About three billion years ago the Earth was very different from today. The continents were not yet formed and the only living things were a few simple plants. Later, tiny boneless animals began to live in the sea. After millions of years these first animals developed into more complicated creatures with shells and skeletons. Eventually certain fish developed organs that meant they could leave the water and breathe on land. They were called amphibians.
Some amphibians developed into huge reptiles called dinosaurs. Just 200 million years ago some dinosaurs were the size of a truck. They had four legs, but some of them only used their two back legs for walking or running - rather like today's kangaroos.
150 million years ago on earth there were plenty of
plants and trees because the climate was so good. Perhaps this is why the
plant-eating dinosaurs grew so big, almost as long as three buses put together.
Their massive size meant that as
well as having a brain in their head, they also needed brain cells in their tail to work their hindquarters. They were gentle creatures, but so huge that few animals ever dared attack them. All meat-eating dinosaurs, except the Allosaurus, were small by comparison and only about two meters long.
Dinosaurs are now extinct. No one is quite sure exactly why these amazing creatures disappeared, but about 65 million years ago, the Earth suffered many earthquakes, and continents began to separate. The weather became much colder too. It would seem that the dinosaurs found it too difficult to adapt to these changes because after about 10 million years, they had all died out.
While dinosaurs were roaming the Earth for millions of
years, what kind of animals were inhabiting the oceans? Between 200 and 60
million years ago, sea creatures became more and more complicated. Among the
most ancient was the 
Dinichthys,
a
huge shark with a bony head. Then there was the Mixosaurus, a reptile that had adapted to living in water, and the Ichthyosaur which gave birth to its young rather than laying eggs. Later on, about 60 million years ago, a speedy swimmer called the Basilosaurus appeared in the seas, an ancestor of the whale.
Ninety-nine per cent of all species that ever existed
are now extinct. 
The
only records that we have about these extinct species lie in the petrified bones
we call fossils. Often these fossils are covered by layers and layers of earth.
However, some of the rocks beneath our feet came from the bottoms of ancient
oceans; and, if we look carefully, we might discover the remains of prehistoric
creatures.