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Dinosaurs – Dinozorlar
114 okunma

Dinosaurs – Dinozorlar

09 Ekim 2015 08:43
Dinosaurs – Dinozorlar



Dinosaur is the name of a group of prehistoric reptiles that ruled the earth for about 160 million years. These animals died out millions of years ago, but they have fascinated people ever since they were first described in the early 1800’s. The name dinosaur comes from the term Dinosauria, which means terrible lizards. But dinosaurs were not lizards, only distantly related to them, and most were not very terrible.
Some of the best-known dinosaurs were terrifying, however. Many were of enormous size. Some dinosaurs towered above and weighed more than any other animal ever to live on land. The largest dinosaurs may have grown as long as 150 feet (45 meters) and weighed as much as 85 short tons (77 metric tons). Such giants would have been more than 10 times as heavy as a full-grown elephant. The only animals that grow to this size today are a few kinds of whales, and they live only in the water. Size was not the only characteristic that made some dinosaurs terrifying. Many large dinosaurs were fierce and deadly meat-eaters.
DINOSAURSThe first dinosaurs appeared on the earth about 230 million years ago. They lived in nearly all natural settings, from open plains to forests to the edges of swamps, lakes, and oceans. Then about 65 million years ago, the dinosaurs died out.
Dinosaurs varied greatly in how big they grew, how they looked, and where they lived. Some of the most famous were such gigantic animals as Apatosaurus «uh pat uh SAWR uhs», Diplodocus «duh PLAHD uh kuhs», and Tyrannosaurus «tih ran uh SAWR uhs». Apatosaurus (formerly called Brontosaurus) grew about 70 feet (21 meters) long. Diplodocus reached an even greater length—about 90 feet (27 meters). Both Apatosaurus and Diplodocus were plant-eaters. Each had a small head and an extremely long neck and tail. Tyrannosaurus was a fierce meat-eater. It stood almost 12 feet (3.7 meters) tall at the hips and had an enormous head and long, pointed teeth. But not all dinosaurs were giants. The smallest kinds were approximately the size of a chicken.
In certain ways, dinosaurs were like many modern reptiles. For example, some dinosaurs had teeth and skin much like those of alligators living today. Many were probably about as intelligent as crocodiles and alligators. However, dinosaurs also differed from present-day reptiles in many ways. For example, no modern reptiles grow as large as the biggest dinosaurs. In addition, many kinds of dinosaurs were bipedal—that is, they walked on their hind legs. Dinosaurs also had a different kind of leg posture. Lizards, turtles, and most other modern reptiles hold their legs out to the sides of their body in a low, sprawling posture. But dinosaurs held their legs under their body, much like those of a bird, a horse, a dog, or a person. This upright posture enabled dinosaurs to walk on all four legs without dragging their bellies on the ground.
Dinosaurs lived during most of the Mesozoic Era. This period in the earth’s history lasted from about 248 million to 65 million years ago. The Mesozoic is sometimes called the Age of Reptiles or Age of Dinosaurs because dinosaurs and other reptiles were the largest animals during that time. Dinosaurs belonged to a group of closely related animals called archosaurs «AHR kuh sawrs» (meaning ruling reptiles). However, not all archosaurs were dinosaurs. Other well-known members of this group included crocodilians (alligators and related animals) andpterosaurs «TEHR uh sawrs» (meaning winged reptiles). By about 65 million years ago, nearly all archosaurs and many other creatures had died out, and the Mesozoic Era came to an end. Scientists do not know why dinosaurs died out. For many years, scientists thought that dinosaurs had left no descendants (offspring). But since the 1960’s, dinosaur research has indicated that birds descended from particular kinds of small, meat-eating dinosaurs. This research has led many scientists to classify birds as living dinosaurs.
Scientists learn about dinosaurs by studying their fossils, which include preserved dinosaur bones, teeth, eggs, nests, tracks, skin imprints, andwaste material. Scientists also study living animals that resemble dinosaurs in some ways.

