Dinosaur

Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria. They lived during the Mesozoic Era, from about 225 million years ago to 66 million years ago. Species of dinosaurs ranged from chicken-sized creatures such as the 2-pound (1-kilogram) predator Compsognathus to colossal, herbivorous animals known as sauropods weighing more than 80 tons (72 metric tons). The sauropods were larger than any terrestrial animals that lived before or since.

Some dinosaurs were enormous, awesomely fierce predators, while others were mild-mannered plant eaters. The word dinosaur is derived from two Greek words meaning "terrible lizard." The name comes from the fact that the remains of the earliest dinosaurs discovered were very large and showed they had a lizardlike appearance.

Biology of the dinosaurs

The dinosaurs shared some common physical characteristics, such as the presence of two openings on opposite sides of their skulls and 25 vertebrae. However, the dinosaurs also differed from each other in many important ways. They displayed an enormous range of forms and functions, and they filled a wide array of ecological niches. Some of the dinosaurs were, in fact, quite bizarre in their shape and, undoubtedly, their behavior.

Most species of dinosaurs had a long tail and long neck, but this was not the case for all species. Most of the dinosaurs walked on their four legs, although some species were bipedal, using only their rear legs for locomotion. Their forelegs were greatly reduced in size and probably used only for grasping. The species that walked on four legs were all peaceful herbivores. In contrast, many of the bipedal dinosaurs were fast-running predators.

The teeth of dinosaur species were highly diverse. Many species were exclusively herbivorous, and their teeth were correspondingly adapted for cutting and grinding vegetation. Other dinosaurs were fierce predators, and their teeth were shaped like serrated (notched) knives. These teeth were undoubtedly used to seize and stab their prey, cutting it into smaller pieces that could be swallowed whole.

Until recently, it was widely believed that dinosaurs were rather stupid, slow-moving, cold-blooded creatures. However, some scientists now believe that dinosaurs were intelligent, social, quick-moving, and probably warm-blooded animals. This question is still rather controversial. Scientists have not yet reached agreement as to whether at least some of the dinosaurs were able to regulate their body temperature by producing heat through metabolic reactions.

Evidence for the existence of dinosaurs

Humans have never lived at the same time as dinosaurs on Earth. Yet, a surprising amount is known about these remarkable reptiles. Evidence about the existence and nature of dinosaurs has been obtained from fossilized traces left by these animals in sediment deposits.

Words to Know

Bipedal: Walking on two feet.

Carnivore (carnivorous): Meat-eating.

Embryo: The earliest stage of animal development in the uterus before the animal is considered a fetus.

Extinct: No longer alive on Earth.

Fossil: Evidence of plant or animal life preserved in earth, usually in rocks.

Herbivore (herbivorous): Plant-eating.

Ornithischian dinosaurs: Dinosaurs with birdlike characteristics.

Predator: An animal that eats other animals.

Saurischian dinosaurs: Dinosaurs with reptilelike characteristics.

Sauropods: A group of large saurischian herbivores.

Terrestrial: Relating to the land.

Thecodonts: Early reptiles regarded as ancestors of the dinosaurs.

The first evidence suggesting the existence of dinosaurs was the discovery of traces of their ancient footprints in sedimentary rocks. Sedimentary rocks are formed when sand, silt, clay and other materials are packed together under great pressure. Dinosaurs left their footprints in soft mud as they moved along a marine shore or riverbank. That mud was subsequently covered over as a new layer of sediment accumulated, and later solidified into rock. Under very rare circumstances, this process preserved traces of the footprints of dinosaurs. Interestingly, the footprints were initially attributed to giant birds. They were somewhat similar to tracks made by the largest of the living birds, such as the ostrich and emu.

The first fossilized skeletal remains to be identified as those of giant, extinct reptiles were discovered by miners in western Europe. These first discoveries were initially presumed to be astonishingly gigantic, extinct lizards. However, several naturalists recognized substantial anatomical differences between the fossil bones and those of living reptiles. The first of these finds were bones of a 35- to 50-foot-long (10 to 15 meters) carnivore named Megalosaurus and a large herbivore named Iguanodon. Fossils of both were found in sedimentary rocks in mines in England, Belgium, and France.

Discoveries of fantastic, extinct oversized reptiles in Europe were soon followed by even more exciting finds of dinosaur fossils in North America and elsewhere. These events captured the fascination of both naturalists and the general public. Museums started to develop extraordinary displays of reassembled dinosaur skeletons.

This initial period of discoveries occurred in the late nineteenth and early twentieth centuries. During this period many of the most important finds were made by North American paleontologists (scientists who study fossils). An intense scientific interest grew over these American discoveries of fossilized bones of gargantuan, seemingly preposterous animals. Unfortunately, the excitement and scientific frenzy led to a rather passionate competition among some paleontologists, who wanted to be known for discovering the biggest, or the fiercest, or the weirdest dinosaurs.

