How Fossils Form     A fossil is any preserved remains or traces of once-living organisms. Fossils include body fossils, which are fossilized remains of body parts, and trace fossils, which include burrows, tracks, feces, or other traces left by an organism. Collections of fossils that are found together are known as fossil assemblages.     The process of a once-living organism or its traces becoming a fossil is called fossilization. It most often occurs when the remains or traces are buried by sediments and then gradually change to rock. Fossilization is very rare.     Only a tiny... Show more

How Fossils Form    
A fossil is any preserved remains or traces of once-living organisms. Fossils include body fossils, which are fossilized remains of body parts, and trace fossils, which include burrows, tracks, feces, or other traces left by an organism. Collections of fossils that are found together are known as fossil assemblages.    
The process of a once-living organism or its traces becoming a fossil is called fossilization. It most often occurs when the remains or traces are buried by sediments and then gradually change to rock. Fossilization is very rare.    
Only a tiny percentage of the organisms that have ever lived have become fossils. Usually, it is only the hard parts of organisms that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine organisms are also common. Organisms that lack hard parts, from bacteria to jellyfish, rarely become fossils.    
Quick burial is almost always necessary for an organism to be fossilized because most decay and fragmentation occur when remains are exposed at the surface. Marine animals that die near a river delta may be buried rapidly by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains. In general, quick burial is rare on land, so fossils of land organisms are less common than  fossils of marine organisms. However, land organisms are sometimes buried by mudslides, volcanic ash, or sand in a sandstorm. They may also be buried by mud in lakes, swamps, or bogs. Rarely, they may be buried by tar in tar pits or the ice of glaciers.    

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