Fossil fuels are formed from the living organisms which were buried inside the Earth for millions of years.
Some of the important fossils which have been studied are those of organisms X, Y and Z, X were marine arthropods which were common between 400 to 600 million years ago. Y were the invertebrate animals (molluscs) with a flat, coiled, spiral shell which lived in the sea about 180 million years ago. Z are the extinct carnivorous or herbivorous reptiles which appeared on the earth about 250 million years ago and became extinct about 65 million years ago. What are X, Y and Z?