Other famous discoveries of fossilized dinosaur bones have been made in the Gobi Desert of eastern Asia. Some of those finds include nests with eggs that contain fossilized embryos (the earliest stage of development). The embryos have been used to study dinosaur development. Some nests contain hatchlings, suggesting that dinosaur parents cared for their young. In addition, the clustering of the nests of some dinosaurs suggests that the animals had led a social life. They may have nested together, for example, for mutual protection against predatory dinosaurs.

By now, fossilized dinosaur bones have been discovered on all continents. Discoveries of fossils in the high Arctic and in Antarctica suggest that the climate there was much warmer when dinosaurs roamed Earth. It also is likely that polar dinosaurs were migratory, traveling to high latitudes to feed and breed during the summer and returning to lower latitudes during the winter.

Although the most important fossil records of dinosaurs involve their bones, other sorts of evidence exist as well. In addition to footprints, eggs, and nests, imprints of dinosaur skin, feces, rounded gizzard stones, and even possible stomach contents have been found. In early 2000, paleontologists announced they had discovered the fossilized heart of a dinosaur that had died some 66 million years ago. Uncovered in South Dakota, the heart was encased in a natural sarcophagus of stone in the chest cavity of a dinosaur's fossil skeleton.

Fossilized plant remains are sometimes associated with deposits of dinosaur fossils. These finds allow scientists to make inferences as to the habitats of these animals. Inferences also can be based on the geological context of the locations of fossils, for example, their nearness to a marine shore, or geographical position, as is the case of polar dinosaurs. All of these types of information have been studied and used to infer the shape, physiology, behavior, and ecological relationships of extinct dinosaurs.

Major groups of dinosaurs

Scientists have only incomplete knowledge of the way in which dinosaurs were related to each other and to other major groups of reptiles. The reason for this fact, of course, is that dinosaurs can be studied only through their fossilized remains. These remains are often rare and fragmentary, especially those that are millions or hundreds of millions of years old. Nevertheless, some dinosaur species bear clear resemblances to each other, while also being obviously distinct from certain other dinosaurs.

The dinosaurs evolved from a group of early reptiles known as thecodonts, which arose during the Permian period (290 million to 250 million years ago) and were dominant throughout the Triassic (250 million to 208 million years ago). It appears that two major groups of dinosaurs evolved from the thecodonts, the ornithischian ("bird hips") dinosaurs and the saurischian ("lizard hips") dinosaurs. These two groups are distinguished largely on the basis of the anatomical structure of their pelvic or hip bones. In general, dinosaurs can be classified as carnivorous (meat-eating) or herbivorous (plant-eating).

Carnivorous dinosaurs. The carnosaurs were a group of saurischian predators that grew large and had enormous hind limbs but tiny fore limbs. Tyrannosaurus rex, perhaps the best known of all dinosaurs, was once considered the largest carnivore that ever stalked Earth's landscape. Its scientific name is derived from Greek words for "absolute ruler lizard." This fearsome predator reached a maximum length of 40 feet (12 meters), and may have weighed as much as 7 to 9 tons (6.5 to 8 metric tons). Tyrannosaurus rex had a massive head and a mouth full of about 60 dagger-shaped, sharp, serrated teeth. Those teeth grew to a length of 6 inches (15 centimeters) and were renewed throughout the life of the animal. This predator probably ran in a lumbering fashion on its powerful hind legs. The hind legs also may have been used as sharp-clawed, kicking weapons. Scientists think that Tyrannosaurus rex may have initially attacked its prey with powerful head-butts and then torn the animal apart with its enormous jaws. Alternatively, Tyrannosaurus rex may have been primarily a scavenger of dead dinosaurs. The relatively tiny fore legs of Tyrannosaurus rex probably only had minor uses. The long and heavy tail of the dinosaur was used as a counterbalance for the animal while it was running and as a stabilizing prop while it was standing.

Tyrannosaurus rex 's distinction as the largest carnivore was taken away in 2000 when a team of scientists announced they had discovered the fossilized bones of a previously unknown dinosaur species that had lived about 100 million years ago. The bones of six of the dinosaurs were unearthed in Patagonia, a barren region on the eastern slopes of the Andes Mountains in South America. The scientists estimated that the needle-nosed, razor-toothed, meat-eating giant measured up to 45 feet (14 meters) in length. Like Tyrannosaurus rex, it had a tail and short front legs, but it was heavier and had slightly shorter back legs. It also was probably more terrifying than Tyrannosaurus rex.

