What are fossils?
Fossils are the petrified remains of the living beings from the past or of their vital traces. They are studied by the science of the Paleontology.
Fossils are commonly found in sediments or sedimentary rocks (limestone, sandstone, mudstone, shale), typically as a result of the burial of the remains of a living being within a layer of sediments. Heavy metamorphism and the extreme temperatures of the magmas (> 700ºC) are likely to destroy any remain or trace of a living being, and so fossils are not found in heavily metamorphosed rocks (schist, gneiss) or igneous rocks (granite, diorite). Only sedimentary rocks that have undergone a gentle metamorphism, such as slates, are likely to contain fossils.
The totality of fossils and their placement in the rocks containing them constitute the fossil record. This placement may be as important as the fossil itself, because it can give a lot of information about the way and the type of ecosystem in which the fossilised organism lived. For instance, if a fossil is found within a conglomerate, which is a rock formed from the sediments deposited by a river, we'll know that it is one of a land organism. Or if an unknown fossil is found in the same rock where we also find fossils of seashells, it will probably be the fossil of a marine living being.
The fossil record is gappy and uneven. It is gappy because fossilisation is a rare event that happens sporadically and irregularly, depending heavily on the environmental conditions in the moment when a living being dies. If a land animal dies in a place that undergoes a landslide a short time after its death, most likely it will be well preserved inside the sediments. But if it dies in a place where strong winds, or heavy river flow or marine currents drag its corpse for a long time before it is left in a quiet place to be buried as sediments are deposited, chances are that the corpse will be destroyed by those currents, and the scavengers, detritivores and decomposers during the long time that it took before it was buried. And it is uneven because not all species have the same chances to leave any kind of fossil remain. As a start, hard structures (bones, teeth, shells) are commonly necessary, because soft tissues decay rapidly when the scavengers and decomposers (chiefly invertebrates, bacteria and fungi) start to predate on them. Organisms such as jellyfish or worms have really low possibilities to leave body fossils; traces such as imprints or burrows in the sediments are amongst their very few chances to leave a sign of their existence.
Types of fossils
- Body fossils are the petrified remains of living beings from the past, and are produced by the substitution of the biomolecules that pertained to the deceased organism by mineral substances that precipitate from the groundwater that circulates through the sediments in which the corpse is buried. This requires a rapid burial of the organism following its death; otherwise, it will be soon destroyed, as explained above. Other times the remains can be destroyed once covered by sediments, but leaving an organism-shaped hole in the rock: this is called an external mould. If this hole is later filled with other minerals, it is called a cast. An internal mould is formed when sediments or minerals fill the internal cavity of an organism, such as the shell of a bivalve or the skull of a vertebrate.
- Trace fossils are the physical remains of the vital activity of living beings from the past, and are produced by their movement (trackways left by trilobites, footprints from hominans), their reproduction (eggs of dinosaurs), their nutrition (coprolites, gastrolites, holes drilled in the shells of the prey), and other living habits (burrows, root cavities, stromatolites...). The oldest physical fossils on Earth fall into this category. They are stromatolites, and the oldest might be the 3.5 by old found in Warrawoona, Australia. Stromatolites are layered rocks generated by communities of microorganisms, usually dominated by cyanobacteria, which produced the precipitation of mineral substances dissolved in the seawater, generating layers of sediments that stacked one on top of another creating a stratified biogenic rock.
- Biochemical fossils are the biochemical remains of the vital activity of living beings from the past. The best examples are carbon-rich rocks (such as the stromatolites) or minerals (such as graphite granules) that have more 12C than usual and less 13C than normal. As the CO2 molecules with 12C weigh 44 u instead of the 45 u of a molecule of CO2 with 13C, the former move faster (they are called "light CO2") than the latter, and have more chances to randomly reach the places of a living being capable of capturing them. This way a plant captures light CO2 in a greater proportion than it is found in the atmosphere, and so the fossilised remains of a plant will contain 12C in a greater proportion than it is found in the atmosphere. The oldest fossils on Earth might be 3.8 by old graphite granules found in Isua, Greenland, that contain a greater than normal proportion of 12C.