Dinosaurs were a group of animals that ruled our world for about 150 million years. They lived during the geologic middle ages of the Earth's history, in a time that is called the Mesozoic era. Dinosaurs were reptiles and belonged to the greatest animals on Earth.
The Mesozoic era is often called the Age of Dinosaurs. It began about 250 million years ago and ended about 65 million years ago. Geologists divide it into 3 parts:
• the Triassic
• the Jurassic
• the Cretaceous
During the Mesozoic period the Earth consisted of only one single mass of land. About 200 million years ago this huge continent, Pangaea, broke into many pieces, which started to drift apart. Continents moved slowly into the position they are today.
The climate during the Age of Dinosaurs was different than today's. It was warmer and there was more vegetation. The land was full of swamps, green plains, rivers and lakes. The first forests, mostly conifers that had cones on them emerged. Ferns and mosses also appeared on the Earth.
Dinosaurs were land animals that lived in almost all parts of the world, from tropical regions to deserts. They came in many different sizes and had different body shapes. Some were huge creatures, up to 40 meters long and a hundred tons in weight. Others were as small as a chicken.
Most dinosaurs had a scaly skin with no hair on their bodies. Although we often think dinosaurs were grey, dull creatures, experts think that some of them could have been really colorful.
Until today scientists don’t really know if dinosaurs were cold- or warm-blooded animals. They had their own way of regulating their body temperature.
Some dinosaurs were able to walk on their two hind legs and stood upright. They had hollow, light bones which enabled them to move quickly. Others had strong bones to support their massive weight. They moved slowly on all four legs.
Some types of dinosaurs had powerful jaws with sharp teeth made for eating meat and tearing apart their prey while others had long necks and ate leaves off of trees and other plants.
Most dinosaurs laid eggs. After they hatched, baby dinosaurs grew very quickly. They reached their full size after 7 or 8 years. It is still unclear how long dinosaurs lived, but some may have survived up to a hundred years. Fossils show that some dinosaurs lived in herds for at least parts of the year.
Dinosaurs were able to protect themselves in many ways. Some of them used their horns for protection; others drove enemies away with their massive size.
Dinosaurs are divided into two basic groups.
Ornithischians were dinosaurs that had bird-like hips and other features that you can find in modern birds. Many had a skin made of plates and most of them were plant eaters. The Stegosaurus, for example, was a plant-eating dinosaur with a small head. It reached a length of up to 10 meters.
Saurischians were dinosaurs that had hips like lizards. Some of them belonged to the largest creatures that ever roamed the Earth.
The Brontosaurus, for example, was an enormous animal. It had a long neck, a small head and ate plants from trees. It moved slowly because it was so heavy. Some Saurischians were meat-eating dinosaurs. The fiercest of all was the Tyrannosaurus rex, the king of lizards. The T. rex was almost 4 meters tall and had sharp teeth. It had short front legs but very strong and massive hind legs. Tyrannosaurs roamed across North America and Central Asia towards the end of the Mesozoic period.
Dinosaurs became extinct towards the end of the Mesozoic era, about 65 million years ago. Although there are still some mysteries left most scientists think that a drastic change in climate killed them. Winters became too cold for dinosaurs to survive.
Another theory says that a huge asteroid hit the Earth at that time. It whirled so much dust and gas into the atmosphere that sunlight was blocked out for millions of years. Dinosaurs probably starved to death because there was no food.
Other scientists maintain that dinosaurs may have evolved into birds. Some modern birds, like ostriches may have come from dinosaurs like the archaeopteryx.