Dinasaurs(PART 1)

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) and pterosaurs «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, and waste 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, they formed 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 and along 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. Land animals, 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 reptile from 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 the body 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 and parallel 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. Adults 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...