The various types of duck-billed dinosaur can be distinguished by modifications of the bones associated with the nostrils. Some were molded into hollow, domelike crests, bizarre swellings of the nasal region, or long, projecting tubular structures that were possibly used to warm the air or to produce sounds. The remaining three groups of ornithischians presumably evolved from the primitive ornithopods.

The earliest of these three groups of highly specialized quadrupeds was the “plated dinosaurs,” or stegosaurs, which first appeared early in the Jurassic period. This large dinosaur was more than 6 meters long. In comparison to its body size, its head was extremely small. Stegosaurus had an average of twenty plates arranged alternately in two parallel rows down the back. The plates were originally thought to have been used for protection, but scientists now believe that the plates could have been used for thermoregulation. Stegosaurus died out in the Early Cretaceous period.

The “armored dinosaurs,” or ankylosaurs, are not very well known, even though their remains have been found over much of the world. Their armor consisted of a mosaic of studs over the body, spikes that protected the legs, and, in some cases, spikes on the tail. They protected themselves by crouching and drawing in their head and legs.

Some of the last dinosaurs to develop were the “horned dinosaurs,” or ceratopsians. The skull was characterized by a beaked snout and a bony frill that extended from the back of the head. The ceratopsians were also distinguished from others by various patterns of horns. The skull of Triceratops, for example, had three sharp horns, one on the snout and one above each eye. The best known of the small ceratopsians was Protoceratops, which was a small, hornless dinosaur from the Gobi Desert in Mongolia.

Studies of dinosaur eggs, nests, trackways, and bone structures have shown that smaller dinosaurs probably had a warm-blooded, or endothermic, metabolism similar to mammals. This is supported by the discovery of small theropods in China that show a covering of feathers, presumably for insulation in cold environments that could not support cold-blooded animals. Large dinosaurs, such as sauropods, may have been more efficient as ectotherms, similar to most modern reptiles. Although research has increasingly supported the theory that dinosaurs were active and endothermic rather than sluggish creatures as originally imagined, there is still much debate over the issue, and it remains an active area of study. A new theory emerged in 2024 regarding the debate over when warm-blooded dinosaurs first roamed the planet. Research revealed that the emergence of the first warm-blooded dinosaurs on Earth was likely earlier than previously reported—approximately 180 million years ago during the Early Jurassic period—which led to new information about dinosaurs' habits, including their distribution across the earth.

Extinction Theories

Several theories regarding the dinosaurs’ extinction were first proposed in the late nineteenth and the early twentieth centuries. According to one popular theory, dinosaurs were wiped out because early mammals of the Cretaceous period ate their eggs. Yet the eggs of many modern reptiles have faced the same threat and have survived, primarily because reptiles lay so many eggs. Another theory suggested that the same animals ate the plants on which the dinosaurs depended. Although that is possible, virtual plagues of mammals would have been required to eradicate the dinosaurs. Some early scientists also believed that the dinosaurs became too big for their environment; that is unlikely, however, because gigantic dinosaurs had been successful for millions of years and small dinosaurs existed as well. Changes in the physical environment also occurred in the Late Mesozoic. Evidence indicates that the sea levels fell. Geologic evidence shows, though, that drastic environmental changes had occurred many times during the dinosaurs’ reign without any sudden effect as detrimental as a total mass extinction.

A theory proposed in early 1979 by Luis Alvarez and Walter Alvarez suggests that the iridium that has been found in several samples of sedimentary layers between the rock of the Cretaceous and Tertiary periods came from an asteroid that struck Earth at that time. Such a catastrophic event could have caused an enormous cloud of dust to circle Earth and cut off the sunlight, destroying the plants and the dinosaurs that depended on them. While this theory fails to explain why so many other animals, such as the mammals, managed to survive, it has long been the most generally accepted idea, sometimes in combination with other factors.

Another modern theory places the blame on the greenhouse effect. It has been argued that the reduction of the seas that occurred during the Cretaceous period caused a reduction of marine plants. As a result, the amount of carbon dioxide in the air increased, trapping heat from Earth’s surface. A similar theory suggests that the eruption of a tremendous volcano produced a fatal amount of carbon dioxide. Neither theory, however, explains why other animals, especially heat-sensitive reptiles, survived.

The main alternative to the extraterrestrial catastrophist explanation is a gradual ecosystem change model. Declines in many groups of organisms that started well before the Cretaceous-Tertiary boundary are seen as being caused by long-term climatic change, as lush tropical environments were replaced by strongly seasonal, temperate climates. The best explanation for the extinction of most dinosaur species may be a combination of the two main theories. However, as scientists in the late twentieth and early twenty-first centuries increasingly came to recognize the connection between dinosaurs and birds, a new consensus began to emerge that dinosaurs, in fact, never went fully extinct. Instead, the theropods lived on as birds, surviving together with mammals and other species through whatever catastrophes or gradual changes wiped out the rest of the dinosaurs.