The world of the dinosaurs
When dinosaurs lived, the earth was much different than it is today. For example, the Alps, the Himalaya, and many other surface features had not yet formed. The first flowering plants did not appear until late in the Mesozoic Era. Mammals, which evolved at about the same time as dinosaurs, were extremely small during the Mesozoic. In addition, many plants and animals that are now extinct or rare were common then.
Land and climate. Scientists believe the earth’s continents have not always been arranged as they are today. About 250 million years ago, theyformed a single land mass surrounded by an enormous sea. During the Mesozoic Era, this land mass began to break apart to form the continents and oceans that we know today. The continents slowly drifted away from each other toward their present locations.
As the continents moved, their surface features and climate changed. For a time, huge, shallow seas covered portions of North America, Europe, Africa, and Asia. Thick forests bordered drier plains, and swamps and deltas lined the seacoasts. Later in the Mesozoic, the seas drained from the continents and the Rocky Mountains began to form.
Throughout the Mesozoic, dinosaurs lived in a climate that was milder and less changeable than the climate today. Areas near the seas andalong rivers and lakes may have had mild, moist weather all year. Inland regions were drier and, in some cases, desertlike. Toward the end of the Age of Reptiles, the climate grew cooler and drier and the change of seasons became more distinct.
Plant and animal life also changed during the Mesozoic Era. During the first half of the era, primitive forms of conifers (cone-bearing trees), cycads (palmlike trees), and ginkgoes were among the most common plants. Other plant life included ferns, giant horsetails, and mosses. Landanimals, in addition to dinosaurs, included crocodilians, frogs, insects, lizards, turtles, and a few kinds of small mammals. Many reptiles lived in the seas, including ichthyosaurs «IHK thee uh sawrs», which resembled porpoises, and plesiosaurs «PLEE see uh sawrs», which had long necks. Other Mesozoic sea creatures included clams, corals, jellyfish, snails, sponges, squids, and starfish, as well as sharks and many other primitive varieties of fish. Flying pterosaurs dominated the skies.
During the second half of the Mesozoic Era, the first flowering plants began to appear. Forest trees included the first modern conifers as well as primitive magnolias, oaks, palms, and willows. Birds had evolved from small meat-eating dinosaurs, and the first snakes appeared. The dominant sea animals were gigantic lizards called mosasaurs «MOH suh sawrs». These animals had flippers instead of legs. Other sea creatures included huge turtles and the first modern bony fish.
Kinds of dinosaurs
Dinosaurs lived throughout most of the Mesozoic Era, which is divided into three periods—Triassic, Jurassic, and Cretaceous. The Triassic Period lasted from about 248 million to 213 million years ago. The Jurassic lasted from about 213 million to 145 million years ago, and the Cretaceous from about 145 million to 65 million years ago.
dinosaurs Dinosaur ancestors. Dinosaurs probably evolved from the same ancestors as crocodilians and other archosaurs. Scientists believe the ancestors were small, meat-eating reptiles. These agile predators hunted prey by chasing after it on their hind legs and sometimes on all four legs. Scientists have discovered fossils of several kinds of archosaurs that lived at the beginning of the Mesozoic.
One possible dinosaur ancestor is Lagosuchus «luh GAHS uh kuhs», whose name means rabbit crocodile. Lagosuchus was a small reptilefrom the Triassic Period whose fossils have been found in what is now Argentina. It was one of the most active predators of its time, and it had many of the characteristics of fully developed dinosaurs. For example, its long neck curved in an S-shape, a feature typical of many dinosaurs. True dinosaurs probably evolved from Lagosuchus and other early archosaurs.
True dinosaurs. Scientists divide true dinosaurs into two major groups: (1) ornithischians «awr nuh THIHS kee uhns» and (2) saurischians «saw RIHS kee uhns». The two groups differed mostly in the structure of their hips and other skeletal features. Ornithischians, whose name means bird-hipped, had a birdlike hip structure. Saurischians, whose name means lizard-hipped, had a hip formation much like that of lizards. Both groups consisted of several recognizable kinds of dinosaurs.
Dinosaur hips Ornithischians were plant-eaters. They had a beaklike bone in front of their lower jaw, and many had bony plates in their skin. During the Cretaceous Period, ornithischians became the most important plant-eating dinosaurs. There were five basic kinds of ornithischians: (1) stegosaurs, (2) ankylosaurs, (3) ornithopods, (4) pachycephalosaurs, and (5) ceratopsians. Each group included many different species.
