Fossils are the preserved remains or traces of animals and occasionally plants from many millions of years ago. Preserved remains of animals that are older than approximately 10,000 years are usually considered to be fossils, though a more strict interpretation would probably require them to be a few million years old. Fossils are found in layers of sedimentary rock and can vary in size from the very very small (such as fossilised bacteria) to the very large such as the skeletons of large animals including dinosaurs. Usually fossils that are found show only a portion of the individual animal that was fossilised. Normally it is only the hard portions of an animals body that are preserved in this way, for example their bones and teeth. Soft tissues tend not to survive and so it is very rare to find any fossil record of them. It's also possible to find fossilised traces of the animal for example footprints or even fossilised faeces - yes you can even get fossilised dinosaur poo!
So how do fossils actually form? There are a number of different processes and different types of fossilisation possible. As an example consider the fossilisation of a shellfish. When the animal dies it will slowly sink to the bottom of the sea and gradually become covered with sand or silt. The animal's body decomposes or is eaten by scavengers. As time passes the shell gets covered by more and more layers of silt gradually becoming deep and deeper under the sea floor. Over a period of many tens of thousands of years the shell gradually undergoes a series of changes. The chemicals in the shell gradually become replaced by minerals (frequently things like calcite, Iron or silica) coming from water that is passing through the shell and layers of silt. This type of fossilisation is called permineralisation. This carries on over many millions of years until eventually it results in the replacement of all of the shell with the minerals. This leaves the fossil with the shape of the original animals shell but now actually entirely composed of rock. Now we have a fossil buried deep in the rock under the seabed.
So how does it eventually come to be found? Over a period of many millions of years geological changes and movements in the Earth's crust result in what was originally the seabed being pushed up until it is no longer the seabed, but new dry land. More geological changes and the further action of the wind and rain (through a process called erosion) eventually lead to the rock bearing the fossil being exposed leaving the fossil ready to be found. The very last stage in its journey back to the light may be with the help of a geologists hammer!
As previously mentioned there are a number of ways an organism can become fossilised. We have already discussed permineralisation where the original soft tissues are replaced with minerals such as silica calcite or iron pyrite. Other ways organisms can become fossilised include carbonisation, unaltered preservation, recrystallisation and authigenic preservation. In unaltered preservation the organism is preserved as it originally was because it is protected for other effects and decomposition in some way. The classic example of this is an insect that becomes trapped in tree sap which over millions of years turns to Amber, preserving the insect in its entirety. Carbonisation (sometimes also called coalification) results in everything but carbon being removed from the organism. Authigenic preservation refers to fossils of molds or casts of organisms that themselves have entirely decomposed leaving just the cast preserved in the rock. These are fossil equivalents of the worm casts that you often find on a sandy beach. Finally recrystallisation describes the process where crystals form inside the structure of the organism. These crystals eventually replace it entirely - leaving just a crystallised copy.
The different types of mineral that replacing the original organisms structure in permineralisation often result in the final fossil having a distinctive appearance. For example in the case of an ammonite when fossilised these can show considerably different appearances depending on the mineral in question. The ammonites you usually find look like the rock they come from but it is also quite common to find pyritised ammonites which show a lovely golden metallic sheen.
Hopefully this is given you an idea of how fossils come to be and what they are. Have a look through the rest of the website or some of our recommended links to find out much more!
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