Fossils open a window in time. The petrified remains of long dead plants and animals, they tell us about the organisms that lived on Earth millions of years ago. The oldest fossils are over 3 billion years old. They are "microfossils", visible only under the microscope. The most famous fossils are probably those of the giant dinosaurs who dominated the earth for 135 million years, until they were suddenly wiped out 65 million years ago. Private collectors of fossils have established a new trade in these objects.