Caught In Time 
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What are fossils and how are they formed?
Fossils are the remains of animals and plants, or the record of their presence, preserved in the rocks of the Earth. Fossilization is the process that turns a once living thing into a fossil. There are lots of fossils to be found, but only a tiny number of all the animals and plants that ever lived have been fossilized. Usually when an animal or plant dies it is completely destroyed. Another animal may eat it or it may decay. But sometimes the remains of an animal are buried before they can be destroyed, and if the conditions are just right, the remains get preserved as fossils. In some very rare cases, scientists have found fossils of bird feathers and dinosaur skin. But usually only the hard parts of an animal, like teeth, bones and shells, become fossilized.
Invertebrates are animals without backbones. Snails, clams, worms, and insects are all invertebrates. In fact, 95% of all living animals are invertebrates. This number was even greater in the past. Although they do not have bones, many invertebrates have a shell or a hard exoskeleton that can be fossilized. The exoskeleton is like a suit of armour, giving the animal support and protection. Modern invertebrates like lobsters, cockroaches, and spiders have exoskeletons.
How are fossils formed? The process is actually very simple. After an animal or plant dies, it may become buried in mud, ash, sediment. The soft body decays and leaves behind bones, exoskeleton, imprints, footprints, etc. Over millions of years, the remains or surrounding materials become hardened rock. Voila, a fossil is born.
Almost all fossils are preserved in sedimentary rocks , rocks that formed when layers of sediment such as clay, mud, silt, or sand became compressed and hardened over millions of years. These types of sediments are laid down in lakes, swamps and oceans. This is one of the reasons why most fossils are the remains of animals who lived in or near the water.
