Fossil
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Fossils are the mineralized or otherwise preserved remains — or other traces (such as footprints) — of animals, plants, and other organisms. The totality of fossils and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record. The study of fossils is called paleontology.
The word fossil is derived from the Latin word fossus, which means "having been dug up".
Fossilization is actually a rare occurrence because natural materials tend to decompose. In order for an organism to be fossilized, the remains normally need to be covered by sediment as soon as possible. However there are exceptions to this, such as if an organism becomes petrified or comes to rest in an anoxic environment such as at the bottom of a lake. There are several different types of fossils and fossilization processes.
Fossils usually consist of traces of the remains of the organism itself. However, fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or feces of a dinosaur or reptile. These types of fossil are called trace fossils, as opposed to body fossils. Finally, past life leaves some evidences that cannot be seen but can be detected in the form of chemical signals; these are known as chemical fossils (for lack of a better term).
The oldest known structured fossils are most likely stromatolites. Now understood to be formed by the entrapment of minerals by mucous-like sheets of cyanobacteria, the oldest of these formations dates from 3.5 billion years ago. Even older deposits (3.8 billion years old) of heavy carbon that are indicative of even earlier life are currently proposed as the remains of the earliest known life on Earth.
Permineralization
Permineralization consists of organic remains being to some degree impregnated by minerals derived from the surrounding sediments or waters. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decaying process. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. Once covered with sediment, these layers slowly become compacted and cemented into rock, and the organic remains are slowly replaced with hard minerals.
Replacement and compression fossils
In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, we call this a mould fossil or typolite. If this hole is later filled with other minerals, it is called a cast fossil and is considered a replacement fossil since the original materials have been completely replaced by new, unrelated ones. In some cases replacement occurs so gradually and at such fine scales that no "hole" in the rock can ever be discerned and microstructural features are preserved despite the total loss of original material.
Compression fossils such as those of fossil ferns are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state.
To sum up, fossilization processes proceed differently for different kinds of tissues and under different kinds of conditions.
Trace fossils
Trace fossils are the remains of trackways, burrows, footprints, eggs and egg-shells, nests, and droppings (among other types of impressions). Fossilized droppings, called coprolites, can give insight into the feeding behavior of animals and can therefore be of great importance.
Resin fossils
Smaller animals, such as insects, spiders and small lizards, can be trapped in resin (amber), which is secreted from trees. These fossils can be found in sandstones or mudstones or washed up on beaches like those around the Baltic Sea.
Pseudofossils
Pseudofossils are visual patterns in rocks that are produced by naturally occurring geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as dendrites, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore (round shapes in iron ore) and moss agates, which look like moss or plant leaves. Concretions, round or oval-shaped nodules found in some sedimentary strata, were once thought to be dinosaur eggs, and are often mistaken for fossils as well.
Living fossils
Living fossil is a term used for any living species which closely resembles a species known from fossils, i.e., as if the fossil had "come to life". This can be a species known only from fossils until living representatives were discovered, such as the coelacanth and the ginkgo tree, or a single living species with no close relatives, such as the horseshoe crab, that is the sole survivor of a once large and widespread group in the fossil record.
See also
- Compression fossil
- Collecting fossils
- Fossils and the geological timescale
- Fossil fuels
- Prehistoric life
- Transitional fossil
External links
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