- A fossil can be anything from a well- preserved body of a plant or animal, to a vague indentation left by that organism in the dirt eons ago. The majority of fossils fall somewhere in between. It isn't uncommon for bones, teeth or exoskeletons to be preserved either partially or completely. Soft tissue, hair and skin are much rarer, however.
- Permineralization is the most common way in which fossils are created. An animal first dies in the water and sinks to the bottom. Its body is eaten away by other animals and microorganisms, leaving only the skeleton, which takes much longer to degrade. Before the bones can break down, the animal is covered by sediment. It could be buried by a mudslide or undersea earthquake, for example, or by silt carried downstream by a river.
- Over millions of years, layers upon layers of sediment pile on top of the remains. The pressure becomes so great that the sediment turns into rock. Eventually, groundwater starts to seep through that rock, carrying mineral deposits. If everything happens just right, those minerals will slowly erode and replace the skeleton with a fossil in the same shape, but made out of mineral deposits and not bone.
- Some fossils are not mineral deposits in the shape of an organism, but actually the organism itself. If an animal dies in an extremely cold environment, it can sometimes be frozen and preserved in ice. Scientists have found woolly mammoths that have been almost perfectly frozen since the last ice age. Similarly, insects can become trapped in tree sap. If that sap is buried under enough sediment, it hardens to amber, perfectly preserving the creature inside. One of the most novel forms of fossil preservation is the La Brea tar pits. Tar seeped up from ancient oil deposits, forming a sticky, treacherous layer on the surface. Any animal unwary enough to venture in would become trapped, slowly sinking into the tar. Scientists have been able to pull millions of bones from the sticky pits.