Not all of the dinosaurian predators were enormous. Deinonychus, for example, was a dinosaur that grew to about 10 feet (3 meters) and weighed about 220 pounds (100 kilograms). Deinonychus was one of the so-called "running lizards." These dinosaurs were fast, agile predators that probably hunted in packs. As a result, Deinonychus was probably a fearsome predator of animals much larger than itself. One of Deinonychus's hind claws was enlarged into a sharp, sicklelike, slashing weapon. The claw was probably used to slash and tear apart its victim.

The most infamous small carnivorous dinosaur is Velociraptor, or "swift plunderer." Velociraptor attained a length of about 6 feet (2 meters). Restorations of this fearsome, highly intelligent, pack-hunting "killing machine" were used in the popular movie Jurassic Park.

Herbivorous dinosaurs. The sauropods were a group of large saurischian herbivores that included the world's largest-ever terrestrial animals. This group rumbled along on four enormous, pillarlike, roughly equalsized legs, with a long tail trailing behind. Sauropods also had very long necks, and their heads were relatively small. Their teeth were peglike and were used primarily for grazing rather than for chewing their diet of plant matter. Digestion was probably aided by large stones in an enormous gizzard, in much the same way that modern, seed-eating birds grind their food.

Perhaps the most famous of all sauropods was Apatosaurus, previously known as Brontosaurus. (The Apatosaurus was the first of the two to be discovered, and what was thought to be a different dinosaur, the Brontosaurus, was discovered later. In the 1980s it was discovered that they were really the same dinosaur, so they are all now referred to as Apatosaurus.) Apatosaurus achieved a length of 65 feet (20 meters) and a weight of 30 tons (27 metric tons).

Diplodocus was related to Brontosaurus, but had a much longer body. A remarkably complete skeleton of Diplodocus has been found that is 90 feet (27 meters) long overall, with a 25-foot (8-meter) neck, a 45-foot (14-meter) tail, and an estimated body weight of 11 tons (10 metric tons). Brachiosaurus was an even larger herbivore, with a length as great as 100 feet (30 meters) and an astonishing weight that may have reached 80 tons (73 metric tons).

What became of the dinosaurs?

Many theories have been proposed to explain the extinction of the last of the dinosaurs about 65 million years ago. Some of the more interesting ideas include the intolerance of these animals to rapid climate change, the emergence of new species of plants that were toxic to the herbivorous dinosaurs, an inability to compete successfully with the rapidly evolving mammals, destruction of dinosaur nests and eggs by mammalian predators, and some sort of widespread disease to which dinosaurs were not able to develop immunity. All of these hypotheses are interesting, but the supporting evidence for any of them is not enough to convince most paleontologists that the dinosaurs became extinct for any of these reasons.

Perhaps the most widely accepted theory today is based on the belief that a planet-wide catastrophe resulted in the extinction not only of the dinosaurs but also of hundreds of other species. Scientists have now found evidence that such a catastrophe may have occurred when a large asteroid struck Earth 65 million years ago off the northern tip of the Yucatan Peninsula of Mexico. In such an impact, huge amounts of dust and debris would have been thrown into the atmosphere. Carbonates and sulfate rocks would have also been vaporized, releasing chemicals into the atmosphere that produced sulfur and the greenhouse gas carbon dioxide. The dust and rocks would have blocked out sunlight for an extended period of time, perhaps for years, which would have killed off plants in large numbers. Deprived of plants, choking on carbon dioxide, and suffering showers of caustic sulfuric acid rain, the dinosaurs would have died out.

[See also Evolution; Fossil and fossilization; Geologic time; Paleontology ]

dinosaurs The name ‘dinosaur’ (‘terrible reptile’) was first coined by Sir Richard Owen, a British anatomist, in 1841. He used the term to denote the large reptiles Megalosaurus and Iguanodon that had been collected and described in the 1820s. A considerable amount of work has been done on dinosaurs since then and we now recognize them as a monophyletic (ancestor and all descendants) group of archosaurian reptiles that are closely related to the flying reptiles, the pterosaurs (Fig. 1). Dinosaurs are characterized by having erect limbs and a pelvis that incorporates at least five vertebrae, characters (using that word in its biological sense) that are related to their active terrestrial lifestyle. The earliest dinosaurs are known from the late Middle Triassic of South America and have been studied and described by Paul Sereno from the University of Chicago. Forms such as Herrerasaurus and Staurikosaurus were small bipedal (moving on two legs) carnivores showing a variety of advanced characters that quickly allowed them to gain ascendancy over the then dominant terrestrial vertebrates, the mammal-like reptiles. In these animals the hind leg was elongated and the foot was functionally three-toed and digitigrade, that is, the animal stood on its toes. The acetabulum (hip-socket) was partly open and the upper rim was buttressed to help support the body on the vertical legs. In addition, the two outer fingers were reduced and the thumb had an offset to bring it into opposition with the other fingers, thus providing a grasping hand. During the Late Triassic, the two major groups of dinosaurs developed. They are separated by characteristics of the structure of the pelvis (Fig. 2). In the Saurischia (lizard-hipped) the pubis points forwards, while in the Ornithischia (bird-hipped) the pubis has rotated backwards to lie parallel to the ischium.

