modes of fossil preservation

• Mode of preservation 1 is replacement:

  Where the original material is dissolved atom by atom and substituted by another. 
• Pyritization is a method of replacement when the original rock is replaced by pyrite. It only happens in anoxic environments and forms due to the action of sulphur bacteria. 
• Silicification is when percolating groundwaters are rich in silica, if wood and bone are present silica fills the pores increasing density. 
• Mode 2 of preservation is carbonisation. It is a common method in the preservation of plants. After burial the specimen is compacted and the hydrogen, nitrogen and oxygen are squeezed out leaving only carbon. 
• The third mode if preservation is entombment in amber. Amber is a sticky, transparent substance produced by conifers. It leads to near perfect preservation of organisms. It mostly preserves insects but can also contain small vertebrates like frogs and lizards.  
• The forth mode of preservation is entombment in peat bogs and tar pits. They slow down the rate of decay, they have cold anaerobic conditions so preserve with great detail. Pollen grains and seed spores can be found in peat bogs. Humans have occasionally been found in peat bogs. 
• Tar pits are large deposits of crude oil on the surface, in los Angeles it has been pooling for 40,000 years. Tar pits trap animals which further attracts scavengers/predators. A wide range of fossils have been found in tar pits, 59 species of mammal, 135 species of bird and 450+ species of plant.