A single organism can be preserved as a variety of fossil types depending on the different types of sediments in which it is buried. After burial, the sediment gradually turns to rock and the fossil will change in various ways depending on the physical and chemical make up of the surrounding sediment. This type of preservation is more common in recent fossils but it has also been found in some fossils of the Cambrian and Ordovician age, up to 500 million years old. Most fossils undergo some alteration, though in many cases it may only be possible to see these changes under a microscope. The changes can be both chemical and physical.
Carbonisation means the organism is 'altered' by being reduced to carbon.
This occurs in oxygen free environments such as deep, stagnant water. One of the best examples of carbon is coal. Coal consists of plants which have been reduced to almost pure carbon. Skeletons of fossil fish can be found with their bones completely covered in carbonaceous matter. This happens when the fossil is buried in wet sediment, such as an ocean bed and the sea water seeps through the surrounding rock and alters the fossil.