Dinosaur

A dinosaur is an extinct vertebrate (an animal with a backbone) reptile. The first dinosaurs appeared on Earth around 220,000,000 years ago and after surviving for 140,000,000 years, suddenly disappeared. Certain dinosaur species, like the Brachiosaurus, were the largest animals ever to have lived. All of the knowledge about dinosaurs is the result of studying the fossil remains that have been discovered in all parts of the world.

Dinosaurs lived during a time in Earth's history called the Mesozoic Era, also called the Age of Reptiles. In many ways, dinosaurs were much like the reptiles we know today—the familiar snakes, turtles, lizards, and crocodiles. Like them, dinosaurs may have been ectothermic or cold-blooded, meaning that their internal body temperature would change according to the temperature of their environment. However, the great physical bulk of some species suggests that it would have taken them a very long time to reach their full size, since ectothermic animals grow very slowly. Like today's reptiles, they varied greatly in size, from those the size of a chicken to others that grew to more than 90 feet (27.4 meters) long. Some dinosaurs were carnivores (meat-eaters) and others were herbivores (plant-eaters). As a reptile, they laid eggs that had tough outer shells and that may have contained enough water and food for the dinosaur embryo to grow and finally hatch.

KNOWLEDGE OF DINOSAURS REVEALED THROUGH FOSSILS

Although a great deal more is known about dinosaurs today compared to when the first fossil bones were discovered in England around 1822, new facts are learned every year as new finds reveal more about their lives and habits. One thing that is known is how dinosaurs actually reproduced. It is known, however, that many probably laid eggs as all reptiles do. Also, it is not known how long they normally lived. Some species may have lived in herds, while others could have been solitary. Another mystery is whether plant-eaters ate underwater plants or leaves on trees, or if carnivores ate other dinosaurs. Among the many things not known

about dinosaurs, certainly the biggest and most important of all is the actual reason for their sudden and total disappearance. It is known that after entirely dominating the Earth, dinosaurs went extinct about 63,000,000 years ago.

The fossilized remains of dinosaurs have been found in all parts of the world because at the time they lived, there was a single land mass or continent called Pangaea. When this single continent slowly broke up and moved apart into the several ones evident today, the fossil remains were scattered along with the land masses. This fossil evidence is available today only because of a particular set of circumstances that occurred millions of years ago. If a dinosaur was stuck in soft mud and died there, or if it died and simply fell in and sank, it would sometimes be covered by more and more sediment (that was moved there by wind, water, or ice). Over even more time, these sediments would be compressed by layers of Earth deposited on top of it until everything was transformed by great pressure into solid rock. After nearly 200 years of collecting and studying fossilized dinosaur bones, fossilized eggs, and footprints left in rock, scientists have been able to reconstruct several species with some degree of certainty. They have also been able to classify them as they would any living animal, and have divided dinosaurs into two main groups according to the structure of their hips. This may sound strange, but when people realize that how their hips and those of animals are shaped and function affects how they move about, it starts to make sense.

DINOSAUR GROUPS

The first group or order called Saurischia had a hip structure that resembled a lizard. The second order, called Ornithischia, had hips that were built like those of a bird. The first group included the largest (and most ferocious) dinosaurs. Thus the well-known Apatosaurus (formerly known as the Brontosaurus) is among this group, as is the fearsome Tyrannosaurus rex. Many believe that this large group consisted mainly of carnivores (meat-eaters), and it is known that they walked mainly on four feet, lived mostly on land, and had barrel-like bodies and legs that looked like columns, as well as long, heavy tails. The second group with birdlike hips are believed to be mainly plant-eaters. The well-known Stegosaurus was a member of this order. It had the smallest brain compared to body size of any dinosaur.

Although there are many things left to learn about dinosaurs to understand their life cycle, habits, and internal functions, what remains the largest gap in the knowledge about these great beasts is the cause of their sudden extinction. Scientists now believe that a mass extinction must have somehow occurred, but they are still not in agreement as to its cause. Scientists have already eliminated theories that say that dinosaurs simply grew too large to hold themselves up. Instead, many now think that dinosaurs were already on the decline when something very big and destructive happened. One theory says that the key event was Earth being struck by a massive piece of debris from space, like an asteroid. This would have caused tons upon tons of dust and soot to clog Earth's atmosphere, causing either prolonged darkness that cooled the planet or a greenhouse effect that trapped warmth and caused the surface to overheat. Others say that climates may have simply changed too fast for dinosaurs to adapt. Whatever the exact nature of the cause or causes, something did happen with which the dinosaurs were unable to cope, and they all eventually disappeared. Science may never know for sure what killed all the dinosaurs.

[See alsoEvolution; Fossil; Geologic Record; Paleontology ]