Ornithischia

The ornithischian dinosaurs were an extremely diverse group of terrestrial herbivores. Apart from the structure of the pelvis they characteristically had an additional bone at the front of the upper jaw, the predentary, which bore no teeth but acted like a beak for cropping vegetation. Their teeth were rather leaf-like in shape, similar to those of the modern Iguana, and they had ossified tendons running along the back and tail. This group includes such familiar forms as the stegosaurs (plated dinosaurs), ankylosaurs (armoured dinosaurs), ornithopods (bird-footed dinosaurs), and ceratopids (horned dinosaurs) (Fig. 1). They were all large animals, between 3 and 9 m long. Whereas the ornithopods were bipedal the other groups were secondarily quadrupedal.

A number of interesting structural adaptations were developed within the ornithischians. The ornithopods included the hadrosaurs or duck-billed dinosaurs, in which the front of the mouth was enlarged to form a broad flat beak and a variety of bony crests were developed on top of the head. The crests were formed by enlargement of the nasal bones so that the nasal passages extended through them. Although at one time it was thought that the crests might have been an adaptation for storing air while swimming and diving, it is now thought that they provided a resonating chamber for the production of loud calls. Stegosaurs bore a double row of diamond-shaped plates along their back and a pair of spikes at the tip of the tail. Although the spikes were clearly defensive, the plates were probably for thermoregulation, enabling the animal to control its internal temperature. Ankylosaurs were broad, short-legged dinosaurs, covered by a mosaic of defensive bony plates while the tail bore a large bony club. Because of this defensive armour they have been called ‘dinosaurian tanks’. In the ceratopians, posterior skull bones extended back to form a large neck, while most also had horns on the nose and above the eyes. Triceratops is a typical example of these forms. The extension of the skull bones provided additional area for the attachment of jaw muscles, and together with the extensive battery of teeth made these animals extremely efficient at shearing and grinding tough vegetation.

Saurischia

The saurischian dinosaurs include both carnivores, the theropods, and herbivores, the sauropods. The theropods were all bipedal and ranged in length from less than 1 m up to 12 m. The smaller carnivores, such as Coelophysis, had small heads, long necks, long arms with grasping hands, and long hind limbs. They were lightly built and were probably opportunistic feeders, eating insects as well as small vertebrates. At the other end of the scale, forms such as Tyrannosaurus were the largest land predators that have ever lived. They possessed very large heads with serrated, blade-like teeth; the hind limbs were long and powerful, but the forelimbs were very much reduced and in some forms the hand had only two fingers. They were probably adapted as ambush predators of large herbivores and were capable of moving very quickly for short distances. A third group of theropods includes Deinonychus, a medium-sized predator in which one hind claw was greatly enlarged for use as an offensive weapon. It is from this group that the birds are thought to have developed.

The sauropods include the largest land animals that have ever existed; they may have reached 24 m in length. They had small heads with weak teeth at the front of the mouth, long necks and tails, and short barrel-shaped bodies supported on massive vertical limbs. At one time they were considered to have been swamp-dwellers, too massive to have been able to move on land, but further analysis has shown that they were terrestrial animals adapted to browse in high trees. As there were no crushing teeth in the mouth, vegetation must have been swallowed and then crushed in a gizzard similar to that found in many birds. The presence of gastroliths (gizzard stones) in the rib cages of some specimens shows that this view is correct.

Dinosaur metabolism

It was long considered that dinosaurs had an essentially reptilian metabolism; that is, they controlled their internal body temperature by activities such as moving into or out of the sun. This is termed ectothermy and differs from the mammalian system in which control is internal or endothermic. In accordance with this view, dinosaurs were often reconstructed as large sprawling lizards with activity levels similar to those of modern reptiles. In 1970 John Ostrom of Yale and Robert Bakker of Harvard University suggested that dinosaur metabolisms may have been more mammalian, thus initiating an argument that still continues. Bakker pointed out that dinosaurs' skeletons show that they had upright postures and gaits similar to those of modern endotherms. In particular, the combination of long hind limbs and flexible limb joints, together with long rigid tails for balance suggests that bipedal dinosaurs were fast and agile. The bone structure of dinosaurs is also similar to that seen in modern endotherms and unlike that of ectotherms. More recent evidence includes the fact that dinosaurs are found in localities much further north or south than ectotherms inhabit now, again suggesting that they were endothermic. Bakker also suggested that fossil predator–prey ratios supported endothermy in dinosaurs. These ratios are based on the fact that endothermic predators consume more than ectothermic predators in order to support their higher metabolic rates, and hence there are fewer of them in proportion to prey organisms. Although some analyses suggest that dinosaurs showed an endothermic ratio, these have been challenged as unreliable because of problems such as preservational biases. In fact, the case for endothermy in all dinosaurs is still not proved, although it is generally accepted for small carnivorous dinosaurs. Other dinosaurs, particularly large herbivores, would have been more efficient as ectotherms because the temperatures of their large bodies would not have fluctuated easily, and they would have needed less food to support an ectothermic metabolism.

