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TABLE 21.1

Summary of the types of fossil preservation
I. Preservation of all or part of the organism
A. Unaltered
1. Soft parts
2. Hard parts
B. Altered
1. Mineralization
2. Replacement
3. Carbon films
II. Preservation of the organism’s shape
A. Cast
B. Mold
III. Signs of activity
A. Tracks
B. Trails
FIGURE 21.3 This fly was stuck in and covered over with
C. Burrows
plant pitch. When the pitch fossilized to form amber, the fly was
D. Borings preserved as well. The entire fly is preserved in the amber.
E. Coprolites
are among the most abundant marine microorganisms. Silica is
the most resistant common substance found in fossils. Chitin
Occasionally, entire organisms are preserved, and in rare is the tough material that makes up the exoskeletons of insects,
cases, even the unaltered remains of an organism’s soft parts are crabs, and lobsters.
found. In order for this to occur, the organism must be quickly Calcium carbonate shell material may be dissolved by
protected from scavengers and decomposers following death. groundwater in certain buried environments, leaving an empty
There are several conditions that allow this to occur. The best mold in the rock. Sediment or groundwater deposits may fill the
examples of this uncommon method of fossilization include pro- mold and make a cast of the organism (Figure 21.5).
tection by freezing, entombing in tree resin, or embalming in tar. Figure 21.6 is a photograph of part of the Petrified Forest
Mammoths, for example, have been found frozen and preserved National Park in Arizona. There are two processes involved in
by natural refrigeration in the ices of Alaska and Siberia. The the making of petrified fossils, and they are not restricted to just
body of a human from the Bronze Age has also been discovered wood. The processes involve (1) mineralization, which is the
frozen in ice. Insects and spiders, complete with delicate append- filling of pore spaces with deposits of calcium carbonate, silica,
ages, have been found preserved in amber, which is fossilized tree or pyrite, (2) replacement, which is the dissolving of the original
resin (Figure 21.3). The bones of saber-toothed tigers and other material and depositing of new material an ion at a time, or
vertebrates were found embalmed in the tars of the La Brea tar pit (3) both mineralization and replacement. Petrified wood is
in Los Angeles, California. In each case—ice, resin, and tar—the formed by both processes over a long period of time. As it
remains were protected from scavengers, insects, and bacteria.
Fossils are more commonly formed from remains of hard
parts such as shells, bones, and teeth of animals or the pollen
and spores of plants (Figure 21.4). Such parts are composed of
calcium carbonate, calcium phosphate, silica, chitin, or other
tough organic coverings. The fish fossil pictured in Figure 21.2
is from Wyoming’s Green River Formation. This freshwater fish
died in its natural environment and was soon covered by fine-
grained sediment. The sediment preserved the complete artic-
ulated skeleton, along with some carbon traces of soft tissue.
Plant fossils are often found as carbon traces, sometimes look-
ing like a photograph of a leaf on a slab of shale or limestone.
Shells and other hard parts of invertebrates are sometimes
preserved without alteration or with changes in the chemical
composition. Some protozoans and most corals, mollusks, and
other shelled invertebrates have calcium carbonate shells, but
some do have calcium phosphate shells. Silica makes up the FIGURE 21.4 Skeletons of recent organisms are often preserved
hard shells of some protozoans, sponges, and diatoms. Diatoms as fossils in sediments. These are the bones of a mammoth.

524 CHAPTER 21 Geologic Time 21-4