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1. Introduction

The word " dinosaur " means " terrible lizard (Greek)". This word was invented by a British scientist called Richard Owen to describe reptiles that lived during the Mezozoic Era, between 65 and 265 million years ago. Dinosaurs were on this earth for about 180 million years.

Not all dinosaurs were alive at the same time during the Mezozoic Era. Some were around earlier and others lived on the earth millions of years later. Therefore, scientists divided this era into three smaller time periods, Triassic, Jurassic, and Cretaceous.

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.

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2. Classification

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 100,000 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.

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3. Extinction

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. 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.

Theories regarding the causes of mass extinctions abound and are the subject of intense study and debate. In general it is believed that the extinctions resulted from drastic environmental changes that followed events such as meteorite or comet impacts or massive volcanic eruptions. For example, the final Permian extinctions have been linked to huge volcanic eruptions in what is now Siberia. These eruptions, which continued for up to 800,000 years (a dramatically short period of time by geological standards), spewed out dust and droplets that blocked the sun, causing global cooling that trapped sea water in the polar ice caps. The levels of inland seas and oceans lowered significantly, eliminating or changing marine habitats. Other theorized causes for the Permian extinctions include a large meteor impact and a supernova that exploded near enough to the earth to bathe it in radioactivity, wiping the dinosaurs from the face of the Earth.