Prehistoric animal refers to any animal that existed more than 5,500 years ago, before human beings began recording their history in writing. Some prehistoric animals greatly resembled their present-day relatives. Many ancient creatures, however, were unlike anything alive today.
Dinosaurs rank among the best-known prehistoric animals. They continue to facinate the imagination of people since their discovery in the early 1800's.

The name dinosaur comes from the term Dinosauria, which means terrible lizards. Dinosaurs were a kind of reptile that ruled the earth for about 160 million years ago, during the Mesozoic Era (which is 70 times longer than people have existed), when all the continents were jammed together and odd fish in the form of amphibians colonized the land. This is why they are found all around the world.
The word dinosaur was coined in 1842 by the British anatomist Sir Richard Owen. The first dinosaur discovered and named was iguanodon in the 1820's in England, from a tooth brought to a medical doctor named Gideon Mantell. Iguanodon grew to more than 20 feet long and had a big spike on its thumb. But the first dinosaur scientists goofed and put the spike on its head. Iguanodon chewed plants. It lived about 120 million years ago or more in the early Cretaceous period.

References to dinosaurs extend as far back as 5 BC. The Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins - legendary beasts that were part eagle and part lion - guarding nests in central Asia. "Dragon bones" mentioned in a 3rd century AD text from China are thought to refer to bones of dinosaurs.

What's Older Than A Dinosaur?

Long before any backboned creatures - whether amphibian or reptile - set foor on land, invertebrates (animals without backbones) crept across the Earth. Among the first were arthropods, animals with a hard outer body casing and joint legs. They included centipedes, millipedes, mites, ticks, scorpions, and insects. With a 400-million year history, land atherpods far oulived the dinosaurs, and still survive in incredible numbers. Today, these small scurriers are sometimes called "mini-beasts." But some mini-beasts reached maxi-sizes - one centipede was longer than your leg!

Different Kinds

Approximately 700 dinosaur species have been named. Dinosaurs are often named for characteristics, like earth shaker lizard, the Greek name for the longest dinosaur, seismosaurus. But the scientist who finds it and describes it can name it for a place, a mythical animal, a friend ... anything.
Recent estimates suggest that about 700 to 900 more dinosaur genera may remain to be discovered. This is still less than one-tenth the number of currently known living bird species, less than one-fifth the number of currently known mammal species, and less than one-third the number of currently known spider species. Not all of these animals lived at the same time. Not all dinosaurs walked on 4 feet either.

Different Sizes

Dinosaurs were all shapes and sizes. Some dinosaurs were the size of chickens. Others were enormous, towering above and weighing more than any other animal ever to live on land. The largest dinosaur may have grown as long as 150 feet (45 meters) and weighed as much as 85 short tons (77 metric tons). Such giants would have been more than 10 times as heavy as a fully-grown elephant. The only animal that grows to this size today are a few kinds of whales, and they only live in the water. (Some dinosaurs spent time in the water, but no dinosaur could swim very well.)

Dinosaur Eggs

Dinosaur eggs are smaller than those of modern birds (relative to the weight of the mother). In fact, the largest known dinosaur eggs are much smaller than those of the extinct elephant bird of Madagascar called Aepyornis (given rise to the legend of the roc, a gigantic mythical bird).

What They Looked Like

Dinosaurs were like many modern reptiles. Some had teeth and skin much like those of alligators living today, and were probably about as intelligent. Instead of chewing their food, some sauropods ground their food between stones in a portion of the digestive tract, much like the gizzard of a modern bird. Also, many dinosaurs walked on their hind legs.
No one knows for sure, though, what exactly dinosaurs looked like, what color they were, or what they sounded like.

Lifespan

How long could a dinosaur live? Animal lifespans relate in part to their body size and metabolism. Dinosaur lifespans probably varied in length from tens of years to hundreds of years. Their possible maximum age can be estimated from the maximum lifespans of modern reptiles, such as the 66-year lifespan of the common alligator. A Black Seychelles Tortoise, which was an adult when captured, lived a record 152 years in captivity (1766-1918) and had an accidental death. These estimates, based on lifespans of cold-blooded animals, would be too long if dinosaurs had metabolisms more similar to modern birds and mammals.

Blood

Dinosaurs have been traditionally classified as cold-blooded reptiles, but evidence suggests that they used a range of metabolic temperature-regulating techniques. They are all thought to have been egg-layers.

How To Order A Dinosaur

Dinosaurs belonged to a group of closely related animals called archosaurs (meaning ruling reptiles). However, not all archosaurs were dinosaurs. Other well-known members of this group included crocodilians (alligators and related animals) and pterosaurs (meaning winged reptiles). Ironically, birds are related not to bird-dinosaurs but to saurischian carnivores.

Dinosaurs are classified into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs (like Allosaurus and Tyrannosaurus), and Ornithischia, or bird-hipped dinosaurs (like Stegosaurus and Triceratops).

Extinction / Fossils

About 65 million years ago, the dinosaurs had died out, and the Mesozoic Era came to an end. It is not known why they died out. Most theories postulate that drastic changes in geography, climate, and sea level, possibly resulting from catastrophic volcanic eruptions or an asteroid or comet impact, were to blame.

For many years, they thought the dinosaurs left no descendants, but research indicated that birds descended from a particular kind of small, meat-eating dinosaur.

Scientists called paleontologists learn about dinosaurs by studying fossils, which include preserved bones, teeth, eggs, nests, tracks, skin imprints, and waste material. Dinosaur bones have been found on all of the continents, but the fossil record is most complete in North America and Asia.

New fossils were found in Madagascar in 1999. Scientists believe the fossils are about 230 million years old, which would make these dinosaur remains the oldest ever found.

Scientists also study living animals that resemble dinosaurs in some way.