Absolute dating fossils
Index fossils are fossils that only occurred during a specific time frame.Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites make the best index fossils.
Another principal used in relative dating that rock layers are deposited sequentially.Replacement and Recrystallization - Replacement occurs when the shell, bone or other tissue is replaced with another mineral.A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite.There are two main types of fossils; body and trace. The absence of heat or compression which might destroy the fossil Fossils are most often preserved within sediments that were deposited in water, such as wetlands, river basins, or the ocean.Body fossils include the remains of organisms that were once living (bones, shells, teeth, eggs, etc), while trace fossils are the signs that organisms were once present (footprints, tracks, burrow, coprolites). Permineralization or Petrification - After an organism is buried, minerals carried by water such as silica, calcite or pyrite replace the organic material in the fossil.
By measuring the ratio of the amount of the original isotope to the amount of the isotopes that it breaks down into an age can be determined.