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Body Fossils
The fossilized remains of actual animals or animal parts are called body fossils or true-form fossils. Body fossils can occur when animals are trapped in amber, tar pits and quicksand. The most common body parts preserved as fossils are bones, teeth and shells. Eggs and embryos; skin; and soft tissues like muscles, tendons and organs are very rarely preserved as fossils because they're more likely to rot before fossilization has a chance to occur.
Mold Fossils
Mold fossils are formed when an animal or plant has dissolved but its imprint is left behind in the surrounding sand or mud. A common way for mold fossils to form is when shells get buried in mud, the mud hardens and turns to stone around the shell over time, and acidic water dissolves the original shell so that all is left behind is the shell's fossilized impression.
Cast Fossils
When the original organism in a mold fossil dissolves, sometimes the hole left behind is filled in with other minerals and the result is a cast fossil - literally, a fossilized copy of an organism.
Trace Fossils
Also called ichnofossils, trace fossils are the impressions left behind by an animal in its environment. Trace fossils can provide valuable clues about movement, diet and behavior. For example, sets of footprints can be preserved as the mud or sand around them turns to rock. Sets of footprints provide information about an animal's speed, stride and bone structure, and whether it walked on two or four legs. Other types of trace fossils include toothmarks on bones, gizzard rocks, nests and feces.