KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.

Specific conditions are needed for fossilization. Only a tiny percentage of living things became fossils.

Fossils can form in several ways. Petrification occurs when minerals in groundwater replace the original organic materials that were covered by layers of sediment with new materials. The substitution of minerals for organic material often results in the formation of a nearly perfect mineral replica of the original organism.

Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

Freezing: Preserved remains form when an entire organism becomes encased in material such as ice.

Tar Seeps form when petroleum oozes to Earth’s surface Tar Seeps form when petroleum oozes to Earth’s surface. Tar seeps are commonly covered by water. Animals who come to drink water can become trapped in the sticky tar. The remains of the trapped animals are covered by the tar and preserved.

Mummification: Mummified remains are often found in very dry places, because most bacteria, which cause decay cannot survive these places.

Types of Fossils Imprints: Carbonized imprints of leaves, stems, flowers, and fish made in soft mud or clay have been found in sedimentary rock. When original organic material partially decays, it leaves behind a carbon-rich film. An imprint displays the surface features of the organisms.

Molds & Casts: Shells often leave empty cavities called molds within hardened sediment. When a shell is buried, its remains eventually decay and leave an empty space. When sand or mud fills a mold and hardens, a natural cast forms. A cast is a replica of the original organism.

Coprolites are fossilized dung or waste materials from ancient animals Coprolites are fossilized dung or waste materials from ancient animals. They can be cut into thin sections and observed through a microscope. Such observations can reveal the feeding habits of ancient animals.

Gastroliths are stones in a dinosaurs stomach to help grind their food Gastroliths are stones in a dinosaurs stomach to help grind their food. Gastrolith fossils can often be recognized by their smooth, polished surfaces and by their close proximity to dinosaur remains.

Trace fossils record the activity of an organism Trace fossils record the activity of an organism. Tracks, footprints, borings, and burrows can still provide information about prehistoric life.

Index fossils are another tool to determine the age of rock layers. Index fossils are used to establish the age of rock layers because they are distinct, abundant, and widespread and existed for only a short span of geologic time.

Radiometric dating provides an accurate way to estimate the age of fossils. Relative dating estimates the time during which an organism lived. It compares the placement of fossils in layers of rock. Scientists infer the order in which species existed.