HOW ARE FOSSILS FORMED?
When an organism dies, it generally decays
away completely, but sometimes leaves a record of
its existence. Those that leave a record do so as a
result of various physical and chemical processes.
Fossils can be divided into two main classes unaltered
remains and altered remains.
Unaltered Remains
Unaltered remains are fossils of organisms or
parts of organisms that are preserved with little or
no change.
- freezing - Ancient organisms are sometimes
frozen in ice or permafrost (frozen soil) at
high latitudes and /or high altitudes. If this
occurs, the organism may be preserved for a
long time. In Siberia, whole mammoths are
found in a near-perfect state of preservation.
Meat from one these extinct animals was even
eaten at a National Geographic Society meeting
in the 1930s.
- drying - In desert environments, animals and
plants have been found in a mummified condition.
The lack of moisture in these areas
lessens or eliminates decay.
- encasement - Organisms that are trapped in
tar or tree resin area often protected from air
and moisture, both of which are agents of
decay. Bones of many extinct animals have
been found preserved in this manner at the La
Brea Tar Pits in California. The forms of some
insects have lasted for millions of years in
resin that has hardened into amber.
Altered remains
Plants and animals may become fossilized
when they are changed by various means.
- carbonization - Organic remains can be
altered by heat and pressure. These agents
force the soft tissues of the organism to give
off hydrogen and oxygen, and leave a residue
of carbon that outlines the shape of the organism.
Delicate black outlines of leaves are commonly
preserved in this manner in the Middle
and Upper Devonian rocks of the Catskill
Mountains of New York, and in the
Carboniferous age coal deposits in
Pennsylvania.
- permineralization (also called petrification) As
water seeps through the hard parts of
organisms, minerals dissolved in the water
may be deposited in the spaces within bones
and wood. These minerals are added to the
bone or woody tissue of the fossil. Thus, petrified
wood and dinosaur bones are still composed
of wood and bone, respectively, but are
much heavier because of the added minerals.
- replacement - In this type of fossilization, the
original hard parts are completely dissolved
by water. The cavity or outline of the shell is
then filled with minerals such as chert or
quartz (silicon dioxide or silica, quartz [the
common mineral in many beach sands) is
coarsely crystalline silica, chert is micro-crystalline
silica), hematite (an iron oxide, one
type of iron ore), calcite (calcium carbonate, or
lime), or pyrite (iron sulphide, often called "fool's gold").
- molds and casts - An external mold is the
imprint of the exterior form of an organism's
shape or shell. An internal mold is composed
of material that fills a cavity (often the hollow
space under a shell or between two shells) . If
the shell is dissolved away, the internal mold
may remain. A cast is made either by filling
the impression of an external mold or by
removing the sediment or rock that composes
an internal mold. In both molds and casts, the
original soft tissue of the organism or its shell
are destroyed.
Trace fossils
Trace fossils are fossilized evidence of behavior.
They include tracks, trails, burrows, and borings
made by animals and the hollows made by
plant roots. Tracks are single impressions of feet
or foot- like structures. Trails or trackways are
lines of tracks or drag marks made by an animal
that moved over a soft surface, usually mud.
These evidences of locomotion are preserved if the
mud is hardened in to rock. Burrows are the tunnels
made by animals in mud or sand that is later
hardened. Fossil evidence of soft-bodied organisms, such as worms, is provided mostly by burrow
fossils. Complete burrow systems made by
ants and termites in soil are known in sandstones
in the western United States. Borings are tunnels
cut by organisms into hard substances such as
wood (termite borings), shells or coral heads, or
even in to hardened rock. Interestingly, a number
of marine (salt water) clams can burrow through
wood (these clams are called "ship worms"), living
coral heads, and even through such hard rocks
as granite. These clams live in the burrows, and
use them for protection.
Chemical fossils
Chemical fossils are carbon-based chemical
remains of organisms. Petroleum and natural gas
are produced from fatty chemicals derived from
dead organisms. Amino acids de rived from the
break-down of protein are known in ancient rocks.
The oldest evidence of life on earth is indicated by
chemical isotopes of carbon in 3.8 billion year-old
rocks found in northern Canada.
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