FOSSILS - MODES OF PRESERVATION

To become preserved as a fossil, some of the normal processes of decay must be permanently arrested. This usually involves isolating the organism from the things that cause decay, and then filling any voids in the hard tissue with additional minerals. Because of this, the vast majority of fossils are found in fresh water or marine sediments where oxygen deprived silt or clay has buried the organism soon after death.
In the case of mummification it is a temporary preservation, as, once exposed to air, the organism will again begin to decay. Although soft tissue usually disappears it can be found where organisms are preserved in ice, or trapped in amber. Silicified wood can produce amazing colour effects.
 
Let's look in more detail at some of these preservation methods.
 
Silicification - very common in the preservation of petrified wood. It can usually be found in sands and silts, and weathering volcanic ash usually supplies the silica, which is incorporated into partially decayed wood. The presence of other minerals produces the spectacular colours found in fossiled wood.
 
Phosphatisation - bones and teeth which would normally dissolve on the sea bed or leach from sediment are much more likely to be preserved if  large amounts of phosphates are present. Phosphatic deposits, such as those in Morocco, are a rich source of well preserved fossils.
 
Limestone - often provides a tomb for the dead organism, forming a calcerous nodule encasing the creature. Limestone produces some stunning fossils showing very fine detail.
 
Pyritisation - iron pyrites replace the normal structure of the organism. This is often found in fossil ammonites.Because this is an ongoing process in moist conditions, specimens need to be stored carefully. Pyritised ammonites are very beautiful and collectable.
 
Trapped in Amber - amber is the fossilised resin of a plant or tree, and sometimes preserves not only the external structure of the creature trapped, but also the internal parts. Hence the idea for Jurassic Park, and the use of DNA from the blood found inside mosquitos.
Insects, spiders, scorpions, and even frogs & lizards can be trapped in amber.
 
Tar & Sand - many creatures have been preserved in this way, often by falling into a tar pit, or a carnivore following a hapless beast already trapped. Normally such preservation alone only protects for thousands of years rather than millions of years. Some amazing specimens have been discovered in the USA.
 
Freezing - many mammoth remains have been preserved in perma frost, including soft tissue, hair, and skin. Care has to be taken when these creatures are recovered as decay resumes once they have been thawed.
 
Mummification - it is only really a pause in disintegration so is really not true fossilisation in the pure sense, but it does give us the opportunity to examine extinct species. Usually bones and some soft tissue are preserved. Dry, arid conditions are necessary for the process.
 
Luckily, enough fossils are preserved, to enable scientists to build up a picture of life on Earth, from the early primitive organisms, to the complex creatures alive today.