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Let's Talk About Dinosaurs

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Two groups of extinct reptiles, dinosaurs (whose name is derived from the Greek for 'terrible lizards'), lived on Earth during the Mesozoic era (about 60,000,000 to 200,000,000 years ago). There were many large reptiles flourishing at that time, mostly belonging to the subclass Archosauria. On land, besides the dinosaurs, were the flying pterosaurs, while, in the sea, lived such reptiles as the plesiosaurs and the ichthyosaurs, which dominated the seas just as the dinosaurs dominated the land.

The earliest fossil remains of dinosaurs have been found in rocks of the Triassic period (circa 180,000,000 to 225,000,000 years ago), discovered on all continents except Antarctica. During the Jurassic period (130,000,000 to 180,000,000 years ago), dinosaurs were most abundant, but they gradually declined and became extinct towards the end of the Cretaceous period, about 65 000 000 years ago. Because they are found only as fossils, palaeontologists must base their theories on their appearance, physiology and mode of life on comparisons made with living animals, although this can often be misleading.

Many early speculations on, for example, the appearance of dinosaurs have been proved wrong; one nineteenth century scientist actually placed the head of one dinosaur at the end of its tail!

Recent theories suggest that dinosaurs may not have been cold-blooded, sluggish reptiles, such as modern lizards (whose temperatures vary with external climatic conditions), as was once popularly believed. Some palaeontologists have even claimed that dinosaurs were hot-blooded and, like birds and mammals, were capable of maintaining a high body temperature by means of their own energy output. They base this claim on such features of the dinosaurs as their upright, non-sprawling stance and the microstructure of their bones, which more closely resembles that of mammals than of modern reptiles.

However, although most palaeontologists stop short of this claim, many believe that dinosaurs were capable of retaining a relatively constant internal temperature despite external temperature changes. The main evidence for this belief is drawn from the large size of most dinosaurs: the ratio between their surface area and volume would have helped them to maintain a reasonably constant temperature.

Appearance

Dinosaurs greatly varied in size and shape. Most were very large; Brachiosaurus, the largest land animal that has ever lived, had a body length of about 28 m and weighed about 50 tonnes. Some were very small; Compsognathus was no larger than a domestic turkey. All, however, had tiny brains, as is evident from their very small heads in relation to their overall body size; it has been estimated that the brain of Brontosaurus weighed a mere 1/100,000th its total body weight. (By comparison, the average weight of the human brain is about 2% its total body weight.)

Classification of DInosaurs

Dinosaurs have been separated into two orders, the Saurischia and the Ornithischia, based on the shape of their hip bones.

The Saurischia had a hip structure similar to that of lizards, whereas the Ornithischia had a hip structure similar to that of birds.

Saurischia

The order Saurischia has been further divided into two groups, the herbivorous (plant-eating) sauropods and the carnivorous (meat-eating) theropods. The accepted theories of the ways in which dinosaurs lived are currently being questioned. Traditionally, sauropods, among which were the largest of the dinosaurs, were believed to have spent much of their lives in lakes or swamps, the water helping to support their heavy bodies. Evidence in favor of an aquatic way of life includes the position of their nostrils, which were placed high up on their skulls near their eyes, as in whales.

This evidence has been disputed, however, and some palaeontologists now suggest that they may, in fact, have been terrestrial.

The carnivorous theropods were certainly terrestrial, walking on their hind legs and using their small front legs to grasp and tear their prey. Among the theropods was Tyrannosaurus, the largest known carnivore, which reached a length of about 15 meters, a height of about 6 meters and weighed about 8 tonnes. Like other theropods, it had large, sharp teeth, which were about 15 cm long.

Ornithischia

The order Ornithischia has been divided into four groups, all of which were herbivorous: the Stegosaurs, the Aknylosaurs, the Ornithopods and the Ceratopsians. Most of them were armored or had horns or bony plates to protect them from predators. It has also been suggested that they may have retreated into water, as a means of escape, when threatened. One group, the Ornithopods, walked on two legs, using their tails for balance. Some Ornithopods had bills rather like those of ducks, although one species, Anatosaurus, had 2000 teeth.

Extinction of DInosaurs

Dinosaurs became extinct in a relatively short period in geological time and were not the only group of animals that became extinct at that time; the pterosaurs, many marine reptiles, certain other groups of animals and many plants also became extinct. The reasons for this widespread extinction are not known, nor is it known why some animals survived while others died. Many explanations have been put forwards in an attempt to solve this mystery.

Perhaps the most convincing explanation lies in the changes in the physical environment that occurred during the Cretaceous period, there being widespread volcanic activity and mountain-building. Swamps dried out, the temperature fell, and many forms of vegetation died. As the plant-eating dinosaurs gradually became extinct through lack of suitable food, so the carnivores that preyed on them also became extinct; similar events caused the extinction of the other creatures that died out.

Other reasons that have been suggested to explain the dinosaurs' extinction include cosmic-radiation disease and the increase in the number of mammals that, it has been claimed, ate the eggs and young of the dinosaurs.

Possibly these and other unknown factors all played their part.

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