The Formation of Fossils

A wasp of 54 to 28 million years old, petrified in amber.

Following 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.

A 20- to 15-million-year-old midge preserved in amber.

The most common, widespread process of fossilization is known aspermineralization 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 place:

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 waters—minerals such as calcite, pyrite, silica and iron, which are far more resistant to erosion and chemical breakdown—gradually 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.

This dragonfly trapped in mud may one day become fossilized and will reach the future generations as evidence that evolution has never happened.

Various situations may be encountered during mineralization:

1. 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 emerges.

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 ago.

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.

1. Reef: Calcareous sea animals that form the reef. 2. Radiolarian: a type of microscopic plankton with skeletons of silica. 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 on sediments. 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.

This fossil fish, 50 million years old, is evidence that fish have always remained as fish.

At times, fragile organisms may also get fossilized under extraordinary conditions. Pictured is a starfish from the Jurassic period (206 to 144 million years ago). There is no difference whatsoever between this fossil and the starfish of our day.

The skin and scales of this fish from the Triassic Period (250 to 203 million years ago) are fossilized with all their details intact. This sample reveals that fish had the same scale structure 250 million years ago.

An 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 GREATEST SPONGE REEF ON EARTH This sponge reef of 145 million years old is a trace of the Tethys Ocean floor. The sponges of our day are no different from those that make up the hill. These sponges make it clear that they have not undergone any evolution.

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.