Study of Dinosaurs

Scientists study dinosaurs mainly by examining fossils, which are animal remains that have turned to stone. If a dinosaur died near a river or in a swamp, it stood a fairly strong chance of being preserved. Its body might sink into the mud, or floodwaters might float it downstream, where it would end up on a sandbar, on the bottom of a lake, or even in the sea. After the flesh decayed, the bones would be covered by sediments, such as mud or sand. The weight of accumulated layers of sediment would slowly compress the remains and turn them into rock: mud into shale, sand into sandstone, limey oozes into limestone or chalk.

The way a fossil is studied is determined by the category to which it belongs. The first category is petrified fossils. They may be preserved in two ways. In replacement, minerals replace the original substance of the animal after water has dissolved the soft body parts. In permineralization, minerals fill in the small air spaces in bones or shells, thereby preserving the original bone or shell. The second group of fossils is composed of natural molds that form when the bodies dissolve. Scientists make artificial casts of these molds by filling them with wax, plastic, or plaster. The third type is prints, which are molds of thin objects, such as feathers or tracks. Sometimes, even skin is preserved. Prints are formed when the soft mud in which they are made turns to stone. Scientists can determine the length and weight of the dinosaur that made a set of footprints by studying the depth, size, and distance between them.

Most fossils are found in sedimentary rocks, which lie beneath three-fourths of the earth. The best collecting areas are places where the soil has worn away from the rocks. Areas in Colorado, Montana, Wyoming, and Alberta, Canada, have been especially rich in fossils. Most of the finds consist of no more than scraps of limb bones, odd vertebrae, loose teeth, or weathered lumps of rock with broken bone showing on the surface. Once a scientist has discovered a few fossilized fragments, he or she combs the area to find the rest of the animal. If the skeleton is embedded, it is extracted with the help of a wide variety of tools, ranging from picks and shovels to pneumatic drills. Loose fragments are glued back into place, and parts that are too soft or breakable are hardened by means of a special resin solution that is sprayed or painted on.

As the fossil is uncovered, it is encased in a block of plaster of paris. (A more modern method uses polyurethane foam instead of plaster.) After the entire surface is covered, the fossil is rolled over, and another layer of plaster is added. After the fossil has been transported to the museum, the plaster is removed. The “development” stage involves the removal of the rock around the bones. The oldest way is by hand, using tools, such as hammers and chisels; a more modern technique uses electrically powered drills similar to dentists’ drills. Sandblasting and chemicals may also be employed. After the fossil is cleaned, it is ready for mounting. The bones are fastened to a steel framework that makes the skeleton appear to stand by itself.

The early 2020s were remarkably successful in terms of dinosaur discoveries. Among these discoveries, scientists identified the first dinosaur built to swim, the Natovenator. Found in modern-day Mongolia, these semiaquatic hunters were small, similar to a duck, and ate small fish and other aquatic creatures. Additionally, a new species that looked like the T. rex was discovered—Meraxes. Discoveries such as these continue to improve the bank of knowledge surrounding these extinct creatures, continually spurring the development of new theories while disproving previously held beliefs.

Life-Earth Interaction

From the dinosaurs, scientists continually learn new lessons about the physiology of such beasts, their relationship to the world in which they lived, their distribution and the bearing of that distribution on the past arrangements of the continents, various aspects of evolution, and the reasons that they became extinct. The dinosaurs played a major part in the shaping of the natural world and, in turn, have shaped human understanding of natural processes. For example, the fossil evidence of the intermediary species Archaeopteryx, a primitive bird that lived during the Late Jurassic period and had a beak containing teeth, provided a powerful example of evolution. Eventually, the direct transition from dinosaurs to birds would be recognized.

The disappearance of a species that seemed to rule the world for more than 100 million years brings into question the notion of a “dominant” species. Most people believe that mammals are now the dominant form of life; however, dinosaurs did not truly “rule,” and neither do mammals. If one were to list the biological organisms whose influence on the planet is such that their removal would produce chaos, then that list would be headed by microorganisms so small that they can be seen only through powerful microscopes. The list would also include the green plants and the fungi. Studying dinosaurs can bring humanity's humble place in the world into perspective.

The extinction of the dinosaurs also brings into question the ability of humans to destroy the world. All species, from the simplest microorganism to the largest plant or animal, modify their immediate surroundings. They cannot avoid doing so. The success of one group, however, does not imply the failure of the groups it exploits. The complexity of individual organisms may increase, but the simpler forms do not necessarily disappear. Life continued after the demise of the dinosaurs and would probably continue to do so if humankind were destroyed.

Principal Terms

Ankylosaurs: A group of later ornithischians characterized by heavy armor

Cerotopsians: A group of later ornithischians characterized by a beaked snout and a bony frill on the back of the head

Ornithischians: One of the two orders of dinosaurs; it comprises the “bird-hipped” dinosaurs

Ornithopods: The early, bipedal ornithischians

Saurischians: One of the two orders of dinosaurs; it comprises the “reptile-hipped” dinosaurs

Sauropods: The herbivorous, quadrupedal saurischians

Stegosaurs: A group of later ornithischians characterized by a row of plates down the back

Thecodonts: An order of Triassic reptiles that were the ancestors of dinosaurs, birds, and crocodiles

Theropods: The carnivorous, primarily bipedal saurischians

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