Fossils are the remains of prehistoric organisms or other evidence those organisms left behind. There are a number of different kinds of fossils, including molds and casts, petrification fossils, whole body fossils, footprints and trackways and coprolites, among others. Fossils that consist of all or part of a plant or animal or an impression of the plant or animal are known as body fossils. A second type of fossil, known as trace fossils, show indications of an organism's activity.
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.