For 183 million years, during a period of the Earth's history called the Mesozoic era, dinosaurs were the main land animals on Earth. Dinosaurs were so dominant that the Mesozoic era was called the Age of Dinosaurs. Their name comes from the Greek words deinos, meaning “fearfully great,” and sauros, meaning “lizard.” The dinosaurs died out at the end of the Mesozoic era, some 65 million years ago. They were reptiles, but they are believed to be the ancestors of modern birds.
Dinosaur fossils, or remains, were first discovered in the early 19th century. After studying several of these remains a scientist named Richard Owen realized that the bones represented a group of large reptiles that were unlike any living varieties. In a report written in 1841 he called them Dinosauria. Since these early finds, more than 1,000 different sites containing dinosaur fossils have been uncovered around the world.
Where and when dinosaurs lived
The Mesozoic era began about 248 million years ago and ended about 65 million years ago. It was divided into three different periods: the Triassic period (248 to 206 million years ago); the Jurassic period (206 to 144 million years ago); and the Cretaceous period (144 to 65 million years ago). The oldest known dinosaur, the Eoraptor, lived during the Triassic period in what is now South America. (See also geologic time.)
Different species of dinosaurs lived and died out during different periods. For example, some dinosaurs living in the Triassic period did not exist in the Jurassic or Cretaceous periods. Therefore, all species of dinosaurs did not live together at the same time, nor did they all become extinct together. Only a few species of dinosaurs were living on the Earth at the time they all died out.
Dinosaurs were terrestrial, or land, animals that inhabited all parts of the Earth. They adapted to different environments and climates. Their habitat covered a range of ecosystems, including tropical forests and drier desert regions. Fossils of dinosaurs have been found in China, Mongolia, Argentina, the United States, Canada, North Africa, Antarctica, Europe, and India. Fossils are the impressions or remains of a plant or animal of the geologic past, preserved or buried in the Earth's crust.
Physical features
Dinosaurs were a diverse group of creatures that came in all sizes and shapes. They all had hairless bodies like modern-day reptiles, but most dinosaurs were not scaly. For the most part, their skin had a bumpy, pebbly surface, instead of scales. Dinosaurs are often pictured as a dull gray in color, like modern elephants and rhinoceroses. Scientists now believe that dinosaurs may actually have been quite colorful, however. One reason for this belief is that modern birds, which are living relatives of the dinosaurs, can see color. The scientists know that modern animals that can see color are often colorful themselves. The colors help them attract mates since many different species live together and each species needs a unique way to advertise for mates. Because their relatives can see color and are colorful, dinosaurs may have had the same features. Dinosaurs were also not completely cold-blooded, like modern reptiles, nor were they completely warm-blooded. They had their own unique way of controlling their temperature.
Some types of dinosaurs stood upright and walked on their two back legs, while others walked on all fours. Usually, dinosaurs that walked upright on two legs had long, muscular back legs and short front legs. They also had short necks and tails and powerful jaws. Their fingers and toes ended in long, sharp claws. They were built to hunt and eat meat. Dinosaurs that walked on all fours had thick, sturdy legs and elephant-like feet. The necks and tails of some of these dinosaurs were very long. They were built to eat plants.
The skeletons of dinosaurs were quite different, too. Large, four-legged dinosaurs, like Apatosaurus (Brontosaurus), had very heavy bones to support their massive weight. Dinosaurs that walked or ran on two legs usually had hollow bones, like those of modern birds. Some larger species even had large holes in their skulls to make them lighter. Lighter bones helped these dinosaurs to either chase prey or to escape from enemies more quickly.
Generally, dinosaurs are divided into two major groups, or orders: the Ornithischia, or “bird-hipped” dinosaurs, and the Saurischia, or “lizard-hipped” dinosaurs. The way the pelvis is shaped determines the order into which a particular dinosaur fits. Some dinosaurs had a pelvis like that of a modern bird, and some had a pelvis like that of a modern lizard. The Saurischian dinosaurs were further dived into the meat-eating theropods and the plant-eating prosauropods and sauropods. The Ornithischian dinosaurs were plant eaters.
