(''DINOSAURS''- PART 2) 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 like Apatosaurus 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 heat and 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 and crocodilians. 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 adult 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 adult 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, and ceratopsians 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 and the 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 and widespread 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. Mammals and 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, and Africa. Today, most dinosaur discoveries are made in China, Mongolia, Argentina, and Australia. Some also occur in the United States and Canada. 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's body 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 and virtually useless. New research and fossil discoveries will help solve such mysteries.
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