the death of a living thing, a fossil comes into being
through the preservation of hard body components an animal
leaves behind, such as bones, teeth, shell or nails. Fossils are generally thought of as parts of a plant or animal
in a petrified state. However, fossils do not come into
being only through petrifaction. Some have survived down
to the present day without any impairment or decay of
their structures, such as mammoths frozen inside ice or
insects and small species of reptiles and invertebrates
preserved in amber.
When a living thing dies, the soft tissues comprising
its muscles and organs soon begin to decay under the effects
of bacteria and environmental conditions. (In very rare
occasions, such as in sub-zero cold or dry heat of deserts,
decay does not take place.)
The more resistant parts of
the organism, usually mineral-containing parts such as
the bones or teeth, can survive for longer periods of
time, allowing them to undergo various physical and chemical
processes. And these processes allow fossilization to
take place. Therefore, most of those parts that become
fossils are vertebrates' bones and teeth, shells of brachiopods
and molluscs, the external skeletons of certain crustacean
and trilobites, the general outlines of coral-like organisms
and sponges, and the woody parts of plants.
organism's surroundings and environmental conditions also
play a major role in fossil formation. One can predict
whether or not fossilization will take place on the basis
of an organism's surroundings. For example, in terms of
fossil formation, underwater environments are more advantageous
than dry land ones.
The most common, widespread process of fossilization
is known as permineralization or mineralization. During
this process the organism is replaced by minerals in the
liquid in the soil in which the body is immersed. During
the process of mineralization, the following stages take
1) First, it is essential that by being covered in soil,
mud or sand, the body of the dead organism should immediately
be protected from contact with the air. Over the following
months, new layers of sediments are laid down over the
buried remains. These layers act as a thickening shield,
protecting the animal's body from external agents and
physical wear. Many more layers form, one atop the previous
ones; and within a few hundred years the animal's remains
lie several meters beneath the surface of the land or
sea or lake bottom.
As more time passes, structures such
as the animal's bones, shell, scales or cartilage slowly
begin their chemical breakdown. Underground waters begin
to infiltrate these structures, and the dissolved minerals
contained in these watersminerals such as calcite,
pyrite, silica and iron, which are far more resistant
to erosion and chemical breakdowngradually replace
the chemicals in the tissues. Thus over the course of
millions of years, these minerals give rise to an exact
stone copy by replacing the tissues in the organism's
body. Finally, the fossil comes to possess the exact shape
and external form as the original organism, although now
converted into stone.
1. Reef: Calcareous sea animals that form
2. Radiolarian: a type of microscopic plankton with skeletons
3. Two-shelled mollusk, shelled with calcium carbonate.
In fossils, such hard organs may be preserved unchanged.
4. Graptolite: Fossils with organic skeletons that generally
left traces on black shale. These creatures lived in groups.
5. Shark teeth: Bones and teeth consist largely of phosphorus,
for which reason they are more resistant, compared with
many soft-tissue organs.
6. Trace fossils: Fossils that are formed by traces seen
7. Ammonite: A specimen whose shell had been replaced
by iron pyrites and fossilized.
8. A petrified tree: In time, the tree's wooden cells
are replaced by silica and fossilized.
9. Amber: Small organisms are preserved in resin. 10.
Carbonized leaves: Plants transformed into carbon fibers.
Various situations may be encountered during
If the skeleton is completely filled with liquid solution
and breakdown takes place at a later stage, then the internal
structure gets fossilized.
2. If the skeleton is totally replaced by a different
mineral from the original, a complete copy of the shell
3. If an exact template or "mould" of the skeleton
forms due to pressure, then the remains of the skeleton's
external surface may remain.
In plant fossils, on the other hand, it is carbonization
caused by bacteria that applies. During the carbonization
process, oxygen and nitrogen are replaced by carbon and
hydrogen. Carbonization takes place by breaking down the
tissue molecules by bacteria through changes in pressure
and temperature or various chemical processes, causing chemical
changes in the structure of the protein and cellulose in
such a way that only carbon fibers remain. Other such organic
materials as carbon dioxide, methane, hydrogen sulphate
and water vapour disappear. This process gave rise to the
natural coal beds that formed from the swamps that existed
during the Carboniferous Period, 354 to 290 million years
Fossils sometimes form when organisms are submerged in
waters rich in calcium and get coated by minerals such
as travertine. As the organism decays, it leaves behind
traces of itself in the mineral bed.
The complete fossilization of a living thing's soft
parts, even including fur, feathers or skin, is encountered
only rarely. Remains of some soft-tissued life forms of
the Precambrian Period (dating back 4.6 billion to 543
million years ago) have been very well preserved. There
are also soft-tissue remains that permit internal structures
from the Cambrian Period (543 to 490 million years ago),
to be examined in addition to hard-tissue remains of living
things right down to the present day. Fossil remains of
animal fur and hairs preserved in amber, and fossil remains
dating back 150 million years are other examples that
permit detailed investigation. Mammoths compacted in Siberian
ice packs or insects and reptiles trapped in amber in
Baltic forests have also become fossilized together with
their soft-tissue structures.
Fossils can vary considerably in terms of size, according to the
type of organism preserved. Very different fossils have
been obtained from the fossilized microorganisms to giant
fossils from animals that lived together as groups or
herds, in a communal lifestyle. One of the most striking
examples of such giant fossils is the sponge reef in Italy.
Resembling a giant hill, this reef is composed of 145-million-year-old
limestone sponges that developed at the bottom of the
ancient Sea of Tethys, and later rose up as the result
of the movement of tectonic plates. It contains specimens
of the life forms living in sponge reefs during the Triassic
Period. The Burgess Shale in Canada and Chengjiang in
China are among the largest fossil beds containing thousands
of fossils from the Cambrian Period. The amber beds in
the Dominican Republic and along the western shores of
the Baltic Sea are other major sources of fossil insects.
The Green River fossil beds in the U.S. state of Wyoming,
the White River fossil beds in Central America, the Eichstatt
beds in Germany and the Hajoula fossil beds in Lebanon
are other examples that can be cited. This article is based on the works of Harun
150 years ago, the British naturalist Charles Darwin
proposed a theory based on various observations
made during his travels, but which could not be
supported by any subsequent scientific findings.
In essence, his theory of evolution consisted of
various scenarios, ...More
WHAT IS A FOSSIL?
the broadest definition, a fossil is the remains
of a living thing that lived long ago and that has
survived down to the present day by being preserved
under natural conditions. The fossils that come
down to us are parts of an organism, or remains
left behind when the living thing concerned was
still alive ...More