Stegosaurs «STEHG uh sawrs» were large plant-eaters with huge, upright bony plates or spines along the back. They lived from the middle of the Jurassic to the middle of the Cretaceous. One of the best-known stegosaurs is Stegosaurus, which lived in what is now North America. Other stegosaurs lived in Africa, Europe, India, and China. Stegosaurs walked on four legs. The largest stegosaurs measured about 30 feet (9 meters) long and about 6 feet (1.8 meters) tall at the hips. They had a small head and a short neck. Stegosaurs’ front legs were much shorter than their back ones. Because of the difference in leg length, stegosaurs walked with their head close to the ground, making them look bent over.
Some stegosaurs had two rows of spines along their back, while others had two rows of stiff, vertical plates. The tail was armed with pairs of bony spikes. Some scientists believe that the plates and spikes may have helped protect the animals from enemies. Other scientists also suggest that the plates helped control the animal’s body temperature. According to this theory, overheated blood was pumped through the thin plates and returned to the rest of the body. Air moving around the stegosaur’s back would have cooled the blood as it flowed through the plates. The plates could also have warmed the blood by absorbing heat from the sun.
Ankylosaurs «AHNG kuh luh sawrs» were the most heavily armored of all dinosaurs. They were low, broad animals and walked on four legs. Most kinds of ankylosaurs grew 15 to 30 feet (5 to 9 meters) long and had a skull 2.5 feet (80 centimeters) long. Heavy, bony plates covered thebody and head of most ankylosaurs. Many of the plates had ridges or spikes. In some ankylosaurs, large spikes also grew at the shoulders or at the back of the head. Some kinds of ankylosaurs had a large mass of bone at the end of the tail. This bone could be used as a powerful club against enemies.
Ankylosaurs lived in many parts of the world from the middle Jurassic to the end of the Cretaceous. These tanklike animals were some of the most successful plant-eating dinosaurs. They most frequently ate the leaves of ferns and low-growing flowering plants.
Ornithopods «AWR nuh thuh pahds» could walk either on four legs or on their two hind legs. One of the first dinosaur fossils ever discovered was that of an ornithopod’s tooth. This animal, called Iguanodon «ih GWAN uh dahn», measured about 30 feet (9 meters) long. Iguanodon had a bony spike on the thumb of each forelimb. Other ornithopods, such as Heterodontosaurus «heht uhr uh DAHN tuh sawr uhs», grew only about 4 feet (1.2 meters) long.
Ornithopods lived throughout the Mesozoic. But they reached their greatest development in hadrosaurs «HAD ruh sawrs», also known as duckbilled dinosaurs. Hadrosaurs lived at the end of the Cretaceous, mostly in what are now Asia and North America. They also inhabited Europe and South America. Hadrosaurs had a broad, ducklike beak at the front of the mouth. They also had jaws with hundreds of teeth farther back in the mouth, which they used to chew tough plant leaves. Their hind legs were strong, and they carried their tails stiffly outstretched andparallel to the ground. Some hadrosaurs were 9 feet (2.7 meters) tall at the hips and more than 30 feet (9 meters) long.
Dinosaurs of the Cretaceous Period Hadrosaurs such as Prosaurolophus «proh SAWR uh lahf uhs» and Edmontosaurus «ehd MAHN tuh sawr uhs» had skulls that were either flat or arched into a bony crest. Other kinds of hadrosaurs, such as Corythosaurus «kaw RIHTH uh sawr uhs»and Parasaurolophus «pair uh sawr uh LAHF uhs», had a showy crest on the top of the head. This crest housed air passages from the animal’s nose. Some scientists think that hollow-crested hadrosaurs could have made loud honking sounds when they exhaled through the air passages. These honks may have resembled sounds made by modern elephants and whales. Scientists also believe hadrosaurs took care of their young, tending the nests and providing the babies with food and protection. Most modern reptiles do not care for their young.
