Fossils are the remains of animals or plants preserved in the rocks. These plants and animals remained buried beneath the Earth due to many upheavals of the Earth, millions of years ago. There are many ways in which plants and animals that lived hundreds of millions of years ago have been preserved. A few fossils are hard parts of animals or plants, such as dinosaur’s bones or shark’s teeth. These remained sunk to the bottom of the seas and were trapped in the sediments, which now form layers of rocks. Many fossils are moulds or casts. The remains of living things slowly dissolved, as they lay buried in sediments. Their exact shape was preserved in the rocks. Minerals or sediments took the place of their remains to form a cast. Some ancient trees were ‘petrified’, as the decaying logs were gradually replaced by minerals dissolved in water. The science related to the study of fossils is known as ‘palaeontology’. Fossil studies reveal that life on Earth originated some 3,500 million years ago. These studies also tell us about the various changes that took place during the evolution. It is chiefly through the study of fossils that we know about the animal life as it existed millions of years ago. For example, it is through the fossils, taken from certain rocks, that we have come to know about the existence of dinosaurs which do not exist today. Fossil studies also tell us about the changes in climates and geography. Fossils may also provide important clues to the geologists in locating coal and oil deposits as well as ore-bearing rocks. Fossils also help in determining the boundaries between time periods. Interesting details can be fossilised too. Footprints, once made on mud which later dried out, have also been recovered. Ammonites are the fossil remains of extinct molluscs. When the mollusc died, it was buried on the seabed. The animal dissolved away to form a hollow fossil mould. When the mould was filled by sediments, a cast was formed.