What are fossils? The word "fossil" comes from the Latin word fóssilis, which means "dug out". Fossils are tangible remains or signs of ancient life forms that have been preserved in sedimentary rocks. Since fossils are evidence of life forms that existed millions of years ago on Earth, they can be considered to be the written history of life on our planet. Like other forms of writing, it is necessary to learn to read them to decipher their message. This is what paleontologists do.
The fossil record
The collection of fossils constitutes the fossil record, which is the basis of the geological time scale. The fossil record is unfortunately incomplete and strongly biased, i.e., there is a greater presence of organisms with shells or bones and teeth than those without. This is because these anatomical parts are more resistant to decomposition and therefore fossilize more frequently.
Consequently, organisms lacking these parts have a poor fossil record. This does not mean that fossils of other anatomical parts do not exist, but they are rare in comparison. Although it is an infinitesimal portion of all life on Earth that has been preserved as fossils, it is still surprising - given the millions of years and all the conditions necessary for their formation - that they exist.
Fossilization is the process that turns the rest of an organism into a fossil. An almost indispensable requirement for fossilization is that the organism, a part of it or a trace of it, is quickly buried, thus avoiding its immediate chemical and biological decomposition.
It is also necessary that it has maintained sufficient equilibrium with the surrounding rocks to ensure that its identity is not altered. This last condition is most often found in sedimentary rocks. In igneous or metamorphic rocks we do not find fossils because the forces that create these rocks (heat and pressure) erase any potential remains of them.
One type of fossilization is that in which substances or compounds of the organism are being displaced by new mineral substances that the water transports, leaving the same form of the structure. This process of mineral penetration is called permineralization.
Thus, for example, the cell wall of plants is formed by cellulose. As the plant dies and is buried, the cellulose is displaced by some mineral compound (for example, calcium carbonate or calcium phosphate), until the structure is completely occupied by it. A similar process occurs with bones.
Although most often only a kind of outline or delineation of the most outstanding parts of the organism remains, there are exceptional cases in which the minerals that have been introduced reproduce with total fidelity to both the internal and external structure of the organism.
Other fossils do not undergo permineralization but are formed when an organism leaves its footprint in soft soil. In this process, the organism that dies decomposes but its "mold" remains. Over time, the soil hardens, leaving its imprint fixed in the rock.
A process often related to the previous one occurs when another rock fills this mold left by the organism and then the fossil remains as if imprinted on it. Some fossils are impressions, which can be described as "flattened molds".
Some carbonizations are carbon residues left on the surface of some impressions after other compounds have been lost by the escape of liquids and gases. Other fossils are indications of the activity of an organism, such as grooves and footprints, and footprints.