Pachycephalosaurs «pak uh SEHF uh loh sawrs» were the dome-headed ornithischians. They lived mostly in western North America and Asia during the second half of the Cretaceous Period. Pachycephalosaurs walked on their hind legs, and most measured from 6 to 25 feet (1.8 to 8 meters) long. These dinosaurs had extremely thick skulls, often covered with bumps and spikes. Scientists believe that pachycephalosaurs used their heads in butting matches, much as male bighorn sheep do today.
Ceratopsians «sehr uh TAHP see uhns» are known as the horned dinosaurs because most of them had horns. Most walked on four feet, resembled rhinoceroses, and ranged in length from about 6 to 25 feet (1.8 to 8 meters). Ceratopsians’ heads were often enormous. They typically had a parrotlike beak and a bony frill extending across the neck from the back of the skull. One ceratopsian, Torosaurus «tawr oh SAWR uhs», had the largest head of any animal ever to live on land. This head measured about 8.5 feet (2.6 meters) long. In another kind, Styracosaurus «sty RAK uh sawr uhs», the margin of the frill had many spikes. Most ceratopsians had horns on the face, usually one on the nose and one over each eye. Centrosaurus «sehn troh SAWR uhs», for example, had one large horn on the nose and two smaller ones over the eyes. Triceratops «try SEHR uh tahps» had horns over the eyes that grew up to 3 feet (90 centimeters) long. Earlier ceratopsians, such as Psittacosaurus «siht uh koh SAWR uhs», Protoceratops «proh tuh SEHR uh tahps», and Leptoceratops «lehp tuh SEHR uh tahps», were hornless. Ceratopsians lived during the Cretaceous Period in what are now Asia and North America.
Saurischians included both the largest and the fiercest dinosaurs. There were two basic kinds of saurischians: (1) sauropodomorphs «sawr uh PAHD uh mawrphs» and (2) theropods «THUR uh pahds». Each of these groups included many different kinds of dinosaurs.
The earliest sauropodomorphs, such as Plateosaurus «PLAT ee oh sawr uhs», lived during the late Triassic Period. These dinosaurs had a long neck and a small head and grew almost 30 feet (9 meters) long. Animals like Plateosaurus were the first diverse and widespread plant-eating dinosaurs. They fed on the tall trees of the period. Early sauropodomorphs could walk on their two hind legs as well as on all four legs.
Sauropods, the later sauropodomorphs, were the giants of the dinosaur world. The largest ones included Seismosaurus «SYZ muh sawr uhs», which may have grown to a length of 150 feet (45 meters), and Supersaurus «SOO pur sawr uhs», which was about 98 feet (30 meters) long. Most sauropods were 30 to 60 feet (9 to 18.3 meters) in length. …..s usually weighed from 10 to 30 short tons (9 to 27 metric tons). Sauropods walked on four stout, strong legs, much like those of an elephant. All sauropods had a long neck, a small head, a long tail, and a huge, deep chest and stomach region. Sauropods were the largest plant-eaters, feeding on the tops of tall trees such as conifers. During the Cretaceous Period, they declined in importance in the Northern Hemisphere. But they remained the dominant plant-eaters in what are now South America, India, and Africa. These regions were almost completely in the Southern Hemisphere at that time.
One of the best-known sauropods is Apatosaurus. More than any other dinosaur, Apatosaurus brings to mind the image of a dinosaur for many people. Its front legs were shorter than its hind legs, and its back sloped down toward the base of its neck. Diplodocus looked much likeApatosaurus but was slimmer and longer. Both Apatosaurus and Diplodocus lived during the Jurassic Period in what is now North America.
Dinosaurs of the Jurassic Period Brachiosaurus «brak ee uh SAWR uhs», another kind of sauropod, lived in Africa and North America at the same time as Apatosaurus and Diplodocus. With its neck stretched upward, Brachiosaurus stood about 50 feet (15.2 meters) tall. It weighed about 85 short tons (77 metric tons). The animal’s front legs were longer than its hind legs, and its back sloped down toward the tail. These features gave Brachiosaurus a stance much like that of a giraffe.