Dinosaur behaviour

Information about the behaviour of dinosaurs has been gleaned from trace fossils, mainly footprints, analysis of various skeletal structures, and the presence of eggs and nests. These suggest that many dinosaurs exhibited group behaviour like that seen in modern large mammals rather than the essentially solitary behaviour observed in modern reptiles. Trackways of dinosaurs commonly show multiple tracks moving in the same direction, indicating herding behaviour. Many finds of dinosaur eggs and nests, particularly those made in Montana by Jack Horner of the Museum of the Rockies in Bozeman, Montana, show that the young must have been cared for by the adults for an extended period, for they hatched out at a stage at which they were not self-sufficient. This indicates co-operative group behaviour of the adults at group nesting grounds on a level similar to that seen in birds, and far beyond that seen in any modern reptiles. Finally, the presence of possible display structures, particularly crests on hadrosaur skulls, suggests sociality among groups of dinosaurs, for such structures would have aided the recognition of potential mates or opponents within a social group.

Dinosaur extinction

Despite their success during the Mesozoic, the dinosaurs became extinct at the end of the Cretaceous. How this happened has become the focus of a highly charged scientific debate. The dinosaurs were only one of the groups that became extinct at the end of the Cretaceous; also hard-hit were the unicellular Foraminifera, the ammonites, and marine reptiles such as the mosasaurs and plesiosaurs. On land the pterosaurs and some mammalian groups became extinct, but many terrestrial organisms were unaffected. This rather selective extinction has been explained by changing climatic conditions, particularly a gradual cooling and drying of the climate during the Late Cretaceous. William Clemens of the University of California at Berkeley has pointed out that in the rare cases where there is a good record of dinosaurs up to the Cretaceous–Tertiary boundary it shows a gradual reduction in diversity that would tend to support the climatic model. However, more recent work by Louis and Walter Alvarez, also from Berkeley, has demonstrated the presence of a clay layer at the boundary in many localities. This contains high levels of elements that are usually rare at the Earth's surface, particularly iridium, together with droplets of shocked quartz which indicate a high-speed impact. They have interpreted this as evidence the Earth was hit by a large extraterrestrial body or bolide, and that the impact generated large amounts of dust, causing global darkness and ultimately extinction of many groups of organisms. Although this appears to be a simple and elegant solution to the problem, it has been criticized for a lack of connection between such a catastrophic event and the selective pattern of extinctions that is seen. No generally accepted scenario has yet been proposed and the debate will certainly continue for some time—although it has been pointed out that as the dinosaurs survived as birds perhaps they did not really become extinct.

David K. Elliott

Bibliography

Lucas, S. G. (2000) Dinosaurs: the textbook. McGraw-Hill, Boston.
Norman, D. (1985) The illustrated encyclopedia of dinosaurs. Crescent Books Ltd, New York.

Dinosaurs

The history of the Dinosauria begins with one of the dinosaur's small extinct ancestors called Petrolacosaurus. Around 270 million years ago, this animal was a member of the group of early land vertebrates called the diapsids that had skulls with two openings behind the eye socket (other evolutionary groups, like fish and amphibians, had one opening or none at all). The diapsids are believed to be the ancestors of the lepidosaurs (modern lizards and snakes) as well as of the archosaurs, the group that led to the dinosaurs. The lepidosaurs achieved great evolutionary success. The extra openings in their skulls led to the interesting structures found in modern snakes, including a light and flexible skull that allows them to catch and eat prey larger than their mouths.

Ancestry: Euparkeria

The first known archosaurs appeared in the Permian Period (319 to 286 million years ago) and they were well on their way to becoming large-sized animals by the early Triassic (about 245 million years ago). When discussing dinosaur ancestry, paleontologists prefer to examine an interesting little archosaur known as Euparkeria. Euparkeria had anatomical characteristics of most archosaurs (and eventually the dinosaurs), including deeply rooted, sharp, serrated teeth; two holes behind the eyes; and a broad space in front of the eye sockets. Their jaws had a distinctive opening that was different in shape and position from other tetrapods, and their spine had small bony plates suggestive of the beginning of armor plating. Perhaps the most important feature of Euparkeria is the arrangement of their hipbones.

