Five of the most common types of fossils include mold and cast, trace, body, compression and petrifaction. Each type of fossil forms differently and is preserved by more than one method.
Mold-and-cast fossils form when the original shells or bones dissolve and leave a depression that is later filled in with other sediments. Squid, snails and clams form some of the most common mold-and-cast fossils because their soft shells dissolve quickly and easily.
Trace fossils record the movements of the dinosaurs. There are many different types of trace fossils, such as footprints, tail furrow marks, nests, burrows and tooth marks. Trace fossils form by a wide variety of means. Footprints remain the most common trace fossils and have been found at more than 1,500 fossil sites.
Body fossils form when an organism's whole body is preserved unaltered. A mosquito perfectly preserved in amber is one example of a body, or true form, fossil.
Compression fossils form when the body is compressed by high pressure. Compression leaves a dark imprint of the organism. Fossilized ferns and leaves are most often found as compression fossils.
Petrifaction fossils form when minerals replace organic matter, filling original pore spaces. The original organism is thereby transformed into stone. Petrified wood is one example of a petrifaction fossil.