Theropods were the only meat-eating dinosaurs. These powerfully built animals walked upright on their two hind legs. Their short, slender forelimbs ended in hands that could grasp objects fairly well. Nearly all theropods had a long, muscular tail, which they carried straight out behind them for balance. Large theropods had a short neck and a large, long head. Small theropods had longer necks and a smaller head. Some theropods were toothless. Others had sharp teeth and strong jaws, which helped make them the fiercest predators of the Mesozoic.
Two important groups of theropods were the allosaurs «AL uh sawrs» and ceratosaurs «suh RAT uh sawrs». Allosaurs were the main meat-eating dinosaurs during the Jurassic Period. Most were about 30 to 40 feet (9 to 12 meters) long. Allosaurs had forelimbs with three fingers on each limb. In contrast, most ceratosaurs were small. One example, Coelophysis «see loh FY sihs», was about 10 feet (3 meters) long. These animals were fast, active predators. By studying them, scientists have learned much about the early evolution of theropod dinosaurs. Many ceratosaurs lived about 230 million years ago, as the Age of Dinosaurs was just beginning.
The best known of all theropods are the tyrannosaurs «tih RAN uh sawrs». They ranked among the most frightening meat-eaters of their time. One famous kind is known by its scientific name, Tyrannosaurus rex, which means king of the tyrant lizards. This giant predator stood nearly 12 feet (3.7 meters) tall at the hips and grew about 40 feet (12 meters) long. Its head measured up to 4.5 feet (1.4 meters) in length, and its teeth were about 6 inches (15 centimeters) long from the base to the sharp tip. The animal had short forelegs with only two fingers, but extremely powerful hind legs. Tyrannosaurs roamed what are now western North America and east-central Asia. They lived during the second half of the Cretaceous Period.
Other dinosaurs of the Cretaceous Period included small but ferocious theropods, such as Deinonychus «dy NAHN ih kuhs» and Velociraptor «vuh lahs uh RAP tuhr». Both grew to about 20 inches (50 centimeters) tall at the hips and 6 feet (1.8 meters) long. On each of the hind feet, there was a large, curved, razor-sharp claw used to slash, kill, and cut apart prey.
A more peaceable theropod, Ornithomimus «awr nihth uh MY muhs», looked much like a featherless ostrich with a long tail. This dinosaur was about the size of a modern ostrich and may have been just as fast a runner. Scientists think that Ornithomimus sprinted as fast as 30 to 40 miles (48 to 64 kilometers) per hour. These ostrichlike theropods lived during the Cretaceous Period in North America and Asia. They probably ate small fruits, the cones of certain trees, and the seeds of flowering plants, as well as eggs, insects, mammals, and lizards.
One of the most intelligent dinosaurs was a small, bipedal theropod called Troodon «TROH uh dahn». It lived during the late Cretaceous in North America. Troodon’s brain was as large, compared with its body weight, as that of many modern birds and small mammals. It had excellent vision and probably hunted for mammals and other prey at night. Troodon grew to about 6 feet (1.8 meters) in length. Theropods also included one of the smallest known dinosaurs, Compsognathus «kahmp SAHG nuh thuhs». This animal was about the size of a chicken.
How dinosaurs lived
For many years, people thought that dinosaurs were clumsy, slow-moving, unintelligent creatures that lived much like modern reptiles. However, fossil evidence suggests that some dinosaurs—especially small theropods—were much more active and intelligent than previously thought. In addition, scientists generally agree that small theropods are the closest known relatives to birds. Thus, scientists can learn much about the life of dinosaurs by studying birds and other modern animals that have some similarity to dinosaurs.
How dinosaurs lived depends partly on whether they were ectothermic (cold-blooded), like modern reptiles, or endothermic (warm-blooded),like birds and mammals. The body temperature of ectothermic animals changes with the temperature of their surroundings. For example, a lizard’s body temperature rises as the air becomes warmer or the sun shines. When the air cools or the sun disappears, the lizard becomes cooler. Lizards become active when they are warm, but they are sluggish when cool. In contrast, endothermic animals generate their own heatand have a constant, fairly warm body temperature. Such animals tend to be more active than their ectothermic counterparts.