As the archosaurs evolved, many species developed hipbones that allowed the angle of their hind limbs to change from a sprawling posture, such as a lizard or crocodile has, to an erect one, like a bird. The limbs came under the body instead of being spread out to the side. This leg position provided a firmer basis of support for a larger and heavier body. Dinosaurs became the largest land animals, and part of their ability to become so large was a direct result of this change in posture. When the legs are spread out to the side they can only support so much weight before the joints that attach them to the body give out. With the legs underneath the body, they form a kind of column that can support a great deal more weight. This means a larger animal can move around without being slow and sluggish or breaking its bones. This change in body posture is one of the main reasons dinosaurs were able to become so large.

One important group of archosaurs was the thecodonts, which included Euparkeria. During the Triassic period, thecodonts continued to evolve and undergo changes in body shape. By the end of the Triassic 213 million years ago), there were two groups of thecodonts, the saurischians and the ornithischians. The saurischians ate both plants and meat. Their pelvis was distinctive in that the three bones that made up the hip—the ilium, the ischium, and the pubis—were joined so that they angled away from each other in a triangular shape. There were two distinct groups of saurischians, the theropods ("beast feet") and the sauropods.

The theropods were carnivorous dinosaurs that walked upright on two feet. Some of the most famous dinosaurs we know today are theropods, including Tyrannosaurus rex, Velociraptor, and other swift and dangerous predators. The other group of saurischians, the sauropods, were herbivores and moved about on four legs. Some sauropods, including Brachiosaurus, Camarasaurus, and Ultrasaurus, were the largest animals that ever lived on Earth. The other evolutionary offshoot of the ancestral thecodonts was the ornithischians. The hip structure of the ornithischians differed from that of the saurischians in that their forward-extending pubis bone was turned toward the back. Because this bone structure also occurs in modern birds, the ornithischians are called bird-hipped dinosaurs. (The ornithischians are not ancestors of birds, they just have a similar hip structure.) The legs, feet, and anklebones of ornithischians were similar to those of the saurischians.

A peculiar adaptation of ornithischians was the beaklike covering of the front of the mouth, which is characteristic of the ceratopsians and duck-billed dinosaurs. Ornithischians also had a complex network of bony rods along their spine which supported the spine. All ornithischians were herbivores. The group contains some of the more distinctive-looking dinosaurs, including Stegosaurus, which had a series of large, triangular, horn-covered bony plates along its back and tail; the heavily armored Ankylosaurus; and the ceratopsians, including Triceratops, with its huge bony hood and horns.

The First True Dinosaurs

The first groups of true dinosaurs, the coelurosaurs, appeared about 210 million years ago in the late Triassic period. These dinosaurs were carnivorous theropods. They were agile and lightly built; most species were smaller than an adult human. Some of the best fossil specimens of the coelurosaur known as Coelophysis were discovered in the United States in New Mexico. Other coelurosaurs were the smaller Ornitholestes, a heavily jawed predator with nostrils that faced upward on its skull, and Coelosaurus perhaps the best-known coelorosaur. All coelorosaur feet had three toes pointing forward and a fourth facing back. The fifth toe was greatly reduced. This pattern persisted throughout the history of all the dinosaurs and is one way to document change within the group.

From the remains of dinosaurs that appear in the fossil record, paleontologists can infer what those animals looked like, how they moved, and what and how they ate. But paleontologists today are also asking: What dinosaur behavior can we infer from fossils? The Maiasaura ("Good Mother" dinosaur) eggs and young found in Montana in 1978 are an excellent example of one way paleontologists can hypothesize dinosaur behavior. In this case, the young dinosaurs at the site were too big to fit in the fossil eggs in nests that were found nearby. Paleontologists think that the parent dinosaurs brought food to the baby dinosaurs and protected them from predators. Also, since many nests were discovered together in a small area, the scientists think that these dinosaurs many have lived together in some sort of herd.

The thinking is that if the baby dinosaurs were too big to fit in the eggs, but were still in the nest they probably remained in nest for some time after hatching. They would have to had food brought to them by the parents until they were large enough to forage for food on their own. Many birds care for their young this way by bringing them food until the fledglings are old enough to feed and fly on their own. This is another piece of evidence that links birds and dinosaurs.

At the end of the Cretaceous era (around 64 million years ago), something happened that caused the remaining dinosaurs to die out. No one knows for sure what the event was. Many scientists believe an asteroid hit Earth, causing harsh atmospheric conditions that led to the dinosaurs' extinction. However, very few species of dinosaurs remained by the end of the Cretaceous. Scientists will probably never know what actually happened to the dinosaurs. Most think they are still here as birds. This is not difficult to imagine when you look closely at the characteristics of birds. It is comforting to imagine that these fantastic and lively creatures are still among us, only much smaller.

