The world is full of all sorts of animals. Dogs, birds, snakes, frogs, whales, and ants are just a few kinds of animals alive today. But the animals we have today did not always exist. They developed from other animals that lived long ago. Those early animals included dinosaurs, tiny ancestors of horses, and gigantic flightless birds.
Those animals and many others became extinct, or died out. They are called prehistoric animals because they lived in prehistoric times. Prehistoric times are all of time until about 5,500 years ago. That is when people began to invent writing and record history. But most extinct animals lived much longer ago than that.
Not all animals that lived in prehistoric times are extinct. Cockroaches, for example, have existed almost unchanged for 320 million years. Crocodiles and sharks are also much the same as their ancestors. Such animals are often called living fossils.
WHAT WERE THE FIRST ANIMALS?
The first animals on Earth lived more than 1 billion (1,000,000,000) years ago. They all lived in the ocean. They included creatures similar to jellyfish and worms.
Other kinds of sea animals slowly evolved, or developed, from those early animals. Some types evolved shells. One common shelled animal was the trilobite, a small, crablike creature that crawled about on the ocean floor. By 400 million years ago, many sea animals had developed backbones. Those were the first fishes.
By 360 million years ago, animals called amphibians had evolved. Amphibians could live both in water and on land. Some amphibians started living more on land than in water. They began to evolve into other kinds of animals. By that time, the land was covered with trees and other plants. Early insects and spiders crawled through the ancient plants.
THE AGE OF DINOSAURS
About 330 million years ago, some amphibians evolved into creatures called reptiles. Today’s reptiles include snakes, alligators, and lizards. The reptiles of long ago included flying reptiles that looked like huge, strange birds. They also included gigantic, sharp-toothed sea lizards up to 30 feet (9 meters) long. But the most famous reptiles of long ago were the dinosaurs.
The first dinosaurs appeared about 230 million years ago. They evolved from animals that looked a lot like crocodiles. Some dinosaurs ate plants. Others ate meat. One of the most ferocious meat eaters was the Tyrannosaurus rex, a terrifying animal up to 41 feet (12 meters) with a huge head and long, pointed teeth.
Some dinosaurs were as small as chickens, but others were even bigger than the Tyrannosaurus. The biggest dinosaurs were plant eaters. The largest of these weighed 80 metric tons or more. That’s heavier than 15 elephants!
THE END OF THE DINOSAURS
Dinosaurs ruled the Earth until about 65 million years ago. Then they all died out. Why that happened is not known for sure. Many scientists think their extinction was caused when a huge rock called an asteroid struck our planet.
But dinosaurs may have left some descendants. Many scientists believe that modern birds evolved from dinosaurs.
THE MAMMALS TAKE OVER
After the dinosaurs disappeared, a group of animals called mammals spread all over the planet. Mammals feed their young with mother’s milk. Cats, dogs, cows, horses, people, and many other animals alive today are mammals.
The first mammals were small animals just a few centimeters long. As long as the dinosaurs were stomping around, mammals stayed small and came out mainly at night. When the dinosaurs were finally gone, mammals started evolving into bigger animals. Some mammals returned to the sea. Those animals evolved into dolphins and whales.
Most mammals, however, stayed on land. Some types got really big. One type of mammal, which lived about 30 million years ago, was a leaf-eater called Indricotherium. It was about 18 feet (5.5 meters) high at the shoulder and weighed about 20 metric tons. The Indricotherium may have been the largest mammal that ever lived on land.
Another large mammal was the woolly mammoth. It lived during the last Ice Age. Woolly mammoths were closely related to today’s elephants but were bigger. Some were more than 14 feet (4.3 meters) high at the shoulder. They had tusks up to 13 feet (4 meters) long. Most mammoths died out about 11,000 years ago. They and other large Ice Age animals may have been hunted to extinction by another mammal: human beings.
Source: Microsoft ® Encarta