Traditionally, dinosaurs were considered ectothermic. People believed they were merely large versions of cold-blooded lizards andcrocodilians. Later, scientists argued that some dinosaurs were more active and thus maintained a more constant body temperature than previously believed. However, they continued to think dinosaurs were ectothermic. Scientists pointed out that large animals, particularly gigantic dinosaurs, lost their body heat slowly. Their great size would enable them to keep a more constant body temperature even if they were cold-blooded.
Since the mid-1970’s, growing numbers of scientists have argued that many, or even all, dinosaurs were probably endothermic. They point out that not all dinosaurs were gigantic, especially the babies. Smaller dinosaurs could not rely on their size to keep them warm. Other dinosaur features, such as the internal bone structure and the chemistry of the bone, also suggest that dinosaurs were endothermic.
Reproduction and growth. Scientists do not know how all dinosaurs reproduced. Fossil dinosaur eggs show that at least some dinosaurs laid hard-shelled eggs, as do modern alligators. The female may have dug a nest in the soil and deposited eggs in it. Some dinosaurs, particularly hadrosaurs, may have cared for their young from the time the babies hatched until they left the nest. Others probably left the young to survive as best they could.
Scientists can only guess how long dinosaurs lived. But they can estimate the time it took for dinosaurs to grow to ….. size. The growth rate depends on whether dinosaurs were ectothermic or endothermic. Endothermic animals grow more rapidly than do ectothermic ones. If sauropods were endothermic, it probably took them about 30 years to reach their average ….. weight of 30 short tons (27 metric tons). If the animals were ectothermic, however, it may have taken them 200 years or longer to grow that large.
group life. Fossil evidence shows that more than 20 kinds of dinosaurs may have occupied a particular area at the same time. Many dinosaurs, including ceratopsians, ornithopods, sauropodomorphs, and perhaps stegosaurs, probably lived in herds the year around. Other kinds, such as ankylosaurs and tyrannosaurs, may have spent most of their life alone or in small groups.
Some scientists think that dinosaurs were brightly colored, like many modern birds, snakes, and lizards. Certain dinosaurs perhaps attracted mates by displaying colorful body parts. For example, the crested head of a hadrosaur and the neck frill of a ceratopsian may have been vividly colored. If dinosaurs also made noises, both sounds and colors may have served to attract mates.
Getting food. Most dinosaurs were plant-eaters. They probably fed on a wealth of leaves, small fruits, and seeds from Mesozoic plants. Sauropodomorphs browsed on the leaves of tall trees, while hadrosaurs chewed on the foliage of lower branches, shrubs, and ferns. Pachycephalosaurs, ankylosaurs, ceratopsians, and stegosaurs fed on low vegetation that grew along the edges of streams and rivers or on open plains.
Nearly all theropods, large and small, were hunters. They preyed on a wide variety of plant-eating dinosaurs and possibly on each other. Some of the small theropods probably ate insects, eggs, mammals, and lizards. All theropods, and particularly the smaller ones, were extremely active and could run quickly when attacking. Such fierce animals as Deinonychus may have hunted their prey in packs as wolves do today. Other theropods may sometimes have been scavengers that picked up meat from dead animals they found.
Protection against enemies. Plant-eating dinosaurs had many forms of protection against predators. The huge size of sauropods probably kept them safe from most enemies, but their smaller offspring had to stay alert to danger. Ankylosaurs had bony plates for protection, andceratopsians and stegosaurs probably used their horns and spikes to fight off predators. Ornithopods, ceratopsians, and other dinosaurs probably gathered in herds to discourage enemies.
Why dinosaurs died out
For about 160 million years, dinosaurs were the largest and most successful animals on land. Then about 65 million years ago, these huge archosaurs died out along with pterosaurs, mosasaurs, and many other reptiles. Mammals then became the dominant animals on the earth.
Scientists have developed many theories to explain dinosaur extinction. The two major theories involve (1) gradual climate changes and (2) the collision of an asteroid with the earth. The first theory argues that, toward the end of the Cretaceous Period, the shallow seas dried up andthe climate became more varied everywhere around the globe. Winters became too cold and summers too hot for dinosaurs to survive. Dinosaurs were too large to hibernate in dens, and they had no fur or feathers for protection against the cold. They also probably had difficulty cooling off in hot weather. Thus, death and extinction came as a result of gradually colder winters and hotter summers.