Recent discoveries of many species of feathered dinosaurs in China have really supported this idea. Most scientists know agree that birds are very closely related to dinosaurs, if not actually dinosaurs themselves. The so-called "missing links" between dinosaurs and birds have been found.

Leslie Hutchinson

Bibliography

Carroll, R. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company, 1988.

Czerkas, S., and E. Olsen. Dinosaurs: Past and Present, vols. I and II. Los Angeles: Natural History Museum of Los Angeles County, 1987.

Farlow, J. O., and M. K. Brett-Surman, eds. The Complete Dinosaur. Indiana University Press, 1997.

Horner, John R. Digging Dinosaurs. New York: Workman Publishing, 1988.

Norman, D. Encyclopedia of Dinosaurs. New York: Crescent Books, 1985.

Sloan, Christopher. Feathered Dinosaurs. Washington, D.C.: National Geographic, 2000.

dinosaur [Gr., = terrible lizard], extinct land reptile of the Mesozoic era . The dinosaurs, which were egg-laying animals, ranged in length from 2 1/2 ft (91 cm) to about 127 ft (39 m). Recognized discoveries of fossilized dinosaur bones date only to the 1820s; Sir Richard Owen, a Victorian anatomist, coined the term dinosaur.

Dinosaur Traits and Classification

Fossil remains of dinosaurs have been found in rock strata of every continent, indicating that they differed widely in structure, habitat, and diet. Their brain sizes varied, with some predators having brain-to-body ratios equivalent to those of some modern birds and animals. Many species built nests. Many theories regarding dinosaurs and their behavior are hotly debated by the experts. These include the debate over the grouping of birds with dinosaurs, the question of whether nonavian dinosaurs were cold-blooded (ectothermic) or warm-blooded (endothermic), the question of whether dinosaurs protected and nurtured their young in the nest after hatching or whether the young were mobile and self-sufficient at birth, and the reason for the disappearance of nonavian dinosaurs.

No complete fossil dinosaur has ever been discovered. Inferences must be made from fragments or pieces that have been compressed and distorted. Information about the diet has been gleaned from stomach contents and coprolites (fossilized dinosaur feces) and by comparing the teeth to those of living animals, for example, relating the large grinding teeth of hadrosaurs to those of living herbivores. Fossilized dinosaur footprints, such as the trackways found at Davenport Ranch in Texas, have been interpreted as evidence that dinosaurs traveled in herds. What is known about dinosaurs is that, far from being evolutionary failures, they dominated their habitats for most of their 160 million years of existence (the human species Homo sapiens has existed for approximately 150,000-200,000 years).

Although all dinosaurs were originally classified in a single order, it was later discovered that the group contained two distinct types distinguished by structural differences. The pelvis in the saurischian (lizard-hipped) dinosaurs resembles that of still-extant reptiles, but in the ornithischian (bird-hipped) dinosaurs the pubic bone of the pelvis has forward and backward extensions that resemble those found in birds. It was later determined, however, that the backward-tilting hips of ornithischian dinosaurs and birds were the result of convergent evolution and not inheritance. Many other shared characteristics have been noted between birds and saurischians, and it is now believed by many paleontologists that modern birds are in fact extant dinosaurs of the saurischian order.

The jaws and teeth of the two dinosaur orders also differ. The saurischian order, which includes both herbivores and carnivores, has teeth around the entire jaw or confined to the front of the mouth. Ornithischians have "cheek teeth" along the sides of the jaw, but never in the front; the bones at the front of the mouth sometimes developed into the horny beaks typical of modern turtles. All known ornithischians were herbivores.

Dinosaurs are further classified into some common groupings. In the saurischian dinosaurs, some were theropods [Gr., = beast feet], a group sharing hind feet with only three functional toes (e.g., the carnivorous bipeds Tyrannosaurus , Velociraptor , Deinonychus , and possibly the living birds); others were sauropods [Gr., = lizard feet] with small heads and long necks (e.g., the herbivorous quadrupeds Apatosaurus [Brontosaurus] and Diplodocus ). Among the ornithischians, there were ornithopods (bird-footed dinosaurs), such as Iguanodon ; thyreophorans (armored dinosaurs), such as Stegosaurus and Ankylosaurus ; and ceratopsians (horned dinosaurs), such as Triceratops . The total number of dinosaur genera that existed is unknown; new species are discovered every year, but some species, on further examination, are found to be redundant with earlier finds. One estimate of the possible number of distinct genera exceeds 1,800.

Similarities of dinosaurs found on what are now different continents have given scientists clues to the breakup of the supercontinent Pangaea, which began about 170 million years ago. For example, the discovery of a 130-million-year-old African dinosaur similar to the North American Allosaurus suggests that the African plate was connected to the northern continents (Laurasia) longer than had been believed previously.

