The formation of fossils



Fossils are formed through a variety of different methods as listed below.
  1. Fossilization
  2. Carbonization
  3. Petrifaction
  4. Replacement
  5. Recrystallization
  6. Organic Trap



The Fossilization Method

external image it120002.jpg The first step to the fossilization process is when the remains of dead organisms fall into the bottom of water bodies. Then, the sediments in the water body forms a protective covering over the remains, which slows down the process of its decay. After thousands of years, the sediments covering the remains harden into rock. Eventually, the remains decay, leaving an empty space inside the sedimentary rock. Minerals then filter down into the empty space and hardens into rock, forming a shape alike the organism.




The Carbonization Method
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Plants are most commonly fossilized by carbonization. When plants are exposed to extreme heat, the mobile oils in their organic matter are dissolved and the remaining matter is reduced to solid carbon. Plants can be preserved in this manner, because they have inner structures of organic walls, which reveals the framework of their original cells.




The Petrifaction Method

tree.jpg In the petrifaction process, silica-rich fluids enter the cells of a plant and crystalize the minerals inside, causing the plant to appear as if it has turned into rock. Petrifaction may also occur in animals which contain minerals such as calcite, silica and iron in their bones. Famous examples of petrifaction can be discovered in the forests of Western United States.





The Replacement Method

Replacement occurs when organisms are buried into mud, where their minerals are subsituted by sulfide or phosphate minerals. During this process, the soft tissue of the organism may be replaced, which preserves the rare details of its anatomy. Although the replacement method is rare, fossils formed via this method are significant to the paleontologists' work on comparing the anatomical details of the prehistoric organisms and the living organisms.



The Recrystallization Method

external image Recrystallization.jpgAnimal shells are mostly composed of mineral aragonite, a form of calcium carbonate that has broken down over millions of years to form the more stable mineral calcite. The recrystallization method of preservation destroys the microscopic details of the animal shell, however, keeps the overall shape of theshell, forming a fossil.




The Organic Trap Method

Amber, also known as natural asphalt or peat are the fossilized remains of tree resin. When resin first flows down from a tree, it is extremely thick and adhesive. Therefore, when it runs down a tree, it may trap insects such as ants and spiders, and occasionally larger animals like lizards.
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  • Why is the fossil record often referred to as "imperfect"?
  • Where are fossils found?
  • Describe some of the important fossil finds that significantly contributed to our understanding of the evolution of living organisms on earth.
  • Explain why each was important.

NB: you can check the philiosophy, instructions and assessment criteria for this exercise.

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