Fossils are the remains of an animal or plant from the past geological eras. There are many different kinds of fossilized materials, like bones, teeth, shells, wood. Then there are trace fossils, like footprints, eggs, nests, animal droppings, leaf impressions, etc.

Fossils are at least 10,000 years old. It is fascinating to see that despite time some things did not perish, but were quite well preserved. Scientists list several ways in which fossils can be formed. Some of them are quite common, while others are rarely seen. Here are the ones that have been discovered:

Unaltered preservation:

1. Amber

Small living creatures like insects, spiders, small lizards as well as plants got trapped in sticky tree sap which hardened in time and turned into amber.

2. Freezing

It happens when an animal becomes suddenly frozen, and stays frozen until it is found. There have been only a few cases discovered in the arctic areas of the earth. For example in Alaska and Siberia archaeologists found a frozen mammoths with their flesh, skin and fur intact. It is suspected the animals were from the latest ice age that ended about 10,000 years ago.

3. Drying or desiccation

Diseased animal’s body dries out intact. There have been several cases of animals and even humans preserved or mummified that way.

4. Tar

Animals bodies fallen into crude oil areas deteriorated, but their bones, teeth or shells reminded well preserved as the oil turned into asphalt. There is a famous Hancock Park down town Los Angeles, CA filled with tar pits, where scientists found many fossils. Because of that the park became known as ‘The La Brea Tar Pits’.

Preservation in the form of rock:

1. Permineralization or petrification

An animal or plant is preserved as a rock. It happens when a buried animal body comes in contact with mineral rich waters and the minerals slowly fill the tissues and cells of the body. The key to a good quality fossil is fast burial of the organism. If the tissue becomes decayed, permineralization will not occur.

2. Carbonization or coalification

Also known as compression fossils. Plant or animal body decays and the only mineral that remains is carbon. The carbon leaves an imprint in rock. Many plant fossils were formed that way.

3. Replacement

An animal body is decomposed, but the hard parts (bones, teeth) are replaced with other minerals, like iron, calcite, silica, etc.

4. Re-crystalization

Bone minerals are not replaced, but rather change their form and become crystals.

5. Authigenic preservation

Organism dies and deteriorates completely, but what’s left is an organism shaped rock. There are two forms of such fossils:

Casts – where the hole in the rock after animal’s death is filled with hard minerals, or

Molds – where remains of an animal leave an impression in the rock.

While millions of fossils have been found, fossilization is considered very rare. Most animals and plants decay or are being eaten quickly after death. A very small percentage is buried in a way, that allows a fossil to form.