The Extinction of the Dinosaurs

Many explanations have been offered for the worldwide extinction of the dinosaurs after 160 million years of existence. The most popular theory is that one or more asteroids or comets hit the earth, lifting massive amounts of debris and sulfur in the air and blocking the sunlight from reaching the earth's surface. The 1991 discovery of the Chicxulub crater on the Yucatán peninsula in Mexico lent support to this idea. The second currently popular theory is that the extinctions followed the huge volcanic eruptions that created the lava flows of the Deccan Traps in what is now India. (See mass extinction for more information.) No theory perfectly describes why dinosaurs, pterosaurs, and many marine organisms were affected by the extinction, when many mammals and other animals (e.g., turtles and crocodiles) survived.

Bibliography

See R. Bakker, The Dinosaur Heresies (1986); D. Lambert, The Ultimate Dinosaur Book (1993); D. Lessem and D. Glut, The Dinosaur Encyclopedia (1993); M. A. Norell et al., Discovering Dinosaurs in the American Museum of Natural History (1995); J. R. Horner, Dinosaur Lives (1997); P. Taquet, Dinosaur Impressions (1994; tr. by K. Padian, 1998); D. B. Weishampel et al., ed., The Dinosauria (2d ed. 2004).

dinosaur Any of a large number of reptiles that lived during the Mesozoic era, between 225 and 65 million years ago. They appeared during the Triassic period, survived the Jurassic, and became extinct at the end of the Cretaceous. There were two orders: Saurischia (‘lizard hips’), included the bipedal carnivores and the giant herbivores; the Ornithiscia (‘bird hips’) were smaller herbivores. Their posture, with limbs vertically beneath the body, distinguish them from other reptiles. Many theories are advanced to account for their extinction. It is possible that, as the climate changed, they were incapable of swift adaptation. A more catastrophic theory is that they died because of the devastating atmospheric effects from the impact of a large meteor. See also apatosaur; Diplodocus; tyrannosaurus

http://www.enchantedlearning.com/subjects/dinosaurs; http://www.nhm.ac.uk; http://www.nmnh.si.edu/paleo/dino

dinosaur An extinct terrestrial reptile belonging to a group that constituted the dominant land animals of the Jurassic and Cretaceous periods, 190–65 million years ago. There were two orders. The Ornithischia were typically quadrupedal herbivores, many with heavily armoured bodies, and included Stegosaurus, Triceratops, and Iguanodon. They were all characterized by birdlike pelvic girdles. The Saurischia included many bipedal carnivorous forms, such as Tyrannosaurus (the largest known carnivore), and some quadrupedal herbivorous forms, such as Apatosaurus (Brontosaurus) and Diplodocus. They all had lizard-like pelvic girdles. Many of the herbivorous dinosaurs were amphibious or semiaquatic.

dinosaur a fossil reptile of the Mesozoic era, often reaching an enormous size; in extended usage, a person or thing that is outdated or has become obsolete because of failure to adapt to changing circumstances.

Dinosaurs were all extinct by the end of the Cretaceous period (65 million years ago), the most popular theory being that the extinctions were in fact the result of the impact of a large meteorite.

The word is recorded from the mid 19th century, and comes from modern Latin dinosaurus from Greek deinos ‘terrible’ + sauros ‘lizard’. It was coined by the English anatomist and palaeontologist Richard Owen (1804–92).

dinosaurs Literally the name means ‘terrible lizards’, but in fact the dinosaurs were not lizards. They were diapsid reptiles whose closest living relatives are the crocodilians and birds. Dinosaurs first appeared in the Middle Jurassic and produced an astonishing array of different types and sizes before becoming extinct at the end of the Cretaceous. The two groups of dinosaurs, Saurischia and Ornithischia, are not usually thought to be more closely related to each other than to other archosaurs, so the concept of ‘dinosaur’ is a heterogeneous one.

di·no·saur / ˈdīnəˌsôr/ • n. 1. a fossil reptile of the Mesozoic era, often reaching an enormous size. 2. a person or thing that is outdated or has become obsolete because of failure to adapt to changing circumstances. DERIVATIVES: di·no·sau·ri·an / ˌdīnəˈsôrēən/ adj. & n. ORIGIN: mid 19th cent.: from modern Latin dinosaurus, from Greek deinos ‘terrible’ + sauros ‘lizard.’

dinosaur •Nassau • hacksaw • heartsore •bedsore • Ensor • fretsaw • chainsaw •Esau, seesaw •jigsaw •ripsaw, whipsaw •eyesore • Warsaw • bowsaw •footsore • Luxor • plesiosaur •stegosaur • Arkansas • Chickasaw •dinosaur • brontosaur