Fossils are the
preserved remains or traces of animals, plants, and
other organisms from the remote past. The totality of
fossils, both discovered and undiscovered, and their
placement in fossil-containing rock formations and
sedimentary layers is known as the fossil record. The
study of fossils across geological time, how they were
formed, and the evolutionary relationships between taxa
are some of the most important functions of the science
of paleontology. Such a preserved specimen is called a
"fossil" if it is older than some minimum age, most
often the arbitrary date of 10,000 years ago.
Hence, fossils range in age from the youngest at the
start of the Holocene Epoch to the oldest from the
Archaean Eon several billion years old. The observations
that certain fossils were associated with certain rock
strata led early geologists to recognize a geological
timescale in the 19th century. The development of
radiometric dating techniques in the early 20th century
allowed geologists to determine the numerical or
"absolute" age of the various strata and thereby the
included fossils.
Fossils vary in size from
microscopic, such as single bacterial cells only one
micrometer in diameter, to gigantic, such as dinosaurs
and trees many meters long and weighing many tons. A
fossil normally preserves only a portion of the deceased
organism, usually that portion that was partially
mineralized during life, such as the bones and teeth of
vertebrates, or the chitinous or calcareous exoskeletons
of invertebrates. Preservation of soft tissues is rare
in the fossil record. Fossils may also consist of the
marks left behind by the organism while it was alive,
such as the footprint or feces of a reptile. These types
of fossil are called trace fossils, as opposed to body
fossils. Finally, past life leaves some markers that
cannot be seen but can be detected in the form of
biochemical signals; these are known as chemofossils, or
biomarkers.
Ever since recorded history began,
and probably before, people have noticed and gathered
fossils, including pieces of rock and minerals that have
replaced the remains of biologic organisms, or preserved
their external form. Fossils themselves, and the
totality of their occurrence within the sequence of
Earth's rock strata, is referred to as the fossil
record.
The fossil record was one of the early
sources of data relevant to the study of evolution and
continues to be relevant to the history of life on
Earth. Paleontologists examine the fossil record in
order to understand the process of evolution and the way
particular species have evolved.
Fossilization is
an exceptionally rare occurrence, because most
components of formerly-living things tend to decompose
relatively quickly following death. In order for an
organism to be fossilized, the remains normally need to
be covered by sediment as soon as possible. However
there are exceptions to this, such as if an organism
becomes frozen, desiccated, or comes to rest in an
oxygen-free environment. There are several different
types of fossils and fossilization processes.
Permineralization
occurs after burial, as the empty spaces within an
organism (spaces filled with liquid or gas during life)
become filled with mineral-rich groundwater and the
minerals precipitate from the groundwater, thus
occupying the empty spaces. This process can occur in
very small spaces, such as within the cell wall of a
plant cell. Small scale permineralization can produce
very detailed fossils. For permineralization to occur,
the organism must become covered by sediment soon after
death or soon after the initial decaying process. The
degree to which the remains are decayed when covered
determines the later details of the fossil. Some fossils
consist only of skeletal remains or teeth; other fossils
contain traces of skin, feathers or even soft tissues.
Casts and Molds
- In some cases the original remains of the organism
have been completely dissolved or otherwise destroyed.
When all that is left is an organism-shaped hole in the
rock, it is called an external mold. If this hole is
later filled with other minerals, it is a cast. An
internal mold is formed when sediments or minerals fill
the internal cavity of an organism, such as the inside
of a snail.
Replacement and Recrystallization - Replacement
occurs when the shell, bone or other tissue is replaced
with another mineral. In some cases mineral replacement
of the original shell occurs so gradually and at such
fine scales that microstructural features are preserved
despite the total loss of original material.
Trace Fossils are
the remains of trackways, burrows, bioerosion, eggs and
eggshells, nests, droppings and other types of
impressions. Fossilized droppings, called coprolites,
can give insight into the feeding behavior of animals
and can therefore be of great importance.
Other
fossil types are MicroFossils, Bioimmuration,
Compression Fossils, Resin Fossils such as those
preserved in amber, and Living Fossils.
Information
obtained from the Wikipedia web site. Wikipedia the free
encyclopedia.
