Fossils are the remains or imprints left behind from plants and animals. They can be whole parts of an animal, or just pieces of their remains. They can also include movement remnants of animals, such as footprints or imprints.
In order for a fossil to form, an animal or plant must be trapped into a material. In many instances, fossils are found in sediment. Layers of sediment will envelop the remains; this can continue for thousands of years. The actual remains that are found aren't the animal or plant at all. Instead, the remains begin to decay within the sediment. Minerals then replace the space that was once the remains. The mineral hardens and makes an impression of the animal or plant. What is left is what is known as a fossil.
Conditions for Fossilization
Fossilization is a rare occurrence. Certain conditions need to have been met in order to increase the likelihood of fossilization. First, the organism should have had hard parts that have a high mineral content. Second, the organism should have been buried rapidly, decreasing chances of being eaten by predators or destroyed by aerobic bacteria. Third, the organism should have died in a biologically inert area where sedimentation rapidly occurs. Many fossils are found in ancient deltas or where an ancient sea once was because the remains are covered quickly and are safe from damage.
Methods of Fossilizaiton
Premineralization occurs when hard water infuses the pores and cavities of a deceased organism. When the remains of the organism is exposed to high pressure, the dissolved mineral solidify, creating a fossil. Molds and casts form when shells or other organisms are buried in sediment. These organisms leave an impression or mold in the sediment. If the interior of the shell fills with sediment and then the shell dissolves, it forms a cast or replica of the organism. An impression fossil forms from an imprint on the silt and clay. Plant fossils form by impression. Freezing is an extremely rare form of fossilization where the entire organism is preserved in a block of ice. Coprolites and gastroliths are the indigestible remains of food. Finally, trace fossils indicate the movement of an organism through footprints and burrows.
Five Different Types of Fossils
Mold and Cast Fossils
When a plant or animal buried under layers of sediment decays, the impression of its body left in the rock forming around it is known as a mold fossil. Sometimes the space left behind will then fill with other sediment, forming a cast fossil. Most dinosaur bones fall into the mold and cast category.
Petrification occurs when groundwater permeates the remains of an organism and leaves minerals behind. In replacement fossils the body dissolves and minerals are left in its place. In permineralization, the water enters the cells of the organism and deposits minerals in the spaces inside them. Petrified wood is a permineralized fossil.
Whole Body Fossils
Whole body fossils occur when an entire organism, including soft tissues, is preserved. Examples include insects entombed in tree sap, which hardens to become amber, and mammoths encased in ice.
Footprints and Trackways
Footprints made by prehistoric animals walking through the mud sometimes harden and become fossils. Several footprints occurring together and made by the same animal are referred to as a trackway. Trackways may also include impressions made by other parts of the animal, such as the tail or snout.
Coprolites are fossilized feces. Their location offers clues about where animals lived. Close examination of coprolites can also yield information about what the animals that produced them ate.