We have all heard of dinosaurs, and we all know they lived a long time ago. But when people talk of dinosaurs they also often use words like Jurassic – for example in the film Jurassic Park. So what do words like this mean, and what have they got to do with dinosaurs? To understand this, we have to know something about the history of the Earth.

The Earth is old, very, very old. About 4.6 billion years old to be precise. At first its surface was molten, then covered with volcanoes. There was no oxygen in the air and no oceans at all. But then things began to change. The Earth got cooler and the water, which had come out of volcanoes as gas, began to form droplets of rain that, over hundreds of millions of years, gave us our oceans.

The first living things

These oceans were not such harsh places to live as the air, and their temperatures were much more steady, so this is where the first living things grew. The earliest were very simple – just microscopic simple celled creatures like bacteria and tiny little plants called algae. They began about 3 billion years ago. Over billions more years, these creatures changed and became more complex. We call this natural change evolution.

The first fossils

By about 600 million years ago, evolution had made it possible for living things to develop hard shells. These shells did not rot away quickly like fleshy parts of bodies, so there was time for them to be changed into stone – to be made into fossils. That is why we suddenly find fossils at this time.

Making the time line

Evolution produces new and different animals as well as causing some to stop living – to become extinct. That is very different from the way that the Earth’s rocks form. They have always formed the same way. So there is not much difference between a sandstone rock that was formed a billion years ago to one formed ten million years ago. This means that if we look for a kind of Earth clock, it’s no good looking to the rocks because they don’t change. But living things do.

About 200 years ago, people began to realise this and so they started to try to work out the Earth’s history and make a time line. They looked hard at the fossils and gradually they found that it was easy to spot some kinds of fossils because they appeared everywhere. They also noticed that each thing lived for a limited range of time. So they started to use fossils to measure time.

Now with fossils used this way, you don’t get an actual date, you get a geological ‘period’. What you notice through the history of the Earth is that there are long times – periods – when things change gradually, followed by sudden great catastrophes when many creatures are killed off and the land opened up for new ones to take their place, almost like wiping the slate clean. So people studying the rocks – geologists – divided the Earth’s history up into periods of calm separated by catastrophes.

Naming the periods

Scientists also found that some places were better for finding fossils than others. So they went to places that were best, and there they started to work. But then they wondered what to call these periods in Earth history that they had found. So they often named the periods after good places for finding the fossils. That is how words like Jurassic came to be. Jurassic is named after the Jura mountains on the France/ Switzerland border where the fossils of that period were first studied.

So now you know how all the funny names came to be. Since then geologists have found ways of knowing how many millions of years the rocks are too, but the period names have stuck.

When the dinosaurs lived

The dinosaurs lived in three of these periods. The oldest period when they lived is called the Triassic, the middle one is called the Jurassic and the most recent one is called the Cretaceous (pronounced kret-ay-shee-us).

Triassic dinosaurs

This period started just after a catastrophe which wiped out two thirds of all living things. But it is an ill wind that blows nobody any good, as they say, and with less competition about, the reptiles evolved quickly into all kinds of shapes and sizes.

Dinosaurs were not the first reptiles to live on Earth. Many species of lizard had already evolved and competed with the first dinosaurs and stopped them from dominating the world.

One of the largest reptile-like creatures still alive was Placerias, a very large plant-eater, up to 4m long and weighing up to 2 tonnes. It had a beak, rather than a toothed mouth and short tusks to defend itself. In some ways it was like an armoured hippo and it spent much of its time in the water, in part, perhaps, to keep away from crocodile-like hunters, such as Postosuchus. Postosuchus was the largest hunting dinosaur of the day, being about 6 metres long and 2 metres tall.

But it was not alone. Eoraptor (which means ‘dawn plunderer’) was also one of the world’s earliest dinosaurs. It is possible that the ancestors of all dinosaurs looked like this. This early dinosaur had five fingers on each hand. Three fingers had claws.

Other early hunting dinosaurs included Celophysis and Utahraptor. Meanwhile ichthyosaurs ruled the oceans and the pterosaurs began to fly in the skies.

At this time much of the land was a tropical semi-desert, much like Arizona or Kenya today. It is not an inviting land unless you are adapted to survive in droughts and eat poor, scrubby plants. Like most places with seasonal rain, most animals range around a reliable source of water, such as a waterhole or river bed. During the dry season, venturing to the waterholes and muddy river beds, or trying to eat some of the few remaining plants is much more dangerous. Plant-eaters have to spend all day eating just to stay alive and that means they are easy for the hunters to ambush. Of course, it all depends on size. If you are a plant-eater and you are simply too big to be eaten by a hunter, then you have a great advantage. Thus, natural selection may have favoured the larger plant-eating dinosaurs.

At the very end of the Triassic period, the single great continent that had existed through early dinosaur times began to split apart. It was at this time that many of the competitors of dinosaurs went extinct, leaving the way for dinosaurs to evolve quickly and fill all of the spaces left by their now dead competitors.