Dinosaurs and Fossils

Giants from the deep past

Long before there were people, enormous animals walked the Earth. They were the dinosaurs. The name means "terrible lizard", though dinosaurs were not really lizards at all — they were a special group of reptiles all their own.

Dinosaurs ruled the land for an astonishing amount of time: more than 150 million years. To picture how long that is, imagine that all of human history is the length of your fingertip — the age of the dinosaurs would stretch the length of a football pitch. They were not a quick chapter in Earth's story; they were a huge one.

How do we know any of this, when no person ever saw a living dinosaur? The answer is fossils.

What is a fossil?

A fossil is the preserved remains or traces of a living thing that died long, long ago. Fossils are nature's way of keeping a record.

Some fossils are body parts that have turned to stone, like bones, teeth, claws and shells. Others are trace fossils — not the animal itself, but marks it left behind. A dinosaur footprint pressed into mud and hardened into rock is a fossil. So is a fossilised egg, or even fossilised dung (scientists call this coprolite, and it reveals what the animal ate!).

Every fossil is a clue. By piecing clues together, scientists build up a picture of animals no one ever photographed.

How a fossil forms

Most living things rot away and leave nothing behind. Becoming a fossil is rare and slow. Here is how it usually happens:

1. An animal dies, often near water like a river, lake or sea.
2. Its soft parts rot, but the hard parts — bones and teeth — are left.
3. Mud and sand quickly bury the remains before they are destroyed.
4. Over thousands of years, more layers pile on top and press down, slowly turning the mud into rock.
5. Water seeps through the buried bone, and minerals fill it in, replacing the bone bit by bit until it becomes stone.

After millions of years, movements of the Earth or wind and rain wear away the rock above — and the fossil is revealed. The whole process can take millions of years, which is why fossils are so precious.

Fossil hunters at work

Scientists who study fossils are called palaeontologists. When they find fossils sticking out of rock, they do not just yank them out. They work slowly and gently, brushing away dust and using tiny tools and even toothbrushes so they do not crack anything.

They carefully record exactly where and in which layer of rock each fossil was found, because deeper layers are older. By comparing fossils from many places, palaeontologists work out which animals lived, when they lived, and how they were related.

What did dinosaurs eat?

Dinosaurs came in incredible variety, and their teeth tell us their diet.

• Herbivores ate plants. Triceratops had a tough beak and grinding teeth for chewing leaves, and the long-necked Diplodocus reached high into the treetops like a giant living crane.
• Carnivores ate meat. Tyrannosaurus rex had banana-sized, knife-sharp teeth and powerful jaws. Velociraptor was small but fast, with a curved killing claw on each foot.

Some dinosaurs were huge, taller than a house. Others were no bigger than a chicken. They were not all monsters — they were a whole world of different animals.

Where did the dinosaurs go?

About 66 million years ago, something dramatic happened. Most scientists believe a giant asteroid — a huge space rock several kilometres wide — smashed into the Earth. The crash threw up so much dust that it blocked out sunlight for a long time. Plants struggled to grow, the planet cooled, and food chains collapsed.

Most of the big dinosaurs could not survive and died out. This is called an extinction.

But here is the surprise: the dinosaurs did not vanish completely. A group of small, feathered, meat-eating dinosaurs survived and slowly changed over millions of years into birds. So when you watch a sparrow or a pigeon, you are looking at a living dinosaur! If you have explored Animal Groups: Mammals, Birds, Reptiles and More, you will see exactly where birds fit in.

Try it yourself: make your own fossil

You can copy how a trace fossil forms using kitchen materials.

1. Mix flour, salt and a little water to make a stiff dough (about 2 cups of flour to 1 cup of salt and 1 cup of water).
2. Flatten a lump of dough into a thick pancake on a plate.
3. Press something hard into it — a clean shell, a leaf, a toy dinosaur foot, or your own thumb. Press firmly, then lift it away.
4. Leave the dough to dry hard for a day or two.

You now have an imprint — just like a fossil footprint preserved in rock! Swap fossils with a friend and try to guess what each other pressed into the dough. Real palaeontologists do the same thing: they read prints to work out what made them.

For another deep look at how Earth changes over vast time, explore Food Chains and Ecosystems to see how a broken food chain can topple even the mightiest animals.