ORIGINAL SOFT PARTS
The fossilization of unaltered soft parts takes place only under exceptional favorable conditions.

FREEZING: Wooly mammoths and rhinoceroses have been found in permanently frozen ground in Alaska and Siberia. These animals were frozen so rapidly and are so perfectly preserved that the food remains unchewed in their mouths.

DRYING: Fossil mummies of ground sloths and camels, virtually in an unaltered state, have been found in caves in southern United States. Even skin and hair retain their original color.

AMBER: Insects were trapped in the sticky gum of certain conifers and then engulfed in more resin. The resin hardened with time and leaving the insect undamaged and in a perfect state of preservation.

PEAT BOGS: Relatively recent fossils of animals preserving flesh, skin and hair have been found in peat bogs where the tannic acid in the water has prevented their decay.

ORIGINAL HARD PARTS
These are often preserved with little or no alteration. The hard bones and teeth of vertebrates are common examples. The shells of certain invertebrates frequently remain intact.

ALTERED SOFT PARTS
On rare occasions the soft parts of plants and animals may also be altered over time and preserved.

CARBONIZATION: As leaves or soft parts of animals slowly decomposed under water the oxygen, hydrogen, and nitrogen were driven off. The carbon molecules, being very stable, remained behind as a thin carbon film.

TRACE FOSSILS: Only a trace or an impression of the organism provides evidence of the plant or animal responsible for it.

MOLDS: A mold is the impression of an organism in the surrounding material. A shell may be buried in sediment and the shell itself may be dissolved away. The impression of the outside of the shell is known as an external mold. If the inside of the shell had filled with sediment than an internal mold or stein-kern may be preserved.

CASTS: All the original buried plant or animal may dissolve away so that only a cavity remains. Later dissolved minerals may fill the cavity forming a natural cast of the original.

TRACKS: Footprints made by animals as they walked over soft sediments. The sediments then hardened to stone preserving the prints.

BURROWS: Tubes or holes in the ground made by an animal for shelter or in search of food. These tubes may later become filled with different sediments and preserved.

BORINGS: Holes made by animals on other organisms for the sake of food, attachment, or possibly shelter. Such holes frequently occur in shells and wood.

COPROLITES: Fossil excrement of anything from worms to large dinosaurs. The study of coprolites provides valuable information pertaining to diet and anatomical structure.

GASTROLITHS: “Stomach stones” are highly polished rounded stones believed to have been an aid in grinding the stomach contents of extinct reptiles.

ALTERED HARD PARTS
The original hard structures of many organisms may undergo considerable alteration with the passing of time.

PERMINERALIZATION: As the hard parts lie buried in the sediment, ground water carrying dissolved minerals infiltrates the microscopic pores in bone, shells, or wood, depositing their mineral content. The original structure is preserved.

REPLACEMENT: The original hard parts are dissolved by chemical action and other minerals are substituted. The size and shape of the fossil are not disturbed but the structure is imperfectly preserved. Two common replacement minerals are silica and pyrite.

PETRIFICATION: Both permineralization and replacement are commonly referred to as petrification which means 'turned to stone'.

TAR PITS: Vast numbers of fossils have been found in the asphalt deposits of Rancho La Brea, California. Prehistoric animals which gathered to drink at certain water pools were trapped in underlying tar seeps. Their accumulated bones have been removed in an excellent state of preservation.

PSEUDOFOSSILS
Many objects of inorganic origin closely resemble forms of organic action.

DENDRITES: Thin branching patterns which superficially resemble a fern or moss. They are produced by certain minerals, crystallizing similar to frost on a window.

CONCRETIONS: Hardened masses of mineral substances which are commonly mistaken for fossils. Under unusual conditions of weathering, these may be sculptured in shapes resembling plants or animals.