Different species of dinosaurs each had different types of teeth. Some teeth were large and had sawlike edges made for tearing apart flesh. Other teeth were small and flat like molars. These were used for grinding tough plant material.
Dinosaurs varied greatly in size. One of the largest creatures was the Seismosaurus, a plant-eating dinosaur that is believed to have been as long as 140 feet (43 meters). The Compsognathus, a predator that walked on two legs, was probably the smallest at less than 3 feet (1 meter) in length.
Certain dinosaurs had unusual features. Some had horns like a modern rhinoceros. Another dinosaur had a huge club for a tail. Some had bony neck frills or crests on their heads. Still others had large, triangular bony plates sticking out of their backs. (See also Stegosaurus; Triceratops.)
Behavior
Some dinosaurs were solitary animals, living and hunting alone. Some species of dinosaurs lived and roamed in groups. The food habits of dinosaurs are still much discussed among scientists. The size, form, and number of teeth of a particular species give some idea of their diet. The study of fossilized dinosaur droppings, called coprolites, also reveals what dinosaurs ate. Studies show that some species were herbivores, or plant eaters. Others were carnivores, or meat eaters.
Herbivorous dinosaurs usually had a small head and teeth. The smaller species often fed on soft, low-growing plants, such as ferns and horsetails. The tall and long-necked dinosaurs, such as Brachiosaurus, probably fed on high branches and treetops. These types of dinosaurs usually stood and walked on four legs and lived together in groups. They are the most massive of all dinosaurs, and are known as sauropods.
Carnivorous dinosaurs had sharp teeth and sharp, curved claws. They ate mainly smaller, herbivorous dinosaurs. Carnivores normally hunted alone, and they often stalked their prey. Sometimes they hunted in packs to bring down larger dinosaurs. The most ferocious in this group was the predator Tyrannosaurus rex. Some of the carnivorous dinosaurs are thought to have been scavengers that fed mainly on dead animals. These types of dinosaurs usually walked or ran on their two back legs. Carnivorous dinosaurs belonged to the theropod group of the lizard-hipped dinosaurs.
Life cycle
Scientists believe that all dinosaurs reproduced by laying eggs. Some dinosaurs built vast nesting colonies to take care of their young. Baby dinosaurs grew very fast, and many reached adult size in seven or eight years. Dinosaurs exhibited courtship behavior much in the way modern animals do. Rival male dinosaurs may have used their natural weapons, such as horns or spikes, in combat against one another during the breeding season. They also may have used these features as a display to attract females. Scientists have not been able to determine how long individual dinosaurs lived, though they believe that large dinosaurs, like Apatosaurus, could live to be almost 100 years old.
Extinction
The dinosaurs died out at the end of the Cretaceous period, about 65 million years ago. The reasons for their extinction are still debated by scientists. Fossil studies provide some clues to the puzzle. Some scientists believe that dinosaurs died out because of a gradual change in climate. A slow cooling of the earth and a fall in global temperatures may have made survival difficult for these animals. Most scientists now believe, however, that dinosaurs died out suddenly.
One of the possible reasons is that a massive meteorite, asteroid, or comet may have collided with the Earth. The dust thrown up in such a collision may have blocked the light from the sun. This would have resulted in a cold and dark period. The loss of sunlight would have caused plants to die. The herbivores would have then died because they had nothing to eat. Their predators, the carnivores, would also have died as a result. An interesting fact that has puzzled scientists is that some animals survived beyond the period when the dinosaurs disappeared. If a disastrous collision did take place it is not clear why some animals were not hurt by it. The ancestors of such modern animals as frogs, turtles, lizards, and snakes found a way to survive. Birds, which are said to be descendants of dinosaurs, also survived. Perhaps these survivors will give scientists the clues they need to find out what exactly happened to the dinosaurs.
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