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Fossil Evidence Fossil Formation Other Fossil Types Permineralization Cast Fossil Phase 1 – Death Having died, the ammonite slowly sinks to the sea floor. Scavengers feed on the fleshy body of the creature, and after only several weeks all that remains is the shell. Phase 2 - Deposition Several months after death the shell gradually becomes covered with silt and sand. These layers continue to build, providing a shield around the shell and protecting it from damage. Time continues to pass and more and more layers are deposited. After a few hundred years the shell is several feet beneath the surface. Phase 3 - Permineralization Gradually the chemicals in the shell undergo a series of changes. As the shell slowly decays, water infused with minerals passes through it, replacing the chemicals in the shell with rock-like minerals (Calcite, Iron or Silica). This process is scientifically known as 'permineralization'. Over millions of years the original shell is completely replaced by the minerals and what remains is a rock-like copy of the original shell. The fossil has the same shape as the original object, but is actually rock. This process also results in loss of original colour. http://www.discoveringfossils.co.uk/Whatisafossil.htm Deb Whittington & Mark Davidson, 2007 Dating Fossils Relative Dating: Some fossils (called cast fossils) are not the remnants of organisms but rocks that have formed around the impressions of the organism in the sediment. A dead organism that is captured in sediment completely decays, but leaves an empty mold in the sedimentary rock. Water filled with minerals fills the mold. The minerals crystallize, forming a cast of the ancient organism . Younger strata (layers of rocks) are deposited on top of older ones. Layers which are deeper are older. Absolute Dating: Gives scientists the approximate date an ancient life form lived. Radiometric dating, the measurement of certain radioactive isotopes in fossils or rocks is the method most often used. Because radioisotopes break down at a constant rate, the ratio of the parent isotope to its break down products found in rock or fossils can be used to determine their ages. http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch04/images/le04_05.gif Trace Fossil Another type of fossil (called a trace fossil), consists of a footprint, animal burrow or other impression left in the sediment by the activities of animals. Trace fossils give paleontologists information about the activities of ancient life forms. Iguanodon track fossil: http://storeforknowledge.com/images/IMG_1100.JPG Organic Preservation http://www.ambermine.com/images/spiders/5sm.jpg If an organism happens to die in a place where bacteria and fungi cannot decompose the corpse, the entire body, including soft parts may be preserved. Examples include: An insect stuck in a drop of resin from a tree 30 million years ago. Mammoths frozen in ice. Saber-toothed cats etc. preserved in La Brea Tar Pits. Prehistoric humans preserved in acid bogs. http://evolution.berkeley.edu/evosite/lines/images/strat_column.gif