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Fossils, the remains or traces of prehistoric life, are important inclusions in sediment and
sedimentary rocks. They are basic and important tools for interpreting the geologic past. The
scientific study of fossils is called palaeontology. It is an interdisciplinary science that blends
geology and biology in an attempt to understand all aspects of the succession of life over the vast
expanse of geologic time. Knowing the nature of the life-forms that existed at a particular time
helps researchers understand past environmental conditions. Further, fossils are important time
indicators and play a key role in correlating rocks of similar ages that are from different places.
Fossils are of many types. The remains of relatively recent organisms may not have been
altered at all. Such objects as teeth, bones, and shells are common examples. Far less common
are entire animals, flesh included, that have been preserved because of rather unusual
circumstances. Remains of prehistoric elephants called mammoths, which were frozen in the
Arctic tundra of Siberia and Alaska, are examples, as are the mummified remains of sloths
preserved in a dry cave in Nevada.
Given enough time, the remains of an organism are likely to be modified. Often fossils
become petrified (literally, “turned into stone”), meaning that the small internal cavities and
pores of the original structure are filled with precipitated mineral matter. In other instances
replacement may occur. Here the cell walls and other solid material are removed and replaced
with mineral matter. Sometimes the microscopic details of the replaced structure are faithfully
retained.
Molds and casts constitute another common class of fossils. When a shell or other
structure is buried in sediment and then dissolved by underground water, a mold is created. The
mold faithfully reflects only the shape and surface marking of the organism; it does not reveal
any information concerning its internal structure. If these hollow spaces are subsequently filled
with mineral matter, casts are created.
A type of fossilization called carbonization is particularly effective in preserving leaves
and delicate animal forms. It occurs when fine sediment encases the remains of an organism. As
time passes, pressure squeezes out the liquid and gaseous components and leaves behind a thin
residue of carbon. Black shales deposited as organic-rich mud in oxygen- poor environments
often contain abundant carbonized remains. If the film of carbon is lost from a fossil preserved in
fine-grained sediment, a replica of the surface, called an impression, may still show considerable
detail.
Delicate organisms, such as insects, are difficult to preserve and consequently are
relatively rare in the fossil record. Not only must they be protected from decay, but they must
also not be subjected to any pressure that would crush them. One way in which some insects
have been preserved is in amber, the hardened resin of ancient trees. The fly in FIGURE 12.4
was preserved after being trapped in a drop of sticky resin. Resin sealed off the insect from the
atmosphere and protected the remains from damage by water and air. As the resin hardened, a
protective, pressure-resistant case was formed.
In addition to the fossils already mentioned, there are numerous other types, many of
them only traces of prehistoric life. Examples of such trace fossils include:
1. Tracks—animal footprints made in soft sediment that was later lithified.
2. Burrows—tubes in sediment, wood, or rock made by an animal. These holes may later
become filled with mineral matter and preserved. Some of the oldest-known fossils are believed
to be worm burrows.
3. Coprolites—fossil dung and stomach contents that can provide useful information pertaining
to food habits of organisms.
4. Gastroliths—highly polished stomach stones that were used in the grinding of food by some
extinct reptiles.

Task 2. Build up a glossary to the most important terms used in the text.