The other major extinction theory claims that a large asteroid hit the earth at the end of the Cretaceous. This asteroid impact would have thrown billions of tons of dust and debris into the atmosphere. Heat from the impact may have caused huge fires worldwide. Together the clouds of smoke and debris would have blocked sunlight from reaching the surface of the earth for many months. Although the seeds and roots of plants had a good chance of surviving this lightless period, the plants themselves stopped growing and died. If the catastrophe was severe andwidespread enough, plant-eating dinosaurs would have starved to death. As the plant-eaters died, so did the meat-eating dinosaurs that fed on them. In addition, the darkened skies caused land temperatures to drop below freezing for 6 to 12 months in many parts of the world. Such low temperatures further damaged the dinosaur populations.
According to the asteroid theory, small mammals and birds survived because they were protected from the cold by fur or feathers. Mammalsand birds also could feed entirely on seeds, nuts, and rotting vegetation. Other survivors may have escaped extinction because they could live at the bottom of lakes or burrow underground.
Most scientists, however, feel that no single theory completely explains why dinosaurs suffered extinction. They argue that a combination of causes contributed to the dinosaurs’ disappearance. In fact, experts now believe that not all dinosaurs became extinct at the end of the Cretaceous. Many scientists regard birds as living dinosaurs that survived extinction.
Dinosaur discoveries
Before the 1800’s, no one knew that dinosaurs had ever existed. People who found a dinosaur tooth or bone did not know what it was. Then, two important events happened. Around 1818, an English scholar, William Buckland, obtained a large lower jaw that contained a number of sharp teeth. After studying this jaw, Buckland came to the conclusion that it was unlike any fossil previously discovered. So he gave it a new name, Megalosaurus (great lizard), in 1824.
At nearly the same time Mary Ann Mantell, an English amateur naturalist, found a large tooth partly buried in a rock. She showed the tooth to her husband, Gideon, a physician who collected fossils. He decided that the tooth came from a huge, iguanalike reptile, which he named Iguanodon (iguana tooth) in 1825.
Within a few years, the remains of several kinds of large, extinct reptiles had been discovered. In 1841, Sir Richard Owen, an English scientist, suggested that these creatures belonged to a group of reptiles unlike any living animals. In 1842, he called this group Dinosauria (terrible lizards). Its members later came to be known as dinosaurs.
During the late 1800’s and early 1900’s, large deposits of dinosaur remains were discovered in western North America, Europe, Asia, andAfrica. Today, most dinosaur discoveries are made in China, Mongolia, Argentina, and Australia. Some also occur in the United States andCanada. During the late 1900’s, extensive research and fossil discoveries greatly increased the number of known dinosaurs. Scientists discover and describe an average of seven new kinds of dinosaurs every year.

Changing interpretations of dinosaurs
Our understanding of dinosaurs has changed dramatically over the years. Scientists once thought these animals were slow-moving, unintelligent creatures that did not adapt well to changing environments. Today, however, scientists believe that dinosaurs were among the most adaptable and diverse animals that ever lived.
Tyrannosaurus rex is one of many dinosaurs about which opinion has changed greatly. People first thought this giant meat-eater was primarily a scavenger, feeding only on the decaying bodies of dead dinosaurs. They also thought it lived a sluggish life, sleeping or basking in the sun between meals.
However, scientists gradually came to believe that Tyrannosaurus had a much more dynamic lifestyle. Scientists now argue that it was an active predator as well as a scavenger. Tyrannosaurus probably stalked its prey. When close enough, it caught the victim by running toward the creature on its powerful hind legs. It killed the prey by clamping its strong jaws on the victim’s neck. It then stripped the meat from the prey’sbody and swallowed it.
Today, scientists are still trying to discover more about Tyrannosaurus. For example, they do not know exactly how the predator used its extremely small arms. Some regard the arms as strong limbs that helped the animal grab prey. Others believe the limbs were weak andvirtually useless. New research and fossil discoveries will help solve such mysteries.

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